Effects of multi-pass friction stir processing on mechanical and tribological properties of Mg-Zn-Zr alloys

Owing to high strength and good biocompatibility, Mg-Zn binary alloys have attracted significant attention as a potential candidate for producing human implants. However due to the formation casting defects and dendritic structure, these alloys show poor mechanical and wear performances. Alloying and applying severe plastic deformation methods are suggested as two techniques for modifying and improving the above properties. In this study, dry-sliding wear and friction behaviors of the Mg-0.5Zn (MZ) alloy were enhanced by micro-alloying with Zr (0.25 wt%) and applying multi-pass friction stir processing (FSP) technique. Adding Zr reduced the average grain size (AGS) from 1.5 & PLUSMN; 0.3 mm in the MZ sample to about 510 & PLUSMN; 50 mm through the formation of evenly distributed Zr-rich particles with the chemical composition of Mg: 8.84, Zn: 0.75, and Zr: 90.41 A% in the structure. Furthermore, the attendance of thermostable Zr particles stimulated the dynamic recrys-tallization (DRX) during FSP and reduced the AGS to less than z2 mm after the 3rd FSP pass. This substantial grain refinement along with the casting defects reduction, and the frag-mentation and redistribution of Zr particles enhanced the hardness and tensile properties of the substrate. Thus, the improved substrate in the FSPed samples, increased the stability of the formed tribolayer on the worn surfaces and changed the wear mechanisms from severe adhesive/delamination in the MZ alloy to mild abrasive/adhesive in the FSPed Zr-modified samples. Therefore, in the optimal condition (3-pass FSPed Mg-0.5Zn-0.25Zr sample), the hardness and total elongation percentage (EL%) increased by 55% and 648% while the mass loss and average friction coefficient (AFC) were reduced by 54% and 39%, respectively, compared to the as-cast Mg-0.5Zn alloy (under a load of 30 N).& COPY; 2023 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/).