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Lucia DENTI

Professore Associato
Dipartimento di Ingegneria "Enzo Ferrari"

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Pubblicazioni

2023 - Beads for Cell Immobilization: Comparison of Alternative Additive Manufacturing Techniques [Articolo su rivista]
Gatto, M. L.; Mengucci, P.; Munteanu, D.; Nasini, R.; Tognoli, E.; Denti, L.; Gatto, A.
abstract

The attachment or entrapment of microbial cells and enzymes are promising solutions for various industrial applications. When the traps are beads, they are dispersed in a fluidized bed in a vessel where a pump guarantees fresh liquid inflow and waste outflow without washing out the cells. Scientific papers report numerous types of cell entrapment, but most of their applications remain at the laboratory level. In the present research, rigid polymer beads were manufactured by two different additive manufacturing (AM) techniques in order to verify the economy, reusability, and stability of the traps, with a view toward a straightforward industrial application. The proposed solutions allowed for overcoming some of the drawbacks of traditional manufacturing solutions, such as the limited mechanical stability of gel traps, and they guaranteed the possibility of producing parts of constant quality with purposely designed exchange surfaces, which are unfeasible when using conventional processes. AM proved to be a viable manufacturing solution for beads with complex shapes of two different size ranges. A deep insight into the production and characteristics of beads manufactured by AM is provided. The paper provides biotechnologists with a manufacturing perspective, and the results can be directly applied to transit from the laboratory to the industrial scale.

2023 - Influence of Trabecular Geometry on Scaffold Mechanical Behavior and MG-63 Cell Viability [Articolo su rivista]
Gatto, Maria Laura; Cerqueni, Giorgia; Furlani, Michele; Riberti, Nicole; Tognoli, Emanuele; Denti, Lucia; Leonardi, Francesco; Giuliani, Alessandra; Mattioli-Belmonte, Monica; Mengucci, Paolo
abstract

In a scaffold-based approach for bone tissue regeneration, the control over morphometry allows for balancing scaffold biomechanical performances. In this experimental work, trabecular geometry was obtained by a generative design process, and scaffolds were manufactured by vat photopolymerization with 60% (P60), 70% (P70) and 80% (P80) total porosity. The mechanical and biological performances of the produced scaffolds were investigated, and the results were correlated with morphometric parameters, aiming to investigate the influence of trabecular geometry on the elastic modulus, the ultimate compressive strength of scaffolds and MG-63 human osteosarcoma cell viability. The results showed that P60 trabecular geometry allows for matching the mechanical requirements of human mandibular trabecular bone. From the statistical analysis, a general trend can be inferred, suggesting strut thickness, the degree of anisotropy, connectivity density and specific surface as the main morphometric parameters influencing the biomechanical behavior of trabecular scaffolds, in the perspective of tissue engineering applications.

2022 - Performance Analysis of Electro-chemical Machining of Ti-48Al-2Nb-2Cr Produced by Electron Beam Melting [Articolo su rivista]
Galati, M.; Defanti, S.; Denti, L.
abstract

Ti-48Al-2Nb-2Cr is a challenging and difficult-to-cut titanium aluminide (TiAl) alloy with several manufacturing issues because of the high sensitivity to crack formation and oxygen picking up. Electron beam powder bed fusion (EB-PBF) made feasible TiAl near net shape components, but the surfaces are particularly rough and present complex surface topographies. In this present investigation, experimental analysis and optimization are proposed for electro-chemical machining (ECM) on as-built Ti-48Al-2Nb-2Cr surfaces manufactured using EB-PBF. Experimental runs are performed under pulsed machining conditions and varying specific process metrics to understand the machining effects on the process efficiency and removal phenomena. In particular, the morphology and isotropy of the surface are studied before and after the machining by scanning electron and confocal microscopies. The results establish the optimal machining conditions and a range for the active machining time that produce, compared to the as-built surface, an extremely smooth and isotropy surface without any detrimental effect on the surface integrity and microstructure.

2021 - Surface roughness prediction model for Electron Beam Melting (EBM) processing Ti6Al4V [Articolo su rivista]
Galati, M.; Rizza, G.; Defanti, S.; Denti, L.
abstract

Electron Beam Melting (EBM) is an Additive Manufacturing technique to produce functional components. Because of the high temperature during the EBM process, the surface texture of the as-built parts is extremely complex and unique. This distinctiveness of the surface depends on many factors and needs to be well understood to predict final surface properties accurately. Chief among these factors is the surface design. A proper surface design makes it possible to tailor a surface with specific properties such as biomimetics. However, predictive models are difficult to determine especially for downskin surfaces. To properly tailor a surface, a full factorial Design Of Experiment (DOE) was designed, and 2D and 3D roughness profiles were collected on an ad-hoc artefact using a profilometer and a confocal profilometer. This reference part comprises several surfaces to investigate the effect on surface roughness of different sloping angles, including upskin and downskin surfaces and cavities. The data are analysed using descriptive and inferential statistical tools, also by distinguishing the role of roughness and waviness in the overall surface texture. A deep investigation of the causes of surface roughness made it possible to obtain analytical predictive models. These models are robust and consistent with respect to the experimental observations. Finally, the accurate design of the artefact allows highlighting the relationship between the roughness and the surface slope.

2020 - Development of a reliable method for contamination detection in raw metal powders for additive manufacturing [Relazione in Atti di Convegno]
Santecchia, E.; Mengucci, P.; Gatto, A.; Bassoli, E.; Denti, L.; Bondioli, F.; Barucca, G.
abstract

The evolution of additive manufacturing draws a growing attention to the quality of the raw materials involved. During Powder Bed Fusion (PBF), impurities can be trapped by the melting action of the laser or electron beam followed by a rapid solidification, resulting in discontinuities of the sintered products where failure is likely to start. To increase the quality of the sintered parts, it is mandatory to minimize the presence of impurities in the final product and therefore the risk of failure under service. This result can be achieved by improving the quality of the raw metal powders, which plays a key role in the overall process. In this study, intentionally contaminated AM metal powders were characterized by coupling scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques with a statistical treatment of the data, obtaining a reliable method for powder impurities detection.

2020 - Disclosing the build-up mechanisms of multi jet fusion: Experimental insight into the characteristics of starting materials and finished parts [Articolo su rivista]
Galati, M.; Calignano, F.; Defanti, S.; Denti, L.
abstract

Multi Jet Fusion (MJF) is an emerging additive manufacturing (AM) technique that enables the production of prototypes and functional parts starting from a thermoplastic-based powder, mainly polyamide 12 (PA12). Layer upon layer, the polymeric particles are selectively impregnated with two different inks and then fused and consolidated by an infrared (IR) lamp. Much faster than other AM techniques for polymers, MJF has shown exciting potentialities. However, little is known about the consolidation mechanisms acting in MJF and about the effect on the properties of the finished parts of the quality of the materials, powder and inks, and of the printing conditions. The present contribution investigates these issues. The study also compares virgin PA12 powder with pure PA12, recycled PA12 for MJF and PA12 for in selective laser sintering (SLS). The powders showed slight differences. The two inks have the same composition, except for the presence of graphitic carbon. Tensile tests showed that the printed parts are isotropic. However, the deformation at break is affected by building direction of the sample. Occasionally, poor inter-layer adhesion is observed and the tensile strength and the deformation at break collapse. Printed tensile specimens are found to be representative of the material behaviour of a printed component, apart from the deformation at break which is systematically overestimated.

2020 - Laser Powder Bed Fusion: Tailoring the microstructure of alloys for biomedical applications [Relazione in Atti di Convegno]
Santecchia, E.; Mengucci, P.; Gatto, A.; Bassoli, E.; Denti, L.; Rutkowski, B.; Barucca, G.
abstract

Additive manufacturing (AM) is particularly attractive for biomedical applications, where complex geometries and a high degree of individualization are required. Laser powder bed fusion (LPBF) is an AM technology exploiting the action of a solid-state laser to locally melt a metal powder according to a computer aided design (CAD) model. In the present study, the EOS Cobalt Chrome SP2 (Co-Cr-Mo-W) and Ti64 (Ti6Al4V) powders were sintered by the system equipped with a Yb fiber laser. During LPBF, the Co-Cr-Mo-W metal powder undergoes total melting followed by rapid cooling, giving rise to athermal martensitic phase transformation from the high-temperature ? (fcc) phase to the low-temperature ? (hcp) phase. This produces an intricate network of thin ?-lamellae inside the ? phase matrix. After the firing cycle this structure is maintained, and a massive presence of coarse precipitates is also observed. Owing to the rapid cooling taking place during LPBF, in Ti6Al4V sintered samples only the acicular martensitic a' phase is present. The firing cycle induces the ß phase formation at the a plate boundaries and this microstructure leads to reduced values of strength, with respect to those of the as-sintered samples. The highlighted behaviors show that by tuning the post-production heat treatments it is possible to tailor the microstructure and the mechanical properties.

2020 - Preliminary assessment of electro-chemical machining for aluminum parts produced by laser-based powder bed fusion [Articolo su rivista]
Defanti, S.; Denti, L.; Vincenzi, N.; Gatto, A.
abstract

Electro-chemical machining (ECM) is a nonconventional machining process based on the anodic dissolution of the workpiece. The peculiar features of this process make it suitable for application in the aerospace, automotive, or medical fields where laser-based powder bed fusion (L-PBF) is consolidating as a manufacturing solution for high-performance components. The roughness of as-built L-PBF parts often requires surface finishing before usage in order to enable a correct operation as well as to prevent early fatigue failure. The viability of ECM on L-PBF components is still scarcely investigated in the literature. In this article, the process was applied to AlSi10Mg parts produced by L-PBF. An experimental plan was designed to select the process parameters and to study their effect on the surface roughness and morphology. Process variables including feed rate, time, voltage, and water pressure were investigated. As a result, it was observed that the ECM performance was different for parts produced by traditional processes or by L-PBF, even for comparable composition. Owing to the presence of satellite particles on L-PBF surfaces, ECM was only effective in the pulsed mode.

