Estimation of local fatigue behaviour in A356–T6 gravity die cast engine head based on solidification defects content

Abstract

Al–Si–Mg cast alloys have widespread applications, especially in the aerospace and automotive industries, due to their excellent combination of castability and high specific strength. Among Al–Si alloys, hypoeutectic A356 (Al–7Si–0·3Mg) ranks as one of the most widely used for the production of a variety of components, including engine blocks and engine heads, due to its excellent castability and good mechanical properties. The microstructure of this alloy greatly depends on chemical composition, solidification conditions, metal soundness and heat treatment. Furthermore, its mechanical properties are strongly affected by solidification microstructure and defects, which can vary greatly in complex shaped castings. Among the different microstructural features, only secondary dendrite arm spacing and percentage defect content can currently be predicted with sufficient accuracy by casting simulation software. This makes the prediction of the fatigue life of complex shaped Al–Si castings very difficult, since it is widely accepted that fatigue behaviour mainly depends on the size of solidification defects (gas pores and cavity shrinkages). In this study, the experimental work was carried out on an industrial A356–T6 gravity die cast engine head, with the aim of finding relationships among the main microstructural features and solidification defect parameters. The goal of this analysis was to correlate the defect size, which is the most important variable affecting the fatigue behaviour, to the other microstructural parameters that can be predicted by casting simulation software. Moreover, by applying literature models for fatigue behaviour prediction, based on maximum defect size, the local expected fatigue life/fatigue limit on a section of the casting will be evaluated and compared with those obtained by rotating bending fatigue tests. This study would demonstrate the effectiveness of a new approach of coengineering design, with a strong synergy between the structural finite element method and the casting simulation process, able to estimate the local fatigue strength in complex shaped A356 castings.