A Review on Friction Stir Welding of Steels

Friction Stir Welding (FSW) is the most promising solid-state metals joining method introduced in this era. Compared to the conventional fusion welding methods, this FSW can produce joints with higher mechanical and metallurgical properties. Formerly, FSW was adopted for low melting metals like aluminum alloys. In recent years, it has made significant progress in friction stir welding of steels since unfavourable phase transformations occurred in welds due to the melting of the parent and filler metals in fusion welding can be eliminated. The main advantage of FSW over traditional fusion welding is the reduction in the heat-affected zone (HAZ), and the joints exhibit excellent mechanical and corrosion resistance properties. This article reviews the progress in the relevant issues such as the FSW tool materials and tool profiles for joining steels, microstructure and mechanical properties of steels joints, special problems in joining dissimilar steels. Moreover, in-situ heating sources were used to overcome the main limitations in FSW of hard metals and their alloys, i.e., tool damages and insufficient heat generation. Different in-situ heating sources like laser, induction heat, gas tungsten arc welding assisted FSW for various types of steels are introduced in this review. On the basis of the up-to-date status, some problems that need further investigation are put forward.