2020 - Technological Feasibility of Lattice Materials by Laser-Based Powder Bed Fusion of A357.0 [Articolo su rivista]
Sola, A.; Defanti, S.; Mantovani, S.; Merulla, A.; Denti, L.
abstract

Lattice materials represent one of the utmost applications of additive manufacturing. The promising synergy between additive processes and topology optimization finds full development in achieving components that comprise bulky and hollow areas, as well as intermediate zones. Yet, the potential to design innovative shapes can be hindered by technological limits. The article tackles the manufacturability by laser-based powder bed fusion (L-PBF) of aluminum-based lattice materials by varying the beam diameter and thus the relative density. The printing accuracy is evaluated against the distinctive building phenomena in L-PBF of metals. The main finding consists in identification of a feasibility window that can be used for development of lightweight industrial components. A relative density of 20% compared with fully solid material (aluminum alloy A357.0) is found as the lowest boundary for a 3-mm cell dimension for a body-centered cubic structure with struts along the cube edges (BCCXYZ) and built with the vertical edges parallel to the growth direction to account for the worst-case scenario. Lighter structures of this kind, even if theoretically compliant with technical specifications of the machine, result in unstable frameworks.

2019 - Effect of Three Different Finishing Processes on the Surface Morphology and Fatigue Life of A357.0 Parts Produced by Laser‐Based Powder Bed Fusion [Articolo su rivista]
Gatto, Andrea; Bassoli, Elena; Denti, Lucia; Sola, Antonella; Tognoli, Emanuele; Comin, Andrea; Porro, Juan Antonio; Cordovilla, Francisco; Angulo, Ignacio; Ocaña, Jose Luis
abstract

A357.0 parts are processed by laser-based powder bed fusion and surface finished via plastic media blasting, ceramic sand blasting, and laser shock processing. The morphological analysis proves that plastic media blasting causes the most effective peak removal, the most efficient decrease in valley depth, and the greatest reduction in surface roughness. All the surface finishing processes enhance the fatigue life, however ceramic sand blasting bring about the greatest increase in the value of σmax for an infinite fatigue life limit of 2106 cycles. The experimental results suggest therefore that the infinite fatigue life value is more sensitive to the residual stress state engendered by ceramic sand blasting than to the reduction in surface roughness. Breakthrough cracks start at the interface between crushed or modified surface particles and the underlying macro-surface. However, at a distance of a few hundred microns from the crack initiation point, the fracture surface morphology become cellular for all the specimens.

2019 - Effect of powder recycling in laser-based powder bed fusion of Ti-6Al-4V [Articolo su rivista]
Denti, L.; Sola, A.; Defanti, S.; Sciancalepore, C.; Bondioli, F.
abstract

Additive manufacturing (AM) has shown promise to process parts for end-use applications, however stringent requirements must be fulfilled in terms of reliability and predictability. The expensiveness of raw materials for AM, especially for metal-based Powder Bed Fusion (PBF), brings about the need for a careful recycling of powder, but the effect of powder reuse on both processing conditions and final part performance is still the focus of intensive research in the open literature. Although ASTM F2924-14 specifies the virgin-to-used powder ratio to be introduced to manufacture titanium-6aluminum-4vanadium (Ti-6Al-4V) components by PBF, a deeper understanding of the effect of powder recycling on the mechanical properties of finished parts is expected to foster a more efficient and safe reuse. The present contribution is therefore addressed to investigate the consequence of Ti- 6Al-4V powder recycling on the flowability, particle size distribution and morphology of the feedstock material as well as on the density and tensile performance of built parts. In order to quantify the recyclability of powders, a new "average usage time" (AUT) parameter is defined to account for both the real usage time of the powder and the virgin-to-used powder mixing ratio. The new parameter, whose applicability can be readily extended to any kind of feedstock powder, offers a significant contribution to achieve a more consistent and economical recycling of raw materials for PBF processing.

2019 - Efficacy of a Copper-Calcium-Hydroxide Solution in Reducing Microbial Plaque on Orthodontic Clear Aligners: A Case Report [Articolo su rivista]
Meto, A.; Colombari, B.; Castagnoli, A.; Sarti, M.; Denti, L.; Blasi, E.
abstract

The aim of this study was to investigate the ability of a copper-calcium-hydroxide-based compound to remove microbial plaque naturally produced onto orthodontic clear aligners. A commercially available dental paste, named Cupral, based on copper-calcium-hydroxide, was used. A healthy volunteer (female, 32 years old), undergoing orthodontic treatment with thermoplastic clear aligners was enrolled. By conventional/confocal microscopy and colony-forming unit (CFU) assay, 2-week used aligners were examined for microbial plaque, prior and following exposure to Cupral. Confocal microscopy revealed abundant plaque irregularly distributed onto the aligner surface. Following Cupral treatment, a drastic decrease occurred in plaque thickness and matrix presence. As assessed by the CFU assay, total microbial load approached 10 9 CFUs/aligner, with slight differences in aerobiosis and anaerobiosis culture conditions; six macroscopically different types of colonies were detected and identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Following Cupral treatment, microbial load dropped to undetectable levels, irrespectively of the conditions considered. Exposure of clear aligners to Cupral results in the elimination of contaminating microorganisms; the antimicrobial activity is retained up to 1.25% concentration. Overall, our data describe a novel use of Cupral, a copper-calcium-hydroxide-based compound, in daily hygiene practices with promising results.

2019 - Enhancing the quality of metal powder feedstock for laser PBF through cross-contamination removal [Relazione in Atti di Convegno]
Santecchia, E.; Mengucci, P.; Gatto, A.; Bassoli, E.; Denti, L.; Gheorghiu, B.; Barucca, G.
abstract

The presence of impurities in metal powder feedstock for laser powder bed fusion (L-PBF) can strongly affect the mechanical properties of the sintered part. As a matter of fact, the contamination particles trapped in the uniform metal matrix of the raw material, act as a discontinuity and, therefore, as a site for fatigue crack nucleation and growth, leading to a dramatic reduction of the expected fatigue life. While cross-contamination detection is a key parameter to establish the quality level of the metal powder feedstock, its removal is crucial to ensure that the sintered parts will fulfil the service requirements. In this study, metal powder feedstock for laser PBF were intentionally contaminated, submitted to a removal process based on their magnetic properties, through suitably developed equipment. The contaminated and processed powders were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques.

2019 - Experimental approach to measure the restraining force in deep drawing by means of a versatile draw bead simulator [Articolo su rivista]
Bassoli, E.; Sola, A.; Denti, L.; Gatto, A.
abstract

The extreme personalization of the sports car industry requires the development of reliable and affordable techniques for the in-house set-up and control of processing conditions in deep drawing of body-in-white parts. In order to cope with this urgent need, a handy draw bead simulator (DBS) is proposed to measure the restraining force exerted on a sheet metal by the draw bead during deep drawing. The apparatus can be integrated into a common tensile testing machine and the male-female parts of the DBS are variable to account for different geometries of the draw bead and for different thicknesses of the sheet metal. For validation, the DBS is operated to reproduce the effect of a draw bead working on an Al6014-T4 strip according to assigned industrial conditions. Tensile tests repeated on the metal strip after drawing open the way to quantify work hardening effects. The new DBS can be applied to measure the restraining force and investigate the specific role of the draw bead in deep drawing, but it may also be used to verify the predictivity of computational models, thus supporting the development of simulations for feasibility studies and tool design optimization.

2019 - Fatigue life and microstructure of additive manufactured Ti6Al4V after different finishing processes [Articolo su rivista]
Denti, L.; Bassoli, E.; Gatto, A.; Santecchia, E.; Mengucci, P.
abstract

Finishing methods of additive manufactured metal parts are becoming a key driver of industrial viability, increasingly with additive processes being challenged in demanding end-product applications. The same scenario stresses the requirements as to fatigue life of parts built by Additive Manufacturing (AsM). The paper addresses fatigue life of Ti6Al4V produced by Powder Bed Fusion in four finishing conditions: as-built, tool machined, after tumbling and after tumbling and subsequent shot-peening. Failure mechanisms at the micro-scale are observed in order to reinforce the mechanical results by identifying the role of different surface morphologies in crack initiation. X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques and microanalysis (EDX) are used to investigate microstructural modifications generated by the different finishing methods. Results show that tumbling alone does not improve fatigue life against the as built condition, whereas tumbling and subsequent shot peening allow matching the fatigue endurance of tool machined specimens. The shot peening process causes surface amorphization and implantation of the peening media turning into subsurface inclusions. Based on the results, an optimized finishing process can be envisaged, consisting in prolonged tumbling up to the removal of a stock allowance at least equal to the powder size, before shot peening.

2019 - Laser powder bed fusion: Tailoring the microstructure of alloys for biomedical applications [Relazione in Atti di Convegno]
Santecchia, E.; Mengucci, P.; Gatto, A.; Bassoli, E.; Denti, L.; Rutkowski, B.; Barucca, G.
abstract

Additive manufacturing (AM) is particularly attractive for biomedical applications, where complex geometries and a high degree of individualization are required. Laser powder bed fusion (LPBF) is an AM technology exploiting the action of a solid-state laser to locally melt a metal powder according to a computer aided design (CAD) model. In the present study, the EOS Cobalt Chrome SP2 (Co-Cr-Mo-W) and Ti64 (Ti6Al4V) powders were sintered by the system equipped with a Yb fiber laser. During LPBF, the Co-Cr-Mo-W metal powder undergoes total melting followed by rapid cooling, giving rise to athermal martensitic phase transformation from the high-temperature γ (fcc) phase to the low-temperature ε (hcp) phase. This produces an intricate network of thin ε-lamellae inside the γ phase matrix. After the firing cycle this structure is maintained, and a massive presence of coarse precipitates is also observed. Owing to the rapid cooling taking place during LPBF, in Ti6Al4V sintered samples only the acicular martensitic α' phase is present. The firing cycle induces the β phase formation at the α plate boundaries and this microstructure leads to reduced values of strength, with respect to those of the as-sintered samples. The highlighted behaviors show that by tuning the post-production heat treatments it is possible to tailor the microstructure and the mechanical properties.

