Papers by Author: Maurizio Vedani

Abstract: Analysis of peculiar properties offered by Al alloys produced according to additive manufacturing techniques, specifically by Selective Laser Melting (SLM), is carried out. Two alloys are considered, derived by casting (AlSi10Mg) and by wrought (ENAW 2618) applications. The SLM processed samples are investigated considering their microstructural and mechanical properties after SLM and compared to cast and wrought counterparts. A strong microstructural refinement induced by SLM processing is observed for both alloys, resulting in excellent hardness properties. Investigation on integrity of samples revealed that small-size microvoids and unmelted regions could be present in SLM parts.

Abstract: In the present work an investigation on primary processing of CuZr based shape memory alloys is proposed. In particular, the study addresses the effect of hot rolling process on the properties of CuZr shape memory alloys, in which the addition of some elements, such as Cr, Co and Ni, is taken into account. These alloys are produced by means of vacuum arc melting furnace under non contaminating conditions. Considering the high reactivity of these alloys, due to Zr element, hot working was undertaken after sealing the ingot cigars in a stainless steel protective can. The characterization of the rolled alloys is performed using DSC in terms of evolution of martensitic transfor-mation temperatures. The analysis is completed with mechanical and the microstructure investigations by means of microhardness and scanning electron microscopy (SEM) observations, respectively. The main result of this work is the evaluation of hot workability property of this system, which can be improved by adding metallic elements to the binary CuZr intermetallic system. Moreover, interesting characteristics, such as high transformation temperatures, thermal stability and reduced thermal hysteresis, can be also improved by means of the addition of Ni and Co at the same time.

Abstract: Thick coatings for heavy duty applications in many industrial fields are produced by generating hard layers of Fe-based complex alloys on the critical surfaces. The composition and structure of these coatings have to be carefully tuned in order to generate a defect-free layer after solidification and to meet the expected service conditions. The present study is aimed at investigating and developing improved Fe-based alloys for the above described thick coatings by a careful definition of the microstructural features required to bear service conditions and by defining the optimal compositions to achieve the desired properties. Primary carbide distribution and size as well as steel matrix composition are the main microstructural features that will be evaluated in order to fine tune coating performance in terms of toughness and hardness. The experimental part of the investigation here presented will be mainly concerned to laboratory casting the above alloys and to assess their microstructure and properties.

Abstract: Metal matrix nanocomposites have been produced by powder metallurgy route. Al and nanoAl₂O₃ powders were grinded through high energy ball milling. Then, the composite powders were sintered by Equal Channel Angular Pressing (ECAP). 12 ECAP passes were carried out in order to improve the dispersion of the hard particles. SEM analysis was performed to investigate the distribution of the ceramic nanoparticles within the matrix. Hardness tests were executed to evaluate the mechanical behavior of the nanocomposites. Finally, mechanical strength values obtained by numerical models were compared with those estimated from hardness measurements. High energy ball milling followed by ECAP process revealed to be a suitable route for the production of metal matrix composites reinforced with well dispersed nanoparticles.

Abstract: The present contribution is aimed at investigating the microstructure evolution of commercially pure silver under severe plastic deformation conditions. ECAP billets have been produced by using a die with channels intersecting at 90° and straining the samples at room temperature. The evolution of the microstructure as a function of imparted strain was evaluated by scanning electron microscopy as well as X-ray diffractometry. Furthermore, tensile properties were measured from ECAP billets in order to evaluate the strengthening and work hardening behaviour of silver as a function of structure evolution. Comparison in terms of grain structure and corresponding properties are also drawn by considering published data about Al-Mg-Si alloy samples ECAP-processed by identical routes and parameters.

Abstract: Two commercial ZM21 and AZ31B alloys were extruded into small-size tubes at 410°C and strain rate 2.78·10-3 s^-1 by a laboratory hot-extrusion system. The series of tubes, with outer diameter in the range of 8-4 mm and inner diameter from 6 to 3 mm were investigated considering microstructure and texture analysis. The tubes featured a homogeneous and refined equiaxed grain structure since dynamic recrystallization was clearly observed during the extrusion process. A typical ‘basal’ type texture was detected in extruded tubes by using EBSD technique.

