Strain-controlled fatigue properties of dissimilar welded joints between Ti–6Al–4V and Ti17 alloys

Abstract

The aim of this study was to evaluate the microstructure, hardness and cyclic deformation behavior of electron beam welded dissimilar joints of Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys. The welding resulted in a significant microstructural change across the joint, with hexagonal close-packed (hcp) martensite α′ and orthorhombic martensite α″ in the fusion zone (FZ), α′ in the heat-affected zone (HAZ) of Ti–6Al–4V side, and coarse β in the HAZ of Ti17 side. A characteristic asymmetrical hardness profile across the dissimilar joint was observed with the highest hardness in the FZ and a lower hardness on the Ti–6Al–4V side than on the Ti17 side, where a soft zone was observed. The dissimilar joint exhibited a lower Young′s modulus and higher cyclic strain hardening exponent than both Ti–6Al–4V and Ti17 base metals (BMs), and had the monotonic and cyclic yield strengths lying in-between those of two BMs with higher values for Ti17 alloy. Both BMs and joint showed essentially symmetrical hysteresis loops and equivalent fatigue life, and exhibited cyclic stabilization at lower strain amplitudes up to 0.6%, while cyclic softening occurred after initial cyclic stabilization at higher strain amplitudes. The initial cyclic stabilization was shortened with increasing strain amplitude. In the Ti–6Al–4V BM fatigued at a high strain amplitude of 1.2%, a short initial cyclic hardening emerged, corresponding to the presence of twinning and its resistance to the dislocation movement. Fatigue failure of the dissimilar joint occurred in the HAZ of Ti17 side where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.