2019 - On the effectiveness of different surface finishing techniques on A357.0 parts produced by laser-based powder bed fusion: Surface roughness and fatigue strength [Articolo su rivista]
Denti, L.; Sola, A.
abstract

Laser-based powder bed fusion (L-PBF) is an additive manufacturing (AM) technique that uses a computer-controlled laser beam as the energy source to consolidate a metal powder according to a layer-upon-layer strategy in order to manufacture a three dimensional part. This opens the way for an unprecedented freedom in geometry, but the layer-wise build-up strategy typically results in a very poor surface finish, which is affected by the staircase effect and by the presence of partially molten particles. Surface finishing treatments are therefore necessary to obtain an adequate surface finish, to improve the fatigue behavior and to meet mechanical and aesthetic needs. The present contribution systematically compares numerous surface finishing techniques, including laser shock processing, plastic media blasting, sand blasting, ceramic shot peening and metal shot peening with steel particles of different sizes (ϕ = 0.2 mm and ϕ = 0.4 mm). The results show that all the proposed methods improve the surface quality and the fatigue life of A357.0 L-PBF parts. However, the achievement of the lowest surface roughness does not necessarily correspond to the best fatigue performance, thus suggesting that multiple mechanisms may be active and that besides surface roughness also residual stresses contribute to increase the fatigue strength.

2019 - Precipitates formation and evolution in a Co-based alloy produced by powder bed fusion [Articolo su rivista]
Santecchia, E.; Gatto, A.; Bassoli, E.; Denti, L.; Rutkowski, B.; Mengucci, P.; Barucca, Giulia
abstract

Metal additive manufacturing is strongly employed in aerospace and biomedical applications, whose high degree of customization and low production volumes are the main characteristics. Cobalt-based alloys have been widely used for dental prosthesis and can be produced via metal additive manufacturing, or rather powder bed fusion, in a more convenient way compared to traditional manufacturing techniques. In the present paper, a comprehensive study of the tight correlation between the microstructure and the static mechanical properties of Co-Cr-Mo-W samples produced by powder bed fusion, is reported. In particular, the formation of precipitates during the sintering process is observed, and the evolution of their size, shape and frequency due to post-processing and to a heat-treatment typical of dental applications, is exhaustively studied. By coupling different characterization techniques such as scanning transmission electron microscopy and small-angle neutron scattering, it was possible to correlate the ductility degradation of the samples with the formation and growth of coarse and elongated precipitates.

2018 - A Study on the Use of XCT and FEA to Predict the Elastic Behavior of Additively Manufactured Parts of Cylindrical Geometry [Articolo su rivista]
Esposito, F.; Gatto, A.; Bassoli, E.; Denti, L.
abstract

Defining general criteria for the acceptability of defects within industrial components is often complicated, since the specific load conditions and the criticality of the given application should be considered individually. In order to minimize the risk of failure, high safety factors are commonly adopted during quality control. However this practice is likely to cause the rejection of components whose defects would be instead acceptable if a more sound knowledge of the component behaviour were achieved. Parts produced by additive manufacturing (AM) may suffer from various defects, including micro- or macro-holes, delamination and microstructural discontinuities. Such processes, which are specially suitable for one-off components, require robust and reliable inspection before a part is accepted or rejected, since the refusal of even a single part at the end of the production process represents a significant loss. For this reason, it would be very useful to simulate in a reliable way whether a certain defect is truly detrimental to the proper working of the part during operation or whether the component can still be used, despite the presence of a defect. To this purpose, the paper highlights the benefits of a synergistic interaction between Industrial X-ray computed tomography (XCT) and finite element analysis (FEA). Internal defects of additively manufactured parts can be identified in a non-destructive way by means of XCT. Then FEA can be performed on the XCT-based virtual model of the real component, rather than on the ideal CAD geometry. A proof of concept of this approach is proposed here for a reference construct produced in an Aluminium alloy by AM. Numerical results of the proposed combined XCT–FEA procedure are contrasted with experimental data from tensile tests. The findings sustain the reliability of the method and allow to assess its full provisional accuracy for parts of cylindrical geometry designed to operate in the elastic field. The paper moves a step beyond the present application limits of tomography as it is currently employed for AM parts and it evidences instead the possibility of extending the usage of tomography to acceptance testing and prediction of operative behaviour.

2018 - Additive manufactured A357.0 samples using the laser powder bed fusion technique: Shear and tensile performance [Articolo su rivista]
Denti, Lucia
abstract

New aluminium alloys, with lower silicon content than in the first-developed formulations, have recently been introduced in the field of Additive Manufacturing and are dedicated to automotive applications. As they are relatively new, mechanical characterization under standard protocols of the automotive field are of utmost scientific as well as industrial relevance. The paper addresses the mechanical properties and microstructure of A357.0. Static tensile and shear tests of samples built by Laser Powder Bed Fusion, with different orientations in the machine work volume, have been performed. The aim was to identify possible anisotropy in the tensile and shear behaviour of this innovative alloy. Particularly for shear, the effect of adhesion between the layers onto shear strength was studied. Results analysis, by means of statistical tools, allows for the affirmation that no tensile modulus or yield strength anisotropy is observed. Instead, a small (yet statistically significant) increase in both shear-and tensile strength and a decrease in ductility are obtained as the direction of the specimens approaches the growth direction. Scanning Electron Microscope (SEM) observation of the failure mechanisms assisted in the interpretation of the results, by relating different failure modes to the relative orientation of loads versus the directions of inherent anisotropy in Laser Powder Bed Fusion processes.

2018 - Assay of secondary anisotropy in additively manufactured alloys for dental applications [Articolo su rivista]
Bassoli, Elena; Denti, Lucia
abstract

Even though additive manufacturing (AM) techniques have been available since the late 1980s, their application in medicine is still striving to gain full acceptance. For the production of dental implants, the use of AM allows to save time and costs, but also to ensure closer dimensional tolerances and higher repeatability, as compared to traditional manual processes. Among the several AM solutions, Laser Powder Bed Fusion (L-PBF) is the most appropriate for the production of metal prostheses. The target of this paper was to investigate the mechanical and microstructural characteristics of Co-Cr-Mo and Ti-6Al-4V alloys processed by L-PBF, with a specific focus on secondary anisotropy that is usually disregarded in the literature. Tensile specimens were built in the EOSINT-M270 machine, along different orientations perpendicular to the growth direction. Density, hardness, and tensile properties were measured and the results combined with microstructural and fractographic examination. For both alloys, the results provided evidence of high strength and hardness, combined with outstanding elongation and full densification. Extremely fine microstructures were observed, sufficient to account for the good mechanical response. Statistical analysis of the mechanical properties allowed to attest the substantial absence of secondary anisotropy. The result was corroborated by the observations of the microstructures and of the failure modes. Overall, the two alloys proved to be high-performing, in very close agreement with the values reported in the datasheets, independently of the build orientation.

2018 - Directed energy deposition of steel 316L: Effects of build orientation [Articolo su rivista]
Bassoli, Elena; Denti, Lucia; Gatto, Andrea; Sewell, Neil Thomas; Johns, Daniel
abstract

Metal Additive Manufacturing for the production of end parts is today the major interest in the field of layer-by-layer fabrication. Even if Powder Bed Fusion is by far the most diffused technology, powder-fed systems retain a specific attractiveness, mainly because they enable an easier manufacture of multi-material parts or even of composition-graded ones. These systems, recently categorized by ASTM standards under the term Directed Energy Deposition (DED), still suffer from scarce knowledge of part characteristics and of process robustness and repeatability. Among DED processes, Laser Consolidation (LC) allows the production of net-shape metal parts with good metallurgical soundness, high strength and ductility. As regards the mechanical performance, the non-coaxial architecture of the LC head poses the question of a secondary anisotropy, within each layer, in addition to the primary one that is due to the layerwise construction. The paper addresses the mechanical response and the microstructure obtained by LC with AISI 316L. The direction dependence of part properties is specifically explored. Remarkably high ductility, combined with high hardness and strength, are obtained. The effect of the relative orientation between the LC head and the part is quantified and associated with the observed microstructure.

2018 - Evaluation of Laser-Sintered CR-CO and TI Alloys for Dental Applications [Articolo su rivista]
Denti, Lucia
abstract

Even if additive layer manufacturing techniques have been introduced in industries since last ’80, the application in medicine is quite recent and tend to remain at research level. In dental implantology the introduction of these technologies in the prostheses production could introduce more precision well time and money savings. Dental prostheses may be built by additive layer manufacturing, specifically by direct metal laser sintering (DMLS). The target of this paper is to investigate the mechanical and microstructural characteristics, comparing samples built along different orientation. Tensile specimens were built in accordance with ASTM E8M both by DMLS (EOSINT-M270 ), using Cr-Co alloy and Ti6Al4V alloy. An experimental plan was designed to evaluate the effect of different built orientation. Density, hardness, tensile performances, rupture surfaces SEM observation, porosity evaluation and microstructure observation were performed on the following group of specimens (4 specimen for each group): were produced (4 for each group) in three orientations with respect to the machine distinctive directions Both alloys have good mechanical performance in terms of tensile strength, elongation and hardness. The specimens revealed a low porosity and of consequence to be quite fully densified. The microstructure observed is very fine and explain the mechanical characteristics of the materials. The statistic analysis doesn’t evidence a unique difference between the different building orientation. DMLS produce parts with excellent mechanical properties independently from building orientation.

2018 - Fatigue behavior of as-built L-PBF A357.0 parts [Articolo su rivista]
Bassoli, Elena; Denti, Lucia; Comin, Andrea; Sola, Antonella; Tognoli, Emanuele
abstract

Laser-based powder bed fusion (L-PBF) is nowadays the preeminent additive manufacturing (AM) technique to produce metal parts. Nonetheless, relatively few metal powders are currently available for industrial L-PBF, especially if aluminum-based feedstocks are involved. In order to fill the existing gap, A357.0 (also known as A357 or A13570) powders are here processed by L-PBF and, for the first time, the fatigue behavior is investigated in the as-built state to verify the net-shaping potentiality of AM. Both the low-cycle and high-cycle fatigue areas are analyzed to draw the complete Wohler diagram. The infinite lifetime limit is set to 2 × 106stress cycles and the staircase method is applied to calculate a mean fatigue strength of 60 MPa. This value is slightly lower but still comparable to the published data for AlSi10Mg parts manufactured by L-PBF, even if the A357.0 samples considered here have not received any post-processing treatment.

