Influence of Cu addition on the heat treatment response of A356 foundry alloy

Strength and high-temperature performance are important properties in Al-Si-Mg alloys for power-train applications such as combustion engine cylinder head components in automotive industry. Addition of Copper (Cu) to Al-Si-Mg alloys is a common practice to obtain high strength as well as high temperature stability. However, Cu addition alters precipitation kinetics and precipitate types during artificial ageing, and consequently properties of final cast products. In this study, the effects of Cu on tensile properties in connection with precipitation and precipitate types, as observed through aging curves and TEM analysis, were investigated in Al-Si-Mg alloys having three different Cu contents in order to optimize Cu contents and heat treatment parameters. Higher solutionizing temperature enabled a significant increased Cu content in solid solution without any incipient melting of Cu-rich phases. A refinement of the precipitate microstructures was observed in the Cu-containing alloys, accompanied with a phase-change of the hardening needle precipitates. different precipitation kinetics and double aging peaks were observed during artificial aging in higher Cu containing alloys, which were correlated to higher strength than in the Cu-free alloy in underaged and peak aged conditions. Optimized Cu contents and heat treatment parameters were found with respect to a combination of strength and elongation.