Nuova ricerca

Barbara REGGIANI

Professore Associato presso: Dipartimento di Scienze e Metodi dell'Ingegneria


Home | Curriculum(pdf) | Didattica |


Pubblicazioni

2021 - Experimental investigation on a novel approach for laser surface hardening modelling [Articolo su rivista]
Orazi, L.; Rota, A.; Reggiani, B.
abstract

Laser surface hardening is rapidly growing in industrial applications due to its high flexibility, accuracy, cleanness and energy efficiency. However, the experimental process optimization can be a tricky task due to the number of involved parameters, thus suggesting for alternative approaches such as reliable numerical simulations. Conventional laser hardening models compute the achieved hardness on the basis of microstructure predictions due to carbon diffusion during the process heat thermal cycle. Nevertheless, this approach is very time consuming and not allows to simulate real complex products during laser treatments. To overcome this limitation, a novel simplified approach for laser surface hardening modelling is presented and discussed. The basic assumption consists in neglecting the austenite homogenization due to the short time and the insufficient carbon diffusion during the heating phase of the process. In the present work, this assumption is experimentally verified through nano-hardness measurements on C45 carbon steel samples both laser and oven treated by means of atomic force microscopy (AFM) technique.


2020 - A novel algorithm for a continuous and fast 3D projection of points on triangulated surfaces for CAM/CAD/CAE applications [Articolo su rivista]
Orazi, L.; Reggiani, B.
abstract

In the present work, a novel algorithm for the continuous projection of point triangles belonging to a triangle mesh is presented. The algorithm uses the normals defined at the vertices of the triangle to perform the projection. The direction of projection is not chosen a priori but depends on the point to be projected: the proposed algorithm, named Fast Continuous Projection method (FCP) lets the projection direction vary continuously on the mesh. Moreover, the direction is coherent with the original surface that is approximated with the triangle mesh. An optimized version of the algorithm is also presented: this uses pre-evaluated matrices to reduce the calculation time. This algorithm can be effectively used when a large set of points has to be projected on a coarse mesh as, for example, to generate the scanning vectors for laser engraving/milling.


2020 - Comparison of experimental methods to evaluate seam welds quality in extruded profiles [Articolo su rivista]
REGGIANI, Barbara; DONATI, Lorenzo
abstract

The wedge and bulge expansion tests were compared in the assessment of the seam welds strength in a tubular profile extruded at two ram speeds. In the wedge test, the expansion was determined by moving a conical punch into the tube until the specimen fracture. In the bulge test, a hydrostatic tensile stress state was applied by expanding the specimen with an internal rubber plug. The two methodologies were compared in terms of load and hoop strain at fracture and by detecting the fracture morphology and location. Then, the effect of a number of design parameters was investigated in order to evaluate the robustness of the standard testing conditions. For both tests, ductile fractures appeared in the seam welds location, but the bulge test was more robust and conservative with respect to the wedge test, showing less scattered data. Thus, the performances of a second die for the tube profile, designed to optimize the seam welds quality, have been successfully assessed by the bulge test and results compared to those achieved by a novel numerical quality index, coming to a final good matching.


2020 - Efficiency of conformal cooling channels inserts for extrusion dies [Relazione in Atti di Convegno]
Pelaccia, R.; Negozio, M.; Donati, L.; Reggiani, B.; Tomesani, L.
abstract

During aluminum extrusion process, critical temperatures can be reached both in the profile and in the tooling set due to the high pre-heating temperatures and to the work spent in overcoming friction at the workpiece/tool interfaces and in deforming the billet that is converted into heat. By considering that especially the rising of temperature in the profile can represents a critical aspect of the process strongly limiting the maximum achievable extrusion speed, liquid nitrogen die cooling is becoming a consolidated industrial practice in order to increase the process productivity. If cooling channels are traditionally manufactured by subtractive technologies on a third plate (the backer), a more flexible and efficient solution is offered by the selective laser melting (SLM) technology. SLM allows the design of free-form channels with the opportunity to efficiently remove the heat as near as possible to the bearing zones where the highest temperatures are reached. In the present work, the design approach and the manufacturing parameters of an SLM printed H13 die insert for the extrusion of a 10 mm round bar are presented. In addition, the extrusion process is simulated by means of the COMSOL code accounting for the nitrogen cooling effect. Experimental extrusion trials have been furthermore performed with ZM21 magnesium and 6063 aluminum alloys to assess the potentiality of the conformal cooling channel design and to validate the developed numerical model. As main result, a good experimental numerical matching has been achieved with peak errors of 7.5% and 14% in terms of temperature and extrusion load respectively.


2020 - FEM validation of front end and back end defects evolution in AA6063 and AA6082 aluminum alloys profiles [Relazione in Atti di Convegno]
Negozio, M.; Pelaccia, R.; Donati, L.; Reggiani, B.; Tomesani, L.; Pinter, T.
abstract

The reduction of scraps related to back end defects (i.e. billet skin contamination) and front end defects (i.e. charge welds) is gaining nowadays an increasing industrial interest in order to obtain greater process efficiency. Today, extrusion industrial practice faces the issue by means of technician's experience, empirical rules or, in most critical profiles, through time consuming and expensive experimental analyses. On the other side, FEM simulation of extrusion dies is becoming a common support tool for the design of new critical dies. Stating this scenario, the possibility to include the prediction of front end and back end defects evolution as simulation output can then be easily obtained at almost comparable computational costs. In this paper the FEM code Altair HyperXtrude® is used for the simulation of 2 industrial cases made by AA6063 and AA6082 alloys following the transient moving boundaries approach. Experimentally, the profiles were extruded, sectioned, polished and etched with caustic soda in order to reveal and measure front and back end development in front of and behind the profile stop mark. The data obtained from experimental analyses are initially discussed referring to billet skin contamination and charge weld evolution, then compared to industrial experience, to theoretical and empirical methods available in literature and to FEM results in order to evaluate pros and cons of each evaluation method.


2020 - Fast LIPSS based texturing process of dental implants with complex geometries [Articolo su rivista]
Orazi, L.; Pelaccia, R.; Mishchenko, O.; Reggiani, B.; Pogorielov, M.
abstract

In the present work, the generation of Laser Induced Periodic Surface Structures (LIPSS) by using femtosecond laser source is investigated in the surface texturing of Ti grade 5 dental implants. The proposed procedure permits a fast treatment of dental implants characterized by complex shape through the combined and synchronous use of a galvo scanner and of workpiece movements. The obtained surface micro- and nanostructures are characterized from a morphological point of view while in-vitro essays are performed to evaluate cellular proliferation. The results indicate the effectiveness of the LIPSS as method to increase cell viability and the efficiency of the proposed procedure to treat complex geometries as dental implants.


2020 - Innovative method for rapid development of shoes and footwear [Articolo su rivista]
Orazi, Leonardo; Reggiani, Barbara
abstract

In the present work, a novel set of software tools for the rapid development of shoes and footwear is presented aimed at strongly reducing the design time and then the time-to-market of the products. These tools are mainly based on an implementation of Constrained Free Form Deformation techniques and on the use of modern surface scanning methods. The proposed approach allows, in many cases, to avoid the mathematical surface reconstruction and to quickly generate the complex geometries that CAM systems will process in order to generate the tool paths for mold manufacturing.


2020 - Modeling and analysis of the visualized gas-assisted laser cutting flow from both conical and supersonic nozzles [Articolo su rivista]
Darwish, Mohamed.; Mrna, Libor; Orazi, Leonardo; Reggiani, Barbara
abstract

A coaxial high-pressure gas jet is normally used to assist and enhance laser cutting quality and capability. However, the gas nozzles are mostly of conical type causing the deterioration of the dynamic characteristics of the exit jet. In this research, the effect of both nozzle type and nozzle dimensions on the uniformity and behavior of the exit jet has been investigated. A total of three different supersonic nozzles have been designed in accordance to gas dynamics theory and manufactured using wire electrical discharge machining (WEDM). The exit jet patterns from these supersonic nozzles and a reference conical nozzle have been numerically modeled and compared with the experimental observations made through Schlieren visualization. The experimental results are found to match and hence validate the simulations. It is concluded that the exit jet from the supersonic nozzle is marked by uniformity, better dynamic characteristics, and longer effective exit jet length compared with conical nozzle.


2020 - Numerical analysis of the gas-assisted laser cutting flow from various supersonic nozzles [Articolo su rivista]
Darwish, M.; Orazi, L.; Reggiani, B.
abstract

In the present paper, the exit jet patterns from four different supersonic nozzles have been numerically simulated using OpenFOAM® Computational Fluid Dynamics (CFD) toolbox. The desired-design operating condition, for each of these nozzles, has been calculated using quasi 1-D gas dynamics theory. Mach number and pressure distributions are presented for each nozzle to illustrate the effect of both the nozzle geometry and the operating conditions on the behaviour of the exit jet. Then, the predicted numerical results have been validated by comparison with the experimental measurements reported in previous literature. As main outcomes, the simulations results are quantitatively in high agreement with the reported experimental observations. The correlations between jet flow stability and divergence angle are discussed and the effect of Mach number and exit diameter on the jet divergence are presented.


2020 - Numerical modeling and Schlieren visualization of the gas-assisted laser cutting under various operating stagnation pressures [Articolo su rivista]
Darwish, Mohamed; Mrňa, Libor; Orazi, Leonardo; Reggiani, Barbara
abstract

The uniformity of the exit jet pattern in high pressure gas-assisted laser cutting represents the main feature in order to achieve high cutting quality and capability. Therefore, the effect of both inlet stagnation pressure and nozzle geometry on the behavior of the exit jet has been investigated in this research. Quasi 1-D gas dynamics theory has been used to calculate the exact-design operating conditions for three different supersonic nozzles that were fabricated by means of Wire Electrical Discharge Machining. The jet flow through these nozzles has been numerically modeled and experimentally checked, using Schlieren visualization, under exact-design, over-expansion and under-expansion operating conditions coming to a good numerical-experimental agreement in terms of flow structure. As main result, the exit jet was found to preserve its uniform distribution with parallel boundaries and low divergence under the exact-design operating condition, differently to what observed for the others two conditions, especially for nozzle with small divergence angle.


2020 - Point Inversion for triparametric NURBS [Articolo su rivista]
Orazi, L.; Reggiani, B.
abstract

In this paper a Point Inversion algorithm for NURBS volumes is presented. This algorithm is an extension to 3D of the classic Newton–Raphson iteration, thus implying the calculation of the NURBS volume partial derivatives. Explicit formulas for the derivatives are deduced and reported, in turn requiring the evaluation of the corresponding derivatives of the rational basis functions. Also these derivatives are inferred and shown together with some examples of applications. The method, applicable to any free-form shape NURBS volume is moreover compared with Fast Mapping, a novel approach presented here that, once applied to simple NURBS boxes give performance 20–50 times higher.


2020 - Scrap assessment in direct extrusion [Articolo su rivista]
Reggiani, Barbara; Pinter, Tommaso; Donati, Lorenzo
abstract

The continuous direct extrusion is an economic and efficient process to manufacture high-quality aluminium profiles. However, because of multiple billets processing, transient defects (coring and charge welds) can be generated affecting portions of the profiles and deteriorating their mechanical properties. In case of structural applications, the reliable prediction of the onset and extend of these portions is therefore mandatory in order to be scrapped by the commercialized length. In this context, the present work has been aimed at examining the dynamic evolution of the coring and charge weld defects in the direct extrusion of an industrial solid profile. Both experimental and numerical activities have been performed and results evaluated in terms of coring and charge welds defect evolution achieving a good agreement for both phenomena. Results have been also compared with the outcomes of the analytical models available in literature. For the charge weld extension, the analytical model returned a poor predictably while a very good match with numerical data was obtained for the coring defect.


2019 - A NOVEL 3D PRINTED COOLING INSERT FOR EXTRUSION DIES [Relazione in Atti di Convegno]
Valli, Giuseppe; Reggiani, Barbara; Todaro, Ivan; Pelaccia, Riccardo; Squatrito, Rosario; Pinter, Tommaso; Mainetti, Enea; Rami, Yoram; Donati, Lorenzo; Tomesani, Luca
abstract

An accurate control and monitor of the temperatures developed during the extrusion process is a key factor for the process success in order to produce sound profiles and extend the die life. To this aim, conformal channels can be used to achieve a targeted process cooling by means of liquid nitrogen and the additive manufacturing technologies offered the best solution for a free form design with almost no geometrical constraints. In this work, a multi die concept for hot aluminum extrusion process is proposed in which the insert with forming zones and cooling channels was manufactured by means of the slm technology while the external insert support by conventional methods. This solution allowed obtaining a free-form cooling channel with the aim to get an optimized and targeted cooling nearby the bearing zones, where the highest temperatures are reached. Inserts were slm printed and complex Finite Element simulations of the extrusion process with nitrogen cooling performed in order to predict the thermal field of the inserts.


2019 - A novel design concept of the extrusion dies [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca; Pelaccia, Riccardo
abstract

In aluminum extrusion, one of the main limit is the developed temperature field that limits the maximum achievable production rate. Liquid nitrogen cooled dies are frequently today used in industry but they remove heat far away from the bearings, where the most critical conditions are reached. The novel concept involves the use of the additive manufacturing technology (SLM) to grant a full cooling channel design freedom, including the possibility to be located nearby the bearings. The insert is fit in a steel-housing die conventionally manufactured in order to limit final die costs. A comprehensive numerical analysis has been performed with the aim to investigate the insert thermo-mechanical performances, including the nitrogen cooling effect. The achieved numerical results encouraged the experimental manufacturing and testing of the insert. Specifically, eight inserts have been produced by SLM technology and the optimal one was successfully tested in the extrusion of an aluminum alloy.


