Tool Materials for Friction Stir Welding of Titanium Alloys

Titanium alloys due to their higher specific strength and superior mechanical properties are attractive for numerous industrial applications. They are particularly appealing to the aerospace, chemical, petrochemical and biomedical industries. However, when it comes to the weldability of titanium alloys, there are a number of problems that make it challenging. TWI has been joining titanium alloys using friction stir welding (FSW) since 1995 and has pioneered the technique called stationary shoulder friction stir welding (SSFSW). This technique overcomes the issue of poor thermal conductivity of titanium alloys as the process provides a more even heat input into the weld compared to conventional FSW techniques. However the choice of tool materials still offers significant technical and economic challenges. The tool must be able to withstand the welding process without losing shape, strength or ductility. TWI has recently assessed the performance of several tool materials using SSFSW technique for welding Ti-6Al-4V that include W-25%Re, W-1.0La and Si3N4 based tooling. A total of three metres weld lengths were made using each tool material, with the probe profile measured after every 0.6 metre of weld length so as to establish the tool wear. Also weld quality was investigated through macroscopic investigations so as to see the relative effect of tool wear on microstructure developments. The results showed the potential to make commercially useful weld lengths and also the potential of low cost tooling. The project results will be discussed during this presentation.