Tool wear mechanisms in friction stir welding of Ti-6Al-4V alloy

Friction stir welding (FSW) of titanium alloy Ti-6Al-4V was performed using three types of tools made of W-1.1%La2O3 and two different grades of WC-Co based materials. Tool wear characterization was carried out by weight loss measurement, pin profile photographic technique and microscopic observations. Severe tool deformation occurred in the W-La2O3 tool with a small conical pin. Deformation was reduced by introducing a larger cylindrical pin design in the W-1.1%La2O3 tool. Among the cermet tools, fracture was observed in CY16 grade WC-Co tool with 8 wt% Co but was not found in WC411 grade WC-Co tool with 11 wt% Co. Adhesive wear mechanism was related to the result of chemical affinity between WC-Co and Ti-6Al-4V alloy. Decarburization of WC occurred at tool surface and an interaction layer was observed. Adhesion layer of processed material influenced heat input during FSW and resulted in a narrower heat affected zone (HAZ) in the material processed with the WC-Co tools as compared to the W-1.1%La2O3 one. Besides, defects were observed at the bottom of the nugget accompanied by α+transformed β microstructure, which has been discussed in terms of the heat input and strain localization.