Abstract: A study on low-cycle and high-cycle fatigue behaviour of 6061-Al₂O₃ composites reinforced with nominal volume fractions of 10% and 20% of Al₂O₃ particulates is presented. The effects of reinforcement geometrical features (volume fraction and size) and of the loading mode experienced during the different kind of fatigue tests (strain controlled and stress controlled tests) were evaluated. A relation with crack growth mechanisms was drawn by analyses on fracture surfaces and on longitudinal sections of specimens subjected to the fatigue tests. The micromechanisms of cyclic deformation and of microstructural damage acting in the materials are discussed and compared to data and observations available from the wide published literature.

Abstract: The main drawback of conventional stenting procedure is the high risk of restenosis. The idea of a stent that "disappears" after having fulfilled its mission is very intriguing and fascinating. The stent mass should diminishing in time to allow the gradual transmission of the mechanical load to the surrounding tissues. Magnesium and its alloys seem to be among the most appealing materials to design biodegradable stents. The objective of this work is to develop, in a finite element (FE) framework, a model of magnesium degradation able to predict the corrosion rate and thus providing a valuable tool to design biodegradable stents. Continuum damage approach is suitable for modelling different damage mechanisms, including several types of corrosion. Corrosion is modelled by a scalar damage field which accounts for the material strength loss due to geometrical discontinuities. As damage progresses, the material stiffness decreases. Corrosion damage results as the superposition of stress corrosion process and uniform corrosion. The former describes the stress-mediated localization of the corrosion attack through a stress-dependent evolution law similar to the one used in analytical models, while the latter affects the free surface of the material exposed to an aggressive environment. The effects of both phenomena described are modelled through a linear composition of the two specific damage evolution laws. The model, developed in a FE framework, manages the mesh dependency, typical of strain-softening behaviour, including the FE characteristic length in the damage evolution law definition. The developed model is able to reproduce the behaviour of different magnesium alloys subjected to static and slow-strain-rate corrosion tests. Moreover, 3D stenting procedures accounting for the interaction with the arterial vessel are simulated.

Abstract: Heat treatable aluminium alloys show their best properties when properly heat treated. Most of the high-strength alloys are usually serviced in the so-called T6 temper consisting of ageing at moderate temperature after a solution treatment and a subsequent quenching. A large extrusion was investigated in this paper. The part was solution annealed at 505°C, water quenched and aged at 160°C for 16 hours. Each stage of the heat treatment is analyzed in this paper in the light of the properties achieved, by experimental investigations and several kinds of numerical simulations. In particular, a thermodynamic calculation (Pandat® software) provides solidification temperatures and equilibrium phases, in order to check whether the solution temperature is adequate. A finite element analysis (performed with COMSOL Multiphysics® software) supplies a simulation of the temperature field during water quenching. The cooling curves obtained are drawn on CCT (Continuous Cooling Transformation) curves, calculated by means of JMatPro® software, to verify if any undesired high temperature precipitation could occur during quenching. Finally, calculated TTT (Time Temperature Transformation) curves can be related to the ageing treatment.Thermal analyses, microstructural investigations and microhardness profile measurements on extrusion sections are also performed to validate calculated results.

Abstract: The ductility behaviour experienced by steels for linepipe (LP) applications in a temperature range generally from 700°C to 1200°C is a widely studied subject in steel research, especially for its implication on cracking during continuous casting and rolling. Hot tensile / torsion tests on as-cast products, conducted until fracture, are normally used to characterise the hot deformability behaviour. Depending on the industrial hot deformation process within which the steel aptitude is being investigated, other types of tests can be more adequate. With the aim to characterize the hot deformability behaviour in terms of the damages each steel presents at moderate strain levels (i.e. far from the onset of necking), a special device for interrupted hot tensile tests, followed by immediate quenching (i.e. to “freeze” the microstructure) was developed. Various industrial steels with different starting microstructures (ad hoc in-lab heat treatments performed before testing) were tested by this method, and subsequent metallurgical investigations of the strained samples were carried out to identify, for each case, the damage mechanism and the microstructure features having the major influence on ductility loss. As a result, it was found that (i) microstructural damages at moderate strain levels can be much better described throughout interrupted hot tensile tests, (ii) different compositions and starting microstructures within the industrial LP scenario lead always to microstructural damages at relatively high deformation temperatures (e.g. 950°C) and moderate strains (e.g. 0.1 to 0.2, very far from the onset of necking), (iii) the common mechanism by which LP steels start voiding is the grain boundary sliding and (iv) the intergranular voids, once formed, grow longer in coarser microstructures.