Microstructure and mechanical properties of K-TIG welded dissimilar joints between TC4 and TA17 titanium alloys

In this study, the authors used keyhole tungsten inert gas (K-TIG) welding to weld together 12-mm-thick dissimilar TC4/TA17 titanium alloy plates without grooves. The microstructural evolution of the dissimilar joints was analyzed, to determine the relationship between their microstructure and their mechanical properties. The results showed that nine identifiable sub-zones were formed in each joint under the thermal action of welding. Significant microstructural changes occurred after welding, with the formation of the α phase, α' martensite and β phase in the heat-affected zone (HAZ), α' martensite in the fusion zone (FZ), and the α phase with a small volume of the β phase in the base metals (BMs). The textural distribution and intensity in the HAZ and FZ changed significantly compared with those in the BMs. Under the action of welding-induced heat, the distorted microstructures reverted, resulting in a marked reduction in the number of low-angle grain boundaries in the HAZs. Due to differences in their chemical composition and microstructure, the microhardness of the joint was characterized by an asymmetric distribution, that was ranked as follows: TC4 HAZ > TC4 BM > FZ > TA17 HAZ > TA17 BM. The ultimate tensile strength and yield strength of the joints were between those of the two BMs, but their elongation was smaller than that of either owing to the large difference in strength between the BMs. Because the TA17 BM had the lowest strength, the joint on its side underwent ductile fracture near the HAZ. In addition, the impact-induced energy absorbed in the FZ of the joint was between those of the BMs and was close to the average of their values. The work here provides a new method for the K-TIG welding of dissimilar titanium alloys in plates of medium thickness.