2019 - Advanced modeling of die cooling with liquid nitrogen [Relazione in Atti di Convegno]
Pelaccia, Riccardo; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The profile temperature monitoring during aluminum extrusion is a an important information performed in order to avoid profile defects such as burn and crack, or an incorrect tempering, or to preserve the die life and to increase the process productivity. In the last decade die cooling systems through liquid nitrogen have been installed on several extrusion plants. However, a comprehensive and systematic assessment of the liquid nitrogen die cooling effect on process parameters is still missing in literature. In this context, aim of the present work is to develop an advanced numerical finite element model of the extrusion that accounts for liquid nitrogen cooling. The numerical model is then validated over ICEB 2011 Industrial Benchmark experimental trials, where the cooling efficiency in a multi-hollow industrial profile was evaluated. The results of the simulations have been compared to experimental investigation both in cooled and uncooled conditions. The assessment showed a good experimental-numerical agreement in terms of temperature map and extrusion load, thus suggesting the reliability of the novel modelling to support the process optimization.


2019 - Assessment on the creep-fatigue performances of tool steels [Abstract in Atti di Convegno]
REGGIANI, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Extrusion dies are typically produced by hot work steels with a great diffusion of AISI H-11 (1.2343) and AISI H-13 (1.2344) steels. Nevertheless different steels for hot working application are available on the market and several new steel grades are proposed every year for this type of application. A main concern for extruder and die maker that are interested into testing new steel grades, is how to evaluate the performances of the new material. If the material is directly applied for manufacturing a die, even if the die is an identical duplication of a previously tested, no reliable data are usually obtained as consequence of process variance. On the other side, standard testing (like tensile testing), even when performed at several high temperatures, offer only a partial view of the mechanical behavior of an extrusion die. Indeed dies are not affected by static loading being fatigue and creep the most damaging mechanisms of an extrusion die properly designed. In the present paper an innovative testing method, (previously proposed by the authors in [1] for investigating separately fatigue, creep and creepftigue interaction), was carried out on a TQ1 (X36CrMoV5-2) hot-work tool steel in cooperation with Constellium R&D center. A creep-fatigue loading type was applied to the specimens, i.e. a cyclic load with a dwell-time, in order to properly reproduce the conditions acting on extrusion dies. The specimens were tested at four different loading conditions (from 26 to 45 kN), wirth two different dwell times (160-300 s) and also with different surface conditions (tempered/tempered plus nitriding). Tests were performed on a 100kN MTS fatigue machine equipped with a heating furnace and the temperature was kept constant at 520°C. Final results were presented in terms of fatigue diagram curve of the TQ1 steel and compared to the performances of the H11 tool steel tested in a previous research by the same authors


2019 - Back.end and front-end scrap in direct extrusion: an estimation by means of finite element analysis [Relazione in Atti di Convegno]
Tommaso, Pinter; Ezio, Sicignano; Pelaccia, Riccardo; Reggiani, Barbara; Lorenzo, Donati
abstract

Finite elements analyses are offering to extrusions a big opportunity: to estimate accurately not only the charge weld extension but also the billet skin contamination. The present work examined the dynamics of the back-end and front-end defect in the direct extrusion of a solid profile through the use of FE analyses. Numerical results have been compared with industrial experiments with the aim to understand the effect of the selected boundary conditions on the accuracy of the predictions.


2019 - Effects of Surface Treatments on the Fatigue Response of High Strength Fasteners [Relazione in Atti di Convegno]
Reggiani, Barbara; Olmi, Giorgio; Orazi, Leonardo; Tomesani, Luca; Fini, Stefano; Barozzi, Lorenzo; Pelaccia, Riccardo; Bandini, Michele
abstract

The aim of the present work was the assessment of the impact of deep-rolling and shot-peening performed in the underhead and in the unthreaded shank of two high strength screws (36 NiCrMo and 42 CrMoV) for fatigue life enhancement. The experimental campaign consisted of six combinations, including the non-treated state. The aforementioned treatments were evaluated alone or with shot-peening performed after deep-rolling in the underhead fillet of the screws. Deep rolling was carried out at the optimal rolling force, whereas two shot diameters were considered for shot-peening (Z100 and UFS70). The results have been evaluated in terms of fatigue limits and factor effects have been assessed by marginal mean plots.


2019 - Experimental Investigation on the Effect of Shot Peening and Deep Rolling on the Fatigue Response of High Strength Fasteners [Articolo su rivista]
Reggiani, Barbara; Olmi, Giorgio
abstract

Shot-peening and deep rolling are mechanical surface treatments that are commonly applied to enhance the fatigue performances of components, owing to their capacity to generate compressive residual stresses and induce work hardening. However, literature is still poor of published data concerning the application of these treatments to high strength steels fasteners, although these represent a class of components among the most widespread. In the present work, the impact of deep rolling and shot-peening performed in the underhead radius of two set of fasteners made of 36NiCrMo and 42CrMoV for fatigue life enhancement has been investigated. The experimental tests consisted of six combinations of shot-peening and deep rolling, including the non-treated state. Two test campaigns have been sequentially carried out with dierent process parameters and treatment sequences. The results always showed a beneficial impact of the deep rolling on fatigue, especially for the 42CrMoV steel. Conversely, the eect of the shot-peening strongly depended on the selected set of parameters, alternatively leading to an improvement or a worsening of the fatigue life in relation to the level of induced surface roughness.


2019 - Extrusion Benchmark 2017: Effect of Die Design on Profile Quality and Distortions of Thin C-Shaped Hollow Profiles [Relazione in Atti di Convegno]
Donati, Lorenzo; Segatori, Antonio; Gamberoni, Andrea; Reggiani, Barbara; Tomesani, Luca
abstract

The Extrusion Benchmark 2017 is focused on the evaluation of different die design strategies for the production of two AA6005A thin C-shaped hollow profiles. The process was monitored in term of press load, profiles speed, profiles exit temperature, die temperatures (with specific focus not only on the die but also on the external die ring) and die deflection. Extruded profiles were analyzed in term of geometrical tolerances, microstructure evolution and profile distortions for both air and water quench; the profiles were also be analyzed for retrieving seam welds positions and charge welds extent. (C) 2018 Elsevier Ltd. All rights reserved.


2019 - Investigation on the Inert Gas-Assisted Laser Cutting Performances and Quality Using Supersonic Nozzles [Articolo su rivista]
Orazi, Leonardo; Darwish, Mohamed; Reggiani, Barbara
abstract

In the present work, three different supersonic nozzles were designed and manufactured to operate at various stagnation pressures during laser cutting. Several cutting experiments were performed on stainless steel samples of various thicknesses (2, 4, 6 mm) using a fiber laser of 3 kW with a head adapted to fit with both the proposed supersonic nozzles and a commercial reference conical nozzle. The flow through these nozzles was numerically modeled and compared with the Schlieren visualization measurements. The mass flow rate, the Mach number, and the pressure distributions were selected in detail in order to analyze the dynamic characteristics of the exit jet and to comparatively assess the achieved cutting quality (roughness perpendicularity) and capability (maximum thickness, cutting speed). The numerical and the experimental results were found to be in high agreement in terms of the flow structure and mass flow rate. In addition, a significant reduction of the assistance gas consumption of up to 65% on average was achieved by using supersonic nozzles as opposed to conical ones, accompanied with the decrease of the operating pressure and the increase of the cutting speed.


2019 - Investigation on the design of a novel selective laser melted insert for extrusion dies with conformal cooling channels [Articolo su rivista]
Reggiani, Barbara; Todaro, Ivan
abstract

The thermal control and the maintenance of a uniform temperature in the extrusion process of aluminum alloys is a crucial task in order to generate sound profiles with high press productivities. This can be accomplished through liquid nitrogen flowing in conformal cooling channels (CCC). The SLM additive technology offer an optimal solution for an unlimited flexibility of the cooling system, thus allowing tailored cooling strategies. In the present work, a smart thermally controlled die made by AISI H13 was designed aimed at maximize and regulate the cooling efficiency by means of CCC. In the novel die concept, the expensive SLM insert with CC channels, has been integrated into a steel housing conventionally machined. A comprehensive numerical investigation has been performed in order to check the insert designs mechanical and thermal performances both in uncooled and cooled conditions. Then, eight inserts were additively manufactured by means of the SLM (Selective Laser Melting) process with the aim to preliminary verify their experimental feasibility and overall quality. As main results, it was numerically proved the capability of the novel insert design to allow a significantly increase of the production rate and it was experimentally demonstrated the insert manufacturability throughout the SLM technology.


2019 - Osteoblast Cell Response to LIPSS-Modified Ti-Implants [Relazione in Atti di Convegno]
Orazi, Leonardo; Pogorielov, Maksym; Deineka, Volodymyr; Husak, Evhenia; Korniienko, Viktoriia; Mishchenko, Oleg; Reggiani, Barbara
abstract

In the present work, the surface of Ti-6Al-7Nb samples was patterned with Laser Induced Periodic Surface Structures in order to improve biocompatibility, increase tissue ingrowth and decrease bacterial adhesion and inflammatory response for applications in dental and orthopedic implants. Polished and sandblasted disks 10 mm in diameter were treated generating LIPSS under two different sets of parameters. The surface morphology and chemistry were investigated both by secondary electrons imaging, EDS analysis and Atomic Force Microscopy. Primary rat osteoblast culture (passage 2) was used to assess cell toxicity and biocompatibility. Alamar Blue assay was used to access cell viability and proliferation on day 1, 3 and 7. The difference between cell adhesion on polished and sandblasted surface as well as between polished and LIPSS-modified surface are described and discussed.


2019 - Prediction and validation of grain shape evolution by 3D FEM simulations of a real industrial profile [Relazione in Atti di Convegno]
Donati, L.; Bandini, C.; Reggiani, B.; Tomesani, L.
abstract

The simulation of microstructural evolution during the extrusion of an AA6063 industrial profile with the commercial Qform VX code is presented and discussed. The description of the grain shape evolution model through specifically developed user-routine is initially presented, then the experimental setup is described in details. Predicted numerical results are compared and discussed with respect to experimental data: relevant outputs like load-stroke diagram, profile speed and thermal distribution in the profile well match with the experimental data. The prediction of the final grain size obtained by the code with air quenching conditions is then compared to experimental measurements. A discussion on potentials and limits of the proposed model is finally presented.


2019 - Prediction of liquid nitrogen die cooling effect on the extrusion process parameters by means of FE simulations and experimental validation [Articolo su rivista]
Reggiani, B.; Donati, L.
abstract

The temperature monitoring during aluminum extrusion is a crucial task performed in order to avoid profile defects, to increase the die life and to optimize the process productivity. Among the different available systems used to control the process temperatures, in the last decade, the die cooling through liquid nitrogen has been installed on several extrusion plants. In order to provide extruders and die makers with a modelling tool able to gain a better understanding of the extrusion and die thermal field, a numerical model of the extrusion that accounts for liquid nitrogen cooling has been developed in the present work. A simplified 1D modelling of the cooling channel was used and validated against an experimental investigation previously specifically carried out to evaluate the cooling efficiency in a multi-hollow industrial profile. Results showed a significant impact of the design aspects of channel on the thermal efficiency of the cooling and an important heat removal when the liquid nitrogen cooling is used. A good experimental-numerical agreement was achieved in terms of temperature map and extrusion load both in cooled and uncooled conditions, thus suggesting the reliability of the developed simulation tool and its potential integration in a flexible procedure to be used for die/cooling system design and process optimization.


2019 - SLM of extrusion dies with liquid nitrogen cooling [Abstract in Atti di Convegno]
Pelaccia, Riccardo; Reggiani, Barbara; Todaro, Ivan; Valli, Giuseppe; Donati, Lorenzo; Squatrito, Rosario; Fortunato, Alessandro; Tomesani, Luca
abstract

Temperature control during aluminium extrusion is a mandatory activity in order to produce defect-free profiles and to optimize process productivity. During the extrusion process, the profile exit temperature increases due to amount of work spent to overcome friction and to plastically deform the workpiece that is converted to heat. In addition, if the ram speed or the extrusion ratio increase, the temperature further raises leading to detrimental surface defects such as hot cracks. The main advantages of dissipate the excessive generated heat in the die and at the end of the forming zone are related to the possibility of increasing only slightly the extrusion force and to reduce the conduction path from the forming zone to the cooling source. In addition, it is possible to cool critical die areas directly by sizing and positioning cooling channels or cooling nozzles adequately to prevent hot cracking. However, the manufacturing of conformal cooling channels close to the main forming zone by means of conventional machining is a difficult task for profiles and dies with highly complex geometries. Thus, the use of additive manufacturing technologies is a promising approach to allow a conformal cooling. The major advantage of additive manufacturing processes against conventional subtractive methods is the geometrical freedom of the produced components and the manufacturing of conformal cooling channels. The chromium hot-work tool steels are the most widely selected materials for forging, die-casting and extrusion dies applications because of their high thermo-mechanical strength, fatigue strength, toughness and relative low cost. Nowadays, thanks to the selective laser melting (SLM) technology, it is possible to produce structures with 99% density, since the material is completely melted and later solidified without forming spherical voids [4]. Since, to date, the additive manufacturing processes are often accompanied with higher manufacturing time and costs than conventional subtractive methods, a new concept for a hybrid extrusion die is presented. In the present work, a multi-die is proposed in which the expensive AM part, the insert with conformal cooling channels, is integrated into a conventional machined steel housing. A profile produced with a short die lifetime and critical issues on thermal field has been identified and selected consisting in a round bar of 10mm diameter made by AA6060.


