Microstructures and mechanical properties of submerged friction stir welding of ME20M Magnesium alloy

Submerged friction stir welding is a new solid-state connection technology, which makes a low heat input in the welding process. It has been applied to the aluminum alloy welding and grain refinement, and has improved the mechanical properties of the joints. There are few reports about friction stir welding of magnesium alloy, especially in ME20M magnesium alloy, which is widely used in aircraft industry. The suitable welding process parameters rotation speed and traverse speed were studied in this paper. Then, friction stir welding of ME20M magnesium alloy in air and water were carried out, respectively. The analysis on the temperature of heat affected zone, joint surface morphology, microstructures, micro-hardness, and shear properties were investigated to study characteristics of SFSW of ME20M magnesium alloy. The result indicated 800 r/min and 60 mm/min were the suitable parameters of the rotation speed and the traverse speed to get good forming surface, The FSW joint in the air had no defects, but the hole defect was generated in the SFSW joint. The peak temperature of the SFSW heat affected zone was lower than that of FSW and the residence time was short. Shear test results showed that the fracture positions were all in the advancing side. Compared with FSW, the shear strength of SFSW was slightly lower, but the elongation was slightly higher. Micro-hardness test showed that the hardness distribution of FSW and SFSW welded joints was "W" type, the hardness of the weld nugget was the highest, the hardness of the advancing side was the lowest, and the average hardness value of SFSW was higher than that of FSW. The fracture morphology showed that it was mixed fracture.