Cyclic deformation of dissimilar welded joints between Ti–6Al–4V and Ti17 alloys: Effect of strain ratio

Cyclic deformation characteristics of electron beam welded (EBWed) joints between Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys were evaluated via strain-controlled low-cycle fatigue tests at varying strain ratios at a constant strain amplitude. The welding led to a significant microstructural change across the dissimilar joint, with hexagonal close-packed (HCP) martensite αʹ and orthorhombic martensite αʺ in the fusion zone (FZ), αʹ in the heat-affected zone (HAZ) of Ti–6Al–4V side, and coarse β in the HAZ of Ti17 side. A distinctive asymmetrical hardness profile across the joint was observed with the highest hardness in the FZ and a lower hardness in the HAZ of Ti17 side than in the Ti17 base metal (BM), indicating the presence of soft zone. The strength and ductility of the dissimilar joint lay in-between those of two base metals (BMs). Unlike wrought magnesium alloys, the Ti–6Al–4V BM, Ti17 BM, and joint basically exhibited symmetrical hysteresis loops in tension and compression in the fully reversed strain-controlled tests at a strain ratio of Rε=−1. At a strain ratio of Rε=0 and 0.5, a large amount of plastic deformation occurred in the ascending phase of the first cycle of hysteresis loops of Ti–6Al–4V BM, Ti17 BM, and joint due to the high positive mean strain values. Fatigue life of the joint was observed to be the longest at Rε=−1, and it decreased as the strain ratio deviated from Rε=−1. A certain degree of mean stress relaxation was observed in the non-fully reversed strain controlled tests (i.e., Rε≠−1). Fatigue failure of the dissimilar joints occurred in the Ti–6Al–4V BM, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by fatigue striations.