2018 - A novel SLM H13 cooling insert for extrusion dies: experimental and numerical investigations [Relazione in Atti di Convegno]
Pelaccia, Riccardo; Reggiani, Barbara; Donati, Lorenzo
abstract

The additive manufacturing technology (AM) is considered to be the forth-industrial revolution allowing to produce «near net» components almost without geometrical constraints. This made the process perfectly fits with the requirement of extrusion dies since usually a maximum of two dies are required and the lack of design constraints allows to easily produce inserts with variable internal cooling channels. In the present work, a multi-die is proposed in which the expensive AM part, the insert with conformal cooling channels, is integrated into a conventional machined steel housing. Two AM inserts have been manufactured with SLM technology with different cooling channels diameters. Complex Finite Element (FE) simulations of the extrusion process have been performed by means of different FE codes in order to properly predict the thermal field gradient, the die stresses under the process thermomechanical loads and the multistate fluid dynamics. Preliminary results of the experimental trials of the insert are also presented in the present work.


2018 - Additive manufacturing for extrusion dies [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo
abstract

Additive manufacturing (AM) technology is considered to be the fourth-industrial revolution, allowing the production of near-net-shaped components almost without geometrical constraints. In contrast to conventional subtractive technologies, AM creates a component by building it up layer by layer, starting from a 3D CAD model.1 Due to the technology’s ability to produce components from high performance materials with added functionalities and increased complexity, AM perfectly fits with the requirements for producing dies, tools, and inserts. The AM technologies first developed in the 1990s were brought into the manufacturing of tools some years later. In this context, the first applications were in low impact processes like plastic injection molding, but were eventually implemented in the tooling for metal working processes like sheet metal forming and die casting, with new developments in extrusion dies underway. Nowadays, AM can process many metallic materials to produce full dense parts with outstanding properties, the most important being at the industrial scale, represented by steels, aluminum, and titanium alloys, as well as Ni-based superalloys and CoCr.3-4 This paper will address the general use of AM in metals and tooling applications, with a specific look at the development of a 3D-printed cooling insert for extrusion dies.


2018 - Analisi e previsione di difetti nei profili estrusi [Abstract in Atti di Convegno]
REGGIANI, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Le saldature trasversali, unitamente a quelle longitudinali, sono difetti generati duranti il processo di estrusione continua di materiali metallici. Se da una parte entrambi i difetti sono ineliminabili conseguenze del processo, diversamente dalle saldature longitudinali quelle trasversali sono caratterizzate da proprietà meccaniche inferiori rispetto al materiale base. Infatti, al termine di ciascuna corsa, la parte terminale del materiale della vecchia billetta, contenuta all’interno della matrice, comincia ad interagire con la superficie frontale della nuova billetta normalmente contaminata da ossidi, lubrificante e impurità generando una zona di transizione di una certa lunghezza che deve essere scartata. Mentre l’indagine sperimentale delle saldature trasversali risulta essere un’attività lunga e complessa, l’impiego di modelli numerici e analitici permette una significativa riduzione delle tempistiche predittive. Ad oggi, due formule sono state proposte in letteratura per l’analisi dell’estensione delle saldature trasversali. La prima formulazione suggerisce di computare l’estensione della saldatura come rapporto del volume delle alimentazioni e delle camere di saldatura sull’area della sezione trasversale del profilo moltiplicato per il numero di luci nella matrice. Una seconda formulazione prevede una modifica della precedente introducendo un fattore moltiplicativo 1.5 che tiene conto sia della presenza di zone morte nella matrice sia dell’effetto dell’attrito che rende il flusso del materiale all’interno della matrice non uniforme. In considerazione di questa limitata letteratura analitica, e in considerazione del fatto che i codici FEM non rappresentano ancora uno standard industriale, la determinazione della lunghezza di profilo da scartare per contaminazione da saldature trasversali avviene ancora sulla base dell’esperienza, di analogie e di regole pratiche. Tra queste, è comune pratica scartare da 1 a 3 mt di profilo in funzione dello specifico rapporto di estrusione. In questo contesto, scopo del lavoro è confrontare i risultati delle indagini sperimentali e numeriche condotte dagli autori su alcuni profili con le formulazioni analitiche ad oggi presenti in letteratura con l’obiettivo di verificarne l’applicabilità a livello industriale.


2018 - Automated approach for optimization of the aluminum extrusion process [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The technical achievement described in this paper is to optimize all the variables in the extrusion of complex aluminum profiles using finite element (FE) simulation, statistical regression analysis, and analytical equations in a software platform to account for these variables and their interactions with each other as a group to achieve an ideal extrusion practice. This approach is done by repetitive computer simulations based on varying die modifications conducted through a CAD model of the die and then applying incremental changes to extrusion parameters (several die variables, extrusion speed, temperature, etc.) via an FE code and regression analysis in an optimization program that takes account of thousands of “tweaks” to the die configuration and extrusion process parameters. Computation time varies depending on extrusion complexity but is reasonable in terms of industrial practice. As has been determined in this paper, this optimization procedure was applied to the extrusion of two sample extrusions, a hollow tube and a complex rectangular hollow with screw bosses at its corners. In each case, die pressure was reduced and weld quality and the level of stress in the die were improved, while maintaining or increasing exit speed.


2018 - Experimental investigation of hot-work tool steels performances under the creep-fatigue regime [Articolo su rivista]
Reggiani, B.; Donati, L.; Ben Tahar, M.; Tomesani, L.
abstract

In the present research, an innovative testing method, specifically developed to characterize the tool steels under creep-fatigue conditions, was carried out on a TQ1 (X36CrMoV5-2) hot-work tool steel in cooperation with Constellium R&D center. The experimental campaign consisted of different testing conditions, and most of the specimens were nitrided to account for the specific surface state of the tools. Tests were performed on a 100 kN MTS fatigue machine equipped with a heating furnace. A creep-fatigue loading type was applied to the specimens, i.e., a cyclic load with a dwell time, in order to properly reproduce the conditions acting on extrusion tools and dies. Under a constant temperature of 520 °C, the effects of four different load levels and two different values of dwell times were evaluated. In addition, selected test conditions were replicated with specimens not nitrided with the aim to evaluate and quantify the influence of the superficial treatment. Final results were presented in terms of fatigue diagram of the TQ1 steel and compared to the performances of the H11 tool steel tested in a previous research by the same authors.


2018 - Experimental, numerical, and analytical investigations on the charge weld evolution in extruded profiles [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo
abstract

Charge welds are unavoidable product defects generated during the continuous extrusion of metallic materials that extend to a certain variable length and that are marked by lower mechanical properties than the base material. The portion of the profile containing the charge welds thus needs to be scrapped and an accurate prediction of this portion becomes mandatory, not only for the final user of the profile, in order to avoid in-service product failures, but also for extruders and die makers in order to increase the process efficiency. In the present work, four case studies carried out in the years by the authors on the prediction of the charge weld extension are reviewed and systematically compared in terms of experimental and numerical results. Data are furthermore compared with the predictions of analytical models reported in literature for the scrap length calculation and of an industrial empirical rule based on the extrusion ratio. Final aim of the work is to highlight potentials and limits of each predictive method and to assess their applicability in the everyday industrial practice.


2018 - SLM printed steel conformal cooled insert for extrusion dies with anti-wear bearings [Abstract in Atti di Convegno]
Todaro, Ivan; REGGIANI, Barbara; Squatrito, Rosario; Fortunato, Alessandro; Donati, Lorenzo; Valli, Giuseppe; Rami, Yoram
abstract

Aim of the project: control and maximize extrusion dies cooling efficiency; increase their life by means of the development of wear resistant (harder) material (i.e. material enriched with Tungsten Carbides WC) through the additive manufacturing technology (AM); develop cost-efficient extrusion dies design


2017 - Evolution of the charge welds: towards a novel analytical formulation [Relazione in Atti di Convegno]
Reggiani, Barbara; Pinter, Tommaso; Donati, Lorenzo; Tomesani, Luca
abstract

Charge welds are, together with the seam welds, defects generated during the continuous extrusion of metallic materials. However, while the seam welds can exhibit the same resistance of the basic material if proper levels of pressure, contact time and temperature are applied, charge welds are always contained parts of the profile with lower mechanical properties. Indeed, at the end of each process stroke, the back end of the old billet material that completely fill the die starts to interact with the front side of the new billet loaded into the press that is usually contaminated by oxides, dust or lubricant thus producing a transition zone that extends to a variable length. It comes then clear that the length of the profile marked by the charge welds need to be scrapped and an accurate prediction and reduction of this portion become mandatory, not only for the final user of the profile, in order to avoid in-service product failures, but also for extruders and die makers to increase process efficiency. In this context, aim of the present work was to compare the experimental-numerical investigations performed by the authors in terms of charge welds extensions with the predictions of the analytical models nowadays reported in literature. The final global aim work was to assess the applicability of these models in the everyday industrial practice.


2017 - Experimental assessment of hot-work tool steels performances under the creep-fatigue regime [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In the present research an innovative testing method, specifially developed to characterize the tool steels under creep-fatigue conditions, was carried out an a TQ1 hot-work tool steel. The experimental campaign consisted of different testing conditions and part of the specimens were nitrided to account for the specific surface state of the tools. Tests were performed on a 10tons MTS fatigue machine equipped with a heating furnace. A creep-fatigue loading type was applied to the specimens, i.e. a cyclic load with a dwell-time, in order to properly reproduce the conditions acting on a hot forging or extrusion tool. Then, under a constant temperature of 520°C, the effects of four different load levels and 2 different values of dwell-times were evaluated. In addition, selected test conditions were replicated with the specimens not nitrided with the aim to evalute and quantify the influence of the superficial treatment. Final results were presented in terms of fatigue curves of the TQ1 and compared to the performances of the H11 tool steel tested in a previous research by the same authors.


2017 - ICEB 2017 - Benchmark settings [Relazione in Atti di Convegno]
Donati, Lorenzo; Segatori, Antonio; Gamberoni, Andrea; Reggiani, Barbara; Tomesani, Luca
abstract

Aim of the benchmark at ICEB conference is to examine FEM codes capabilities and users’ knowledge in the simulation of extrusion trials which were carried out and monitored by conference organizers. The comparison of the “blind” simulations by the participants with the experimental results allows users to check if their simulation settings are generally adequate to replicate the problem and software house to verify the sensitivity of their solving methods. It is very important for us to remind that, due to the complexity of this matter, it would be useless to consider the benchmark as a contest: it is, instead, an opportunity to fix some points about the everyday simulation practice, each participant with his own particular interest.


2017 - ICEB-International Conference on Extrusion and Benchmark [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Establishing numerical tools able to reliably analyze a manufacturing process takes a long time not only for their development, but also for proper validation through experimental trials in a wide range of processing conditions. In order to provide the extrusion industry and community with a place to discuss numerical techniques and to train users in FEM simulations, the ICEB – International Conference on Extrusion and Benchmark conference series was established in 2007.1 The aim of the conference was, and is still today, to merge the typical conference format (with presentation of papers from academia as well as industry) with a benchmark session where participants are asked to analyze a specific extrusion problem, the results of which are disclosed only during the conference. Within the benchmark session, in order to provide a solid point of comparison between experiments and simulations, it is extremely important that experimental trials provide clear indications on critical aspects of the process. For these reasons, different die and process settings are designed at every benchmark.2-7 For the 2017 edition, the benchmark session was focused on the evaluation of the effect of different die design strategies on profile quality and distortions of two thin C-shaped hollow profiles. Due to the success of the past 2015 edition,8 the 2017 ICEB conference was once again merged with the 10th Aluminium Two Thousand World Congress. The event was held in the city of Verona, Italy, from June 20-24, 2017 in conjunction with the Metef aluminum exposition. The numbers of the Verona 2017 event were surprising, with 315 participants coming from 36 countries. A total of 100 technical papers were presented and the conference was attended by academic researchers and managers from global players in the aluminum market together with die makers, extruders, aluminum producers, and software developers. American, German, and Italian delegates were the most numerous participants. However, the presence of experts from Europe, China, Asia, and North and South America was proof of the importance of this unique event. Technical papers were selected and evaluated by members of wide scientific and industrial committees.


2017 - Multi-goal optimization of industrial extrusion dies by means of meta-models [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In the design of complex extrusion dies, a number of different and potentially conflicting goals are involved in the process optimization such as profile tolerances, mechanical properties, aesthetical surfaces, and die life. Thus, a robust and comprehensive approach to investigate the problem is required that must be also compatible with the industrial timing. In this context, the aim of the present work was to perform a multi-objective optimization of industrial porthole dies for the maximization of the profile quality, expressed in terms of seam weld strength, of the production rate, that means balanced exit flow and maximized velocity, and of the die strength by means of meta-models. Two aluminum profiles were investigated. First, a thick round tube manufactured by means of a porthole die with three legs was selected as a starting case to define, fix, and validate the procedure. The gain achieved by the optimal design was compared with the experimental evidences of an already optimized configuration showing an additional increase in terms of welding strength and flow balance. In the second step of the work, the virtual optimization performed by means of meta-models was applied to solve a more complex industrial case of a hollow rectangular profile with four eyelets. For this, most significant improvements were attained not only for the seam weld quality but also for the die strength if compared to the performances of a starting configuration.


