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Elena COLOMBINI

Professore Associato
Dipartimento di Ingegneria "Enzo Ferrari"

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Pubblicazioni

2023 - Fluid–Structure Interaction of a thin cylindrical shell filled with a non-Newtonian fluid [Articolo su rivista]
Zippo, A.; Iarriccio, G.; Bergamini, L.; Colombini, E.; Veronesi, P.; Pellicano, F.
abstract

This paper presents the results of an extensive experimental campaign on the dynamic interactions between an elastic structure and a non-Newtonian fluid. The structure consists of a thin circular cylindrical shell, with the bottom end clamped to a shaking table, and the top end carrying a heavy mass. The fluid is a mixture of water and cornstarch, also known as oobleck. The system dynamics has been analyzed in the presence of different fluid levels (i.e., empty, partially, and full-filled). The experimental modal analysis has been carried out to identify the modal properties of the system. High energy tests have been performed by means of a seismic excitation consisting in a stepped sine sweep, spanning the forcing frequency within the neighborhoods where strong resonance phenomena take place. Different excitation amplitudes have been considered in order to induce phase transitions in the fluid, and the onset of complex dynamics has been detected using Fourier spectra and bifurcation diagrams of the Poincaré maps: when the fluid–solid​ transition occurs, the entangled non-Newtonian fluid rheology results in a complex dynamic scenario where period-doubling cascades, quasiperiodic and chaotic responses can be observed.

2023 - High-Throughput Nanoindentation Mapping of aMicrosegregated CoCrFeNi Multi-Principal Element Alloy(MPEA): Challenges and Limitations [Articolo su rivista]
Gualtieri, Eva Magdalena; Colombini, Elena; Testa, Veronica; Bolelli, Giovanni; Giovanardi, Roberto; Veronesi, Paolo
abstract

2023 - Improvement in Wear Resistance of Grade 37 Titanium by Microwave Plasma Oxy-Carburizing [Articolo su rivista]
Veronesi, P.; Balestri, A.; Colombini, E.
abstract

Grade 37 titanium is widely used in racing applications thanks to its oxidation resistance up to 650 °C, but it suffers from poor wear and fretting resistance, especially at high temperature. In this paper, different surface modification techniques, namely, carburizing, coating by PVD-ZrO2 and a novel microwave plasma oxy-carburizing treatment, are investigated in terms of hardness, wear resistance and scratch hardness, compared to the untreated substrate. Numerical simulation allowed optimization of the design of the microwave plasma source, which operated at 2.45 GHz at atmospheric pressure. The proposed microwave plasma oxy-carburizing treatment is localized and can serve to improve the tribological properties of selected regions of the sample; compared to untreated Grade 37 titanium, the oxy-carburized layer presents a decrease in the wear rate at 450 °C against alumina of 54% and an increase in scratch hardness of more than three times.

2023 - Influence of Different Filler Metals on the Mechanical and Microstructural Characteristics of Arc-Welded Joints Made of Dissimilar Titanium Alloys [Articolo su rivista]
Gaiani, S.; Gozzi, M.; Ferrari, E.; Menozzi, A.; Lassinantti Gualtieri, M.; Colombini, E.; Veronesi, P.
abstract

In the motorsport industry, the choice of material for manufacturing the heat resistant components often falls on titanium alloys. In most cases, the production flow for this kind of part involves CNC machining and subsequent assembly by welding process, to other parts obtained by cold plastic forming and possibly made using different titanium alloys. Hence, the alloying element-content in the joint area can be extremely heterogeneous and variable point-by-point. To investigate this topic further, dissimilar welding of the alpha/beta alloy Ti6Al4V and of the oxidation-resistant alpha alloy KS-Ti 1.2 ASN-EX was made by GTAW technology and using different filler metals. Chemical and mechanical properties of the welds were investigated by XRD, SEM-EDS, microhardness maps, and tensile and bending tests. Results show that, despite the different alloying elements present in the two filler wires investigated, static properties of the welds are similar. Results also show that the local V/Al content ratio affects the microhardness as it is responsible for the creation of supersaturated alpha phases during the cooling of the weld beads.

2023 - Recycling Of Spent Powders From Additive Manufacturing Processing Of Inconel 625 For The Synthesis Of Cocrfenimoxnb0.4x (X=0-0.1) Multi-Principal Element Alloys (Mpeas) By Spark Plasma Sintering (SPS) Of Mechanically Alloyed Powders [Relazione in Atti di Convegno]
Veronesi, P.; Gualtieri, M. L.; Feltrin, A. C.; Akhtar, F.; Colombini, E.
abstract

Sieve residues from the powder recycling stream in Laser powder bed fusion (L-PBF) processing of Inconel 625 are currently disposed of as hazardous waste, which conflicts with circular economy thinking. Here, the synthesis of metal matrix composites (MMCs) based on carbide- and oxide-strengthening phases finely dispersed in a Multi-Principal Element Alloy (MPEA) matrix was explored as a recycling option for these powders. In particular, mixtures of virgin metal powders and a spent Inconel 625 powder were mechanical alloyed and consolidated by Spark Plasma Sintering (SPS). The process control agent (PCA, ethanol) used during mechanical alloying acted as a source for C and O in the subsequent crystallization of nano-sized carbides and oxides during sintering. By carefully controlling the powder mixture composition, MPEA matrices with different contents of Nb and Mo in the face-centered cubic structure were obtained, as revealed by X-ray Powder diffraction. The bulk samples were further characterized by Scanning Electron Microscopy (SEM) and preliminary mechanical analyses using instrumented indentation. The results showed that spent powder of Inconel 625 was a valuable source of 4d transition metals for the synthesis of MMCs based on a MPEA matrix with enhanced solid solution strengthening and finely dispersed ceramic phases.

2023 - Recycling Of Spent Powders From Laser Powder Bed Fusion Processing Of Inconel 625 For The Mechanical Synthesis Of Cocrfenimoxnb0.4x (X=0-0.1) Multi-Principal Element Alloys (Mpeas) [Relazione in Atti di Convegno]
Colombini, E.; Gualtieri, M. L.; Paggetti, S.; Veronesi, P.
abstract

2023 - Synthesis of Multi-Principal Element Alloys by a Conventional Powder Metallurgy Process [Relazione in Atti di Convegno]
Valsecchi, G.; Colombini, E.; Gualtieri, M. L.; Mortalo, C.; Deambrosis, S. M.; Montagner, F.; Zin, V.; Miorin, E.; Fabrizio, M.; Veronesi, P.
abstract

The development of tailored microstructures of Multi-principal element alloys (MPEAs) is currently a hot topic in physical metallurgy. The most targeted systems are equimolar alloys composed of 3d transition metals including the so-called Cantor alloy (i.e. CoCrFeMnNi) and derivatives such as CoCrFeNi and CoCrFeNiAlx. Powder metallurgy is a promising route for this purpose and include manufacturing techniques such as hot pressing of mechanically activated or prealloyed powders or the less popular press-sinter route of mixed powders. In this work, cold pressing followed by fast vacuum sintering (1h) at various temperatures (Tmax =1100-1300 °C) of mixed powders of CoCrFeNi and CoCrFeNiAl0.4 were explored for the synthesis of structurally and chemically homogeneous alloys. This approach is promising for the synthesis of bulk alloys of higher purity with respect to hot pressing of mechanically prealloyed powders. Microstructural investigations were performed by X-ray Powder diffraction (XRPD) and Scanning electron microscopy (SEM). It will be shown that the reactive sintering kinetics of the investigated systems require a Tmax of 1200 °C for effective alloying at the short holding time employed for CoCrFeNi. Instead, 1300 °C is needed for CoCrFeNiAl0.4.

