Precipitation Hardening at Elevated Temperatures above 400 °C and Subsequent Natural Age Hardening of Commercial Al-Si-Cu Alloy

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400 degrees C were systematically investigated in the commercially available AC2B alloy with a nominal composition of Al-6Si-3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480 degrees C. The hardness change is attributed to the precipitation of metastable phases associated with the alpha-Al-15(Fe, Mn)(3)Si-2 phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400 degrees C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable theta-Al2Cu and Q-Al4Cu2Mg8Si6 phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al-Si-Cu-Fe-Mn-Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400 degrees C to 500 degrees C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al-Si-Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400 degrees C.