Microstructural investigation on aluminium castings for automotive applications

In the diesel engine cylinder head, especially to resist to high combustion pressure, it has been demanded to develop high strength materials. The traditional cast iron cylinder head meets most of the requirements. However, cast iron has the disadvantage of being heavy and having a low thermal conductivity. To improve the fuel efficiency and the drivability of vehicles, light weighted cylinder heads made of aluminium alloys have been developed. In recent production, Al-Si-Cu-Mg cast alloys have been used so far because of their good performance of castability and mechanical strength with age hardening. This work analysed the microstructural properties of the mentioned cast aluminium alloys on two real complex-shaped castings like diesel cylinder heads, as cast and T6 heat treated, experimentally produced by Teksid Aluminum. The secondary dendrite arm spacings (SDAS) and the size and morphology of eutectic silicon were considered with this study under quantitative aspects. It is well known how finer SDAS, obtained with high solidification conditions, result in more uniform microstructure and create the condition for more closely distributed and finer, smaller particles, like eutectic silicon and intermetallics. In this study the cast aluminium alloy (A354) adopted to cast cylinder heads underwent a heat treatment (T6) in order to obtain an optimum combination of strength and ductility. The mechanical properties of this alloy are generally dependent by the followings conditions: distribution and size of the precipitates, β-Mg2Si and θ-Al2Cu, structure and size of eutectic silicon particles. Therefore, it is evident how the mechanical properties of the casting are severely influenced by the combined effect of SDAS and eutectic silicon.