2022 - A Multi-Physic Modelling Insight into the Differences between Microwave and Conventional Heating for the Synthesis of TiO2 Nanoparticles [Articolo su rivista]
Poppi, G.; Colombini, E.; Salvatori, D.; Balestri, A.; Baldi, G.; Leonelli, C.; Veronesi, P.
abstract

Microwave-assisted synthesis of nanoparticles usually leads to a smaller and more uni-formly distributed particle size compared to conventional heating (e.g., oil bath). Numerical simulation can help to obtain a better insight into the process in terms of temperature distribution or to evidence existing different temperature profiles and heating rates between the two techniques. In this paper multi-physics numerical simulation is used to investigate the continuous flow synthesis of titanium oxide nanoparticles starting from alkoxide precursors. Temperature-dependent permit-tivity of reactants has been measured, including the effects of permanence at the maximum synthesis temperature. A temperature homogeneity index has been defined to compare microwave and conventional heating. Results show that when using microwave heating at 2450 MHz, in the inves-tigated conditions, a much higher temperature homogeneity of the reactants is reached. Moreover, reactants experience different heating rates, depending on their position inside the microwave ap-plicator, while this is almost negligible in the case of conventional heating.

2022 - In-house synthesis of CoCrFeNi ingots using an electric furnace [Articolo su rivista]
Colombini, E.; Lassinantti Gualtieri, M.; Mortalo, C.; Deambrosis, S. M.; Veronesi, P.
abstract

Bulk Multi-Principal Element Alloys (MPEAs) are generally synthesized by casting, a process needing specific equipment. Here, a standard laboratory electric furnace was used to synthesize bulk CoCrFeNi by melting of prealloyed powders followed by natural cooling. The use of prealloyed powders guaranteed atomic-level mixing. In accordance with the literature, the resulting ingot had a face-centered cubic structure. A typical dendritic-interdendritic microstructure was obtained which was explained by partitioning during solidification and grain boundary wetting phenomenon. Post-annealing treatment improved chemical homogeneity without crystallographic phase change. This work shows the feasibility of melt-aided synthesis of CoCrFeNi HEA under static conditions using a conventional laboratory furnace.

2022 - Powder Metallurgy Route for the Synthesis of Multiprincipal Element Alloys Sputtering Targets [Articolo su rivista]
Colombini, E.; Lassinantti Gualtieri, M.; Mortalo, C.; Deambrosis, S. M.; Montagner, F.; Zin, V.; Miorin, E.; Valsecchi, G.; Fabrizio, M.; Veronesi, P.
abstract

Sputter deposition of multiprincipal element alloys (MPEAs) is a relatively new field of research with high functional potential. The multicomponent design space is immense and practically unexplored. An important obstacle for academic research of such sputtered films is the availability of single-alloy targets and technical difficulties in using cosputtering of multiple metal targets or powder targets. This article focuses on the development of a simple powder metallurgy route, including cold uniaxial pressing of powder mixtures followed by pressureless sintering, for the preparation of targets made of two common base alloys forming simple solid solutions, i.e., FeNiCrCo and FeNiCrMn. In addition, targets of the former one containing 10 at% Al are also prepared. The sintered pellets are composed of randomly oriented crystallites with face-centered cubic structures and an optimum chemical homogeneity. Oxide inclusions and residual porosity, inherent to consolidation and sintering of metal precursors, are observed and possible solutions to overcome these challenging problems are discussed. Nevertheless, encouraging results from preliminary deposition tests of FeNiCrCoAl0.4 using both direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) are presented.

2021 - Microwave Processing of PET Using Solid-State Microwave Generators [Articolo su rivista]
Veronesi, P.; Colombini, E.; Salvatori, D.; Catauro, M.; Leonelli, C.
abstract

The present study addresses the possibility of using a frequency-controllable microwave source, that is, a solid-state microwave generator, to rapidly and efficiently heat PET. Based on dielectric properties measurement of PET, numerical simulation has been used to model dedicated microwave applicators, suitable for the rapid reheating of PET granules or preforms. Numerical simulation, validated by experimental activity, demonstrates that using the proper frequency change as PET is being processed allows to maximize heating efficiency or homogeneity. Two examples of microwave applicators, one for small-scale and the other for large-scale production are presented, specially addressing the effect of using different metals for the modeling and construction of the microwave cavity.

2021 - NEW APPLICATORS FOR THE PROJECT SIMPLIFY: SONICATION AND MICROWAVE PROCESSING OF MATERIAL FEEDSTOCK [Relazione in Atti di Convegno]
Veronesi, P.; Colombini, E.; Poppi, G.; Rosa, R.; Dami, V.; Cioni, A.; Lorenzi, G.; Baldi, G.; Leonelli, C.
abstract

2021 - The effect of zr addition on melting temperature, microstructure, recrystallization and mechanical properties of a cantor high entropy alloy [Articolo su rivista]
Campari, E. G.; Casagrande, A.; Colombini, E.; Gualtieri, M. L.; Veronesi, P.
abstract

The effect of Zr addition on the melting temperature of the CoCrFeMnNi High Entropy Alloy (HEA), known as the “Cantor’s Alloy”, is investigated, together with its micro-structure, mechanical properties and thermomechanical recrystallization process. The base and Zr-modified alloys are obtained by vacuum induction melting of mechanically pre-alloyed powders. Raw materials are then cold rolled and annealed. recrystallization occurred during the heat treatment of the cold-rolled HEA. The alloys are characterized by X-ray diffraction, electron microscopy, thermal analyses, mechanical spectroscopy and indentation measures. The main advantages of Zr addition are: (1) a fast vacuum induction melting process; (2) the lower melting temperature, due to Zr eutectics formation with all the Cantor’s alloy elements; (3) the good chemical alloy homogeneity; and (4) the mechanical properties improvement of re-crystallized grains with a coherent structure. The crystallographic lattice of both alloys results in FCC. The Zr-modified HEA presents a higher recrystallization temperature and smaller grain size after recrystallization with respect to the Cantor’s alloy, with precipitation of a coherent second phase, which enhances the alloy hardness and strength.

