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

Abstract

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 8wt% Co but was not found in WC411 grade WC–Co tool with 11wt% 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.