2018 - Investigation on the effectiveness of through-hole replicas of deep small holes [Articolo su rivista]
Bassoli, Elena; Denti, Lucia
abstract

When a micro-hole of high aspect-ratio is required, in addition to machining problems, special attention should be paid to controlling the quality of the manufactured products. Dimensional and surface metrology in the field of micromachining can be as critical as machining itself, therefore several new measurement methods have been developed. However, many of these methods suffer from application limits when used in the case of a deep hole. Replication can be a useful approach, which has well-proven validity in the time-consuming case of the sectioned hole. The method of through replicas, to be pulled out of the unsectioned hole, still needs verification. In the present paper, the surface roughness of deep small electrodischarge drilled holes is measured, and the effectiveness of the use of both open- and through-replicas is evaluated, versus direct measurements on the microdrilled surfaces. Through-hole replicas, by means of injection and extraction of a silicone, are proven reliable for reproducing the surface morphology of holes down to 0.8 mm diameter with an aspect ratio of 12.5. The findings show that the operative range of the considered techniques may be extended with respect to the previous cases mentioned in literature.

2018 - Molla elicoidale perfezionata [Brevetto]
Gatto, Andrea; Gatto, Francesco; Bassoli, Elena; Denti, Lucia
abstract

2018 - Repercussions of powder contamination on the fatigue life of additive manufactured maraging steel [Articolo su rivista]
Gatto, A.; Bassoli, E.; Denti, L.
abstract

A wide range of materials is suitable for processing by powder bed fusion (PBF) techniques. Among the latest formulations, maraging steel 18Ni-300, which is a martensite-hardenable alloy, is often used when both high fracture toughness and high strength are required, or if dimensional changes need to be minimised. In direct tooling, 18Ni-300 can be successfully employed in numerous applications, for example in the production of dies for injection moulding and for casting of aluminium alloys; moreover, it is particularly valuable for high-performance engineering parts. Even though bibliographic data are available on the effects that parameters, employed in PBF processes, have on the obtained density, roughness, hardness and microstructure of 18Ni-300, there is still a lack of knowledge on the fatigue life of PBF manufactured parts. This paper describes the fatigue behaviour of 18Ni-300 steel manufactured by PBF, as compared by forging. Relevant negative effects of the cross-contamination of the raw material are originally identified in this paper, which emphasizes the inadequacy of current acceptability protocols for PBF powders. In the absence of contamination, endurance achieved by PBF is found equal to that by forging and consistent with tooling requirements as set out by industrial partners, based on injection moulding process modelling.

2018 - Towards additive manufacturing of ramified scaffolds of the thyroid vascular system: A preliminary fractal analysis [Articolo su rivista]
Bassoli, E.; Denti, L.; Gatto, A.; Spaletta, G.; Sofroniou, M.; Parrilli, A.; Fini, M.; Giardino, Raffaele; SANDUZZI ZAMPARELLI, Alessandro; Zini, N.; Barbaro, F.; Bassi, E.; Mosca, S.; Dallatana, D.; Toni, R.
abstract

Fractal properties have been demonstrated in literature for several human vascular systems. In the frame of the investigation of additive manufacturing (AM) as a viable solution to prototype single arterial branches of human soft tissue organs, the paper provides a fractal analysis of the arterial tree of the human thyroid gland. The possibility that the thyroid arterial structure may be described as auto-similar is investigated, by studying injection-corrosion casts of the cadaveric gland. Vessel branching is analyzed by measuring branch diameters, ramification angles, and vessel lengths with the use of an optical microscope. Metrological results are made dimensionless by applying, as a scaling parameter, the caliber of major arteries. Data are then studied on a cumulative basis and processed to infer general rules for vessel branching. High resolution microtomography (mCT) is used to determine the spaces occupied by vascular branches and calculate their planar fractal dimension. Finally, the vascular tree has been simulated by a mixed, stochastic / deterministic algorithm based on diffusion limited aggregation (DLA), in which mean values of vascular variables are set as constraints. The purpose of this research is to understand if fractality can be reliably assumed for computational modeling of the organ anatomy, in order to be able to produce, by AM, more representative physical prototypes and scaffolds. The finding allow to affirm that the human thyroid arterial structure exhibits a degree of auto-similarity.

2017 - Comparative study of three yttria-stabilized zirconia formulations in colored vs natural shades [Articolo su rivista]
Denti, Lucia
abstract

Recently the attention in the field of fixed dental restorations has focused on structural ceramics. Yttria-stabilized tetragonal zirconia (Y-TZP) is proposed as an alternative to conventional ceramic-metal prostheses, since it merges excellent aesthetic quality with outstanding toughness. Before Y-TZP integral structures spread in dental practice, much needs to be studied as regards the mechanical response, the influence of color and of the manufacturing process, the occurrence of aging phenomena. This work aims at a comparative study of three Y-TZP commercial formulations in five shades; the variations introduced by polishing and their recovery after annealing are also addressed. Flexural strength and microhardness are investigated, basing on international standards. Significant differences between the groups and parts' reliability are evaluated through statistical data processing and Weibull analysis. Results show low flexural strength (500-800MPa), at least 45% inferior to technical specifications, and low Weibull modulus. Hardness is instead higher than expected (1500-1700 HV1). The main finding of the research is that the effect of color on mechanical properties is significant in many cases, hence esthetical requirements must be merged with mechanical ones. Y-TZP shows an extreme variability with manufacturing conditions, so nominal characteristics should be assumed with caution and higher reliability is still required.

2017 - DREAM: Driving up reliability and efficiency of additive manufacturing [Relazione in Atti di Convegno]
Sciancalepore, Corrado; Bondioli, Federica; Gatto, Andrea; Defanti, Silvio; Denti, Lucia; Bassoli, Elena
abstract

The DREAM project, financed by the EU Commission (H2020, Work program: FOF-13-2016: Photonics Laser-based production) is an end-user driven action which aligns the research and development of Additive Manufacturing (AM) technologies to the specific needs of its three industrial end users, Ferrari SpA, Adler Ortho France SARL, and RB Srl. The Action brings together experts in the field of AM technologies, powder and material characterization, component engineering, laser-matter interaction, to deliver an optimized approach that will be developed and demonstrated to the requirements of the end users. The first results of the project are here reported.

2017 - Effects of build orientation and element partitioning on microstructure and mechanical properties of biomedical Ti-6Al-4V alloy produced by laser sintering [Articolo su rivista]
Mengucci, P.; Gatto, Andrea; Bassoli, Elena; Denti, Lucia; Fiori, F.; Girardin, E.; Bastianoni, P.; Rutkowski, B.; Czyrska Filemonowicz, A.; Barucca, G.
abstract

Direct Metal Laser Sintering (DMLS) technology was used to produce tensile and flexural samples based on the Ti-6Al-4V biomedical composition. Tensile samples were produced in three different orientations in order to investigate the effect of building direction on the mechanical behavior. On the other hand, flexural samples were submitted to thermal treatments to simulate the firing cycle commonly used to veneer metallic devices with ceramics in dental applications. Roughness and hardness measurements as well as tensile and flexural mechanical tests were performed to study the mechanical response of the alloy while X-ray diffraction (XRD), electron microscopy (SEM, TEM, STEM) techniques and microanalysis (EDX) were used to investigate sample microstructure. Results evidenced a difference in the mechanical response of tensile samples built in orthogonal directions. In terms of microstructure, samples not submitted to the firing cycle show a single phase acicular α’ (hcp) structure typical of metal parts subject to high cooling rates. After the firing cycle, samples show a reduction of hardness and strength due to the formation of laths of the β (bcc) phase at the boundaries of the primary formed α’ plates as well as to lattice parameters variation of the hcp phase. Element partitioning during the firing cycle gives rise to high concentration of V atoms (up to 20 wt%) at the plate boundaries where the β phase preferentially forms.

2017 - Evaluation of Performance of Cast and Laser-Sintered cr-co Alloys for Dental Applications [Articolo su rivista]
Denti, Lucia
abstract

Dental prostheses may be built by additive layer manufacturing, specifically by direct metal laser sintering (DMLS). This innovative process allows a high percentage of unmanned work and the direct fabrication of parts from CAD data. Even if , in the product and production development field, these techniques are studied since last ’80, up today there is a lack of knowledge about mechanical performance in the medical application. The target of this paper is to investigate the mechanical and functional characteristics, with respect to traditional cast parts. Tensile specimens were built in accordance with ASTM E8M both by DMLS (EOSINT-M270 ) and traditional lost-wax casting, using the same Cr-Co alloy. An experimental plan was designed to evaluate the effect of all process phases. Tensile performances, hardness, roughness and dimensional measurement, rupture surfaces SEM observation and porosity evaluation were performed .Sintered parts are rougher and proved to have Rockwell hardness values higher confront to cast ones. No significant dimensional variations were noticed among different processes. Some of the cast specimens present defects (macro porosity) that are absent in the sintered ones.UTS of all sintered specimens (~1400MPa) is almost double than of cast ones, whereas are more fragile (εb~5 vs 20%). The porosity in zone free of defects are comparable. Cr-Co specimen produced by DMLS show excellent strength and absence of defects with respect to traditional casting. Low εb values are not critical, since deformation of the final prostheses is limited by the ceramic layer fragility.