2020 - A novel microwave and induction heating applicator for metal making: Design and testing [Articolo su rivista]
Colombini, E.; Papalia, K.; Barozzi, S.; Perugi, F.; Veronesi, P.
abstract

The use of microwave heating in primary metallurgy is gaining an increasing interest due to the possibility to selectively process ores and to volumetrically heat large amounts of low-thermal conductivity minerals. In this paper the study, development and testing of a new applicator combining the use of microwave and induction heating for rapid reduction of metal containing oxides is described. Numerical simulation was used in order to achieve the proper control over heat generation, considering the use of microwave solid state generators. A prototype, with a capacity up to 5 liters of standard input feed but with the predisposition for continuous processing has been designed, built and tested on reference loads like iron oxide powders and pellets. Results on the microwave heating part of the applicator indicate that it allows to efficiently and rapidly process these kinds of loads, which change from dielectric to conductors as reduction proceeds. The use of variable frequency solid state microwave generators allows to maximize energy efficiency and to controllably change the heating pattern inside the load.

2019 - Coating of Titanium Substrates with ZrO2 and ZrO2-SiO2 Composites by Sol-Gel Synthesis for Biomedical Applications: Structural Characterization, Mechanical and Corrosive Behavior [Articolo su rivista]
Catauro, Michelina; Barrino, Federico; Bononi, Massimiliano; Colombini, Elena; Giovanardi, Roberto; Veronesi, Paolo; Tranquillo, Elisabetta
abstract

The use of metallic materials as implants presents some major drawbacks, such as their harmful effects on the living organism, especially those induced by corrosion. To overcome this problem, the implant surface of titanium implants can be improved using a coating of bioactive and biocompatible materials. The aim of this work is the synthesis of SiO2/ZrO2 composites with different percentages of zirconia matrix (20, 33 and 50 wt.%), by the sol-gel method to coat commercial Grade 4 titanium disks using a dip coater. Attenuated total reflectance Fourier transform infrared (ATR/FTIR) spectroscopy was used to evaluate the interactions between the inorganic matrices. Furthermore, the mechanical properties and corrosive behavior of the SiO2/ZrO2 coatings were evaluated as a function of the ZrO2 content. The bioactive properties of the substrate coated with different composites were evaluated using simulated body fluid (SBF). The antibacterial activity was tested against gram-negative and gram-positive Escherichia coli and Enterococcus faecalis, respectively, to assess the release of toxic products from the different composites and to evaluate the possibility of using them in the biomedical field.

2019 - High entropy alloy manufactured by SLM: characterization and development [Articolo su rivista]
Garzoni, A; Colombini, E; Giovanardi, R; Veronesi, P; Casagrande, A; Todaro, I
abstract

The aim of this study was to investigate an alternative manufacturing process to produce a High Entropy Alloy (HEA) starting from mechanically prealloyed powders, compared to the arc and induction melting. In this work the microstructural developments of the equimolar CoCrFeMnNi high entropy alloy processed by SLM were studied. The influence of process parameters and powders properties on the microstructure was characterized. X-ray diffraction and SEM-EDS semi-compositive investigations carried out on the HEA revealed a single crystalline structure (FCC). Preliminary evaluations of the mechanical properties showed higher microhardness values compared to the same alloy produced with conventional manufacturing processes, due to the finer microstructure morphology, the high dislocation density near subgrain and the precipitation of fine particles.

2019 - Improving Durability of Titanium for Biomedical Use by Composite Ceramic Coatings [Capitolo/Saggio]
Tranquillo, Elisabetta; Bononi, Massimiliano; Colombini, Elena; Giovanardi, Roberto; Veronesi, Paolo; Barrino, Federico; Catauro, Michelina
abstract

2018 - Al, cu and zr addition to high entropy alloys: The effect on recrystallization temperature [Relazione in Atti di Convegno]
Colombini, Elena; Garzoni, Andrea; Giovanardi, Roberto; Veronesi, Paolo; Casagrande, Angelo
abstract

The equimolar Cr, Mn, Fe, Co and Ni alloy, first produced in 2004, was unexpectedly found to be single-phase. Consequently, a new concept of materials was developed: high entropy alloys (HEA) forming a single solid-solution with a near equiatomic composition of the constituting elements. In this study, an equimolar CoCrFeMnNi HEA was modified by the addition of 5 at% of either Al, Cu or Zr. The cold-rolled alloys were annealed for 30 minutes at high temperature to investigate the recrystallization kinetics. The evolution of the grain boundary and the grain size were investigated, from the as-cast to the recrystallized state. Results show that the recrystallized single phase FCC structures exhibits different twin grains density, grain size and recrystallization temperatures as a function of the at.% of modifier alloying elements added. In comparison to the equimolar CoCrFeMnNi, the addition of modifier elements increases significantly the recrystallization temperature after cold deformation. The sluggish diffusion (typical of HEA alloys), the presence of a solute in solid solution as well as the low twin boundary energy are responsible for the lower driving force for recrystallization.

2018 - High entropy alloys obtained by field assisted powder metallurgy route: SPS and microwave heating [Articolo su rivista]
Colombini, E.; Rosa, R.; Trombi, L.; Zadra, M.; Casagrande, A.; Veronesi, P.
abstract

The aim of this work was to investigate the field assisted powder metallurgy route for producing HEAs at equimolar composition, i.e. FeCoNiCrAl, starting from metal powders. Both mixed, mechanically activated and mechanically alloyed powders have been used. The powders obtained by mechanical alloying were synthesized only by SPS, whereas the remaining ones were sintered by SPS or microwave heating. The investigated field assisted sintering techniques allowed an extremely short alloying time, high energy density on the load and negligible contamination by the surrounding environment. Both the conducted sintering-synthesis technology resulted not definitive to produce chemical homogeneity and to obtain a single stable structure. Thus a subsequently heat treatment was required. The post heat treatment, indeed, led to a single crystalline structure (FCC) and the material was fully recrystallized. After heat treatment samples are isomorphic: they exhibit two different phases with the same FCC cell, but different chemical composition, in detail Fe-Cr richer and Al-Ni richer. SPS-ed samples present a reduced porosity, while microwave processed ones are much more porous and this is reflected in the mechanical properties.

2018 - SPS-assisted Synthesis of SICp reinforced high entropy alloys: reactivity of SIC and effects of pre-mechanical alloying and post-annealing treatment [Articolo su rivista]
Colombini, E.; Lassinantti Gualtieri, M.; Rosa, R.; Tarterini, F.; Zadra, M.; Casagrande, A.; Veronesi, P.
abstract

In this work a traditional high entropy alloy (FeCoNiCrAl) was reinforced by uniformly distributed reactive silicon carbide (SiC) particles by a powder metallurgy synthetic route, using as precursors simply mixed powders or mechanically prealloyed ones. The reactive sintering produced a single isomorphic BCC structure. The sample microstructure resulted equiassic, more homogenous in samples based on prealloyed powders. The instability of SiC in the presence of metal precursors resulted in the formation of more stable carbides and silicides, as well as in carbon diffusion in the high entropy alloy matrix and partially unreacted SiC particles. The formation of these newly formed fine precipitates, as well as the presence of residual SiC were useful to increase the hardness of the alloy.

