Achieving ultra-fine grains in Ti-6Al-4V alloy welds through pre-weld friction stir processing

Titanium Ti-6Al-4V alloy, recognized for its exceptional strength, is extensively employed in the aerospace, biomedical, and automotive industries. Friction Stir Processing (FSP) has been found to enhance the overall performance, while friction stir welding (FSW) is recognized as the most effective technique for joining the Ti-6Al-4V alloy. This study focusses on the implementation of friction stir welding on a Ti-6Al-4V alloy that had previously undergone friction stir processing. The objective was to analyse the microstructure and mechanical characteristics. The examination using Electron Backscatter Diffraction revealed notable alterations in the microstructure, such as variations in grain size, misorientation angle, and grain boundaries. The proportion of high angle grain boundaries (HAGBs) on the advancing side and stir zone of the friction stir treated Ti-6Al-4V were 59 % and 66 %, respectively. Signifying grain refinement, grains measuring sizes between 0.83 μm and 1.05 μm were achieved as result of processing. Subsequent, friction stir welding resulted in about 50 % further decrease in grain size compared to base metal, with HAGBs comprising 71 % and 52 % at the advancing side and stir zone, respectively. As a result the Vickers micro hardness values increased to 397 ± 13Hv upon friction stir processing to 444 ±7Hv upon subsequent friction stir welding respectively. • Synergy of Friction Stir Welding of Friction Stir Processed Ti-6Al-4V shows significant development in material processing. • Ultrafine grains formed during friction stir processing and retained during friction stir welding (0.51μm-1μm). • Enhanced hardness via friction stir processing and welding, with microhardness values of 397±13 Hv to 444±7 Hv.