E. Colombini

Microwave ignited combustion synthesis as a joining technique for dissimilar materials: Modeling and experimental results

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.

Laser Quenching of Ionic Nitrided Steel: Effect of Process Parameters on Microstructure and Optimization

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.

Optimization of laser welding of dissimilar corrosion resistant alloys

The use of multi-materials components has the benefit of coupling each materials 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.

SPS-assisted Synthesis of SICp reinforced high entropy alloys: reactivity of SIC and effects of pre-mechanical alloying and post-annealing treatment

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.

Laser hardening of steel sintered parts

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.

Microwave processing of capsule-less powdered beverages

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.