2018 - The effect of alkaline earth carbonates on the microstructure and mechanical properties of impermeable and lightweight ceramics [Articolo su rivista]
Lassinantti Gualtieri, M.; Colombini, E.; Mazzini, D.; Alboni, C.; Manfredini, T.; Siligardi, C.
abstract

Lightweight impermeable ceramic bodies were designed by combining pore templating and controlled viscous sintering through in-situ crystallization. Various amounts of limestone were added to a glass-fluxed low-temperature stoneware tile formulation. Closed porosity was created by decomposition of carbonates prior to sintering, thus leaving voids that were not completely filled by the viscous melt. The resulting oxides chemically modified the liquid phase and promoted the crystallization of β-wollastonite, diopside and anorthite. Hence, viscous sintering was affected. The addition of limestone brought on several advantages: the temperature of maximum sintering rate was decreased (<900 °C); the dimensional stability range was extended; the matrix was reinforced by newly-formed crystals that compensated for the global structure weakening evoked by increased porosity; an increase in whiteness was observed in concomitance to crystallization, reaching values only obtained when using zircon as opacifier (L*=87)

2017 - A new multi principal element alloy synthesized by microwave powder metallurgy technique [Relazione in Atti di Convegno]
Colombini, E.; Rosa, R.; Poletti, M. G.; Battezzati, L.; Casagrande, A.; Veronesi, P.; Leonelli, C.
abstract

Multi principal element alloys (MPEAs), consisting of five or more alloying elements at near equiatomic concentrations and forming bcc and/or fcc solid solution phase, were recently developed. According to the literature research, the synthetic route to produce MPEAs should guarantee short alloying time, efficient cooling and capability to operate in controlled atmosphere. Such conditions can be achieved using high frequency electromagnetic fields, like in microwave heating. Microwave assisted combustion synthesis of pure metal powders mixtures as reactants has already been used during the last decade by the authors to prepare intermetallics, functionally-graded materials, and recently to produce multi-principle alloys. Moreover the addition of SiC in these alloys increases the mechanical properties and oxidation resistance at high temperature, as demonstrated in a recent paper by the authors. In this work a new MPEA (Al15Mn20Fe25Co15Ni25) was prepared with and without the addition of SiC, using powder metallurgy route and exploiting microwave as way of generating heat inside the precursors and hence to start the reactive sintering. Results show that direct microwave heating at 2450 MHz of the powder precursors leads to the ignition conditions, with an evident exothermal event ascribable to the mixing enthalpy, and then self-sustaining of the synthesis occurs. The temperature and duration of the microwave-assisted process resulted much lower than other conventional powder metallurgy routes, but at the cost of a higher residual porosity. Sample characterization confirmed that the powder metallurgy approach is suitable to retain the shape of the load imparted during forming by uniaxial pressing.

2017 - Laser hardening of steel sintered parts [Relazione in Atti di Convegno]
Veronesi, P.; Sola, R.; Colombini, E.; Giovanardi, R.; Parigi, G.
abstract

The possibility of applying rapid and localized laser hardening to near-net shape parts, like the ones deriving from powder metallurgy (P/M) is investigated, demonstrating that even low alloyed steels (Fe + 2% Cu + 0,7% C) can be successfully heat treated with minimal or no dimensional variations. Laser hardening conditions have been selected on the basis of the results of the previous research, carried out by means of an Nd-YAG high power system [1]. To avoid some carbon loss, observed on previous activities, the samples have been protected by neutral atmosphere. The microstructural features of the laser hardened steels have been analyzed by optical microscopy, whereas the surface micro-geometry has been characterized by scanning electron microscope. Hardened depth (HD), hardened width (HW) and hardened area (HA) have been measured as well. As expected, the micro-hardness profiles present a sharp drop at low distance from the hardened surface. The typical splitting between hardened zone and heat-Affected zone (HAZ), well known from laser hardened fully dense steels, has been observed also on low-Alloy sintered steels. The use of a protective atmosphere has been helpful to control surface decarburization and to prevent oxidation. The research confirm that Laser transformation Hardening (LTH) is a suitable hardening process of P/M components, through the action of a scanning laser beam. The short heating time and the modest volume fraction structurally modified can contribute to avoid part distortion, in comparison with other hardening methods.

2017 - Microwave processing of high entropy alloys: A powder metallurgy approach [Articolo su rivista]
Veronesi, Paolo; Colombini, Elena; Rosa, Roberto; Leonelli, Cristina; Garuti, Marco
abstract

Microwaves at the ISM frequency of 2450 and 5800 MHz have been exploited to prepare FeCoNiCrAl-family high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave assisted near-net-shape technology, using powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the compound. Microwave processing, exploited both for the ignition and sustaining of such reactions, has been compared to reactive sintering in laboratory furnace and mechanical alloying in a planetary ball milling. Results demonstrate that microwave required the shortest time and lowest energy consumption, thus it is promising time- and cost-saving synthetic route.

2017 - Microwave processing of high entropy alloys: A powder metallurgy approach [Relazione in Atti di Convegno]
Veronesi, P.; Rosa, R.; Colombini, E.; Leonelli, C.; Garuti, M.
abstract

Microwaves at the ISM frequency of 2450 and 5800 MHz have been exploited to prepare FeCoNiCrAl-family high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave assisted near-net-shape technology, using powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the compound. Microwave processing, exploited both for the ignition and sustaining of such reactions, has been compared to reactive sintering in laboratory furnace and mechanical alloying in a planetary ball milling. Results demonstrate that microwave required the shortest time and lowest energy consumption, thus it is promising time- and cost-saving synthetic route.

2017 - Microwave-assisted preparation of multi principal element alloys by powder metallurgy approach [Relazione in Atti di Convegno]
Colombini, E.; Borromei, I.; Trombi, L.; Casagrande, A.; Veronesi, P.
abstract

According to literature, the synthetic route to produce High entropy alloys (HEAs) should guarantee short alloying time, efficient cooling and capability to operate in controlled atmosphere. Such conditions can be achieved using high frequency electromagnetic fields, like microwave heating. In this work FeCoNiCrAl and FeCoNiCuAl, both equiatomic and reinforced by the 10% wt. of SiC were prepared by microwave assisted techniques. Results show that direct microwave heating of the powder precursors occurs, until the ignition conditions are reached. The temperature and duration of the microwave-assisted process result much lower than other conventional powder metallurgy routes, but at the cost of a higher residual porosity. Sample characterization confirmed that the powder metallurgy approach is suitable to retain the shape of the load imparted during forming by uniaxial pressing. The homogeneity of the samples resulted in being good in all cases, without the dendritic segregation typically occurring by liquid phase processing. © 2017 European Powder Metallurgy Association (EPMA)

2017 - Numerical Simulation and Experimental Validation of MIG Welding of T-Joints of Thin Aluminum Plates for Top Class Vehicles [Articolo su rivista]
Bonazzi, Enrico; Colombini, Elena; Panari, Davide; Vergnano, Alberto; Leali, Francesco; Veronesi, Paolo
abstract

The integration of experiments with numerical simulations can efficiently support a quick evaluation of the welded joint. In this work, the MIG welding operation on aluminum T-joint thin plate has been studied by the integration of both simulation and experiments. The aim of the paper is to enlarge the global database, to promote the use of thin aluminum sheets in automotive body industries and to provide new data. Since the welding of aluminum thin plates is difficult to control due to high speed of the heat source and high heat flows during heating and cooling, a simulation model could be considered an effective design tool to predict the real phenomena. This integrated approach enables new evaluation possibilities on MIG-welded thin aluminum T-joints, as correspondence between the extension of the microstructural zones and the simulation parameters, material hardness, transient 3D temperature distribution on the surface and inside the material, stresses, strains, and deformations. The results of the mechanical simulations are comparable with the experimental measurements along the welding path, especially considering the variability of the process. The results could well predict the welding-induced distortion, which together with local heating during welding must be anticipated and subsequently minimized and counterbalance.