2017 - QUALITY ASSESSMENT AND OPTIMIZATION OF AN INDUSTRIAL EXTRUDED PROFILE [Relazione in Atti di Convegno]
Reggiani, Barbara; Andrea, Gamberoni; Antonio, Segatori; Tommaso, Pinter; Lorenzo, Donati; Luca, Tomesani
abstract

The extrusion process is an economic maunfacturing method used to produce profiles with a constant section. However, even when simple components are considered, many process and product issues can emerge such as low mechanical properties or distorsions of the exit profile, die premature failure and/or low production rates. These problems are related to the complex nature of the process that has to guarantee, at the same time, proper conditions for a good welding (in case of hollow profiles), absence of profile distorsions, an acceptable die stress and as much as possible fast extrusion. In addition, all these conditions have to be reached in the short extrusion time of each single billet and can emerge to be conflicting one each other’s. Usually, the optimization of the extrusion process is performed by the operators on the basis of consolidated experience and empirical rules. However, in this way, it is extremely difficult to control all the process and geometrical parameters involved and then to control the final outputs. This explains the number of works in literature aimed to develop numerical and analytical models for the extrusion process optimization. In this context, the authors recently presented a novel procedure for the multi-goal optimization of extrusion dies based on meta-models that was preliminary validated on the experimental results available for a simple round tube profile. In this work, two additional test cases are presented with the aim to extended the validation cases of the proposed procedure to a more complex industrial profile


2017 - The extrusion process towards Industry 4.0: a multi-objective simulation approach [Poster]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The hot extrusion of aluminum profiles is a manufacturing process able to achieve tight tolerances and high mechanical properties. However, due to the high number and complexity of the phenomena involved in the process, it emerges the difficulty in meeting all the set-out requirements that are very often conflicting one each others. The solution can only be find in a multi-objective approach to the process


2016 - A Numerical Modelling Approach for Time-Dependent Deformation of Hot Forming Tools under the Creep-Fatigue Regime [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The present study was aimed at predicting the time-dependent deformation of tools used in hot forming applications subjected to the creep-fatigue regime. An excessive accumulated plastic deformation is configured as one of the three main causes of premature failure of tools in these critical applications and it is accumulated cycle by cycle without evident marks leading to noncompliant products. With the aim of predicting this accumulated deformation, a novel procedure was developed, presented, and applied to the extrusion process as an example. A time-hardening primary creep law was used and novel regression equations for the law's coefficients were developed to account not only for the induced stress-temperature state but also for the dwell-time value, which is determined by the selected set of process parameters and die design. The procedure was validated against experimental data both on a small-scale extrusion die at different stress, temperature, load states, and for different geometries and on an industrial extrusion die which was discarded due to the excessive plastic deformation after 64 cycles. A numerical-experimental good agreement was achieved.


2016 - A novel comprehensive approach for multi-objective optimization of the aluminum extrusion process [Abstract in Atti di Convegno]
REGGIANI, Barbara; DONATI, LORENZO; TOMESANI, Luca
abstract

ABSTRACT The process design and optimization of aluminum extrusion profiles can be extremely challenging since a number of different and potentially conflicting goals are involved such as strict profile tolerances, high mechanical properties, aesthetical surfaces and extended die lifetime. In this context, aim of the present work was to carry out a multi-objective optimization of industrial porthole dies for the maximization of the profile quality (seam welds strength), of the production rate (flow balance and ram speed) and of the die strength. Two industrial profiles were investigated, a thick round tube and a more complex industrial case of a hollow rectangular profile. In the former, the gain achieved by the optimal design was compared with the experimental evidences of an already optimized configuration showing ad additional increase in terms of welding strength and flow balance. For the rectangular profile, most significant improvements were attained not only for the seam welds quality but also for the die strength if compared to the performances of a starting configuration.


2016 - Development and Validation of a Dynamic and Static Recrystallization Model for Microstructural Prediction of AA6060 Aluminum Alloy with Qform [Relazione in Atti di Convegno]
Bandini, Claudia; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The aim of the present work is to develop a model within Qform, a Lagrangian FE Code environment to predict the grain evolution of 6XXX aluminum alloys during deformation and subsequent static recrystallization. Experimental data were obtained from backward extrusion campaigns (AA6060-O and AA6082-O) that were performed at different conditions of temperature and strain rate. All the specimens were subjected to microstructural analysis in terms of grain size: before extrusion (initial grain size), immediately quenched after extrusion (evaluation of dynamic recrystallization) and after a full annealing heat treatment (evaluation of a fully static recrystallization). The regression between experimental data and FEM computation allowed to validate a theoretical model for the grain size and shape evolution during dynamic recrystallization of 6060 and 6082 alloys previously developed on a different experimental campaign and to implement and evaluate a static recrystallization model proposed in literature (by considering 100% of recrystallization only) on AA6060 alloy.


2016 - Multi-Goal Virtual Optimization of Industrial Extrusion Dies [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In the design of complex extrusion dies, a number of different and potentially conflicting goals are involved in the process optimization such as profile tolerances, mechanical properties, aesthetic surfaces, and die life. The aim of the present work was to carry out a multi-objective virtual optimization of industrial porthole dies for the maximization of profile quality (seam welds strength), of the production rate (flow balance and ram speed), and of die strength. Two industrial profiles were investigated, a thick round tube and a more complex industrial case of a hollow rectangular profile. In the first case, the gain achieved by the optimal design was compared with the experimental evidence of an already optimized configuration, showing an additional increase in terms of welding strength and flow balance. For the rectangular profile, the most significant improvements were attained not only for the seam weld quality, but also for the die strength, if compared to the performances of a starting configuration.


2016 - Sviluppo di user routines con Qform per la predizione della microstruttura delle leghe di alluminio durante processi di estrusione [Capitolo/Saggio]
Bandini, Claudia; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Scopo del presente lavoro è quello di implementare in Qform8®, codice FEM, per la simulazione di processi di deformazione plastica, un modello già presente in letteratura in grado di prevedere l'evoluzione microstrutturale nelle leghe di alluminio, serie 6XXX che si attuano se si sottopone il materiale a processi di estrusione Dati sperimentali sono stati ricavati da prove di microestrusione portate a termine su provini di lega AA6082 sottoposti a deformazione in diverse condizione di temperatura, velocità e rapporto di estrusione.Il codice , già validato in precedenza dagli autori per quanto riguarda condizioni di attrito e di scambio termico, è stato utilizzato per effettuare una campagna di simulazioni numeriche, i cui risultati sono stati confrontati con quelli sperimentali e la regressione ottenuta ha permesso la determinazione dei parametri presenti nelle formulazioni matematiche reperibili in letteratura. L’implementazione mediante una user routine, delle formule così ottenute, da la possibilità all’utente di prevedere anche attraverso l’interfaccia di QForm l’evoluzione della lunghezza e larghezza dei grani sottoposti a elevati tassi di deformazione. Il modello consente quindi già in fase di progettazione e definizione del processo di prevedere lo stato fibroso dei grani che subiranno una ricristallizzazione dinamica.


2015 - BENCHMARK - INDUSTRIAL SETTINGS and EXPERIMENTAL RESULTS [Relazione in Atti di Convegno]
Gamberoni, Andrea; Donati, Lorenzo; Reggiani, Barbara; Tomesani, Luca
abstract

The paper reports information in term of simulation settings and output results related to the Industrial benchmark 2015 : extrusion benchmark is an event where participants from software houses, industries and academia are re quested to simulate an extrusion process case which main experimental data are initially unknown and disclosed only after the submission of simulati on results.


2015 - Code Validation and Development of User Routines for Microstructural Prediction with Qform [Relazione in Atti di Convegno]
Bandini, Claudia; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Aim of the present work is to validate the metal forming FE code QForm and to develop user routine for the prediction of microstructure evolution in 6XXX aluminum alloys. Preliminary simulations are carried out in order to select optimal friction models and coefficients among the several formulations available in the code. Numerical results are compared to grid-based visioplasticity experiments: the comparison is performed in term of grid deformation at the billet-tools interfaces, load-stroke behavior and temperatures evolution of die and profile. The optimized friction model and coefficient are then applied in second series of simulations in order to develop the prediction of microstructure evolution. A theoretical model for the grain size and shape evolution of 6XXX aluminum alloys is finally implemented through the use of user routine and compared with experimental observations. The model is found able to properly predict the deformed state of the grains in the fibrous condition.


2015 - Industrial Benchmark 2015: Process Monitoring and Analysis of Hollow EN AW-6063 Extruded Profile [Relazione in Atti di Convegno]
Gamberoni, Andrea; Donati, Lorenzo; Reggiani, Barbara; Haase, Matthias; Tomesani, Luca; Erman Tekkaya, A.
abstract

The paper reports information in term of simulation settings and output results related to the Industrial benchmark 2015: extrusion benchmark is an event where participants from software houses, industries and academia are requested to simulate an extrusion process case which main experimental data are initially unknown and disclosed only after the submission of simulation results. The industrial benchmark 2015 is focused on the extrusion of a multi-cavities hollow profile with EN AW-6063 aluminum alloy. Thermal field was monitored by means of contactless pyrometers installed on the press and five thermosensors were inserted in key positions in the die. Several extrusion data were continuously acquired including the profile speed, the puller force and the extrusion load. After extrusion the profiles are analyzed in order to determine the position of the seam weld and the microstructure inside the profile cross section after air or water quenching.


2015 - Multi-cycles deformation modeling of hot forming tools under creep-fatigue regime [Capitolo/Saggio]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Hot forming processes (extrusion, die ca sting and forging) allow the production of a wide variety of products, both in terms of worked material and achievable shapes. However, critical working conditions are involved for the tools subjected to severe thermo- mechanical loads, thus requiring an accurate design. Among the different classifications proposed in literature for hot forming die failures, one focus on separating manufacturing and in service failures. The latter category is additionally split, as proposed by the authors, in static failure, damage and deformation/deflec tion failures. Static failure appears after e reduced number of extruded billets as a consequence of an overload or of a poor initial die design. Damage and deflection failures are ind eed induced by the synergic detrimental action of creep and fatigue phenomenon. In disc riminating the relative dominant role of creep and fatigue in leading to the final die discard, the level of temperature and of the applied load as well as of the dwell-time are d ecisive. In extrusion, the latter is the time in which a constant load acts on the die and represents the time required to extruded each single billet function of both ram speed and billet length. Fatigue and creep can be seen as limit cases with zero and infinite dwell-time. A novel model is proposed for the prediction of the deformation undergone by hot forming t ools in the creep-fatigue regime after multiple cycles. The model is presented as applied to extrusion dies and is based on a modified version of a simple creep law already implemented in all the FE codes. The model is validated against small scale dies used in physical experiments and then against industrial extrusion dies.


2015 - Multi-objective Optimization of the Extrusion Process [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In the paper a multi-objective optimization of a porthole extrusion die used to manufactured a thick AA7003 round tube is presented. Eight competitive objective functions were selected aimed at increasing the seam weld quality, the die lifetime, the production rate and at restraining the die yielding and the peak process load with respect to an initial already optimized solution. Six geometric input variables were included in the optimization procedure (welding chamber and bridges height, ports width, undercut on ports, die entry angle, mandrel-bridges fillet radius) together with the ram speed. The multi-objective optimization was performed by means of modeFRONTIER® using meta-models generated over a selected set of experimental and numerical training designs.


2015 - Numerical Assessment of the Influence of Process and Geometric Parameters on Extrusion Welds and Die Deformation after Multiple-cycles [Relazione in Atti di Convegno]
Pinter, Tommaso; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

A comprehensive investigation on the correlation between die design and process parameters with extrusion welds (seam and charge) prediction and die deformation after multiple-cycles is presented. In particular, the influence of the number of legs (2,3 and 4) as well of some commonly adopted industrial best design practices in the extrusion of a round tube profile are examined by the ALE code HyperXtrude® for the AA6005A alloy. Then, for the same three designs, the die deformation after multiple-cycles is computed by applying a developed user sub-routine that account for the pressure and temperature maps and for the dwell-time that represents the time required to extrude each single billet. A sensitivity analysis on the influence of process parameters (ram speed, billet length and alloy) on the achieved die deformation after a fixed extruded amount is also performed.


2015 - Scientific Benchmark 2015: Effect of Choking and Bearing Length on Metal Flow Balancing in Extrusion Dies [Relazione in Atti di Convegno]
Selvaggio, A.; Donati, L.; Reggiani, B.; Haase, M.; Dahnke, C.; Schwane, M.; Tomesani, L.; Tekkaya, A. E.
abstract

The Extrusion Benchmark is a biannual event where experts in the field of aluminum extrusion processing come together, not only in order to verify the increase of performance and accuracy of finite element (FEM) simulations for process optimization, but also to share advanced knowledge on the matter. The scientific benchmark 2015 was focused on bearing design and on the effect of choking and bearing length during extrusion of EN AW 6060 round bars. The trials were conducted on a 10 MN extrusion press at the Institute of Forming Technology and Lightweight Construction (IUL), which is equipped with advanced instrumentation for monitoring of process data. The tests were monitored in term of final profiles lengths, extrusion load, thermal evolution of the die in proximity of the six bearings and die deflection.


