The main research activities can be classified as follows:
- Plastic deformation and welding of metal matrix
composite (MMC)
- Relationships between microstructure and mechanical properties
of casting aluminium alloys
- High temperature behavior of aluminium alloys
- Fracture toughness of titanium alloys
PLASTIC DEFORMATION AND WELDING OF METAL MATRIX COMPOSITE (MMC)
Particle reinforced metal matrix composites (MMC) are very attractive materials; due to their high stiffness (high strength and modulus with low density) high temperature stability and superior wear resistance, compared to the unreinforced alloys. However, at present, MMCs have not reached widespread industrial application because of the problems related to the secondary manufacturing processes, such as forging, extrusion, forming and welding.
The activities were focused on:
(i) Evaluating the influence of microstructure on: tensile and low-cycle fatigue behaviour of Al-based MMCs
(ii) Understanding the effects of high strain rate superplastic deformation (HSRS) and forging on microstructure of Al-based MMCs
(iii) New solid-state joining techniques, the friction stir welding (FSW) and linear friction welding (LFW) processes, and their applicability to Al-based MMC.
The studies on microstructural and mechanical properties showed that:
• MMCs ensure higher elastic modulus and tensile strength but lower elongation to failure, respect to the unreinforced alloys, at room and high temperature. The tensile ductility is affected by the material inhomogeneity and fracture mechanisms.
• Fracture mechanisms of MMCs are strongly influenced by: particles size, shape, distribution, volume fraction and by matrix-particle interface strength. At room temperature reinforcement fracture occurs in presence of large particles or in regions with clusters, while high temperatures (150-250°C) promote voids nucleation and interfacial decohesion
• Particles cracking can lead to cyclic softening of MMCs during low-cycle fatigue tests at room temperature.
The results of the researches on forged and superplastically formed MMCs highlighted that:
• It is possible to obtain a superplastic behaviour of MMC only if a significant grain refinement of the aluminium alloy matrix is achieved
• The presence of a liquid phase, at the interfaces matrix/reinforcement and grain boundaries, could play an important role as an accommodation mechanism for the grain boundary sliding (GBS) during superplastic deformation
• The presence of reinforcement enhances the tendency to cavitation of the MMCs during HSRS
• Hot forging, carried out with low deformation ratio, low strain rate and high temperature, do not induce damage and do not affect the particle clustering. However it reduces porosity and matrix grain size
• Hot forging brakes and orients the intermetallic compounds in the direction of the plastic flow, favouring their partial dissolution during heat treatment and the grain size control at high temperature
• Microstructural modifications, in the forged composite, induce an increase in the tensile properties of the material both at room and high temperature.
The researches on the solid-state joining techniques showed that:
• The FSW and the LFW processes can be successfully applied to Al based MMCs and to dissimilar joints Al-Al MMCs. An advantage of LFW respect to the FSW is the absence of the pin, which is subjected to severe wear, while a limitation is its applicability only to flat-edged components
• FSW induces substantial refinement of the reinforcement particles and matrix grain size, as well a better particle distribution in the welded zone. These microstructural modifications are due to the high abrasive action of the hard tool on reinforcement particles and to the dynamic recrystallization induced by the plastic deformation and frictional heating
• FSW joints, without any post-weld heat treatment and surface polishing, reveal a high tensile efficiency and an increase of the impact toughness with respect to the base material. The slight reduction in the low-cycle fatigue resistance is mainly due to the higher surface roughness.
• LFW induces the development of an ultra-fine microstructure with a uniform particle distribution in the joint centre, while in the thermo-mechanically affected zone (TMAZ) the fibrosity of the material follows the plastic flow caused by the process. No effects were observed on particle size and shape.
• The tensile, fatigue and impact toughness efficiency of the LFW joints, without any post-weld heat treatment, are comparable with those of FSW joints.
RELATIONSHIPS BETWEEN MICROSTRUCTURE AND MECHANICAL PROPERTIES OF CASTING ALUMINIUM ALLOYS
The Al-Si casting alloys are widely used in the transport field, due to their excellent castability, high strength-to-weight ratio, corrosion resistance. However, the mechanical properties of aluminium alloy cast components are strongly dependent on the local microstructure, which is directly related to the chemical composition and the solidification conditions imposed by the casting system (mould, cores, cooling circuit, coolers, etc.)
The studies, on Al-Si-Mg and Al-Si-Cu casting alloys, were focused on the evaluation of relationships among casting parameters, microstructure and mechanical properties. The goal is to develop reliable experimental equations, which can successfully predict the local tensile and fatigue properties in complex castings, on the basis of microstructural parameters. These equations could link the post-processing results of the casting simulation software to the pre-processing phase of the structural ones, with an approach of co-engineered design.
The research activities on Al-Si-Mg alloys, with low Fe content, highlighted that:
• Hardness, induced by strengthening precipitates formed after heat treatment, mainly influences tensile strength
• Porosity (without distinction between gas porosity and shrinkage cavities) affects elongation to failure, ultimate tensile strength and, above all, fatigue life
• Damaging effect of pores is related to local stress concentration induced by the single pore and to the total amount of pores
• The effect of SDAS, size and shape of Si particles and Fe based intermetallics, on elongation to failure, tensile strengths and fatigue life, becomes appreciable when the porosity is negligible (as in the case of castings subjected to hot isostatic pressing, HIP).
