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

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 mm 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 mm 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 redis-tribution of Zr-rich and Si-rich phases, particularly after the 2nd FSP pass, enhanced the dynamic recrystallization (DRX) through the particle stimulated nucleation (PSN) mecha-nism and restricted the grain growth through the Zener pinning effect. Therefore, the AGS of 2 +/- 0.1 mm, UTS of 182 Mpa, and EL of 36% were achieved.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).