2015 - Shape and process optimization of an industrial porthole extrusion die under multi-objective functions [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The increasing demand for complex and of high quality extruded profiles has led to the development of even more intricate and difficult to optimize process conditions and dies shapes. The consequence is that different and potentially conflicting objectives are involved in the optimization of the process as the compliance with the required profile tolerances and mechanical proprieties and the endurance of the die itself. The fulfilment of all these requirements necessary demands for an automatic, robust and comprehensive methodology to investigate the problem. In the present work, a multi-objective optimization of the extrusion process is presented by applying it to an industrial round tube profile. Six geometric die parameters and the ram velocity were set as input variables. The objective functions were the peak pressure in the welding chamber, the minimum mandrel deflection and peak principal stress and the minimum flow unbalance at the die exit. The optimization was performed by combining a response surface methodology with a multi-objective genetic algorithm based on a 2-level full factorial numerical and experimental designs. The optimal solution extracted from the Pareto front was compared to experimental data of manually optimized solutions achieving higher product performance in terms of welds quality and die solicitation.


2015 - Validazione del codice Qform per l’analisi del processo di estrusione di leghe di alluminio [Capitolo/Saggio]
Bandini, Claudia; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Scopo del presente lavoro è stato qu ello di validare un codice FEM di nuova generazione, QForm®, per giungere all’ implementazione di una user-routine per la previsione dell'evoluzione micr ostrutturale durante processi di deform azione plastica, in particolare di estrusione. È stata quindi c ondotta una campagna preliminare di simulazioni al fine di validare i modelli di attrito disponibili nel codice così come le condizioni di scambio termico e i parametri che influenzano la discretizzazione della maglia e il processo. I risultati numerici sono stati co nfrontati con indagini sperimentali di visioplasticità riportate in letteratura [1] in te rmini di griglie di deformazione, andamento corsa-carico sul pistone, temperature d el profilo estruso e della matrice. La determinazione del mod ello ottimale di attrito e dei suoi fattori caratterizzanti, reso possibile dal confronto numerico-sperimentale, ha permesso in seguito l’implementazione di una routine per la previs ione dell'evoluzione micros trutturale nel codice Qform.


2014 - Comparison of Bulge Test vs. Conical Expansion Test for Hollow Extruded Profile Characterization [Relazione in Atti di Convegno]
Reggiani, Barbara; Segatori, Antonio; Donati, Lorenzo; Tomesani, Luca; Terenzi, Alberto; Salice, Alberto
abstract

An experimental campaign on a tubular hollow profile, extruded in industrial environment at two ram speeds, was performed to compare two testing methodologies used for the assessment of the seam welds strength: the cone expansion and the bulge tests. In the former, a cone-shaped punch is driven into the tube causing the expansion till the specimen fracture; in the latter, an internal rubber plug is used to expand the specimen allowing to apply an hydrostatic tensile state. Results and repeatability of the two tests were analyzed in terms of loads and tube radius elongations at fracture; location and morphology of the fracture were also inspected. In each condition, and for both tests, ductile fractures appeared at seam weld location. The bulge test showed a significant reduced data scattering if compared to the cone test and provided more conservative outcomes in terms of elongation at fracture; in addition, it marked more prominently the effect of the increased ram speed that promoted a weld strength decay.


2014 - Optimization of the extrusion process by means of a novel comprehensive approach [Relazione in Atti di Convegno]
Reggiani, Barbara; Broccoli, Massimiliano; Donati, Lorenzo; Tomesani, Luca
abstract

In light alloy extrusion process, different and conflicting objectives need to be achieved during die design stage in order to obtain an efficient and robust process. Among these, the production of extruded profiles within the required tolerances is of primary importance. A secondary aim, in case of profiles used for structural applications, is to guarantee adequate mechanical characteristics. In addition, the die has to resist without yielding under the selected process conditions. All these requirements are borne by the proper setting of process parameters and by die design. An extensive number of works are reported in literature for process and die design optimization. However, all these publications show the impact of a limited number of input parameters on a specific output that optimize the profile quality or, alternatively, the die life. Just one publication reports the multi-objective optimization of an extrusion process but the methodology is not clearly reported. Aim of this work is to extend this contribution to account for several input and output variables. To this aim, COMSOL Multiphyisics ® has been integrated with modeFRONTIER and the optimized solutions extracted from the Pareto frontier.


2014 - Prediction of Fibrous and Recrystallized Structures in 6xxx Alloy Extruded Profiles [Relazione in Atti di Convegno]
Antonio, Segatori; Reggiani, Barbara; Lorenzo, Donati; Luca, Tomesani; Mohamad El, Methedi
abstract

The final microstructure of extruded profiles is of great importance for final mechanical properties and, consequentially, the ability to control and predict it is of extreme interest for Academic and Industrial researchers. In the paper a combined model, able to discern recrystallized areas respect to fibrous structures within the same profile, is initially proposed then validated through FEM implementation on an experimental campaign performed by Parson [1]. The model was tested under different die geometries and process conditions and a qualitative comparison with final microstructure obtained in the extrusion of a simple aluminum rod was performed.


2013 - A comparison between different approaches in the evaluation of extrusion die life: static strength, fatigue and creep [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The demand of high quality extruded products involves the requirement of an increasing efficiency of the production systems leading to severe operational conditions for the die. Among different classifications proposed in literature for die failures, one focus on separating manufacturing and in service failures. The latter category is additionally split, as proposed by the authors, in static failure, damage and deformation/deflection failures. Static failure appears after e reduced number of extruded billets as a consequence of an overload or poor initial die design. Damage and deflection failures are indeed induced by the synergic detrimental action of creep and fatigue phenomenon. All the failure mechanisms are die design and in-service conditions dependent. In particular, in discriminating the dominant role of creep and fatigue in leading to final die discard, a considerable role is played by the dwell time. This is the time of constant load acting on the die and represents the time required to extruded each single billet function of both ram speed and billet length. Fatigue and creep can be seen as limit cases with zero and infinite dwell time. A novel model is proposed for the prediction of the deformation undergone by extrusion dies in the creep-fatigue regime after multiple cycles (i.e. multiple extruded billets) based on a modified version of a simple creep law already implemented in all the FE codes. The starting point is a physical experiment reproducing the thermo-mechanical conditions of a die. The model allows to account also for the dwell time. The model has been validated against small scale dies. A test was also performed on an industrial hollow die used in a controlled experiment.


2013 - Extrusion of Magnesium Hollow Profiles for Automotive Applications [Capitolo/Saggio]
Segatori, Antonio; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The use and adoption of magne sium extruded profiles in structural application is limited because magnesium extrusion technology is at its first. Once extruded hollow sections will be available at a reasonable cost, a tremendous in crease of market demand is going to be expected due to the lowest weight of the magnesium alloys in comparison with aluminum based ones. The press productivity re present a key factor in this scope and the die, due to the strong incidence of strain and temperature in the extrudability, will play a basic role to achieve the required stem speed. This paper present a new approach for designing extrusion dies specifically for magnesium alloy ZM21 by means of FEM simulation. The goodness of the new design has been validated extruding an hollow profile using an industrial press with data logging.


2013 - Prediction of charge welds in hollow profiles extrusion by FEM simulations and experimental validation [Articolo su rivista]
Reggiani, Barbara; Segatori, Antonio; Donati, Lorenzo; Tomesani, Luca
abstract

In direct extrusion of aluminum alloys, billets are discretely loaded into the press and joined by the high hydrostatic pressure field. The contamination of the billet-to-billet interface by oxides, dust, or lubricant produces a welded zone (charge weld) with reduced mechanical properties that requires profile discharge. For an efficient material scrapping, both the position of the transition zone and its extent in the profile must be accurately identified. In industrial practice, in relation to a lack of experimental and numerical studies on this specific matter, the determination of the zone to be discarded is still performed mainly by experience or labor-intensive analyses. The aim of the present study is to bridge this gap by investigating the evolution of the charge welds inside an industrial multi-profiles and determining their exact position and extension by experimental microstructural analyses coupled with comprehensive 3D FE simulations performed with the Arbitrary Lagrangian-Eulerian code HyperXtrude®. Skin and rest defects are also experimentally investigated and a numerical sensitivity study on the influence of the friction model selection is performed. Comparison between numerical and experimental results shows a good agreement both in terms of general trend and exhausting points of the charge welds. The results prove that the FE code is a reliable tool in assisting and driving the die and process design stages, not only for process optimization as reported in literature but also for the scrap length determination. Finally, a process efficiency index is defined and, for the specific case study, it is found to be increased from 82.6 %, as resulting from the actual industrial practice, to a 88.3 % as optimized by the performed coupled experimental and numerical activities.


2013 - Prediction of grain shape evolution during extrusion and annealing of 6xxxx alloy [Relazione in Atti di Convegno]
Segatori, Antonio; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In the extrusion of aluminum alloys the process chain involves different steps of thermo-mechanical stresses of the material leading to deformation of the grains during plastic forming and several degrees of static recrystallization during cooling. In defective cases, peripheral grain coarsening may occur thus causing profile scrapping. The implementation of a predictive tool able to simulate the whole complex sequence of phenomena involved in the extrusion process, consequently results of considerable interest. Experimental and numerical analysis were performed to investigate the grain shape evolution of 6XXX aluminum alloy during the extrusion process in order to develop a prediction model. A first experimental campaign was carried out in order to investigate the texture deformation modes and retrieve the microstructure evolution model. This model was coupled with the analytical static recrystallization formulations available in literature for the final implementation in a commercial FE code. A unified model was therefore implemented through user-routine, thus carrying the innovation of a model able to simulate grain evolution during the entire process: deformation and following ageing with an integrated approach. A second experimental campaign was used to validate the model on a wide industrial range of Zener-Hollomon values, comparing the experimental and numerical results at the end of the dynamic evolution and of the consequent static recrystallization. The results were analysed also in terms of grain growth effect, although this phenomenon is still not well captured by the model.


2012 - A survey on the predictability of the die deflection in aluminum extrusion by means of experimental investigation and numerical analysis [Capitolo/Saggio]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

The direction of the market demand in extrusion is to push towards higher quality products. With this aim, is it nowadays a common practice to make use of numerical methods, especially of the finite element simulations in order to get information on the material flow, temperature distribution, process load and other process related issues. Therefore, in parallel to the requirement of higher quality products there is the demand of more accurate, stable and reliable simulation codes. The International Conference on Extrusion and Benchmark (ICEB) works as a meeting point between finite element codes developers and extruders and die makers to asses and compare the capability of each code to predict the parameters of interest in the field of extrusion. The 2009 edition of the conference has been focused on the die deflection evaluation, also accounting for the experimental complexity to monitor this data for the high temperature involved in the process as well as for the reduced direct accessibility to the die. Aim of the present work is to present the results of this activity in which a controlled extrusion experiment has been carried out and to compare the recorded data with the prediction of three process simulations codes in terms of profiles exit velocity and temperature, die temperature, process load and die deflection in the extrusion direction. The codes are based on different approaches: ALTAIR HyperXtrude® based on the Arbitrarian Lagrangian Eulerian formulation, COMSOL® Multiphysics based on a pure Eulerian approach and finally DEFORM 3D based on a pure Lagrangian approach.


2012 - APPLICATION OF DIFFERENT DEFORMATION MODELS TO EXTRUSION DIES: ELASTO-VISCOPLASTIC WITH KINEMATIC AND ISOTROPIC HARDENING AND MODIFIED TIME-HARDENING CREEP LAW [Relazione in Atti di Convegno]
Tomesani, Luca; Reggiani, Barbara; Donati, Lorenzo
abstract

In the present work, two numerical models were evaluated for the prediction of the level of deformation achieved by an extrusion die during the process after multiple cycles: the Chaboche elasto-viscoplastic model and a modified version of the time-hardening creep law. The latter was developed by the authors in order to include the effect of the temperature and stress levels as well as of the billet length in the computation of the achieved deformation. Both models were then applied to a small scale mandrel used as specimen in a novel physical experiment and tested under different loading, temperature and stress conditions.


2012 - Constitutive laws for the deformation estimation of extrusion die in the creep-fatigue regime [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Tools are exposed to severe working conditions during the hot extrusion process. In particular, dies and mandrels can be subjected to an excessive amount of deformation as a result of the developed high cyclic loads and temperatures. In this scenario, a physical experiment reproducing the thermo-mechanical conditions of a mandrel in a porthole die was performed with the Gleeble machine on the AISI H11 tool steel with the aim to investigate the mechanisms that influence the die deformation. The design of experiment consisted of 4 levels of temperature, 3 levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all the testing conditions, a comparable pattern of the mandrel displacement-time curve was found reproducing the 3 stages of softening typical of the strain evolution in a standard creep test but with a marked primary phase. Thus, with the aim to identify an easy-applicable equation to estimate the die deformation, the time hardening creep law was chosen. Coefficients of the time-hardening law were optimized, for each testing condition, on the basis of experimental data starting from values for similar alloys taken from the literature. Results in terms of mandrel displacement were then compared to experimental data for the creep-fatigue condition at different stress and temperature levels. The values found were validated against additional experimental data performed with different specimen geometries. A good average agreement was found between experimental and numerical results. The developed procedure was then applied to an industrial die.


