Fatigue Behavior Up to Very High Cycle Regime of Laser Beam Welded Ti60 Joints

Abstract

Ti60 titanium alloy laser beam welding (LBW) joint exhibits significant microstructural heterogeneity, with the base metal (BM) having equiaxed α and intergranular β phases, the heat-affected zone (HAZ) containing acicular α′ martensite and retained α phase, and the fusion zone (FZ) composed of acicular α′ martensite. This leads to uneven nanohardness and microhardness distributions, with the highest in the FZ. The fatigue strength of the welded joints is significantly lower than the BM up to a very high cycle regime, at 180 MPa, with failure modes including surface welded depression, welded pores in the FZ, and facet cleavage fracture in the BM. Dislocation distribution analysis reveals a mismatch between the soft and hard phases of α and β, with stress concentration around welded pores inducing dislocation nucleation and entanglement, ultimately leading to martensite laths breaking and recrystallizing into nanograins under fatigue loading.