Strain-controlled fatigue properties of linear friction welded dissimilar joints between Ti–6Al–4V and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloys

The purpose of this study is to evaluate the microstructure, microhardness and fatigue properties of linear friction welded (LFWed) dissimilar joints between Ti–6Al–4V (TC4 according to Chinese classification) and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) titanium alloys. A significant microstructure change across the dissimilar joint occurs after linear friction welding (LFW), with martensite in the weld zone (WZ) and small recrystallized grains in the thermo-mechanically affected zone (TMAZ) on the TC4 side. A characteristic asymmetrical hardness profile across the dissimilar joint is observed with significantly higher hardness values in the WZ, and no soft zone is present in the dissimilar joint. The LFWed dissimilar joint exhibits essentially symmetrical hysteresis loops and an equivalent fatigue life to the base metals, which increases with decreasing strain amplitude. While cyclic stabilization appears at lower strain amplitudes up to 0.6% for the joint, cyclic softening basically occurs at higher strain amplitudes. In the joint fatigued at a high strain amplitude of 1.2%, a short initial cyclic hardening occurs, corresponding to the presence of twinning and the resistance to the dislocation movement. Fatigue failure of the dissimilar joint occurs on the TC4 side and is far from the weld line, suggesting that a highly durable and sound dissimilar joint is achieved via the present solid-state LFW. Fatigue crack initiation occurs from the specimen surface or near-surface defect, and crack propagation is mainly characterized by fatigue striations which are perpendicular to the crack propagation direction, in conjunction with some secondary cracks.