Influence of Cu addition on microstructure and hardness of near-eutectic Al-Si-xCu-alloys

The influence of Cu content on the microstructure and hardness of near-eutectic Al-Si-xCu (x = 2%, 3%, 4% and 5%) was investigated. After melting Al-based alloys with different Cu contents, alloys were cast in green sand molds at 690 degrees C and solidified. The solution treatment was performed at 500 degrees C for 7 h and then the specimens were cooled by water quenching. The samples were respectively aged at 190 degrees C for 5, 10 and 15 h to observe the effect of aging time on the hardness of matrix. Also differential thermal analysis was used to obtain the transition temperature of the equilibrium phases at cooling rate of 30 K/min and to determine the effect of Cu content on the formation of quaternary eutectic phases and the melting point of alpha(Al) + Si. The results show that as Cu content in the alloy increases, the hardness of matrix increases due to precipitation hardening, the melting point of alpha(Al) + Si decreases and the amount of these eutectic phases increases, quaternary eutectic phase with melting point of 507 degrees C forms when Cu content is more than 2%.