Predictive equations of the tensile properties based on alloy hardness and microstructure for an A356 gravity die cast cylinder head

Excellent castability and high strength-to-weight ratio, which increases performance and fuel economy, have made A356 alloy one of the most suitable materials for various applications in the automotive industry, such as cylinder head and engine block. The main limitation of cast Al-Si alloys is, however, the considerable influence of the solidification and heat treatment conditions on the final microstructure and consequently on the mechanical properties, that can thus exhibit a wide range of variability in the same cast component. Literature data often relate the static and fatigue properties of cast Al alloys to only one microstructural parameter, such as solidification defects or secondary dendrite arm spacing, without taking into account their simultaneous effect. In addition, the experimental tensile and fatigue data are often obtained on specimens cast apart or obtained under controlled laboratory conditions, substantially different from those of industrial cast components. For these reasons, the main problem in the design phase is the lack of knowledge of the true local mechanical properties, that often make a very conservative approach necessary, with a consequent increase of the cross sections and consequently of the weight. The aim of this research was to study a complex cast component, produced under industrial conditions, in order to verify the range of variability of the microstructural parameters and the tensile properties. The component was an A356 gravity die cast cylinder head, heat treated at the T6 condition, and the effect of the delay between quenching and aging on the alloy hardness was also evaluated. Mathematical models, able to successfully predict the local tensile properties in the cast, from the concurrent effect of the main microstructural parameters and alloy hardness, are proposed. These models allow the designer to predict the local tensile behaviour, without any tensile tests, and can 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.