Effect of Friction Stir Processing Procedures on Microstructure and Mechanical Properties of Mg-Al-Zn Casting

Cast Mg-Al-Zn (AZ80) alloy was subjected to friction stir processing (FSP) with three different FSP procedures: single pass, single pass with a pre–solution treatment (pre-ST), and two pass. FSP resulted in remarkable grain refinement, significant breakup, and dissolution of the coarse, networklike Mg₁₇Al₁₂ phase. While the single-pass FSP procedure produced a heterogeneous microstructure with Mg₁₇Al₁₂ particle-rich bands, both the pre-ST FSP and the two-pass FSP procedures resulted in the generation of a uniform microstructure. The pre-ST FSP and two-pass FSP samples exhibited significantly enhanced strength and ductility due to remarkable grain refinement and dissolution of coarse Mg₁₇Al₁₂ phase. A post-FSP aging resulted in the precipitation of fine Mg₁₇Al₁₂ particle, thereby increasing the strength of the FSP samples. Among three FSP samples, the aged two-pass FSP sample exhibited the highest ultimate tensile strength (UTS) of 356 MPa and an elongation of 17 pct.