Effect of steel ball diameter in the surface mechanical attrition treatment (SMAT) on microstructure, roughness, hardness and wear behavior of AZ31 magnesium alloy

Surface Mechanical Attrition Treatment (SMAT) is recognized as an effective technology for enhancing hardness and surface abrasion resistance. This study examined the influence of SMAT on the microstructure, surface roughness, hardness, and wear behavior of the AZ31 magnesium alloy. For the experiments, steel balls of different diameters – 3.2mm, 4mm, and 5.6mm – were used to perform the SMAT process, which was consistently timed at 8.5 minutes. Detailed analyses of the resultant microstructures were then conducted using tools such as a scanning electron microscope and X-ray diffraction. The hardness and wear were measured using the Vickers and the disk pin methods, respectively. It was observed that the SMAT process significantly reduced the grain size on the sample surfaces. For example, when a steel ball with a diameter of 5.6mm was utilized, the grain size was reduced from 225nm to just 96nm. The process also led to a substantial increase in hardness, with measurements rising from 65 HV to 190 HV, once again when a steel ball of 5.6mm diameter was utilized. Furthermore, the SMAT process, when executed with a tool diameter of 5.6mm, eliminated weight loss in the wear test, which had been previously recorded at 0.26mg, indicating an increase in surface abrasion resistance. An observed correlation suggested that as the tool diameters increased, the abrasion resistance of the surface improved. Abrasion was adhesive on untreated samples; on treated samples, it was ridged and scratched. However, as the tool diameter—and consequently, the surface hardness—increased, the scratches were seen to reduce gradually.