2012 - Deformation Prediction of Porthole Dies After Multiple Extrusion Cycles [Relazione in Atti di Convegno]
Donati, Lorenzo; Reggiani, Barbara; Tomesani, Luca
abstract

Extrusion dies work under the combination of cyclic loads and high temperatures. Accounting also for the billet-die contact time during each single extrusion, the die deflecting mechanism is placed in the creep-fatigue interaction. In order to analyze the die deformation, a physical experiment reproducing the thermo-mechanical conditions of an extrusion die was performed on a Gleeble simulator. The specimens were made of AISI H11. The design of the experiment consisted of four levels of temperature, three levels of stress, and three types of load (creep, fatigue, and creep-fatigue with a dwell-time). It was found that creep is the mechanism that greatly affects die deformation, and that the time-displacement histories of the specimens showed a marked primary softening phase for each tested condition. Thus, a modified version of the time-hardening creep law was chosen to estimate the die deformation. Coefficients of the law were optimized on the basis of experimental data, starting from values found in the literature. Novel equations were formulated to express the dependency of the coefficients on the stress-temperature state, as well as on the dwell-time after multiple cycles. The presented procedure was validated against additional experimental data, performed with different specimen geometries and on an industrial extrusion die. A good average agreement was found between experimental and numerical results.


2012 - Deformation behaviour of a ferritic hot-work tool steel with respect to the microstructure [Articolo su rivista]
Krumphals, Friedrich; Reggiani, Barbara; Donati, Lorenzo; Wlanis, Thomas; Sommitsch, Christof
abstract

A ferritic hot-work tool steel was investigated, which is in use as die for extrusion processes and exposed to cyclic load spectra with overlapping of both, thermal and mechanical loads. The occurring stresses and temperatures in the tool material were used to simulate the material behaviour of the tool steel in service by means of Abaqus StandardTM v.6.8–3 software in conjunction with Z-Mat package. A sensitivity analysis of the used elastic–viscoplastic Chaboche model was performed to characterize the most influencing parameters. To describe the microstructure evolution during thermo-mechanical loads via a physical based approach and to compare it with the phenomenological predictions, precipitation kinetics simulations as well as a dislocation density model for thermal creep using the rate theory, were applied to characterize the hardening and softening behaviour.


2012 - Effect of liquid nitrogen die cooling on extrusion process conditions [Relazione in Atti di Convegno]
Donati, Lorenzo; Segatori, Antonio; Reggiani, Barbara; Tomesani, Luca; Bevilacqua, Pietro Alfredo
abstract

In the paper, a die for the production of a complex hollow profile made by AA6060 alloy on an industrial 2500 ton press has been manufactured and tested under strict monitored conditions. In particular 5 thermocouples were placed in proximity of interesting positions inside the die: 3 next to the bearings and two near the welding chambers. A self-calibrated pyrometer was used for the temperature monitoring of profile. Press loads, ram speeds and container temperatures were continuously recorded directly from the press system. Six billets were initially extruded in order to reach a steady state condition being the last three used as industrial benchmark for the 2011 edition of the ICEB conference. Then the nitrogen was completely and partially opened and the evolution of the temperature in the die and in the profile recorded together with the process load. The effect on bearing temperature was extreme, in particular in proximity of nitrogen inlet, while almost no change in welding chamber thermocouples and in the process load was revealed.


2012 - Effect of process parameters on seam weld quality of ZM21 tubes [Relazione in Atti di Convegno]
Segatori, Antonio; Reggiani, Barbara; Donati, Lorenzo; Pinter, Tommaso; Rami, Yoram; Tomesani, Luca
abstract

The increasing attention to magnesium alloys in extruded profiles, especially in the transportation industry, is related to their low density associated with good mechanical properties and complete recyclability. This allows to push towards both increasing efficiency and pollution restrictions. However, these advantages are negatively balanced by the production rates drop in relation to dangerous profile temperatures increasing that force to keep low velocities. In this context, a novel porthole die has been purposely designed for magnesium alloys allowing an increasing of the process velocity up to four times with respect to past solutions. The mandrel consisted of three ports made by 120° bridges that created an equal number of seam welds. The extruded tubes, made in ZM21, were 50 mm in diameter and 2 mm in thickness and were tested under different process conditions. In the present work, the quality of the seam welds has been investigated in relation to each process condition by means of the rubber plug testing method that allowed to apply an hydrostatic tensile state.


2012 - Finite element modelling of the charge welds evolution in a porthole die [Relazione in Atti di Convegno]
Reggiani, Barbara; Segatori, Antonio; Donati, Lorenzo; Tomesani, Luca
abstract

Although the extrusion process is regarded as a continuous process, in practice billets are discretely loaded into the press. The joining of two consecutive billets is guaranteed by the high hydrostatic pressure of the process. Nevertheless, mechanical proprieties of the profile in the welded region are lower than those of the adjacent area thus leading to the discarding of the welded segment. Extension of the segment is mainly affected by the interaction of seam and charge welding phenomena and, nowadays, its prediction is still a tricky task involving rough empirical relations or labour intensive analyses. Aim of this work was to identify the starting and exhausting points of the charge welds evolution inside an industrial multi-profiles die, in order to determine the exact position and the minimal length to be scraped. The extrusion of four AA6060 hollow profiles through a multi-hole die were preliminary monitored in an industrial press by accurate recording of the process loads, temperatures and speeds during the process. The four profiles were sectioned starting from the ‘stop mark’ then grinded and etched in order to investigate the location and the dimension of the zone to be ridded. While the experimental activity is detailed elsewhere, this paper is focused on the finite element modelling of the process. The numerical simulation was performed by means of the Altair HyperXtrude code and predictions compared to experimental data with a particular focus on the computation of the charge evolution. It was found a good agreement both in terms of general trend and prediction of the exhausting points so thus proving the code to be a reliable tool for an accurate determination of the scraps.


2012 - Prediction of Position and Extent of Charge Welds in Hollow Profile Extrusion [Relazione in Atti di Convegno]
Segatori, Antonio; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In billet-to-billet direct aluminum extrusion, the high hydrostatic pressure field produces joining of the two consequent billets; however, the mechanical properties of the transition zone are lower, thus requiring the scrapping of the charge-welded segment. The localization of the segment to be discarded is a complex matter. This work aims to investigate the interaction between the different welding phenomena inside an industrial multi-profile die, in order to determine the exact position and the minimal profile length to be discarded. Experimental investigations were performed on an industrial extrusion of four hollow profiles in EN AW-6060 alloy through a multi-hole die. Location and dimension of the charge weld was retrieved and compared with finite element (FE) simulations of the process, thus evaluating the possibility of using the code as a reliable tool for the localization and determination of the scrap, particularly if new die design strategies must also be evaluated in terms of reduction of the zone extension.


2011 - ANALYSIS OF CHARGE WELD EVOLUTION FOR A MULTI-HOLES EXTRUSION DIE [Relazione in Atti di Convegno]
Segatori, Antonio; Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Extrusion process presents two types of welding mechanism inside the die: seam and charge weld. The first type is unavoidable in the production of hollow profiles while the second is generated during billet to billet transition. Differently from seam weld, charge weld represent an unacceptable defect that require profile discard. Nevertheless charge weld extent and location on extrudates is not evident, thus requiring scrap definition by means of time-consuming trials or to repetitive tests on each batch. In the present paper an industrial four holes porthole die for hollow profiles production was investigated during billet to billet transition. Extrusion of four billet was performed under strict parameter control. The profiles were microscopically analyzed in order to determine weld evolution: in particular start and end points of weld for each profile were determined.


2011 - Creep behaviour modeling of the AISI H11 tool steel for extrusion dies applications [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Aim of an extrusion die is to allow the production of the profile with the required dimension tolerances and quality level. One of the main impediment to achieve this aim could be an excessive die deformation due to the high cyclic loads and temperatures acting on the die during the extrusion process. In order to investigate the mechanisms that influence the die deformation, a physical experiment reproducing the thermo-mechanical conditions of a die was performed on a martensitic tool steel used for extrusion tools (AISI H11). The design of experiment consisted of 4 levels of temperature, 3 levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all cases, the same pattern of the mandrel displacement-time curve was found consisting of 3 stages as those typical of the strain evolution in a standard creep test with a marked primary phase. Thus, with the aim to define an easy-applicable equation to estimate the die deformation, an exponential type law was chosen. In order to obtain the temperature gradient within the specimen coupled thermo-electric simulations were previously performed. The nodal temperature have been then imported within the structural model and the mechanical properties assigned to the each element as a function of these values. Coefficients of the exponential law were optimized, for each testing condition, on the basis of experimental data starting from values for similar alloys found in literature and novel equations formulated to express their dependency on temperature and stress. A good average agreement was found between experimental and numerical results.


2011 - Creep deformation modeling of a tool steel with a tempered martensitic structure used for extrusion dies [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

Aim of an extrusion die is to allow the production of the profile with the required dimension tolerances and quality level. One of the main impediment to achieve this aim could be an excessive die deformation due to the high cyclic loads and temperatures acting on the die during the extrusion process. In order to investigate the mechanisms that influence the die deformation, a physical experiment reproducing the thermo-mechanical conditions of a die was performed on a martensitic tool steel used for extrusion tools (AISI H11). The design of experiment consisted of 4 levels of temperature, 3 levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all cases, the same pattern of the mandrel displacement-time curve was found consisting of 3 stages as those typical of the strain evolution in a standard creep test with a marked primary phase. Thus, with the aim to define an easy-applicable equation to estimate the die deformation, the time hardening creep law was chosen. In order to obtain the temperature gradient within the specimen coupled thermo-electric simulations were previously performed. The nodal temperature have been then imported within the structural model and the mechanical properties assigned to the each element as a function of these values. Coefficients of the time-hardening law were optimized, for each testing condition, on the basis of experimental data starting from values for similar alloys found in literature. The values found were validated against additional experimental data performed with different specimen geometries. A good average agreement was found between experimental and numerical results.


2011 - Experimental investigation and numerical analysis of the die deflection in aluminum extrusion [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In order to assess the capability of numerical simulation in industrial hot aluminium extrusion, an extensive comparison was performed between data recorded during the experiment performed at 2009 ICEB conference and the process simulations performed by three FE codes: ALTAIR HyperXtrude®, COMSOL® Multiphysics and DEFORMTM 3D. In the experimental trials, two U-shaped profiles made of AA6082 alloy were extruded under monitored conditions: profiles exit temperature and velocity, extrusion load, die temperature and die deflection by means of an innovative laser-based method were continuously recorded during the trials. Numerical simulations were performed by means of the decoupled approach: at first, the material flow was computed with each code (DEFORM with an Updated Lagrangian approach, HYPERXTRUDE with an Arbitrarian Lagrangian Eulerian formulation, COMSOL based on a pure Eulerian approach); then the pressure was mapped and applied on the die face for a subsequent die stress analysis.


2011 - INVESTIGATION AND PREDICTION OF CHARGE WELDS IN ALUMINUM EXTRUDED PROFILES [Capitolo/Saggio]
Segatori, Antonio; Reggiani, Barbara; Donati, Lorenzo; Bagagli, Floriano; Tomesani, Luca
abstract

Aluminum extruded profiles are today extensively used in several type of applications like buildings and construction, engineering, transport, up to airplane primary structures. The proprieties and quality of the profiles are strictly related to their final application: if for aesthetical applications (i.e. windows frames or interior design structures) the surface aspect is of primary importance, in more severe loaded conditions (like automotive or aerospace) the mechanical proprieties of the profiles remains of critical importance. Aim of the present study was to investigate the interaction between the different welding phenomena inside an industrial multi-profile die, in order to determine the exact position and the minimal profile length to be discarded. Finite element simulations of the process were then carried out by means of the commercial code Altair® HyperXtrude® to evaluate the numerical code capabilities for a real industrial application. Numerical predictions were then compared to experimental data with a particular focus on the computation of charge weld evolution.


2011 - Matrici per estrusione: Riflettori puntati sull’acciaio AISI H11 [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo; Zhou, Jie; Tomesani, Luca
abstract

Il presente studio è finalizzato all’analisi del comportamento dell’acciaio da utensili per lavorazioni a caldo AISI H11 (X37CrMoV5-1) nel regime di funzionamento di creep-fatica proprio delle matrici per estrusione industriale. Allo scopo, viene presentato un innovativo test tecnologico nel quale il provino replica, in piccola scala, il mandrino delle matrici a ponte ottimizzato nella geometria e nella definizione dei carichi meccanici e termici mediante analisi a elementi finiti. Il piano prove sperimentale prevede 4 livelli di temperature, 3 livelli di sollecitazione, che coprono l’intervallo di valori riscontrati in una matrice durante un processo di estrusione, e 3 tipi ci carico: pura fatica, puro creep e creep-fatica, ovvero carico ciclico con tempo di mantenimento di 3 minuti. Tutte le prove sono eseguite sul simulatore termo-meccanico Gleeble 1500D. Il test si dimostra un utile strumento per investigare l’interazione tra i diversi meccanismi di deformazione nelle matrici per estrusione.


2010 - Creep-fatigue interaction in the AISI H11 tool steel [Relazione in Atti di Convegno]
Reggiani, Barbara; D'Ascenzo, Marco; Donati, Lorenzo; Jie, Zhou; Tomesani, Luca
abstract

The effect of process parameters on the creep-fatigue behavior of a hot-work tool steel for aluminum extrusion die was investigated through a technological test in which the specimen geometry resembled the mandrel of a hollow extrusion die. Tests were performed on a Gleeble thermomechanical simulator by heating the specimen using joule’s effect and by applying cyclic loading up to 6.30 h or till specimen failure. Displacements during the tests at 380, 490, 540 and 580°C and under the average stresses of 400, 600 and 800 MPa were determined. A dwell time of 3 min was introduced during each of the tests to understand the creep behavior. The results showed that the test could indeed physically simulate the cyclic loading on the hollow die during extrusion and reveal all the mechanisms of creep-fatigue interaction.


