Microstructural, mechanical and tribological performance of a magnesium alloy AZ31B/Si3N4/eggshell surface composite produced by solid-state multi-pass friction stir processing

This study focused on the development of an AZ31B/Si3N4/Eggshell surface composite via solid-state multi-pass friction stir processing. The study aimed to characterize the developed surface composite based on its structural, microstructural, mechanical, and tribological performance at one, two and three successive tool-passes. The structural and compound analysis was conducted using an XRD pattern. Microstructural characterization and grain refinement studies were conducted using FESEM images and EBSD analysis. Furthermore, the mechanical properties were investigated through tensile tests equipped with digital image correlation (DIC) images and fractography analysis via FESEM images. In addition, tribological characteristics, such as COF, friction forces, and volumetric wear losses, were examined using a universal tribometer, and the 3D profile of the wear track was analyzed through surface profilometry. The results revealed the formation of structural compounds such as Mg2Al4, Mg2SiO4, Mg2Ca, and Ca2Mg4Zn13 in the stir zone. The morphological study revealed plenty of white -grey (ESP) and dark grey (Si3N4) sub-micron precipitates uniformly dispersed into the AZ31B matrix alloy. The EBSD map exhibited an average grain size of 2.95 +/- 1.83 mu m. The ultimate tensile stress at one, two and three successive tool-passes were found to be 357 MPa, 366.5 MPa and 375.4 MPa, respectively, which was comparatively 20-25% higher than the base alloy. The FESEM images of the fractured surface exhibited ductile striations and tear ridges with flat shear bands mixed with brittle fracture. The lowest COF obtained in this study was 0.29 for three too-passes at an average frictional force (Fx) of 2.17 N with a volumetric wear loss of 0.0769 mm3.