In Al-Si-Cu, with different contents of Fe and Mn, was observed that:
• The increase in Fe and Mn content induces a slight increase in the yield strength, but a negligible effect on ultimate tensile strength
• During plastic deformation cracks nucleate along the harmful needle-like b-Al5FeSi phase, but also the a-Al15(Fe,Mn)3Si 2 and p-Al8Mg3FeSi, for their size and shape, can favour both crack initiation and propagation, reducing elongation to failure
• Large Fe-rich intermetallic particles facilitate the formation of fatigue crack in pores free samples. The high Fe and Mn contents promote, consequently, fatigue crack initiation, lowering the total fatigue life. In samples with pores, which act as crack initiation, the presence of Fe-intermetallics reduces the crack growth rate and enhances the fatigue life because it leads both to an increase of yield strength and to a deflection of the crack path around the particles, especially in the a-Al15(Fe,Mn)3Si 2 compounds
• In short lifetime regime, when fatigue life strongly depends on micro-crack propagation, Fe-intermetallics reduce the crack growth rate, leading to a branching of the crack around the particles.
To date, multivariable regression analysis, performed on the collected data, led to the development of e xperimental relationships between:
• Most important microstructural features, the hardness and the tensile properties
• Volume fraction of pores in a fatigue sample (predictable by the new solidification simulation software), and area fraction, size and shape of pores on the fatigue fracture surfaces
The promising results of the proposed equations suggest that these can be integrated in a new design methodology in which the knowledge of hardness and main microstructural parameters, evaluated by solidification models, could allow the estimation of the local mechanical properties in complex cast components.
HIGH TEMPERATURE BEHAVIOUR OF ALUMINIUM ALLOYS
In recent years more attention has been devoted to decrease pollution. This led car manufacturers to reduce engine size (downsizing), enabling mass reduction and fuel saving. Nevertheless, such design solutions generally induce an increase of operating temperatures and thermo-mechanical stresses in engine components. Unfortunatly the mechanical properties of heat treatable alloys (e.g. Al-Si-Mg or Al-Si-Cu-Mg) are negatively affected by prolonged exposure to temperature higher than 200°C, due to the coarsening of strengthening precipitates (i.e. overaging). As a result new alloys with increased stability at high temperature are needed.
The research activities were mainly focused on:
(i) microstructural and mechanical characterization, both at room and high temperature, of aluminium alloys used for engine production, in order to evaluate the influence of microstructure features on their mechanical behaviour
(ii) developing innovative alloys with high stability in temperature by adding Zr, Er or Mo to traditional alloys.
The first activity was carried out both on Al-Si-Mg and Al-Si-Cu-Mg casting alloys (A356, A357, A354 and C355) and wrought alloys (AA2618 and AA4032)
Experimental results relating to casting alloys highlighted that:
• SDAS has a negligible influence on mechanical behaviour of the alloys at high temperature
• Fe based intermetallics have a deleterious effect on both elongation and fatigue resistance both ad room and high temperature
• The higher thermal stability of Al-Si-Cu-Mg respect to the Al-Si-Mg alloys is mainly due to the presence of quaternary precipitates AlCuMgSi such as Q phase
Studies on wrought alloys, instead, allowed defining correlation among temperature, exposition duration and mechanical properties of the materials. In particular experimental relationships between residual hardness, tensile strengths and plastic behaviour have been developed. Moreover, the microstructural evolution of the alloys during their exposition at high temperature was observed by means of optical and scanning electron microscopy.
The addition of Er to the A356 alloy induces the modification of the eutectic silicon morphology and the formation of both intermetallics and strengthening precipitates. This leads to:
• an increase of tensile strength at high temperature
• a reduction of the elongation to failure
• a reduction of the castability of the alloys
The addition of Zr and Er leads to a further microstructure refining, and for Zr content of about 0,5 wt% also to the development of a globular microstructure in the A356 alloy. Respect to the alloy with only Er the addition of Zr increases the tensile strength of the material both at room and high temperature.
Mo was added to the A354 alloy. After proper heat treatment Mo induces an increase of tensile strength at room temperature, while at high temperature its effect is negligible. From a microstructural point of view Mo reduces the modification effect of Sr on eutectic silicon and prevents the formation of Fe based acicular intermetallics (beta intermetalics)
FRACTURE TOUGHNESS OF TITANIUM ALLOYS
Titanium alloys are widely used in the aerospace industries for their remarkable strength, weight ratio and resistance to high temperature creep.
The research, carried out during the PhD studies, was focused on the evaluation of the effects of microstructure, hydrogen content and innovative welding technique on impact toughness, KIC and DKth, of a+b Ti alloys. The goal was the development of semi-empirical correlation between the KIC and the data obtained from an instrumented Charpy impact test.
The results of the research highlighted that:
• The bi-modal microstructure offers the best compromise between tensile strength and fracture toughness while, as expected, hydrogen reduces toughness of the alloys independently from microstructure
• The innovative magnetically impelled-arc narrow-gap GTA welding is attractive for joining Ti-6Al-4V plates. However the joints show lower fracture toughness, respect to the base material, due to the presence of fine Widmanstaetten a-plates, partially martensitic microstructure, and some microcracks
Data, collected during the research, led to the development of preliminary experimental relationships able to predict KIC of the Ti-6Al-4V alloys with a good accuracy. These equations correlate KIC with nucleation, propagation energy and dynamic yield strength, obtained from an instrumented Charpy impact test.