2017 - Optimization of laser welding of dissimilar corrosion resistant alloys [Relazione in Atti di Convegno]
Cavallini, Michele; Veronesi, Paolo; Lusvarghi, Luca; Colombini, Elenca; Giovanardi, Roberto; Rigon, Lorenzo
abstract

The use of multi-materials components has the benefit of coupling each material's benefit where it is needed, thus achieving, for instance, heat, wear and corrosion resistance. Joining of dissimilar metals is a challenging task due to the large differences in properties. In this framework, the use of (Quasi Continuous Wave (QCW) Fiber Lasers) proved to be effective in multi-materials components assembly thanks to the high power density but low energy-input. In this paper the laser welding of dissimilar materials, namely stellite and stainless steel, is investigated and optimized in terms of corrosion resistance, comparing the results to conventional brazing joining. Results demonstrate that the optimized laser welding conditions, with the use of a proper filler material, lead to the obtainment of defect-free joints, with minimum alteration of the base materials.

2016 - Laser hardening of low-alloyed PM steels [Abstract in Atti di Convegno]
Bocchini, G. F.; Colombini, E.; Mannini, A.; Parigi, G.; Poli, G.; Sola, R.; Veronesi, P.
abstract

The good results obtained by laser-hardening of high-strength P/M steels drove to extend the research to low-alloy material (Fe-Cu-C; 6,6 g/cm3 density). Process conditions have been selected on the basis of previous results. To avoid carbon loss, observed on previous tests, the samples have been also protect-ed, by neutral atmosphere, during any critical step. The microstructural features of the laser hardened Cu-C P/M steels have been analyzed by LOM, whereas the surface micro-geometry has been characterized by SEM. Hardened depth (HD), hardened width (HW) and hardened area (HA) have been measured. As expected, the micro-hardness profiles present a sharp drop at low distance from the hardened surface. The typical splitting between hardened zone and heat-affected zone (HAZ), well known from laser hard-ened fully dense steels, has been observed also on low-alloy sintered steels. The addition of a protective atmosphere has been helpful to control decarburization of thin surface layers or surface oxidation. The re-search confirm that Laser transformation Hardening (LTH) is a suitable process to get hard, wear resistant surface, or defined spots on P/M components. The short heating time and the modest volume fraction structurally modified can contribute to avoid part distortion, in comparison with induction hardening. The possibility of very selective and precise hardening treatment has been confirmed. The results of the re-search open new possibilities to widen the use of high-property P/M parts, based on low-alloy materials, for wear-resistant and high precision applications.

2016 - Microwave assisted synthesis of Si-modified Mn25FexNi25Cu(50−x) high entropy alloys [Articolo su rivista]
Veronesi, Paolo; Colombini, Elena; Rosa, Roberto; Leonelli, Cristina; Rosi, Fabrizio
abstract

Rapid microwave heating at 2450 MHz of metallic powders mixtures compacts was performed under Ar flux in a single mode applicator in order to produce Si-modified Mn25FexNi25Cu(50−x), (x=25, 30, 35, 40) high entropy alloys. Microwave heating was conducted in presence of a SiC auxiliary absorber, so that the compacts are subjected to both direct heating by microwave absorption and indirect heating by the auxiliary absorber. Due to the extremely rapid processing times, including the cooling stage, depletion of the most oxygen-reactive elements was moderate, considering the not perfectly protective atmosphere used. FCC solid solutions have been obtained and the role of Si is discussed as a microstructure modifier and as increaser of the microhardness.

2016 - Sintesi assistita da microonde per ottenere leghe ad alta entropia a diverso tenore di alluminio [Articolo su rivista]
Colombini, Elena; Rosa, Roberto; Trombi, Lorenzo; Veronesi, Paolo
abstract

In questo studio si è analizzato l’effetto della variazione del contenuto di alluminio nella sintesi e microstruttura della lega ad alta entropia di composizione: FeCoNiCrAlx, dove x può assumere i valori x=1,1.5,2,3. Il ruolo atteso dell’alluminio durane la sintesi è di creare una fase liquida che acceleri le reazioni, che altrimenti avverrebbero unicamente allo stato solido, mentre i rimanenti elementi comprendono almeno un elemento ferromagnetico per migliorare l’assorbimento delle microonde durante la sintesi. Dalle caratterizzazioni effettuate si evince che all’aumentare del contenuto di alluminio si ottengono leghe composizionalmente più omogenee, una promozione della struttura cristallina CCC e l’aumento della durezza media. La lega equimolare FeCoNiCrAl è stata poi confrontata con la lega FeCoNiCuAl. Dalla diffrazione è possibile vedere la tendenza della lega di sposarsi verso una struttura cristallina CFC, grazie alla presenza di rame.

2016 - Tempra laser di acciai sinterizzati basso-legati [Articolo su rivista]
Bocchini, G. F.; Colombini, Elena; Mannini, A.; Parigi, G.; Poli, Giorgio; Sola, Ramona; Veronesi, Paolo
abstract

An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed and diffusion-bonded powders. Eight different process parameters have been applied to optimize the process afterwards they have been set to two different compositions. The independent variables investigated have been: surface temperature, T, and laser travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth both via hardness test and via optical measures were evaluated as well and used as responses for the ANOVA of the experimental data gathered. This work reveals the possibility to apply a selective and precise hardening treatment, like Laser Transformation hardening (LHT), to high-property P/M steel parts. Thanks to ANOVA analysis the correlation between energy density (ED) and optical dimension of treated zone has been identified. By increasing temperature and speed the density energy ED decreases, indeed at high temperature and speed the heated zone is larger. At the same speed, a temperature increase leads to a surface hardness decrease. A similar value of hardness depth (about 0.8 mm) has been detected by means of hardness profiles. The result has been strengthened by scratch test. The parabolic zone presents a very fine martensitic structure on the top of laser trail, and bainite microstructure on transition zone, as literature related. The transition zone between the laser affected zone and the base metal exhibits a micro hardness gradient. It is also possible to notice some Cu diffusion from outer to inner zone. By comparison among LHT specimens and sintering-hardened and stress-relieved one, (characterized by similar chemical compositions) penetration depth and hardness values appear similar. Transformation Hardening (LTH) is a suitable process for producing hard surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with sinter hardened. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.

