Tensile and fatigue properties of electron beam welded dissimilar joints between Ti–6Al–4V and BT9 titanium alloys

The microstructure, hardness, tensile and fatigue properties of electron beam welded dissimilar joints between Ti–6Al–4V and BT9 (Ti–6.5Al–3.5Mo–1.5Zr–0.3Si) titanium alloys were studied. A marked microstructural change occurred after welding, with martensite in the fusion zone (FZ) and in the heat-affected zone (HAZ) of Ti–6Al–4V side, and martensite plus some retained α phase in the HAZ of BT9 side. While an asymmetrical hardness profile across the dissimilar joint was observed with significantly higher hardness values in the FZ, no soft zone was present in the HAZ of the Ti–6Al–4V/BT9 dissimilar joints. The ultimate tensile strength, monotonic and cyclic yield strength, and cyclic strain hardening exponent of the dissimilar joints lay in-between those of two base metals (BMs). The joint and BMs had almost the same monotonic and cyclic stress–strain curves and fatigue life, while the cyclic strain hardening exponent was slightly higher than the monotonic strain hardening exponent. Although cyclic softening occurred at higher strain amplitudes, cyclic stabilization appeared at lower strain amplitudes up to 0.6% for both BMs and joints. Fatigue failure of the dissimilar joints occurred in the BM of Ti–6Al–4V, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by fatigue striations along with secondary cracks.