Microstructural and mechanical property modelling for the processing of Al-Si alloys
The components of a modem internal combustion engine are required to give extreme reliability over extended periods of operation and none is exposed to more arduous conditions than the piston, especially in the pin boss and crown regions of pistons for diesel engines. The increasing emissions requirements and performance targets demanded of direct injection diesel engines has resulted in steep increases in both specific powers and maximum cylinder pressures. This has in turn lead to greater temperatures and pressures being felt by the piston. The adaptation of the piston design to these increasingly demanding load and temperature conditions has required a continuous improvement and innovation in the field of materials and process technologies. The vast majority of the internal combustion engine pistons produced globally are made by a gravity die casting process using Al-Si based alloys. Although Al-Si alloys have been the subject of a great deal of research over the last 30 years, the majority of work has been based on fairly rudimentary characterisation of the microstructures as a function of alloy chemistry and cooling rate. Most of the attention has been paid to the silicon morphology and distribution rather than on a fundamental knowledge of the development of the complex microstructures and intermetallic phases that arise in commercial alloys. However, the properties of cast near-eutectica: luminium-silicon alloys are very strongly influenced by the microstructure, i.e. the primary aluminium, and the interdendritic microconstituents, such as secondary phases, intermetallics, inclusions and porosity. A fine and uniform grain size is often desired as it improves mechanical properties of castings such as tensile strength, ductility and fatigue resistance, and at the same time aids castability, improves porosity distributions and reduces hot tearing susceptibility. A thorough phase characterisation has been carried out using a number of techniques including optical and electron microscopy with electron backscatter diffraction (EBSD), and image analysis. Use was also made of thermodynamic modelling to predict the volume fraction and distribution of phases within the microstructure as a function of chemical composition and process parameters. From this analysis a detailed understanding of the phases occuffing in multicomponent Al-Si alloys was established. Furthermore, additions associated with grain refining, i.e. Ti, Zr and V, have been investigated systematically using commercial and model alloy systems. All three additions were observed to refine the structure of the castings through the formation of the phase A13Ti, although combined additions with Zr were found to be less efficient due to a 'poisoning' effect on the A13Ti. It was also established that there is a strong competition between the effects of grain refiners and P, with the formation of AbTi reducing the nucleating efficiency of AIP to silicon. The nucleation and growth of the primary silicon phase were thus examined by EBSD. AIP was confirmed as nucleating the silicon epitaxially, after which growth continues by surface nucleation, although the presence of twins were seen to influence the shape of the crystal. Finally, suggestions have been made as a consequence of this work for the future development of piston alloys.