2015 - Microwave-Assisted Preparation of High Entropy Alloys [Articolo su rivista]
Veronesi, Paolo; Rosa, Roberto; Colombini, Elena; Leonelli, Cristina; Veronesi, Paolo
abstract

Microwaves at the ISM (Industrial, Scientific and Medical, reserved internationally) frequency of 2450 or 5800 MHz have been used to prepare FeCoNiCuAl, FeCrNiTiAl and FeCoCrNiAl2.5 high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave-assisted near-net-shape technology, using a powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). High entropy alloy compositions have been selected so as to comprise at least one ferromagnetic element and one highly reactive couple, like Ni-Al, Ti-Al, Co-Al or Fe-Al. Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the load. Microwaves have been applied both for the ignition and sustaining of such reactions, showing that by the proposed technique, it is possible to control the cooling rate of the newly-synthesized high entropy alloys. Results showed also that microwave heating in predominant magnetic field regions of the microwave applicator is more effective at controlling the cooling rate. The herein proposed microwave-assisted powder metallurgy approach is suitable to retain the shape of the load imparted during forming by uniaxial pressing. The homogeneity of the prepared high entropy alloys in all cases was good, without the dendritic segregation typical of arc melting, even if some partially-unreacted powders were detected in the samples.

2015 - Recycling of aluminum and titanium chips by microwave augmented SHS [Relazione in Atti di Convegno]
Veronesi, Paolo; Rosa, Roberto; Colombini, Elena; Lancellotti, Isabella; Giovanardi, Roberto; Leonelli, Cristina; Montecchia, L.; Cavatorta, G.
abstract

Recycling of titanium and aluminium chips to manufacture wear and corrosion-resistant intermetallic-based coatings is achieved by microwave assisted Self Propagating High Temperature Synthesis (SHS). A multi-physics model implementing both reaction kinetic and microwave heating and heat transfer has been developed in order to optimize the synthesis condition. Numerical simulation allowed to investigate the preferable applicator geometry and to quantify the excess heat generated, which could be useful to promote further reactions with an iron-based substrate, thus realizing intermetallic-coated steel-based products.

2015 - Tempra laser di acciai sinterizzati omogenei o disomogenei [Laser hardening of homogeneous and inhomogeneous p/m steels] [Articolo su rivista]
Colombini, Elena; Bocchini, G. F.; Parigi, G.; Sola, Ramona; Veronesi, Paolo; Poli, Giorgio
abstract

An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed and diffusion-bonded powders. Eight different process parameters have been applied to optimize the process afterwards they have been set to two different compositions. The independent variables investigated have been: surface temperature, T, and laser travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth both via hardness test and via optical measures were evaluated as well and used as responses for the ANOVA of the experimental data gathered. This work reveals the possibility to apply a selective and precise hardening treatment, like Laser Transformation hardening (LHT), to high-property P/M steel parts. Thanks to ANOVA analysis the correlation between energy; density (ED) and optical dimension of treated zone has been identified. By increasing temperature and speed the density energy ED decreases, indeed at high temperature and speed the heated zone is larger. At the same speed, a temperature increase leads to a surface hardness decrease. A similar value of hardness depth (about 0.8 mm) has been detected by means of hardness profiles. The result has been strengthened by scratch test. The parabolic zone presents a very fine martensitic structure on the top of laser trail, and bainite microstructure on transition zone, as literature related. The transition zone between the laser affected zone and the base metal exhibits a micro hardness gradient. It is also possible to notice some Cu diffusion from outer to inner zone. By comparison among LHT specimens and sintering-hardened and stress-relieved one, (characterized by similar chemical compositions) penetration depth and hardness values appear similar. Transformation Hardening (LTH) is a suitable process for producing hard surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with sinter hardened. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.

2014 - Influence of process parameters and alloying type on properties of laser quenched PM-steels [Relazione in Atti di Convegno]
Colombini, Elena; Bocchini, G. F.; Parigi, G.; Sola, Ramona; Veronesi, Paolo; Poli, Giorgio
abstract

Different alloyed PM steels have been laser quenched in industrial equipment laser diodes (4 kW, controlled by material surface temperature). The aim of this work is to investigate their responses to different process condition and different alloying metals, i.e. Cu, Ni, Mo, Cr and C. Furthermore the microstructure of hardened layer, heat affected zone (HAZ) and bulk zone Pre-alloyed, diffusion bonded and hybrid raw materials have been used. Design of Experiments has been the approach for evaluating the effect of treatment parameters (i.e temperature, spot size and speed) and to develop predictive models, correlating such parameters to hardening depth and scratch hardness number. Results demonstrated which valuable properties could be achieved, even through relatively low alloying. The promising results are encouraging since they allow to forecast a possible positive combination of high local hardness and wear resistance of high precision PM part.

2014 - Laser hardening of homogeneous or not-homogeneous PM steels [Relazione in Atti di Convegno]
Bocchini, G. F.; Colombini, Elena; Parigi, G; Sola, Ramona; Veronesi, Paolo; Poli, Giorgio
abstract

An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed, admixed or diffusion-bonded powders. The surface temperature of treated zone has been measured by pyrometer (to avoid local melting or surface damaging) and the travel speed on scanning the surface of test samples has been an investigated variable. A special attention has been given to specimen clamping, to avoid any misalignment with the beam movement. The independent variables investigated have been: surface temperature, T, travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth (HD), hardened width (HW) and overall cross-sectional hardened area (HA) were measured as well and used as responses for the ANOVA of the experimental data gathered. The microhardness profiles present a sharp drop at low distance from the hardened surface. The typical splitting between hardened zone and heat-affected zone (HAZ), well known from laser hardened fully dense steels, holds also for P/M (porous) steels. The research showed that Laser Transformation Hardening (LTH) is a suitable process for producing hard, wear resistant surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with induction hardening. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.

2014 - Laser Quenching of Ionic Nitrided Steel: Effect of Process Parameters on Microstructure and Optimization [Articolo su rivista]
Colombini, Elena; Sola, Ramona; Parigi, GIAN BATTISTA; Veronesi, Paolo; Poli, Giorgio
abstract

The surface properties of 40CrMnMo7 steel have been increased by ionic nitriding thermo chemical treatment followed by laser quenching. Nitriding treatment increases the wear resistance and decreases the wear coefficient, guaranteeing an uniform, though shallow, hardening depth. Combining nitriding with laser quenching has been shown to increase of the hardening depth, while retaining the advantages obtained by the first treatment. In this work the effect of laser quenching parameters on microstructure and mechanical properties of 40CrMnMo7 steel have been investigated and predictive models developed suitable for further optimization of the process. DoE has been exploited to reduce the number of experiments and for evaluating by statistic methods the optimized process parameters.

