Effect of Zirconium micro-addition and multi-pass friction stir processing on microstructure and tensile properties of Mg–Zn–Si alloys

Abstract

The aim of this study is to investigate the microstructure and tensile properties of Mg-0.5Zn-0.2Si-XZr (X: 0, 0.25, 0.5 wt.%) in the as-cast and multi-pass friction stir processed (FSPed) conditions. Micro addition of Zr element resulted in the average grain size (AGS) reduction from 1.65 ± 0.3 mm in Mg-0.5Zn-0.2Si alloy to less than 700 μm through the formation of Zr-rich particles evenly distributed in the structure and constitutional supercooling effect. However, the refinement efficiency of Zr was reduced by the formation of Si-containing intermetallic phases through the Si poisoning effect. Thus, agglomeration of these brittle compounds, specifically at the grain boundaries, resulted in quasi-brittle fracture mode in the as-cast samples. On the other hand, applying the FSP on Zr-free alloy improved both microstructure and mechanical properties due to grain refinement and porosity content reduction. In other words, in the optimum condition, the AGS reduced to about 5 μm and yield strength (YS), ultimate tensile strength (UTS) and total elongation percentage (EL%) improved up to 70%, 90%, and 566%, respectively, compared to the as-cast sample. On the other hand, in the Zr-modified alloys, the fragmentation and even redistribution of Zr-rich and Si-rich phases, particularly after the 2nd FSP pass, enhanced the dynamic recrystallization (DRX) through the particle stimulated nucleation (PSN) mechanism and restricted the grain growth through the Zener pinning effect. Therefore, the AGS of 2 ± 0.1 μm, UTS of 182 Mpa, and EL of 36% were achieved.

Keywords

• Mg–Zn–Si–Zr alloys
• Magnesium alloys
• Multi-pass friction stir processing
• Grain refinement
• Tensile properties
• Si poisoning effect