RELATIONSHIP BETWEEN MICROSTRUCTURE, MECHANICAL PROPERTIES AND WEAR BEHAVIOR OF FRICTION STIR PROCESSED AZ31B ALLOY UNDER VARIOUS MEDIUM

Plates of AZ31B were friction stir processed in three different environments, such as liquid nitrogen, water and air with a rotational speed of 1200 rpm and feed rate of 90 mm/min. Microstructural studies were carried out using optical and electron microscopes, phase identification was performed using X-ray diffraction, and all specimens were tested for hardness, tensile and wear. An examination of wear debris morphology was carried out. Under cryogenic, water and air mediums, grain refinement was achieved at 1.89, 2.23 and 18.23 mu m, respectively. The specimen using cryogenic medium displayed greater tensile strength and elongation than the water medium. Dissolution of consistently refined grains and secondary Mg17Al12 particles was observed after FSP underwater and cryogenic medium, resulting in a significant increase in microhardness. This surface hardening produced a significant benefit for the friction and wear behavior of the magnesium as measured by pin-on-disc configuration using a universal tribometer. With a cryogenic cooling medium, the friction coefficient is reduced by approximately 23% and the wear rate is reduced by 8% in comparison to AFSP.