Modification—a Parameter to Consider in the Heat Treatment of Al-Si Alloys

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Abstract
The effect of modification on the heat treatment response of 356 alloy has been investigated on both gravity-diecast (permanent mould) and sand-cast samples. The differences between unmodified and modified microstructures are more important in sand-cast samples than in gravity-diecast ones. After one hour of solution treatment at 540°C both gravity-diecast unmodified and modified microstructures become similar in terms of silicon particle size and sphericity. The processes which lead to this are different. Silicon platelets in the unmodified structures segment while silicon particles in the modified alloy coarsen. The final result is however the same. In sand-cast alloy, the initial microstructural differences persist after up to two hours of solution treatment. Porosity caused by modification can negate many of the microstructural benefits by decreasing tensile strength and percent elongation. The hardness of modified alloy is found to be consistently less after T6 temper than in unmodified alloy. This is reflected in a lower yield strength of modified 356 alloy.