Friction stir welding (FSW) was invented in 1991 as a new solid-state welding process. It enjoyed early success in the joining of aluminum (Al) and other soft metals. Over the years, technological advancements in tool material have enabled this process to be applied to hard metals such as steel and titanium (Ti). These advanced tool materials are better able to withstand the harsh welding environment that accompanies the FSW of hard metals. During the FSW of steel, the combination of high welding temperatures (900°C) and high material flow stress can cause significant degradation of the tool. Studies aimed at identifying suitable tool materials for the FSW of steel have demonstrated a tool material should have the following properties: high fracture toughness, excellent yield strength at high temperatures, a stable microstructure, excellent wear resistance, and be inert to the workpiece material. One such tool material that meets these requirements are refractory-based alloys, specifically tungsten (W) based.
Understanding how W based tool materials degrade during a friction stir weld is imperative to advancing the technology and the success of FSW in hard metals. By reducing how much a tool degrades, the tool life will increase driving down the capital cost associated with FSW. Due to certain W alloys ductility at room temperature, the largest factor governing tool life is the degradation rate during welding. Previous studies investigated gross geometrical changes a tool experiences during FSW. Most of these works characterized the tool wear by dimensionally tracking how much the tool material has worn for a given length of weld. Other studies identified two mechanisms of tool degradation which include deformation and wear. Further investigations divided tool wear into two categories, abrasive and adhesive wear. These previous works highlight a need for an investigation into the microstructural changes that occur which drive tool degradation. This study has characterized the pre- and post-weld microstructures of three W-based tool materials, Material A (99% W, 1% La2O3), Material B (75% W, 25% Re), and Material C (70% W, 20% Re, 10% HfC). These three materials weld high strength steel under the same conditions and tool designs. After characterization of the tool material microstructures along with investigations of selected weld cross sections, tool degradation mechanisms were identified. Severe plastic deformation dominated the degradation of Material A. Material B primarily degraded by twinning and Material C degraded predominantly by intergranular failure.