2017 - GAS AND REFRIGERANT ASSISTED INJECTION MOULDING PROCESS [Articolo su rivista]
Denti, Lucia
abstract

Assisted Injection Moulding (AIM) is part of a family of technologies that are conducted with plastic processing methods to improve product quality and significantly reduce costs. These technologies include the injection of gas or water, at high pressure, into the molten polymer within the injection mould. This process cores out sections of the part, and leaves hollow areas. The fluid-assisted injection moulding technology, which includes gas-assisted moulding and water-assisted moulding, has been used widely to manufacture plastic parts in recent years, due to the achievement of lightweight products, the relatively low resin cost per part, the fast cycle time, the uniform distribution of the packing pressure and the elimination of sink marks. Gasassist and water-assist technology may also be combined in sequence to achieve other benefits in certain applications. The basic idea of the proposed process is that the evaporation of a small quantity of water leads to a notable decrease in the cooling time and an increase in the dimensional tolerance of the injected part. Even though several patents pertaining to the use of gas and a refrigerant to obtain a hollow component exist, there is still a lack of knowledge regarding the effectiveness of the process. The present research has evaluated the effect of a co-injection of micro quantitates of water together with nitrogen.

2017 - Reinforcement effectiveness on mechanical performances of composites obtained by powder bed fusion [Articolo su rivista]
Gatto, A.; Bassoli, E.; Denti, L.
abstract

New material formulations to be used in Additive Manufacturing machines are one of the major interests in this fast growing field. The possibility to tune functional and mechanical properties, by the addition of reinforcements to a polymeric matrix, is hindered by the low provisional capability of the additive manufactured composite. The inherent anisotropy of layer manufacturing combines with mechanisms of filler dispersion and of filler/matrix adhesion in a complex scenario. The paper entails a critical evaluation of mechanical properties measured for several polymeric composites produced by Powder Bed Fusion, in the perspective of provisional models commonly accepted for composite materials. The models are reviewed versus experimental and literature data. The provisional effectiveness is generally good, except for the case of nanometric or surface treated fillers, or of specific anisotropic microstructures obtained by layer manufacturing.

2016 - Bioartificial endocrine organs: At the cutting edge of translational research in endocrinology [Capitolo/Saggio]
Toni, R.; Bassi, E.; Barbaro, F.; Zini, N.; Zamparelli, A.; Alfieri, M.; Dallatana, D.; Mosca, S.; Della Casa, C.; Gnocchi, C.; Lippi, G.; Spaletta, G.; Bassoli, E.; Denti, L.; Gatto, A.; Ricci, F.; Tazzari, P. L.; Parrilli, A.; Fini, M.; Sandri, M.; Sprio, S.; Tampieri, A.
abstract

2016 - Effects of thermal treatments on microstructure and mechanical properties of a Co-Cr-Mo-W biomedical alloy produced by laser sintering [Articolo su rivista]
Mengucci, P; Barucca, G.; Gatto, Andrea; Bassoli, Elena; Denti, Lucia; Fiori, F.; Girardin, E.; Bastianoni, P.; Rutkowski, B.; Czyrska Filemonowicz, A.
abstract

Direct Metal Laser Sintering (DMLS) technology based on a layer by layer production process was used to produce a Co-Cr-Mo-W alloy specifically developed for biomedical applications. The alloy mechanical response and microstructure were investigated in the as-sintered state and after post-production thermal treatments. Roughness and hardness measurements, and tensile and flexural tests were performed to study the mechanical response of the alloy while X-ray diffraction (XRD), electron microscopy (SEM, TEM, STEM) techniques and microanalysis (EDX) were used to investigate the microstructure in different conditions. Results showed an intricate network of ε-Co (hcp) lamellae in the γ-Co (fcc) matrix responsible of the high UTS and hardness values in the as-sintered state. Thermal treatments increase volume fraction of the ε-Co (hcp) martensite but slightly modify the average size of the lamellar structure. Nevertheless, thermal treatments are capable of producing a sensible increase in UTS and hardness and a strong reduction in ductility. These latter effects were mainly attributed to the massive precipitation of an hcp Co3(Mo,W)2Si phase and the contemporary formation of Si-rich inclusions.

2016 - Influence of electrode size and geometry in electro-discharge drilling of Inconel 718 [Articolo su rivista]
Bassoli, Elena; Denti, Lucia; Gatto, Andrea; Iuliano, Luca
abstract

Electro-discharge machining (EDM) of thin deep features widens processing opportunities of difficult-to-cut materials, such as Ni-based alloys, towards attractive industrial applications. Conversely, the complex interaction between electrical, thermal, and chemical phenomena in EDM thwarts process modeling and prediction. As a matter of fact, recent experimental discoveries encourage setting forth a new theory of discharge ignition in the gap, based on the role of debris, which lays foundations for a recursive mathematical model showing a chaotic evolution. The paper reports on electro-discharge drilling of small deep holes in Inconel 718. Process performances are measured with varying electrode size and geometry, attesting a pivotal effect of gap pollution on productivity and on the onset of a secondary detrimental removal of material by intergranular corrosion. All findings support the emerging model for discharge ignition via debris bridges, and, most notably, debris chains are documented for the first time in the field of metals, second only to a previous case for a ceramic composite. On the whole, the paper provides conclusive validation of the role of debris in the ignition of discharges.

2015 - Multi-disciplinary approach in engineering education: learning with additive manufacturing and reverse engineering [Articolo su rivista]
Gatto, Andrea; Bassoli, Elena; Denti, Lucia; Iuliano, Luca; Minetola, Paolo
abstract

Purpose - The purpose of this paper is to report an interdisciplinary, cooperative-learning project in a second-year course within the "Enzo Ferrari" Master of Science Degree in Mechanical Engineering. The work aims to raise awareness of the educational impact of additive manufacturing and reverse engineering. Design/methodology/approach - Students are asked to develop, concurrently, the design and the manufacturing solution for an eye-tracker head mount. A digital head model is reverse engineered from an anatomical mannequin and used as an ergonomic mock-up. The project includes prototype testing and cost analysis. The device is produced using additive manufacturing techniques for hands-on evaluation by the students. Findings - Results of the presented case study substantiate the authors' belief in the tremendous potential of interdisciplinary project-based learning, relying on innovative technologies to encourage collaboration, motivation and dynamism. Originality/value - The paper confirms a spreading conviction that the soon-to-be engineers will need new practice-oriented capabilities to cope with new competitive scenarios. Engineering education must adapt to the social, rather than industrial, revolution that is being brought about by additive fabrication.

2015 - Structural characterization of biomedical Co-Cr-Mo components produced by direct metal laser sintering [Articolo su rivista]
Barucca, G.; Santecchia, E.; Majni, G.; Girardin, E.; Bassoli, Elena; Denti, Lucia; Gatto, Andrea; Iuliano, L.; Moskalewicz, T.; Mengucci, P.
abstract

Direct metal laser sintering (DMLS) is a technique to manufacture complex functional mechanical parts from a computer-aided design (CAD) model. Usually, the mechanical components produced by this procedure show higher residual porosity and poorer mechanical properties than those obtained by conventional manufacturing techniques. In this work, a Co-Cr-Mo alloy produced by DMLS with a composition suitable for biomedical applications was submitted to hardness measurements and structural characterization. The alloy showed a hardness value remarkably higher than those commonly obtained for the same cast or wrought alloys. In order to clarify the origin of this unexpected result, the sample microstructure was investigated by X-ray diffraction (XRD), electron microscopy (SEM and TEM) and energy dispersive microanalysis (EDX). For the first time, a homogeneous microstructure comprised of an intricate network of thin ε (hcp)-lamellae distributed inside a γ (fcc) phase was observed. The ε-lamellae grown on the {111}γ planes limit the dislocation slip inside the γ (fcc) phase, causing the measured hardness increase. The results suggest possible innovative applications of the DMLS technique to the production of mechanical parts in the medical and dental fields.

2015 - Tolerance analysis for cast vs machined dental implants [Relazione in Atti di Convegno]
Atzeni, E.; Bassoli, E.; Denti, L.; Gatto, A.; Iuliano, L.; Minetola, P.; Salmi, A.
abstract

The paper addresses the dimensional tolerance and assembly accuracy of prosthetic components obtained by different manufacturing processes. The success of single tooth implant replacements hangs on the stability of the hexagonal connection, but no standard control procedures are available for its evaluation. The research aims at proposing a new protocol for the dimensional assessment of implant-Abutment connections, based on non-contact measurement and statistical data processing. The procedure is applied to machined-And cast-on abutments, as well of the matching implants. Samples are measured using an optical measuring microscope and data are processed to obtain the international tolerance (IT) grade. The rotational misfit is then calculated using the apothems of the external and the internal hexagon. As to the results, all the components are classified between IT8 and IT9 and the maximum rotational misfit is around 4 for all the assemblies, inferior to the critical limits for the screw joint stability. An objective dimensional characterization of prosthetic components and assemblies is reported, which is the basis for their reliability in clinical applications. From a wider perspective, an original measuring protocol is proposed, independent of parts assembly and based on international tolerances.

2014 - A planar fractal analysis of the arterial tree of the human thyroid gland: implications for additive manufacturing of 3D ramified scaffolds. [Relazione in Atti di Convegno]
Bassoli, Elena; Denti, Lucia; Gatto, Andrea; G., Spaletta; M., Sofroniou; A., Parrilli; M., Fini; R., Giardino; A., Zamparelli; N., Zini; F., Barbaro; E., Bassi; S., Mosca; D., Dallatana; R., Toni
abstract

It is currently known that a number of human vascular systems have a fractal geometry. Since we have recently developed a technique to prototype single arterial branches of human soft tissue organs by additive layer manufacturing (AM), we have explored the possibility that auto-similarity in vessel branching represents a key variable for accurate computational modeling of the organ three-dimensional (3D) macro / microscopic anatomy, and its reproduction by inverse engineering. To this purpose, ramification features of the intralobar arteries of the human thyroid were studied using injection-corrosion casts of the cadaveric gland. Vessel diameters, ramification angles, and branch lengths were measured by light microscopic, computer-aided optical metrology. Distribution of morphological variables was considered on a cumulative basis, and special focus was given to the branching laws. To reduce the bias of vascular distortion due to the pressure of intravascular resin injection, measures were made dimensionless through the use of a scaling parameter set on the vascular caliber of major afferent arteries. In addition, using high resolution microtomography (mCT) equipped with CTAn software and the Otsu algorithm for segmentation, spaces occupied by vascular branches (referred to as Volume of Interest, VOI) were selected, and their planar fractal dimension calculated. Finally, a computational simulation of the vascular tree was achieved using a mixed, stochastic / deterministic algorithm based on diffusion limited aggregation (DLA), constrained by mean values of vascular variables. Ratios among decreasing cast calibers, ramification angles, and branch lengths, respectively, were found strictly interrelated, mCT-VOI depicted fractal dimensions, and DLA simulation led to a fractal-like organization consistent with real data morphometrics. In summary, thyroid arterial geometry reliably exhibited a degree of auto-similarity, suggesting that fractality is a key feature for computational modeling and eventual AM of 3D vascular networks of the human thyroid.

