Friction-stir processing (FSP) is an emerging severe plastic deformation technology that is based on friction stir welding (FSW). In FSP, the tool plays a very important role in producing a defect free processed zone. The shoulder of the tool, which is the main source of heat generation, is typically either flat or tapered in shape. The typical shape of the shoulder aids in material consolidation during processing by forcing the softened material to be retained in the processed zone, as the tool traverses along the length of the work piece. The pin also plays a very important role, in refining the grain size along the through-thickness of the work piece. In this work, high carbon steel of and AZ31B magnesium alloy (6 mm & 1.5 mm thick) was chosen as a tool material and work piece material respectively. The tool shoulder diameter of 18 mm and 24 mm were chosen to friction stir process the work piece material in a single pass. The possibility of Increase in the tool shoulder diameter beyond 18 mm but less than 24 mm, a defect free processed zone was observed for variation in the process parameters in 6 mm thick plate. Also as the thickness of work piece is reduced, the defects in the friction stir processed zone of 1.5 mm thick plate is completely eliminated. A fine equiaxed grain of average grain size less than 10 μm were during friction stir processing, thus creating the material to exhibit superplasticity.