2010 - EVALUATION OF DIFFERENT FE SIMULATION CODES IN THE STRESS ANALYSIS OF EXTRUSION DIES [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In hot extrusion, die is subjected to complex working conditions as a combination of both high loads and high temperatures. In order to compute the level of damage reached after a fixed number of cycles or the number of cycles to failure, a realistic prediction of the stress entity and distribution is mandatory. In the present work, the comparison of two finite element codes in the predictability of the stress level induced in a die during hot extrusion was presented. The comparison was also supported by experimental investigation. The FE code, DEFORM 3D®, was firstly used to simulate the extrusion process and to predict the force components exerted by the billet on the die. Then, the die stress analysis was performed within the same code. However, only linear tethraedral elements and elastic material model were available. This justified the requirement of a code specifically dedicated to structural analyses in which a wider range of element types and material models could be chosen. Boundary conditions have been transfer between the two FE codes by means of a purposely developed subroutine and the results compared in terms of predicted die deflection and peak Von Mises stress.


2010 - Evaluation of process speed effect in aluminium extrusion by experiment and simulations [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In the first part of the paper, experimental trials performed in the extrusion of 4 L-shaped profiles made by AA6082 aluminium alloy were presented and analyzed in order to investigate the effect of different pockets concepts on process results like extrusion load, profile lengths, die and profile temperature. The trials were performed at 0,5 and 5 mm/sec ram speed thus producing unexpected differences especially in term of profile lengths. Then two FE codes, COMSOL® Multiphysics and DEFORMTM 3D, were applied in the simulation of the problem in order to validate FEM simulations by comparison with the experimental trials. In particular the two codes adopt different formulations for process analysis: eulerian the first updated lagrangian the second, thus producing also an interesting comparison of the solving methods for the prediction of process outputs like loads, final profiles speeds and exit temperatures.


2010 - Extrusion research activities at the University of Bologna-Italy [Relazione in Atti di Convegno]
Donati, Lorenzo; Reggiani, Barbara; Tomesani, Luca
abstract

A review of the activities recently performed at the University of Bologna in the field of aluminum extrusion is here presented. A brief description on the state of the extrusion industry in Italy is initially drawn with a particular focus on the Extrusion district located in the north of Italy. There, University of Bologna is the Italian reference point for the research activities in the field of extrusion technology, process optimization and products characterization. Four main research directions are actually developed at University of Bologna in cooperation with companies and academia: seam and charge welding, process simulation, die design optimization and microstructural evolution of aluminum alloys during and after hot deformation. The paper summaries each activity with a specific focus on the main results, the applications and the possible developments for each topic.


2010 - Preliminary investigation on creep-fatigue regime in extrusion dies [Capitolo/Saggio]
Reggiani, Barbara; Donati, Lorenzo; Zhou, Jie; Tomesani, Luca
abstract

Aim of this work was to investigate the effect of process parameters on the creepfatigue behaviour of a hot-work tool steel for aluminium extrusion die. To this aim, a technological test in which the specimen geometry resembled the mandrel of a hollow extrusion die was developed. Tests were performed on a Gleeble thermomechanical simulator by heating up the specimen using joule’s effect and by applying cyclic loading up to 6.30 h or till specimen failure. During the tests performed at 380°C, 490°C, 540°C and 580°C and under the average stresses of 400, 600 and 800 MPa the displacements of the mandrel were determined. A dwell time of 3 min was introduced in the fatigue cycle to simulate the time required to extrude a single billet and to understand the creep behaviour. The results proved the capability of the test to physically simulate the loading conditions acting on hollow dies during extrusion and revealed the main mechanisms of creep-fatigue interaction.


2010 - The AISI H11 creep-fatigue behaviour: an innovative experimental design [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Zhou, Jie; Tomesani, Luca
abstract

During the hot-extrusion manufacturing process, a number of damage and deformation mechanisms act simultaneously to produce cumulative damage to the tools, thus causing the increasing deviations from the original geometry or the final breaking [1-3]. Due to the severe cyclic thermo-mechanical loads, mandrel, i.e. the part of the hollow die that defines the internal shape of the profile, is the most critical component in the extrusion of an hollow profile. Indeed, the high pressure generated during the process creates severe friction conditions that results in longitudinal tensile stress and significant bending stresses can arise in the bridges of the mandrel during extrusion. In addition to the mechanical cyclic load, the total loading/unloading time for the whole batch and the temperature that the die is exposed to are great enough to necessitate the consideration on the creep behaviour of the die material, particularly for hollow dies. Hence, the combination of dynamic, heavy loading and high temperature determines a hostile working condition for the mandrel that is normally designed on the basis of static loading at elevated temperatures on hot-work tool steels that are tempered to reach an adequate balance of hot hardness and toughness. Premature failure may occur after a certain number of loading and unloading cycles as a result of creep-fatigue interaction. The new technologies developed for aluminium extrusion aim to minimize the tool system-material flow interference and optimize the mechanical performance of the die that is related both to design and tool steel. Aim of the present work is to illustrate the steps followed to design an innovative experimental test purposely developed to investigate the deforming mechanisms of the AISI H11 tool steel in the creep-fatigue regime. The specimens replicate the geometry and the loading scheme of a mandrel on a smaller scale and are manufactured following the same working scheme. In such a way the test is able to account for realistic stress and strain distributions and superficial roughness of a real mandrel as well as to investigate different material and heat treatments.


2010 - The optimization of parts within complex assemblies [Articolo su rivista]
Cosmi, Francesca; Reggiani, Barbara
abstract

Optimization problems are frequently not straightforward. This is due, among other reasons, to the predefined design constraints and simultaneous target goals that have to be satisfied. In addition, uncontrollable variations can influence the robustness and stability of the design. Moreover, engineers often have to optimize mechanical components that are part of complex assemblies, in which it is difficult to extrapolate and define the boundary conditions to be applied for component optimization. The aim of this work is to present a step-procedure for the multi-goal optimization of components that are integrated in multi-part engineering systems. In order to illustrate the method, optimization of a carry-mould, part of a blowing machinery composed by several components, was used as a test case. Minimization of the carry-mould moment of inertia and the global mass were imposed as the target goals of the optimization process, while the maximum allowable displacement was set for a number of control points. The proposed procedure was capable of a time efficient and cost saving extensive investigation of the optimization design space.


2010 - The role of creep and fatigue in determining the high-temperature behaviour of AISI H11 tempered steel for aluminium extrusion dies [Articolo su rivista]
Reggiani, Barbara; Donati, Lorenzo; Zhiu, Jie; Tomesani, Luca
abstract

The present study was aimed to analyze the effect of loading cycles on the behaviour of the AISI H11 tool steel commonly used for aluminium extrusion dies working at high temperatures and under high, cyclic stresses. A technological test method in which the specimen geometry resembled the mandrel of a hollow extrusion die was developed. Finite element analyses were performed to aid in determining specimen geometry and dimensions as well as the levels of stress to be applied to the specimen so as to replicate the conditions typically encountered by industrial hollow extrusion dies. Tests were performed on a Gleeble thermomechanical simulator by heating the specimen using Joule’s effect and by applying loading for up to 6.30 h or till specimen failure. Displacements during the tests at 380, 490, 540 and 580 ◦C and under the average stresses of 400, 600 and 800MPa were determined. The specimens were tested under creep (with the load held at a fixed value), fatigue (cyclic loading) and creep–fatigue (cyclic loading with a 3 min dwell-time) loading, thereby allowing a direct comparison between different deforming mechanisms. The results showed that the test could physically simulate the cyclic loading on the hollow die during aluminium extrusion and that the creep condition represented the most severe working condition. In addition, the tests could reveal the interaction between creep and fatigue mechanisms.


2010 - Thermal-electric simulations for the temperature setting in a creep-fatigue test [Relazione in Atti di Convegno]
Reggiani, Barbara; Donati, Lorenzo; Tomesani, Luca
abstract

In a previous works [1] an innovative technological test has been presented developed by this research group. Analytical models available in the literature to describe the material behaviour in the creep-fatigue regime are currently under investigation to be validated against this experimental campaign. Aim of the present work is to present the first step of this investigation in which the thermal distribution of the specimen has been replicated by means of thermo-electrical simulations. Preliminary comparison between experimental and numerical results on the creep behaviour are also presented.


2009 - Multi-goal optimization of a carry-mould [Articolo su rivista]
Reggiani, Barbara; COSMi, Francesca
abstract

A common engineering task is the optimization of components that are part of multibody assemblies, in which it is difficult to extrapolate and define the boundary conditions to be applied for the component optimization. This work presents a procedure for multi-goal optimization of components that are integrated in multipart engineering systems. The efficiency of the procedure is illustrated by means of a test case, a carry-mould that is part of a multicomponent blowing machinery. Target goals of the optimization process were the minimization of moment of inertia and of global mass and the maximum allowable displacement in a number of control points.


2008 - Application of optimization techniques to components of complex assemblies [Abstract in Atti di Convegno]
Cosmi, Francesca; REGGIANI, Barbara
abstract


2008 - Effetto supporto, tensione locale e gradiente relativo di tensione nella predizione della vita a fatica [Articolo su rivista]
Reggiani, Barbara; Freddi, Alessandro
abstract

Obiettivo ultimo delle ricerche nelle quali si inquadra questo studio è superare, o almeno limitare, la necessità di complicate e costose prove sperimentali a favore di altri metodi predittivi della vita a fatica. Scopo specifico del presente lavoro è, dopo una analisi critica delle diverse soluzioni proposte in letteratura, la valutazione di una formulazione atta a predire, tramite analisi agli Elementi Finiti (FEM), la vita a fatica locale, nella regione ad alto numero di cicli, di componenti di forma complessa con una distribuzione irregolare di tensione. Uno dei concetti base di questa teoria è che il fenomeno della vita a fatica sia governato dal gradiente relativo di tensione nella zona maggiormente sollecitata del componente. Le simulazioni proposte per il calcolo della pendenza delle curve S-N e del limite di fatica sono verificate tramite il confronto con dati sperimentali tratti da letteratura. Mentre la formulazione per il calcolo del limite di fatica mostra un buon accordo con i dati sperimentali (errore medio percentuale del 7%), la pendenza, valutata mediante l’equazione proposta, risulta in alcuni casi sovrastimata (errore medio percentuale oltre il 100%). Nel presente lavoro si propone quindi una nuova formula che permette di ottenere un errore medio percentuale sulla pendenza dell’ordine del 20% nel confronto con i dati sperimentali.


2008 - Sensitivity of the primary stability of a cementless hip stem to its position and orientation [Articolo su rivista]
Reggiani, Barbara; Cristofolini, Luca; Taddei, Fulvia; Viceconti, Marco
abstract

Abstract: Using computed tomography (CT)-based preoperative planning software, we can define with good accuracy the position of a cementless hip stem inside the host bone, but previous studies suggest that the pose the surgeon achieves during freehand surgery may differ from the planned one even by some millimeters. Advances in simulation now make it possible to predict the primary stability of the stem in a given position during the preoperative planning, but is the stability predicted for the planned pose indicative of that we can expect for the achieved pose? The aim of the present study was to verify how this prediction is affected by the differences observed between the planned and the achieved poses. Two finite element models of an implanted femur were generated, one with the stem in the planned pose, and one with the stem in the achieved pose, as defined from postoperative CT scans. When compared to experimental measurements, the model with the achieved position was clearly more accurate (0.6 vs. 12% error over measured peak micromotion); however, the predictions of induced micromotions were different between the two models for less than 13%. It is thus concluded that while the implant position does have an effect on primary stability, the estimate of micromotion we can get from the planned position remains a clinically relevant indicator.


2008 - Step procedure for multi-goal optimum design of a carry-mould [Relazione in Atti di Convegno]
Reggiani, Barbara; Cosmi, Francesca
abstract

The optimum design of mechanical components is the one meeting all requirements specified by engineers, so resulting to be as effective as possible in terms of performance and reliability. Frequently, optimization problems are not so straightforward to solve due to different reasons. For instance, in many real problems, minimization of a single function may not be the only goal, but attention must also be directed to satisfaction of predefined constraints placed on the design. Moreover, simultaneous optimization of more than one objective function may be required. Besides, mechanical components are often part of complex assemblies, thus making it difficult to extrapolate and define the boundary conditions to be applied in the component for its design optimization. In the present work, a stepprocedure for the multi-goal optimization of a benchmark component is presented.


2007 - Predicting the subject-specific primary stability of cementless implants during pre-operative planning: preliminary validation of subject-specific finite-element models [Articolo su rivista]
Reggiani, Barbara; Cristofolini, Luca; Varini, Elena; Viceconti, Marco
abstract

Pre-operative planning help the surgeon in taking the proper clinical decision. The ultimate goal of this work is to develop numerical models that allow the surgeon to estimate the primary stability during the pre-operative planning session. The present study was aimed to validate finite-element (FE) models accounting for patient and prosthetic size and position as planned by the surgeon. For this purpose, the FE model of a cadaveric femur was generated starting from the CT scan and the anatomical position of a cementless stem derived by a skilled surgeon using a pre-operative CT-based planning simulation software. In-vitro experimental measurements were used as benchmark problem to validate the bone–implant relative micromotions predicted by the patient-specific FE model. A maximum torque in internal rotation of 11.4Nm was applied to the proximal part of the hip stem. The error on the maximum predicted micromotion was 12% of the peak micromotion measured experimentally. The average error over the entire range of applied torques was only 7% of peak measurement. Hence, the present study confirms that it is possible to accurately predict the level of primary stability achieved for cementless stems using numerical models that account for patient specificity and surgical variability. r 2006 Elsevier Ltd. All rights reserved.


