Effects of an overlapping multi-pass friction stir process and rapid cooling on the mechanical properties and microstructure of AZ31 magnesium alloy

• Using overlapping passes created a FSPed surface of uniform thickness.
• Rapid cooling sped up the overlap process.
• Two directional overlap increased the uniformity of the grain distribution and decreased its size.
• Rapid cooling increases the hardness with respect to the single pass FSP.

Applications for magnesium as the lightest structural metal are increasing; however, its low ductility at room temperature could be considered a limitation. The friction stir process (FSP) is a solid state process that can overcome this limitation by producing a fine-grained material and uniform texture. This study used overlapping passes FSP to create a fine-grained area on the surface of AZ31 magnesium alloy. Rapid cooling was employed during the process to eliminate the thermal effects of successive passes. Two different modes of overlap were created. In the first mode, the surface of the sample was FSPed overlapping in one direction. In the second mode, the first overlapped surface of the sample was overlapped again in a direction perpendicular to the primary process. The results showed that mechanical strength in the process direction increased for both modes. Elongation was greater for the first mode than that of the raw material and, for the second mode, was less than for the raw material. In the direction perpendicular to the process, both the mechanical strength and ductility of the material were less than those of the raw material. The particle distribution in the second mode was more uniform than that for the first mode.