A study on the relationship between solidification conditions and microstructural characteristics of a complex shaped A356 gravity die cast cylinder head

Al-Si-Mg cast alloys have widespread applications, especially in the aerospace and automotive industries, due to an excellent combination of castability and high specific strength. Among these alloys, hypoeutectic A356 (Al7Si0.3Mg) is one of the most widely used for the production of a variety of components, including engine blocks and cylinder heads. The microstructure of this alloy greatly depends on chemical composition, solidification conditions, metal soundness, and heat treatment. Most of the literature data about the microstructure of this alloy are generally obtained from laboratory specimens, under highly controlled production conditions, often very different from those of complex shaped industrial components. In the present study, the experimental work was carried out on an industrial A356 gravity die cast cylinder head, with the aim of relating the local microstructural parameters to the different solidification conditions and of finding correlations between the main microstructural features, such as secondary dendrite arm spacing and solidification defects (gas pores and shrinkage cavities) content. Fatigue resistance of Al-Si-Mg castings greatly depends in fact on microstructure and, first of all, on the size of solidification defects. Casting simulation software can nowadays give a good prediction of percentage defect content distribution but cannot predict the defects size. An important finding of this study is indeed the correlation between the equivalent diameter of the maximum pore and the percentage fraction of solidification defects measured in the casting. This finding, added to the predictive potential of casting simulation software, can so lead to an estimation of the local fatigue resistance of Al-Si-Mg castings, already in the design phase of the production.