Microstructure evolution and mechanical characterization of friction stir welded titanium alloy Ti-6Al-4V using lanthanated tungsten tool

Friction stir welding (FSW) exhibits significant advantages to join titanium and its alloys compared to other welding methods. FSW of 3 mm thick titanium alloy Ti-6Al-4V sheets was carried out using a lanthanated tungsten alloy tool. The traverse speed was varied from 40 mm/min to 200 mm/min in steps of 80 mm/min by keeping other parameters constant. The microstructure evolution was observed using optical and scanning electron microscopy. Advanced characterization techniques such as electron backscatter diffraction and transmission electron microscopy were applied to observe the evolved microstructure in detail. The micrographs revealed a recrystallized structure in all the joints. The joint at 40 mm/min showed a fully developed lamellar structure. An increase in the β phase was observed at HAZ while TMAZ showed a distorted structure. The increase in traverse speed caused a reduction in average grain size. No tool wear debris was observed in the stir zone while a worm hole defect was noticed at 200 mm/min. Ti-6Al-4V hardened after FSW due to grain refinement and an increase in dislocation density. The joint strength was closer to the strength of the base metal and the joints failed outside the joint area except at 200 mm/min.