Friction stir butt welding of magnesium alloy to steel by truncated cone-shaped stirring pin with threads

Abstract A truncated cone-shaped stirring pin with threads was designed to enhance friction stir butt welding (FSBW) quality of AZ31B magnesium alloy and Q235 low carbon steel. The effects of welding speed and rotation speed on the microstructure and properties of joints were studied. Microstructure characteristics and element distribution along the Mg/steel interface were analyzed. The results showed that the truncated cone-shaped stirring pin with threads effectively improves the metal flow behavior, especially the fluidity of steel. A steel-strip was formed at the bottom of the Mg side weld nugget during the welding process. The stirring pin effectively improved the interfacial reaction at the Mg/steel dissimilar metal. A thin accumulation layer of the Al was observed along with the interface. Discontinuous oxides of Mg appeared in the interface and were mainly distributed in the upper and bottom interfaces of the joint. As the heat input increased, the metallurgical bonding of the Mg/steel interface became stronger, and the joint strength increased. But when the heat input increased to a certain level, the oxidation of the Mg/steel interface became serious. The aggregation of MgO in the Mg/steel interface led to cracks, which decreased the joint strength. When the welding speed was 75 mm/min and the rotation speed was 850 rpm, the highest tensile strength of the joint was 189.4 MPa, which is 75.8 % of magnesium alloy. The joint fractured in the Mg side near the Mg/steel interface.