2014 - Microwave processing of capsule-less powdered beverages [Relazione in Atti di Convegno]
Veronesi, Paolo; Colombini, Elena; Garuti, M.; Botti, C.
abstract

Microwave rapid and selective heating is successfully applied to produce freestanding capsule-less doses of powdered beans or leaves for hot beverages preparation. 1 or 3-person doses, suitable for use in conventional hot beverage preparation machines are obtained in less than 5 seconds, without any additives except for water. Microwaves at 2.45 GHz are applied to allow shape retention and surface hardening of the pressed powders, thanks to the rapid steam generation. Load is contained inside a PTFE-lined applicator presenting micro-holes on two sides, to control vapor outlet. The use of rapidly generated steam, preferentially on the outer layers of the load, leads to weak bonding of the powders, without affecting the final taste of the brewed beverage. Modeling of the temperature- and moisture-dependent dielectric properties is used to optimize the applicator geometry, so that the outer layers of the pressed powders are preferentially heated, thus creating a rigid 'shell' hosting the remaining pressed powders.

2013 - Microwave assisted combustion synthesis in the system Ti-Si-C for the joining of SiC: Experimental and numerical simulation results [Articolo su rivista]
Rosa, Roberto; Veronesi, Paolo; S., Han; V., Casalegno; M., Salvo; Colombini, Elena; Leonelli, Cristina; M., Ferraris
abstract

Microwaves at 2.45GHz have been applied to ignite the combustion synthesis of compacted Ti-Si-C powders mixtures, having 1:1:1 atomic ratio, in order to join SiC-based components. A mixture of different refractory phases such as TiC and TiSi2 were obtained. Depending on the synthesis conditions, no residual silicon in the joint was detected, suggesting the suitability of the here proposed experimental joining approach for nuclear plants and high temperature applications. A simplified model was developed with the aim of obtaining a deeper understanding of the here proposed rapid, almost pressure-less and localized heating joining method. Experimental and numerical simulation results demonstrate that joining of SiC can be rapidly obtained with minimization of heat affected zones in the SiC substrates. Maximum apparent shear strength values of the joints ranged from 9.9 to 45.1MPa, depending on the process conditions.

2013 - Plastic behaviour and cristallographic texture of alpha titanium alloys for heat resistant applications [Anisotropia ed orientamento cristallografico preferenziale di leghe di titanio alfa per impieghi a caldo] [Articolo su rivista]
Gaiani, Silvia; Colombini, Elena; Veronesi, Paolo; Poli, Giorgio; I., Pribošič
abstract

As known, metallic materials presenting HCP crystallographic structure show a marked anisotropic behavior during plastic deformation, especially deep drawing. This attitude is due to the intrinsic asymmetry of hexagonal crystal lattice and is also depending on the c/a ratio of the cell itself. In addition to this aspect, also the tendency of materials such as titanium, magnesium and zirconium, to give rise to a preferential crystallographic orientation (texture) during the rolling phases needs to be taken into account. The most important aspects which contribute to texture formation are: nature and quantity of the alloying elements, reduction ratio during rolling, and heat treatments made on the alloys. Subject of present study are two alpha titanium alloys, specifically developed for heat resistant applications. Despite being quite similar in terms of chemical composition, oxidation resistance and field of application, these two alloys exhibit dissimilar plastic deformation mechanisms. The feature which makes the use of these materials very demanding on an industrial scale is the modification of their plastics properties, in particular plastic strain ratio (R-value), during the deformation progress. On the aforementioned alpha titanium alloys, a complete microstructural and mechanical characterization has been performed analyzing the material according to different orientation, whit the main purpose of identifying the trend of R-value toward strain. In addition to that, a study of the original texture of the two alloys has been made using different measuring methods (X-Ray diffraction, Electron Back-Scattering Diffraction). The purpose of this activity is finding a correlation between the macroscopic behavior of the alloys during deformation and their crystallographic orientation on microstructural scale, for better understanding the specific role played by the individual sliding systems and their spatial orientation on the plastic properties of the materials.

2013 - Processing and Mechanical Property Characterization of Aligned Carbon Nanotube Carbon Matrix Nanocomposites [Relazione in Atti di Convegno]
Stein, Itai Y.; Vincent, Hanna M.; Steiner, Stephen A. III; Colombini, Elena; Wardle, Brian L.
abstract

Materials comprising carbon nanotube (CNT) aligned nanowire (NW) polymer nanocom- posites (A-PNCs) are emerging as next-generation materials for use in aerospace structures. Enhanced operating regimes, such as operating temperatures, motivate the study of CNT aligned NW ceramic matrix nanocomposites (A-CMNCs). Here we report the synthesis of CNT A-CMNCs through the pyrolysis of CNT A-PNC precursors, thereby creating carbon matrix CNT A-CMNCs. Characterization reveals that the fabrication of high strength, high temperature, lightweight next-generation aerospace materials is possible using this method. Additional characterization and modeling are planned.

2013 - Processing and mechanical property characterization of aligned carbon nanotube carbon matrix nanocomposites [Relazione in Atti di Convegno]
Stein, I. Y.; Vincent, H. M.; Steiner III, S. A.; Colombini, E.; Wardle, B. L.
abstract

Materials comprising carbon nanotube (CNT) aligned nanowire (NW) polymer nanocom- posites (A-PNCs) are emerging as next-generation materials for use in aerospace structures. Enhanced operating regimes, such as operating temperatures, motivate the study of CNT aligned NW ceramic matrix nanocomposites (A-CMNCs). Here we report the synthesis of CNT A-CMNCs through the pyrolysis of CNT A-PNC precursors, thereby creating carbon matrix CNT A-CMNCs. Characterization reveals that the fabrication of high strength, high temperature, lightweight next-generation aerospace materials is possible using this method. Additional characterization and modeling are planned. © 2012 AIAA.

2013 - Recycling of alpha-titanium technological scrap for exhaust system parts manufacturing [Articolo su rivista]
Veronesi, Paolo; S., Gaiani; Colombini, Elena; Poli, Giorgio; R., Tisu
abstract

n this paper, five different titanium alpha alloys obtained by investment casting, and recycled titanium scrap which originated from other technological operations have been studied in order to investigate their basic properties and to determine the industrial feasibility of the manufacturing of recycled titanium exhaust system parts on an industrial scale. Chemical analysis after a first and a second melting step, a complete mechanical characterization and oxidation resistance test at different temperatures have been performed. The direct influence of the alloying elements, like Si and Al, on the oxidation resistance of titanium at high temperature has been studied, in order to identify the best blended alloy for investment casting applications. An important aspect which has also been evaluated is the increase of oxygen content after remelting of the blend alloys and its impact on the mechanical properties. Results show that it is possible to use the identified blend alloys to obtain cast components suitable for applications requiring exposure at a temperature up to 700 C, and for the Si-richer blends, up to 800 C

2012 - Microwave ignited combustion synthesis as a joining technique for dissimilar materials [Articolo su rivista]
Rosa, Roberto; Colombini, Elena; Veronesi, Paolo; Poli, Giorgio; Leonelli, Cristina
abstract

Microwave energy has been exploited to ignite combustion synthesis (CS) reactions of properly designed powders mixtures, in order to rapidly reach the joining between different kinds of materials, including metals (Titanium and Inconel) and ceramics (SiC). Beside the great advantage offered by CS itself, i.e., rapid and highly localized heat generation, the microwaves selectivity in being absorbed by micrometric metallic powders and not by bulk metallic components represents a further intriguing aspect in advanced materials joining applications, namely the possibility to avoid the exposition to high temperatures of the entire substrates to be joined. Moreover, in case of microwaves absorbing substrates, the competitive microwaves absorption by both substrates and powdered joining material, leads to the possibility of adhesion, interdiffusion and chemical bonding enhancements. In this study, both experimental and numerical simulation results are used to highlight the great potentialities of microwave ignited CS in the joining of advanced materials.

