STUDY OF THE INFLUENCE OF THE CASTING PROCESS ON THE MICROSTRUCTURAL FEATURES AND ON THE FATIGUE PROPERTIES OF FOUNDRYALUMINIUM ALLOYS

摘要

The use of aluminium alloy castings in automotive field has seen an increasing development in the last years, thanks to the reduction of the times for the production of components, in respect of other forming techniques (i.e. forging) and thanks to the possibility of a lightening of the weight of the cars. These advantages have lead to a strong interest towards aluminium castings for applications requiring high mechanical properties. In such contest new forming processes for aluminium alloys have been developed in order to obtain high integrity and reliability similar to those obtainable by means of the traditional forging. The quality of aluminium cast parts depend on the volume fraction, on the morphology and on the distribution of the porosity, which reduces the mechanical properties, like the toughness, the fatigue and tensile resistance. The presence of pores decreases the resistance to static and dynamic stresses and, above all, it determines a remarkable statistical dispersion of the mechanical characteristics. This work regards the study of the microstructural features and of the fatigue properties of foundry aluminium alloys in automotive field. In particular this research concerns the study of the influence of the casting process anf of the LHIP process (Liquid Hot Isostatic Pressing) on the fatigue properties of aluminium alloys. The fatigue tests have been carried out on plane or cylindrical samples, from suspension arms and cylinder heads. These castings have been manufactured by means of different casting techniques: low pressure permanent mould (A356 suspension arms), low pressure Disamatic (A356 suspension arms), lost foam (A319 and A356 cylinder heads) and rheocast (A356 and A357 suspension arms). The main feature of lost foam process is the use of models made of polymer foam to produce the final component. In lost foam process the dendrite arm spacings are usually higher in respect of permanent mould castings; then the microstructure of lost foam castings is not so fine as the microstructure of components, which has been cast in a metallic mould. Consequently the mechanical properties are lower, also because of a higher amount of internal porosity. The Liquid Hot Isostatic Pressing (LHIP) is a process that subjects a component to elevate temperature and pressure in a suitable vessel, containing a bath made of melt salts. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and weld up. This process is currently under development with the aim to upgrade the quality of castings. In fact castings may have internal defects such as gas porosity (due to trapped gases, such as hydrogen or air), interdendritic microporosity (due to shrinkage) and microcracks that form during solidification and that have an adverse effect on performance. Such pores tend to concentrate stress and therefore act as yield and crack initiation points. The more sharply angled a pore, the more the stress will be concentrated. The LHIP process heals these defects first through creep and plastic deformation and then by diffusion bonding of the surfaces of the collapsed area, creating a casting with a fully dense omogeneous microstructure. During processing the treatment temperatures are maintained with the plastic range of the material concerned, or more precisely, high enough for the diffusion bonding to occur but at the same time low enough to avoid undesidered microstructural modification such as grain growth. The higher integrity and homogeneity of the casting after LHIP improves its mechanical characteristics with the subsequent increase of fatigue resistance but also of tensile and yield strengths, ductility and, creep life. It also reduces the scatter of these properties. As the volume normally occupied by defects is limited, LHIP does not significantly modify the dimensions or the shape of the casting. The rheocasting process is a semisolid forming technique based on the use of a alloy wit