2006 - Asessment of a local S/N curve theory for the fatigue life calculation on experimental data [Relazione in Atti di Convegno]
Reggiani, Barbara; Zambonelli, Luca
abstract

ABSTRACT: A theory for the fatigue life calculation based on local S/N curves, computed from the finite element results, has been analysed by the comparison with experimental data. In a first analysis, a good agreement in the general trend of data as function of the relative stress gradient, c’, has been observed. This means that an exponential decreasing of the slope K and the number of cycle at the fatigue limit ND over the stress gradient was observed. Nevertheless, an absolute error following an exponential low came out from the comparison of the experimental versus the predicted values of K and ND. This low should be computed for each analysed data set, requiring, however, more than two generally data set as indicated in the original theory, and used to refine the proposed formulas to calculate K and ND.


2006 - Evaluating the Flexural Stiffness of Compliant Hinges Made with Close-wound Helical Springs [Relazione in Atti di Convegno]
Lotti, Fabrizio; Zucchelli, Andrea; Reggiani, Barbara; Vassura, Gabriele
abstract

Advantages and problems related to the use of compliant hinges in articulated robotic structures are briefly discussed. A novel kind of elastic joint made with close-wound helical springs is then described. It is capable of large angular displacement so that it can be conveniently applied in manipulation devices like the humanoid robot hand developed at the University of Bologna. The results of this application encourage a systematic investigation on the properties of this kind of joints, not previously described in the literature, aiming at mechanical characterization and definition of design criteria. To this purpose, the paper outlines a general investigation programme, where theoretical models, Finite Elements analysis and experiments jointly contribute to the evaluation of the hinge stiffness and to the identification of influential design parameters. Preliminary results related to evaluation of the stiffness about the principal bending axis are then examined and discussed, comparing the results obtained from experiments with those achieved by means of a simplified mathematical model and the correspondent FE analysis. An auxiliary parameter is finally introduced in order to define a general criterion for the design of spring-based hinges subjected to large deflection.


2006 - Finite element analysis of a total ankle replacement during the stance phase of gait. [Articolo su rivista]
Reggiani, Barbara; Leardini, Alberto; Federico, Corazza; Mark, Taylor
abstract

Total ankle replacement (TAR) designs have still several important issues to be addressed before the treatment becomes fully acceptable clinically. Very little is known about the performance, in terms of the contact pressures and kinematics of TAR when subjected to daily activities such as level gait. For this purpose, an explicit finite element model of a novel 3-component TAR was developed, which incorporated a previously validated mechanical model of the ankle ligament apparatus. The intermediate mobile polyethylene meniscal bearing was modelled as an elastic-plastic continuum while the articulating surfaces of the tibial and talar metal components as rigid bodies. Overall kinematics, contact pressures and ligament forces were analysed during passive, i.e. virtually unloaded, and active, i.e. stance phase of gait, conditions. Simulation of passive motion predicted similar kinematics as reported previously in an analytical four-bar linkage model. The meniscal bearing was observed to move 5.6 mm posteriorly during the simulated stance and the corresponding antero-posterior displacement of the talar component was 8.3 mm. The predicted pattern and the amount (10.6 degrees ) of internal-external rotation of the ankle complex were found to be in good agreement with corresponding in vivo measurements on normal ankles. A peak contact pressure of 16.8 MPa was observed, with majority of contact pressures below 10 MPa. For most ligaments, reaction forces remain within corresponding physiological ranges. A first realistic representation of the biomechanical behaviour of the human ankle when replaced by prosthetic joints is provided. The applied methodology can potentially be applied to other TAR designs.


2006 - Finite-element modeling of bones from CT data: sensitivity to geometry and material uncertainties [Articolo su rivista]
Taddei, Fulvia; Martelli, Saulo; Reggiani, Barbara; Cristofolini, Luca; Viceconti, Marco
abstract

The aim of the present work was to investigate the influence that the uncertainties related to the segmentation process and to the material assignment procedures may have onto the results of a finite element analysis of a human femur generated from Computer Tomography (CT) data. Starting from a CT dataset of a normal femur, the finite element model was generated using an already validated procedure. The errors associated to the segmentation algorithm adopted, to the CT dataset calibration and to the material assignment procedure were estimated. Simplified loading conditions were applied to the femur to generate a compressive bending and torque loading. The distribution of the mostly used biomechanical indicators was analyzed. The results showed that the level of influence of the input variable is dependent on the load case and on the considered output variable. The coefficients of variation were, however, always smaller than 10%, for all variables.


2006 - Verification of a stress gradient theory for the fatigue life predicition of components in case of pure push-pull and pure torsion conditions [Relazione in Atti di Convegno]
Reggiani, Barbara; Freddi, Alessandro
abstract

A relevant number of theories has been proposed in the last 50 years for calculation of the fatigue limits of components containing notches or more general geometrical discontinuities. Beyond the most classical ones, some methods correlate the life to the relative stress gradient, allowing to overcome the limits of the factors Kt and Kf not definable for generally complex shaped components. On this direction, a recent and very promising formulation has been proposed [11,12] to predict the local S/N curve on the base of a Finite Elements Analysis of the stress gradient and on an empirical formulation of the support factor nx.Aim of the present study is to validate the present theory in the case of shafts in pure torsion and in push-pull, comparing the predicted values with experimental data obtained in two different test campaigns.


2005 - Accuracy of the planned vs achieved position of a cementless hip stem: a finite element study [Relazione in Atti di Convegno]
Reggiani, Barbara; Viceconti, Marco; Cristofolini, Luca
abstract

The implant of an hip stem into the femoral medullary cavity consists of three steps: the resection of the femoral head, the rasping of the femoral canal and the placement of the stem. Priority for cemtless hip implants, in order to achieve a good level of primary stability, is the accuracy with which the stem is positioned in the host bone. An erroneous initial positioning could lead to the implant instability promoting the ultimate failure of the implant 1. Initial excessive relative micromotions at the bone-implant interface may inhibit the bony in-growth and secondary long term fixation 2,3. The final objective of the early researches along this line is to arrange a set of instruments to predict the primary stability in the pre-operative planning moving toward a less and minimally invasive surgical technique. Nevetheless, even assuming a perfect surgical planning, there is still the practical problem of correct positioning of the stem in the femur during surgery. Aim of the present study was to asses the sensitivity of the relative bone-implant micromotions, stresses and strains to the implant position as planned and achieved by the surgeon respectively before and after the operation. For this purpose, the subject-specific finite element (FE) model of a cadaveric femur, accounting for patient and surgeon, was derived from pre-operative and post-operative CT scans. The overall aim was to verify if the pre-clinical planning correctly matches the achieved implant stability conditions and hence if it can be considered as a powerful tool to train the surgeon in taking the appropriate clinical decisions.


2005 - How accurately pre-operative planning matches the achieved surgery: a finite element study. [Relazione in Atti di Convegno]
Reggiani, Barbara; Viceconti, Marco
abstract

Pre-operative planning trains the surgeon in the workshop environment. The final objective of the early researches along this line is to arrange a set of instruments to predict the primary stability in the pre-operative planning moving toward a less and minimally invasive surgical technique. Nevetheless, even assuming a perfect surgical planning, there is still the practical problem of correct positioning of the stem in the femur during surgery. An erroneous initial positioning could lead to the implant instability promoting the ultimate failure of the implant [1,2]. Initial excessive relative micromotions at the bone-implant interface may inhibit the bony in-growth and secondary long term fixation [3,4]. To achieve a good level of primary stability the surgery technique play therefore a fundamental role. Aim of the present study was to asses the sensitivity of the relative bone-implant micromotions, stresses and strains to the surgical parameters as planned and achieved by the surgeon respectively before and after the operation. For this purpose, the subject-specific finite element (FE) model of a cadaveric femur, accounting for patient and surgeon, was derived from pre-operative and post-operative CT scans. The specific aim was to verify if finite element models based on pre-clinical planning correctly match the achieved implant stability conditions.


2005 - MAL-POSITIONING OF A NOVEL TOTAL ANKLE REPLACEMENT: A SENSITIVITY STUDY [Relazione in Atti di Convegno]
Reggiani, Barbara; Leardini, Alberto; Corazza, Federico; Taylor, Mark
abstract

Overall biomechanical behaviour of replaced ankle joints is dependent also on the position and orientation of the prosthesis components relative to the bone and the soft tissues. Mal-positioned compoenents could lead to instability promoting the ultimate failure of the implant. The aim of the present work was investigating the sensitivity of a mobile bearing ankle replacement to prosthesis component final position. For this purpose, a previosuly validated explicit finite element (FE9 model of a novel ankle prosthesis, which includes also eight major ligaments was used to asses the influence of prosthesis mal-positioning relative to the ligaments to joint kinematics, polyethylene contact pressure and stress ligament forces.


2005 - MODELLAZIONE NUMERICA DI ARTICOLAZIONI CEDEVOLI PER DITA DI MANI ROBOTICHE [Relazione in Atti di Convegno]
Reggiani, Barbara; Zucchelli, Andrea; Vassura, Gabriele
abstract

Abstract In the present work, the flexural stiffness of a close-wound cylindrical spring for a novel joint of robotic hands is developed by means of the finite element (FE) method. In the first step, the FE model of the spring has been generated and verificated to guarantee its numerical accuracy. Experimental measurements and theoretical results on close-wound springs in the same geometrical configurations here analysed were used as benchmark to compare predictions of the developed FE models. A good agreement was found in terms of stiffness (Nmm/rad) between numerical versus experimental and theoretical analysis at the same rotation (90°) with a root mean square error respectively of 11% and 8%.


2005 - Subject -specific FE model for the prediction of the relative micromotion in a total hip implant: verification and validation [Relazione in Atti di Convegno]
Reggiani, Barbara; Cristofolini, Luca; Varini, Elena; Viceconti, Marco
abstract

The most common reason for the aseptic loosening of cementless hip stem is the lack of primary stability as the presence of an excessive relative micromotion at the boneimplant interface [1]. Despite pre-clinical validation has remarkably improved over the last few years, some important factors affecting implant biomechanics have still to be considered. Most of the previous numerical studies on hip arthroplasty were based on models of composite femurs. However, subject-specific factors (skeletal anatomy, mechanical properties, implant position) are needed to produce helpful outcomes for clinical practice. In addition, these models were developed using time consuming structured meshes frequently lacking of careful verification and validation phases. Aim of the present work is the development of a Finite Element (FE) model of an implanted femur taking into account the specificity of subject as well as the planned surgery technique parameters. Specific aims are the verification of the numerical accuracy of the FE model and the analysis of the predicted relative micromotions compared to experimental results (validation).


2004 - Analysis of simulated gait for a novel total ankle replacement using explicit FEA [Relazione in Atti di Convegno]
Reggiani, Barbara; Leardini, Alberto; Federico, Corazza; Vincenzo Parenti, Castelli; Mark, Taylor
abstract

The polyethylene stresses at a replaced ankle are dependent on the kinematics of the joint, which in turn are dependent on the implant design, position and orientation of the prosthesis relative to bone and soft tissue tensions. In order to assess the polyethylene stresses the kinematics of the joint must be accounted for. Previous studies of total knee replacement have shown that explicit FE analysis is able to predict the kinematics and stresses simultaneously. The aim of the present work was develop an explicit FE model of a mobile bearing total ankle replacement able to predict joint kinematics, polyethylene stresses and ligaments forces during walking.


2004 - Sensitivity analysis of uncertainties to geometry and material properties in finite element models [Relazione in Atti di Convegno]
Reggiani, Barbara; Taddei, Fulvia; Baleani, Massimiliano; Viceconti, Marco; Curioni, Sergio; Cristofolini, Luca
abstract

Finite element (FE) method is widely used to analyse the structural behaviour of the most mechanical systems. These are affected by a natural variability of geometric parameters and material properties, especially dealing with biomechanical system. Therefore, input variables should be defined not by a single value but trough a suitable statistical distribution. Although several authors agreed on the resulting of these uncertainties on the FE model prediction, to date only few probabilistic studies has been performed. In this paper, a sensitivity analysis was carried out on an FE model of a bone cement specimen subjected to a static tensile test analysing the influence that geometric and material properties uncertainties have onto the maximum tensile stress pointed out in three region of interest. Results were compared with the analytical exact solution of the problem from the simple beam theory. The aim of the present work was to validate a statistical procedure of incorporating potential sources of errors into a finite element analysis and to quantify their influence on the accuracy of the output parameters.


2003 - Finite element modelling of human bones from ct data: a sensitivity study on the influence of geometry and material uncertainties [Relazione in Atti di Convegno]
Taddei, Fulvia; Reggiani, Barbara; Martelli, Saulo; Bordini, Barbara; Viceconti, Marco
abstract

The aim of the present work was to investigate the influence that the uncertainties related to the segmentation process and to the material assignment procedures may have onto the results of a finite element analysis of a human femur generated from Computer Tomography (CT) data. Starting from a CT dataset of a normal femur, the finite element model was generated using an already validated procedure. The errors associated to the segmentation algorithm adopted, to the CT dataset calibration and to the material assignment procedure were estimated. Simplified loading conditions were applied to the femur to generate a compressive bending and torque loading. The distribution of the mostly used biomechanical indicators was analyzed. The results showed that the level of influence of the input variable is dependent on the load case and on the considered output variable. The coefficients of variation were, however, always smaller than 10%, for all variables.