Effect of number of tool shoulders on the quality of steel to magnesium alloy dissimilar friction stir welds

Due to magnesium and iron’s immiscibility, joining magnesium alloy and steel with modern welding procedures like friction stir welding (FSW) is still complicated. Insufficient chemical bonding and mixing of raw materials in the stir zone are the main problems of joining magnesium alloy and steel. Accordingly, this paper aims to use tools with different numbers of shoulders to boost the properties of the final joint. The different tools with 1, 2, 3 and 4 shoulders were produced and used between magnesium alloy and steel during FSW. The thermal changes during the process are monitored, material flow and mechanical properties are investigated, and fractography is done on the tensile test samples. The results show that increasing shoulder numbers enhances frictional heat generation but not leads to more mechanical interlocking of base metals at interfaces. The maximum heat was generated when welded by two shoulders (600 °C), and the lowest heat was generated in one shoulder joints (540 °C). The number of shoulders for achieving the best mechanical properties has limitations. The results show that the final quality of the joint improves from one shoulder tool to three, but shows decreases at joints that were welded by more than three shoulders. The most robust tensile strength was recorded in two shoulder samples (210 MPa) with brittle fracture behavior.