Effect of three-stage heat treatment on the composite waveform and variable amplitude fatigue properties of TC4 titanium alloy pulsed laser-arc hybrid welded joints

Titanium alloy welded structures are often subjected to cyclic loading with composite waveform and variable amplitude during actual service, exacerbating the damage to the joints and leading to low fatigue life. Therefore, a three-stage heat treatment was adopted in this work to enhance the fatigue life of TC4 titanium alloy pulsed laser-arc hybrid welded joints, and its microstructure evolution and fracture mechanism were investigated. The results show that the high-density phase boundary formed by the finely dispersed secondary α phase precipitated after heat treatment was the main reason for the increase of life by 3 times. The crack initiation was mainly due to the accumulation of Pyramidal < c + a > dislocations and base < a > dislocations. The difference was that, combined with molecular dynamics calculations and characterization by TEM and EBSD, it was found that the heat-treated cracks underwent dislocation accumulation, deformation twinning, and low-angle grain boundaries before the initiation of the lamellar α-concave position.