Effect of Local Solidification Conditions on the Microstructure and Mechanical Properties of AlSi7Cu1.3Mg0.6 Alloy: Comparison Between Sand-Cast HIP-Treated Cylinder Head and Separately Cast Specimens

The design of complex-shaped aluminum castings, such as cylinder heads, is commonly based on mechanical data obtained from separately cast specimens. This approach may lead to inaccurate stress and fatigue life predictions by overlooking the effects of local solidification conditions and resulting heterogeneous microstructures formed in industrial casting conditions. This study investigates microstructural and mechanical properties of a sand-cast AlSi7Cu1.3Mg0.6 alloy, comparing separately cast specimens with those extracted from critical regions of a hot isostatic pressed cylinder head. Tensile and hardness measurements were related to local microstructural and fractographic features. Regions subjected to high cooling rates (e.g., the combustion chamber) showed fine secondary dendrite arm spacing (SDAS) and a uniformly fibrous eutectic Si morphology, comparable to that of separately cast specimens. In contrast, areas with lower cooling rates (e.g., the bolt columns) exhibited SDAS values up to 2.8 times larger, coarse plate-like eutectic Si particles, and elongated intermetallic phases up to an order of magnitude larger. These microstructural differences resulted in a reduction in tensile strength by 31% and a decrease in elongation to failure and toughness up to 96%. The area fraction of intermetallics on fracture surfaces strongly correlated (R2 ≈ 0.92) with tensile strength and ductility, while yield strength remained relatively unaffected, reflecting the dominant contribution of the precipitation hardening mechanism. Results highlight the need to consider local casting conditions and resulting microstructures in mechanical assessments, providing quantitative input for more accurate process-structure-property models in the design of complex aluminum castings.