2014 - Additive manufacturing as a cost-effective way to produce metal parts [Relazione in Atti di Convegno]
Atzeni, E.; Iuliano, L.; Marchiandi, G.; Minetola, P.; Salmi, A.; Bassoli, E.; Denti, L.; Gatto, A.
abstract

Recent developments in additive manufacturing technologies for the production of metal parts make this processes very attractive to high technology industries. Especially the geometrical freedom allow to produce components optimized for their functionality, disregarding limitations imposed by the manufacturing process. However, production costs should be evaluated to assess the convenience of additive manufacturing (AM) with respect to traditional technologies. Thus in this paper a break-even analysis is described, comparing the Selective Laser Melting (SLM) additive process with High Pressure Die Casting (HPDC) and 5-axis machining. Costs models are discussed for each process and benefits of applying redesign for additive manufacturing are illustrated. An aeronautical component is selected as case study to estimate the production volume for which additive manufacturing is cost-effective. The study proves the economical convenience of additive manufacturing with respect to traditional technologies from prototypes to small batch production. © 2014 Taylor & Francis Group.

2014 - Electro-Discharge Drilling on DMLS parts in Co-Cr-Mo alloy [Relazione in Atti di Convegno]
Gatto, Andrea; Bassoli, Elena; Denti, Lucia; E., Atzeni; L., Iuliano; G., Marchiandi; P., Minetola; A., Salmi; F., Calignano
abstract

Cobalt-base alloys are generally used in applications that require wear, corrosion and heat resistance. In particular, Cobalt-Chromium-Molybdenum (Co-Cr-Mo) alloy is adopted in the aerospace and medical field. These applications are characterized by low volumes and high personalization, so they could take advantage by the employment of additive technologies, such as Direct Metal Laser Sintering (DMLS). This technology still has limitations for the manufacture of small cavities, such as holes with diameters below 0.6mm and high aspect ratio, which require drilling of the additive manufactured part. Scope of this research is to investigate the effects of Electro-Dicharge Drilling (EDD) on surface and subsurface features of CoCrMo samples built by DMLS. High aspect ratio holes are produced with diameter of 0.6 mm and depth fifteen times higher. The attractiveness of ED machining for thin deep geometries stems from then absence of physical contact between tool and workpiece, which makes machining forces negligible and minimizes tool deformation. Micro features can be produced with a high accuracy irrespective of the material hardness and strength. Material Removal Rate (MRR), Electrode Wear (EW) and surface roughness are investigated versus the process parameters. Results are supported by the study of material removal mechanisms and surface morphology in the microscale, to account for the macroscopic trends.

2014 - On the Effect of Electrodischarge Drilling on the Fatigue Life of Inconel 718 [Relazione in Atti di Convegno]
Bassoli, Elena; Baldini, Andrea; Gatto, Andrea; Strozzi, Antonio; Denti, Lucia
abstract

Difficult-to cut-materials are associated with premature tool failure, most likely in the case of complex geometries and this shapes. However, Nickel-based alloys are commonly used in high-temperature and aerospace applications, where thin deep holes are often required. Then, the only viable manufacturing solution relies on non-contact processes, like electrodischarge (ED) drilling. Morphology of ED machined surfaces is significantly different than obtained by metal-cutting operation and is known to jeopardize fatigue strength, but the extent needs to be gauged and related to the process parameters. Aim of the paper is to study the effect of holes (0.8 mm diameter, aspect ratio 10) produced by ED drilling on the fatigue life of Inconel 718. Rotating bending fatigue tests are carried out on specimens drilled under two ED setups, as well as with a traditional cutting tool. Specimens free from holes are fatigued under the same conditions for comparison. Based on previous studies, extremal ED parameters are selected, giving best surface finish versus highest productivity. S-N curves show that the ED process causes a decrease of the fatigue resistance with respect to traditional drilling, whereas the effect of different ED setups is negligible. Maximum productivity can thus be pursued with no threat to fatigue performance. The fatigue limit variation is quantified by using the superposition effect principle: ED drilling causes an increase of the stress concentration factor around 25% if compared to traditional drilling. The macroscopic fatigue behavior is integrated with a study of the effects of the different drilling processes in the micro-scale, by means of a microstructural and fractographic analysis.

2013 - Bridges of debris in the edd process: going beyond the thermo-electrical model [Articolo su rivista]
Gatto, Andrea; Bassoli, Elena; Denti, Lucia; Iuliano, L.
abstract

Electro-Discharge (ED) processes depend on the contemporaneous effect of many factors, which complicates process control/predictability and induced many authors in the last 60 years to work on explicative models. Studies split into two main approaches: theoretical and empirical. Theoretical works are based on the thermo-electrical theory and try to describe process phenomena by a physical model, with unavoidable assumptions and simplifications that cause a move away from veracity. On the contrary, experimentalists establish empirical models based on statistical analysis of results and optimization methods, but the findings are limited to domain-specific applications. In addition, numerous papers focus on single-spark analysis, failing in considering the interaction between successive discharges, or of transient phenomena as the presence of bubbles and debris in the gap. At present the scientific debate involves the ignition model, with two different points of view regarding the discharge-driving phenomenon: the debris bridge effect (pollutants in the dielectric drive the performances), and the dielectric strength effect (properties of the dielectric drive the performances). The paper addresses this dispute by investigating the debris formed during small-hole ED drilling of a 72 wt% Al2O3 - 28 wt% TiC composite. Particles are found to hollow out and pack within the gap, joining by necks. The first experimental evidence is given of the presence of chains and clusters of debris, towards a new model for electrodischarge processes that goes beyond the thermoelectric theory.

2013 - Investigation into the failure of Inconel exhaust collector produced by laser consolidation [Articolo su rivista]
Bassoli, Elena; Neil, Sewell; Denti, Lucia; Gatto, Andrea
abstract

Among layer manufacturing techniques, Laser Consolidation (LC) finds its ideal application in the production of thin-walled metal parts for industrial niches characterised by high innovation and product complexity. To fully exploit the technological potential, developments must be made to assess LC’s repeatability and reliability. Previous studies proved that high strength parts of fine microstructure are obtained if appropriate build strategies are used. The aim of this research is to analyse a racecar exhaust collector, built in Inconel by LC, relating the failure modes and microstructure to the construction plan. The exhaust collector component was built using a custom strategy and was run on a dynamometer before failing after approximately 1200 km. An investigation of the failure mechanism was carried out by a primary macroscopic analysis, aided by an X-ray control and dye penetrant test. Metallographic sections were then sampled from the critical areas to study the microstructure and relate it to the manufacturing process. Results proved that the distribution of primary microcracks associated with internal residual stresses caused by the build strategy and aggravated by differential shrinkage during the test thermocycles was responsible for ultimate failure. This issue was a result of build strategy and the non-coaxial laser head, therefore, alternative pathways could be developed capable of removing most, if not all, of the contributory factors. The analysed case suggests that LC build strategy selection is as critical to ensure low cycle fatigue resistance as component design and confirms that LC is effective in the production of advanced technological parts with high geometrical complexity given the correct build strategy.

2013 - Studies on electro discharge drilling of an Al2O3 –TiC composite [Articolo su rivista]
F., Calignano; Denti, Lucia; Bassoli, Elena; Gatto, Andrea; L., Iuliano
abstract

Ceramic matrix composites (CMCs) can be attractive for structural applications, but their machining by conventional methods is expensive and often critical. Complex geometries on advanced ceramics require contactless processes, such as electro discharge machining (EDM) or -drilling (EDD). These proved to be viable for CMCs with electrical resistivity below a critical value in the range of 1-3 Ωm. The condition is complied with by many CMCs: an example is alumina with titanium carbide. Material removal of ceramics by electrical discharges is a complex process involving different mechanisms, depending on the process setup. The paper describes an experimental study on EDD of 0.4 mm diameter holes with aspect ratio of 20 in Al2O3-TiC, using copper electrodes. Peak current (Ip), pulse-on time (ton) and pulse-off time (toff) are varied as independent variables. Four performance indicators are measured: material removal rate (MRR), electrode wear rate (EWrate), overcut (OC) and surface roughness (Ra, Sa). Empirical models are proposed to describe t the effect of process parameters on the output indicators. The analysis is supported by the observation of the surface and subsurface morphology, with the aim of investigating the material removal mechanisms and attaining a full comprehension of macroscopic results. It is found that removal mostly occurs by melting and evaporation and that surface morphology is determined by two phenomena ruled by pulse power. A process description is proposed, built around power as the ruling factor.

2012 - A combined Additive Layer Manufacturing / Indirect Replication method to prototype 3D vascular-like structures of soft tissue and endocrine organs [Articolo su rivista]
Bassoli, Elena; Denti, Lucia; Gatto, Andrea; G., Spaletta; A., Paderno; N., Zini; A., Parrilli; R., Giardino; V., Strusi; D., Dallatana; S., Mastrogiacomo; A., Zamparelli; M., Iafisco; R., Toni
abstract

We describe an innovative methodology combining Additive Layer Manufacturing and indirect replication to reconstruct reticular-like, three-dimensional structures mimicking the vascular network of soft tissue and endocrine organs. Using a fractal-like algorithm capable of modeling the intraparenchymal vascular distribution of these viscera, single intraglandular branches of the human thyroid arteries were prototyped with synthetic resin, based on the algorithmic STL output and ALM techniques. Satisfactory dimensional accuracy was obtained for these models, which were used as masters to evaluate protocols for their indirect replication, through both single and double procedures. Additional studies were conducted using casts of the human kidney arteries, obtained by injection / corrosion of the isolated organ. Satisfactory 3D reproduction of the external morphology of the kidney vessels was achieved. We conclude that our approach has the potential to develop up to the reconstruction with biomaterials of an entire, intraparenchymal vascular tree of soft tissue and endocrine organs.

