Effect of Precipitation Hardening Temperatures and Times on Microstructure, Hardness and Tensile Properties of Cast Aluminium Alloy A319

Abstract

In the present work, the effect of T6 precipitation hardening on microstructure, hardness and ultimate tensile strength of 5 wt%Si-3.5 wt%Cu-Al (A319) alloy was studied. As-cast A319 samples were solution treated at 503°C for 2-24 hours followed by aging at 150, 170, 200, 230°C for 1-48 hours. Microstructures were studied by OM and SEM-EDS. The phases present were determined by XRD. In the as-cast condition, the microstructure consists of a primary dendritic α-Al, acicular-plate or globular forms of Si eutectic and intermetallic phases comprising globular Al₂Cu and plate-like Al₅FeSi in the interdendritic regions. The microstructures after solution treatment and aging are similar to that of the as-cast condition, except that some spheroidisation of the eutectic Si has occurred. After solution treatment at 503°C for 8 hours plus aging at 170°C for 1, 6, 12, 24, 36 and 48 hours, the area fractions were decreased to about 11.25 %, 10.35 %, 9.70 %, 9.50 %, 9.90 % and 9.85 %, respectively. The precipitation hardening treatment increased the macro-hardness, micro-hardness, ultimate tensile strength and 0.2 %proof stress compared to the as-cast condition. Peak hardness and maximum levels of tensile strength were obtained after solution treatment at 503°C for 8 hours plus aging at 170°C for 24 hours. At lower aging temperatures or shorter times, the hardness and tensile strength were decreased due to incomplete precipitation. However, at higher aging temperature or longer time, the hardness and tensile strength were decreased most probably due to precipitate coarsening and associated loss in coherency effects.