Microstructure evolution and mechanical properties of rheo-squeeze casting AZ91-Ca alloy during heat treatment

Abstract

The rheo-squeeze casting (RSC) process is a newly-developed casting process for high-performance components. In order to further improve the mechanical properties of magnesium alloys, AZ91-2wt.%Ca (AZX912) alloy was prepared by the RSC process and then subjected to heat treatment. The microstructure evolution and mechanical properties of AZX912 alloy during heat treatment were investigated. It was found that during solid solution treatment at 410 °C, β-Mg17Al12 phase with low melting point dissolves into α-Mg matrix, while the connected network-like Al2Ca phase with high melting point tends to separate gradually, and the tips of Al2Ca phase is partially spheroidized. With the increase of solid solution time, the yield strength (YS) of AZX912 alloy decreases gradually while the ultimate tensile strength (UTS) and elongation to failure (Ef) increase continuously. Isothermal ageing at 225 °C promotes the precipitation of β-Mg17Al12 phase in the matrix of AZX912 alloy. The hardness reaches the peak after ageing for 96 h and the increase in hardness is about 24.8%. The precipitation of β-Mg17Al12 phase during ageing treatment is beneficial to YS but harmful to Ef. The mechanism of microstructure evolution during heat treatment and its effect on mechanical properties are discussed.

Keywords

• magnesium alloys
• semisolid processing
• heat treatment
• microstructure
• mechanical properties