Wear of Ni-Based Superalloy Tools in Friction Stir Processing of Commercially Pure Titanium

Conventional methods for joining titanium alloys often provide a relatively low quality of joints impaired by high residual stresses. A possible solution to this problem can be offered by friction stir welding, which has been increasingly used for aluminum alloys. However, the friction stir welding of titanium alloys is complicated by severe tool wear due to high loads and temperatures in the process. Good results were reported for a tool made of ZhS6U superalloy, but tool life still needs to be improved. Here, we study the wear of a tool made of ZhS32 Ni-based superalloy, which has higher heat resistance than ZhS6U, and the wear of a liquid-cooled ZhS6U tool in the friction stir processing of commercially pure titanium. The effect of tool wear on the strength characteristics of the processed material is discussed. The total processing path length traversed by the tools without failure was 2790 mm. In both cases, the most severe wear was observed at the pin root. Liquid cooling significantly reduced the wear. Based on the obtained results, tool wear is proposed to occur by an adhesion-diffusion mechanism during friction stir processing.