2011 - Dimensional tolerances and assembly accuracy of dental implants and machined- versus cast-on abutments [Articolo su rivista]
Malaguti, Giuliano; Denti, Lucia; Bassoli, Elena; I., Franchi; Bortolini, Sergio; Gatto, Andrea
abstract

Background. The clinical application of prosthetic components obtained by different manufacturing processes lacks technological foundation: the dimensional tolerance of individual parts and their assembly accuracy are not known. The rotational misfit of the hexagonal connection is critical in single tooth implant restorations, but no standard control procedures are available for its evaluation.Purpose. The research aimed at proposing a new protocol for the dimensional assessment of implant-abutment connections, based on non-contact measurement and statistical data processing. The procedure was applied to machined- and cast-on abutments, as well of the matching implants.Materials and Methods. Three groups of 5 abutments each were studied: machined titanium abutments, premachined calcinable abutments before casting procedures and the same specimens after casting. A group of 5 corresponding implants was considered as well. Twice the apothem was measured on each hexagon through an optical measuring microscope. The data were processed to obtain the international tolerance (IT) grade. The rotational misfit was then calculated using the apothems of the external and the internal hexagon.Results. All the components were classified between IT8 and IT9 and the maximum rotational misfit was around 3-4° for all the assemblies, inferior to the critical limits for the screw joint stability.Conclusion. An original measuring protocol was developed, independent of parts assembly and based on international tolerances. An objective dimensional characterization of prosthetic components and assemblies has been achieved, which is the basis for their reliability in clinical applications.

2011 - New approaches to prototype 3D vascular-like structures by additive layer manufacturing. [Relazione in Atti di Convegno]
Bassoli, Elena; Denti, Lucia; Gatto, Andrea; A., Paderno; G., Spaletta; N., Zini; V., Strusi; D., Dallatana; R., Toni
abstract

In this paper preliminary results are summarized on the use of a combined Additive Layer Manufacturing (ALM) and indirect replication methodology to reconstruct reticular-like, three-dimensional (3D) structures mimicking the 3D vascular network of the adult human thyroid gland. In a first step, we developed a fractal-like algorithm capable of modeling the native arterial distribution of the adult thyroid lobe, allowing for vascular growth within its geometrical domain. Although some arbitrary simplifications were adopted, yet the vascular density of the computational simulation showed good consistency with that of a native thyroid lobe. In a second step, single vascular branches were prototyped based on the STL output of the algorithm and ALM techniques, up to the achievement of a model having satisfactory geometrical/morphological accuracy. In a third step, the problem of reproducing the vascular geometry with a biocompatible polymer was ad-dressed, and different protocols of replication technology were evaluated. Limits and possible methodological solutions are discussed.

2011 - Thermoplastic Resin Transfer Moulding in a rapid manufactured mould [Relazione in Atti di Convegno]
E., Atzeni; F., Calignano; L., Iuliano; P., Minetola; A., Salmi; Bassoli, Elena; Denti, Lucia; Gatto, Andrea
abstract

Resin Transfer Moulding (RTM) is a low-cost process for the production of composite parts with thermosetting resins. However, the economic convenience is lost in the case of large components, such as aeronautical panels, because of the high tooling cost. Step milling of a resin master for the electro-deposition of a nickel shell is proposed in this study as a valid alternative for the fabrication of a resin transfer mould. This solution allows overcoming the limitations regarding part size and cutting down tooling costs, but the expected dimensional accuracy needs to be assessed. On the field of materials, innovative thermoplastic resins are now available that can be used in the formof lowviscosity oligomers for optimal mould filling and fibre impregnation. Subsequent in situ polymerization provides high toughness composites. Cyclic Butylen Terephtalate (CBT), which polymerizes into PBT (Poly-Butylen Terephtalate), is studied in this work. The research focuses on two objectives: quantifying the dimensional accuracy of the technological chain and setting up the process of in situ polymerization.

2010 - DMLS DENTAL ALLOYS VS TRADITIONAL TECHNIQUE [Abstract in Rivista]
Bortolini, Sergio; Gatto, Andrea; Natali, Alfredo; Y., Ucar; G., Montin; Bassoli, Elena; Denti, Lucia; Consolo, Ugo; L., Iuliano; T., Akova
abstract

Additive Layer Manufacture (ALM) of dental prostheses allows the direct fabrication of parts from CAD data, replacing a human-sensible process with a faster reliable automatic one. Direct Metal Laser Sintering (DMLS) of Cr-Co alloys produces parts to be directly finished with ceramic and employed. PURPOSE: This study evaluated (1) the application of DMLS process for obtaining final dental prostheses in Co-Cr alloy, to compare mechanical characteristics of cast parts and layer-manufactured ones produced with different alloys and processes; and (2) the effect of porcelain firing cycles on mechanical properties. METHODS: This study was conducted on three classes of tensile specimens in five groups of six specimens: class 1 – 12 specimens built by DMLS (PM100T – Phenix System, France) with Co-Cr powder NOBIL 4000S (NOBIL METAL - Italy) divided in two groups (6 specimens were processed with ceramic heat treatment); class 2 – 12 specimens built by traditional lost-wax casting from Co-Cr bars for fixed prostheses V-CERAM MAGI LINE (SHERA - Germany) divided in two groups (6 specimens were processed with ceramic heat treatment); class 3 – 6 tensile specimens built by traditional lost-wax casting from Cr-Co bars for partial removable prostheses REMANIUM 800+ (Dentaurum - Germany). RESULTS: UTS of all sintered specimens (1174±165 MPa) was higher than of cast ones (503±45 MPa, 499±62 MPa, 712±51 MPa, respectively, for as cast Co-Cr, heat treated Co-Cr, and Remanium) where many porosities were found. UTS decreases after heat-treatment both for cats and DMLS parts. Yield strength was also highest for the laser sintered group (1148±161 MPa).No significant idfference was found for the elastic modulus of compared groups (P=0.398). CONCLUSIONS: DMLS of Co-Cr alloys produces excellent strength and absence of defects compared to traditional casting. This result is due to the fine microstructure obtained by DMLS (high thermal rate, heterogeneous nucleation).

2010 - FRACTURE RESISTANCE ANALYSIS OF SINTERED AND CASTED COBAL/CROMIUM ALLOYS [Abstract in Rivista]
Natali, Alfredo; Bortolini, Sergio; G., Montin; Gatto, Andrea; L., Iuliano; Bassoli, Elena; Denti, Lucia; Y., Ucar; A., Tolga; Consolo, Ugo
abstract

THIS STUDY WANTS TO EVALUATE THE APPLICATION OF SLS PROCESS FOR OBTAINING FINAL DENTAL PROSTHESIS IN CR-CO ALLOY, DEVELOPING EXPERIMENTAL PLAN WITH THE PURPOSE TO MADE A COMPARATIVE ANALYSIS OF MECHANICAL CHARACTERISTICS BETWEEN CASTS PARTS AND SINTERED ONES PRODUCTED WITH DIFFERENT ALLOYS AND PROCESSES.

2010 - Microwave rapid debinding and sintering of MIM/CIM parts [Relazione in Atti di Convegno]
Veronesi, Paolo; Leonelli, Cristina; Poli, Giorgio; Denti, Lucia; Gatto, Andrea
abstract

Microwave assisted thermal debinding of parts obtained by Metal Injection Moulding (MIM) or Ceramic Injection Moulding (CIM) could benefit from the heating selectivity, having the organic binder to preferentially absorb microwaves, thus accelerating the conventional process, which has to rely on heating by conduction. This is particularly useful when dealing with ceramic powders having low thermal conductivity, but also more conductive materials can be treated faster if the maximum temperature difference inside each part is kept low.Once the organic binder is removed, microwaves at 2.45 GHz can be used to rapidly sinter the obtained brown parts, minimizing grain growth.In this work, microwave assisted debinding and sintering of MIM/CIM parts, made of stainless steel, alumina and titania is studied, using numerical simulation to investigate the presence of localised effects ascribable to the electromagnetic field distribution in the powder compacts.

2009 - CAST VERSUS LASER-SINTERED CR-CO ALLOYS: STUDY ON MECHANICAL CHARACTERISTICS [Abstract in Rivista]
Natali, Alfredo; Bassoli, Elena; Denti, Lucia; Berzaghi, Andrea; M., Franchi; Bortolini, Sergio; Consolo, Ugo
abstract

Objectives:Additive Layer Manufacture of dental prostheses allows the direct fabrication of parts from CAD data, replacing a human-sensible process with a faster reliable automatic one. Direct Metal Laser Sintering (DMLS) of Cr-Co alloys produces parts to be directly finished with ceramic and employed. Their clinical application requires a solid know-how on mechanical and functional characteristics, with respect to traditional cast parts.Methods:Tensile specimens were built (ASTM E8M) both by DMLS and traditional lost-wax casting, using the same Cr-Co alloy. An experimental plan was designed to evaluate the effect of all process phases. Standard parameters were used on EOSINT-M270 to fabricate the laser-sintered specimens. A group was left as-sintered (DMLS_L) and the others were heat-treated for stress-relief (DMLS). Wax parts were built by drop-on-demand, invested in ceramic and burnt away before casting (CAST). Then, two groups of specimens were subjected to the high temperature treatment used in ceramic covering without deposition (DMLS_H, CAST_H). On all specimens (6 for each of 5 groups) thickness was measured through a measuring microscope. Roughness was determined on Lm=4,8mm. Specimens were tensile tested obtaining UTS and percent elongation.Results:No significant dimensional variations were noticed among different processes. Sintered parts are on average rougher than cast ones (Ra~3.8vs2.6µm). UTS of all sintered specimens (~1400MPa) is almost double than of cast ones, where many porosities were found. Unlike cast parts, heat-treatment increases UTS for DMLS. Laser-sintered specimens are also much more fragile than cast ones (εb~5 vs 20%).Conclusion:DMLS of Cr-Co alloys produces excellent strength and absence of defects with respect to traditional casting. Low εb values are not critical, since deformation of the final prostheses is limited by the ceramic layer fragility. Slightly higher roughness of DMLS parts is likely to improve mechanical bonding with the ceramic layer or might be smoothed.

