Microstructure and tribological properties of as-cast and multi-pass friction stir processed Mg-0.5Zn-0.5Zr/SiC composite fabricated by stir casting technique

Abstract

In this investigation, the Mg-0.5Zn-0.5Zr/4%SiC composite fabricated by the stir casting method was subjected to multi-pass friction stir processing (MFSP) to evaluate its microstructure, hardness, dry-sliding wear, and friction properties in comparison to those of the as-cast composite (ACC). Microstructural observations revealed the formation of a Mg matrix with average grain size (AGS) of 107 ± 12 μm, uneven dispersion (agglomeration) of SiC particles in the structure, and the formation of Zr-rich phase with the chemical composition of Zr:78.42, C:19.46, Mg:1.86, and Zn:0.26 wt%, ZrC, as well as some new formed carbide-based phases mostly propagated along the grain boundaries. On the other hand, the MFSP implementation led to a significant microstructural modification, i.e., in the optimum circumstance, the AGS and SiC particle size in the stir zone was reduced by 97.28 % (107–2.9 μm) and 67.69 % (21.33–6.89 μm), respectively, and the density of the composite increased from 1.669 to 1.774 gr/cm3. According to dry-sliding wear test results, applying FSP and increasing the pass number to three reduced the wear rate and coefficient of friction (COF) of the composites by about 45 % and 24 %, respectively, compared to those of ACC. The wear resistance enhancement was ascribed to wear mechanisms changing from delamination/adhesion/severe abrasion to mild abrasion wear, which stemmed from the formation of a stable O-rich tribolayer on the worn surface of FSPed samples.