2012 - Microwave ignited combustion synthesis as a joining technique for dissimilar materials: Modeling and experimental results [Articolo su rivista]
Colombini, Elena; Rosa, Roberto; Veronesi, Paolo; M., Cavallini; Poli, Giorgio; Leonelli, Cristina
abstract

Microwave energy has been applied to ignite the combustion synthesis (CS) in Ni + Al (50: 50 at %) powder mixtures in order to join dissimilar materials, in particular high-temperature metals. A numerical simulation was performed in order to obtain the data otherwise difficult to be experimentally measured and to develop a simplified predictive model of microwave ignited and sustained CS in metal powder compacts for joining applications. Experimental and numerical simulation results demonstrate that joining can be rapidly obtained by microwave ignition of the combustion synthesis characterized by a minimum extension of heat affected zones in the joined substrates.

2012 - Nanoporous micro-element arrays for particle interception in microfluidic cell separation [Articolo su rivista]
Grace, D. Chen; Fabio, Fachin; Colombini, Elena; Brian, L. Wardleb; Mehmet, Toner
abstract

The ability to control cell-surface interactions in order to achieve binding of specific cell types is a major challenge for microfluidic immunoaffinity cell capture systems. In the majority of existing systems, the functionalized capture surface is constructed of solid materials, where flow stagnation at the solid-liquid interface is detrimental to the convection of cells to the surface. We study the use of ultra-high porosity (99%) nanoporous micro-posts in microfluidic channels for enhancing interception efficiency of particles in flow. We show using both modelling and experiment that nanoporous posts improve particle interception compared to solid posts through two distinct mechanisms: the increase of direct interception, and the reduction of near-surface hydrodynamic resistance. We provide initial validation that the improvement of interception efficiency also results in an increase in capture efficiency when comparing nanoporous vertically aligned carbon nanotube (VACNT) post arrays with solid PDMS post arrays of the same geometry. Using both bacteria (∼1 μm) and cancer cell lines (∼15 μm) as model systems, we found capture efficiency increases by 6-fold and 4-fold respectively. The combined model and experimental platform presents a new generation of nanoporous microfluidic devices for cell isolation.

2012 - Rapid microwave sintering of protective ZrO2 coatings on reactive metal powder compacts [Relazione in Atti di Convegno]
Veronesi, Paolo; Rosa, Roberto; Colombini, Elena
abstract

Electrophoretic deposition was used to create protective coatings of sub-micrometric ZrO2 particles on substrates made of conductive powders mixture (Ni+Al). In order to achieve the required mechanical properties, such coating requires a sintering stage. However, the rapid microwave sintering of thin zirconia layers usually requires some form of pre-heating or auxiliary heating of the material, in order to increase its loss factor. In this study, the heat released by the exothermal reactions of combustion synthesis occurring in the powders compact, is used to concurrently synthesize high-temperature rated aluminides and pre-heat and sinter the overlaying zirconia coating. Finite elements numerical simulation, fully coupling electromagnetic, heat transfer and chemical reactions application modes is used to investigate the temperature profile and power density distribution during the microwave sintering process, Experimental results show that the concurrent synthesis allows to form a thin alumina-based bond coat, which is expected to increase the high temperature resistance of the zirconia-coated aluminides.

2011 - Microwave ignited Combustion synthesis of intermetallic compounds, modelling and experimental results [Articolo su rivista]
Colombini, Elena; Rosa, Roberto; Veronesi, Paolo; Casagrande, A.
abstract

The process of Combustion synthesis (CS) is based on the highly exothermic reaction by reactants, which, if properly ignited, spontaneously turn into products. The aim of this work is to study the CS of β-NiAl formed starting from Ni and Al (1:1 at. %) powders activated by microwaves at 2.45 GHz. Numerical simulation is used to obtain data otherwise difficult to be measured experimentally and to develop a predictive model of microwave ignited and sustained CS of metal powder compacts. The simulation couples an electro-thermal model with a chemical model, required to study the exothermic reaction between powders. A simplify model was obtained and validated, neglecting volume changes, to study compositional and temperature change and reaction kinetics during the CS. It allowed to demonstrate how microwave application, during and after, synthesis could control the cooling rate of products and hence the microstructure of the newly formed intermetallics.

2011 - Microwave ignited combustion synthesis of metal and intermetallic matrix composites [Relazione in Atti di Convegno]
Rosa, Roberto; Sola, Ramona; Colombini, Elena; Veronesi, Paolo; Leonelli, Cristina
abstract

Aim of this work is to present the results concerning the application of microwave assisted combustion synthesis (MACS) approach in the production of different kind of cermets and intermetallic matrix composite (IMC). Combustion synthesis allows taking advantages from the heat generated from exothermic reactions, which can derive from the synthesis of the intermetallic matrix, of the reinforcement, or both. Microwaves were used to ignite such combustion reactions exploiting their heating selectivity. The use of a single mode applicator allowed also investigating any "specific" microwave effects ascribable to separate electric and magnetic fields.

2010 - Microwave Assisted Combustion Synthesis of Non-equilibrium Intermetallic Compounds [Articolo su rivista]
Veronesi, Paolo; Rosa, Roberto; Colombini, Elena; Leonelli, Cristina; Poli, Giorgio; Angelo, Casagrande
abstract

A simplified model of the microwave-assisted combustion synthesis of Ni and Al metal powders to form the NiAl intermetallic on titanium and steel substrates is presented. The simulation couples an electro-thermal model with a chemical model, accounting for local heat generation due to the highly exothermic nature of the reactions between the powders.Numerical results, validated by experimental values, show that the capability of microwaves to convey energy, and not heat, can be used to alter the temperature profiles during and after the combustion synthesis, leading to unique intermetallic microstructures. This phenomenon is ascribed to the extended existence of high temperature liquid intermetallic phases, which react with the metallic substrates at the interface. Moreover, microwave heating selectivity allows to maintain the bulk of the substrate metallic materials to a much lower temperature, compared to combustion synthesis in conventionally heated furnaces, thus reducing possible unwanted transformations like phase change or oxidation.