2009 - Comparative Mechanical Evaluation Of Three Y-TZP Formulations In Five Shades [Abstract in Rivista]
Berzaghi, Andrea; Denti, Lucia; Bassoli, Elena; A., Natali; M., Franchi; Bortolini, Sergio; Consolo, Ugo
abstract

Objectives: As integral structures in Y-TZP spread in dental practice, verifying the mechanical characteristics reported in literature and if they are affected by coloration becomes imperative. This study aims at determining flexural strength and hardness of three Y-TZP types in five different colours. Methods: Three commercial formulations of Y-TZP (A, B, C) were considered: as-received natural white and 4 different shades. Specimens were ASTM C1161, five for each typology (75 parts in 15 groups). Blocks of pre-sintered material were CAD/CAM dry machined, then some were coloured by dipping, before the last step of sintering (Tmax=1450°C). Three point flexural tests were performed to obtain flexural strength (S). After polishing Vickers hardness (HV1) was measured (ISO 14705). T-test for independent samples was carried out on the data, to evaluate the presence of significant differences (positive t-test) between the groups. Results: Mean S was between 500 and 800MPa, much lower than declared by the producers (1200 MPa). For B and C S tends to decrease with colour, but not significantly. For A S increases with colour and two cases present positive t-tests compared to natural white. There aren't significant differences between white specimens of the three companies. As to other shades, the t-test is positive between B and C for three colours over four and between A and B for one colour. HV1 results between 15.8 and 17GPa (in technical specifications HV=12.2 GPa). Colours significantly influence hardness, shifting towards higher values, for nearly all the studied comparisons. Conclusion: Material properties resulted greatly different from technical specifications. Flexural strength values suggest a careful design of frameworks connectors, especially in posterior prostheses. The studied materials resulted harder than expected, in particular when coloured. Colour proved to affect the performances in many cases, requiring esthetical and mechanical requirements to be merged

2009 - Microwave Rapid Sintering of Nanostructured Stainless Steel Metal Powders [Relazione in Atti di Convegno]
Veronesi, Paolo; Leonelli, Cristina; Poli, Giorgio; Bassoli, Elena; Denti, Lucia; Gatto, Andrea
abstract

Nanostructured AISI430 stainless steel powders, obtained by high energy milling have been uniaxially pressed in order to obtain 8 mm diameter cylindrical compacts. Microwave sintering of the green compacts has been performed in a single mode microwave applicator, at 2.45 GHz. Maximum sintering temperature of each sample, monitored using a sapphire optical fibre, was varied in the 1000-1200°C range, with heating rates in the 20-30 K/s range, applying 1200 W of microwave forward power. Dense nanostructured samples, with less than 1% porosity were obtained, presenting small local variation of Cr content in proximity of the necking region. Rapid microwave sintering helps maintaining the nanostructure, despite a non homogenous densification due to the uneven electromagnetic field distribution in the single-mode applicator

2009 - “Microwave Rapid Sintering of Nanostructured Stainless Steel Metal Powders”,, Vol. 3, , Denmark, 2009, 143-148, ISBN [Relazione in Atti di Convegno]
Veronesi, Paolo; Leonelli, Cristina; Poli, Giorgio; Bassoli, Elena; Denti, Lucia; Gatto, Andrea
abstract

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2008 - High strength brown parts produced by fast microwave-assisted debinding [Relazione in Atti di Convegno]
Veronesi, Paolo; Denti, Lucia; Colombini, L.
abstract

The binder removal from 430L-steel MIM parts was performed in a two-step procedure using chemical and thermal debinding. During the first step, almost 80% of the binder is removed and a porous structure is created, which then favours the subsequent step. The second step was performed in a multi-mode microwave applicator, at 2.45 GHz ISM frequency. Numerical simulation allowed to optimise the microwave applicator loading conditions, in terms of heating homogeneity and efficiency. Experimental validation confirmed that using microwaves it is possible to reduce conventional debinding times by a factor of 3, and that the resulting brown parts possess a 4 times higher compressive strength. This phenomenon can be ascribed to local over-heating of the regions surrounding neighbouring metal particles, triggered by the high frequency electric field. This promotes the formation of necks among the particles, leading to pre-sintering, despite the overall measured samples temperature, which never exceeded 600°C.

2008 - Microwave assisted debinding and sintering of nanostructured metal powders: modeling and experimental results [Abstract in Atti di Convegno]
Leonelli, Cristina; Veronesi, Paolo; Denti, Lucia; Gatto, Andrea
abstract

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2008 - Microwave assisted sintering of green metal parts [Articolo su rivista]
Leonelli, Cristina; Veronesi, Paolo; Denti, Lucia; Gatto, Andrea; L., Iuliano
abstract

Microwave processing has gained worldwide acceptance as a novel method for heating and sintering a variety of materials, as it offers specific advantages in terms of speed, energy efficiency, process simplicity, finer microstructures, and lower environmental hazards.Microwave assisted post-treatment of green metal parts (AISI 420 metal powder with 2–3 wt% polymeric binder) proved to be a new method, suitable for near-net-shape and net-shape manufacturing.Green parts exposed to high intensity microwave fields, using either single mode or multimode microwave applicators operating at 2.45 GHz ISM frequency, were investigated in terms of densification behaviour, microstructure and joining micromechanism. The results, in terms of microstructure and shape retention are comparable to the conventional heating ones, reported in literature, but they have been obtained in much shorter time.

2007 - Pre-Sintered MIM Brown Parts Obtained by Microwave-Assisted Binder Remova [Relazione in Atti di Convegno]
Veronesi, Paolo; D., Belviso; Denti, Lucia; Gatto, Andrea; Leonelli, Cristina
abstract

Microwave assisted thermal debinding of parts obtained by Metal InjectionMoulding (MIM) was studied using Design of Experiment techniques to reducethe number of virtual experiments needed to gather information regardingbinder removal. The optimization, in terms of speed, heating homogeneityand energy efficiency of the early stages of this process in an existing2-feeds multimode applicator operating at 2.45 GHz has been reached withnumerical simulation used to perform the virtual experiments. The obtainedmodel was used to determine the optimum debinding conditions, which wereexperimentally tested in a laboratory multimode applicator. Samplecharacterization, before and after sintering, in terms of strength andfinal density, with respect to an equivalent industrial production, wasused to assess the optimized process. A process time reduction of morethan 6 times was achieved. In case of MIM parts made of 430L steel,material not prone to high temperature oxidation, the preferentialmicrowave absorption by the metallic powders resulted also in apre-sintering treatment which improves the brown parts strength and favorstheir handling during the subsequent sintering stage.

2007 - Pre-sintered mim brown parts obtained by rapid microwave-assisted binder removal [Relazione in Atti di Convegno]
Veronesi, P.; Belviso, D.; Denti, L.; Gatto, A.; Leonelli, C.
abstract

Microwave assisted thermal debinding of parts obtained by Metal Injection Moulding (MIM) was studied using Design of Experiment techniques to reduce the number of virtual experiments needed to gather information regarding binder removal. The optimization, in terms of speed, heating homogeneity and energy efficiency of the early stages of this process in an existing 2-feeds multimode applicator operating at 2.45 GHz has been reached with numerical simulation used to perform the virtual experiments. The obtained model was used to determine the optimum debinding conditions, which were experimentally tested in a laboratory multimode applicator. Sample characterization, before and after sintering, in terms of strength and final density, with respect to an equivalent industrial production, was used to assess the optimized process. A process time reduction of more than 6 times was achieved. In case of MIM parts made of 430L steel, material not prone to high temperature oxidation, the preferential microwave absorption by the metallic powders resulted also in a pre-sintering treatment which improves the brown parts strength and favors their handling during the subsequent sintering stage.

2007 - Studies of rapid microwave sintering of green parts [Relazione in Atti di Convegno]
Veronesi, Paolo; Leonelli, Cristina; Denti, Lucia; Gatto, Andrea; L., Iuliano
abstract

This paper regards the application of microwave for the sintering of green parts produced by selective laser sintering (SLS). Heating tests were performed on 420L stainless steel cylindrical (20 mm diameter; 20 mm height). The parameters of the electromagnetic field distribution during microwave sintering of the specimens were optimized for the heating treatments. The dates obtained from preliminary tests were used to carry out further heating treatments. This paper shows the possibility of achieving rapid consolidation in single mode and multimode applicators operating at 2.45 GHz, in less than 20 minutes. This phenomenon is confirmed by a micro-scale numerical modelling of the distribution of the electromagnetic field around the metal particles that evidence a favourable necking stage promoted by local electric field concentration among the spherical particles. The consolidated parts, however, depending on the microwave heating conditions, can present sensitisation due to the formation of chromium carbides, favoured by the C abundance provided by the decomposing organic binder.

2006 - Preliminary Studies of the Rapid Microwave Sintering of Green Parts Made of 420L Stainless Steel [Relazione in Atti di Convegno]
Veronesi, Paolo; Leonelli, Cristina; Denti, Lucia; Gatto, Andrea; L., Iuliano
abstract

Electromagnetic field distribution during microwave sintering ofmetal compacts, obtained by MIM or SLS was performed, evidencing afavourable necking stage promoted by local electric field concentrationamong the spherical particles. Microwave heating tests on 420L stainlesssteel cylindrical (20 mm diameter; 20 mm height) green parts obtained bySLS showed the possibility of achieving rapid sintering in single modeand multimode applicators operating at 2.45 GHz, in less than 20 minutes.The sintered parts, however, depending on the microwave heatingconditions, can present sensitisation due to the formation of chromiumcarbides, favoured by the C abundance provided by the decomposing organicbinder. This phenomenon constitutes a lower limit to the overall sinteringtime of the examined SLS green parts, which can be overcome only alteringthe sintering atmosphere or performing a preliminar de-binding step