Effect of three-stage heat treatment on mechanical properties and micro-deformation mechanism of TC4 titanium alloy welded joint

Abstract

The influence of three-stage heat treatment on the microstructure and mechanical properties of pulsed laser-arc hybrid welded TC4 titanium alloy joints was studied by experiments and molecular dynamics calculations. The results showed that after the three-stage heat treatment, a large number of fine and diffusely distributed secondary α phases were found in the welded joint, which increased the content of the phase boundary. The TEM characterization and calculation results show that the phase boundary had a strong hindering effect on the dislocation slip. As a result, under three-stage heat treatment conditions, the hardness of the welded joint was increased by 50%, and the yield strength and tensile strength of the welded joints were increased by 24.3% and 13.9%, respectively. The impact toughness of the base metal significantly improved after heat treatment, which was 31.6% higher than that before heat treatment due to the increase in the equiaxed α phase from the weld metal to the base metal. A large number of stacking faults were generated during the tensile deformation of the welded joint before heat treatment due to the low stacking fault energy of the α phase, which could effectively release the stress and hinder the dislocation slip. After heat treatment, the stacking fault of the welded joint decreased owing to the obvious increase in stacking fault energy. The micro-cracks propagated rapidly along the lamellar α phase to fracture. Therefore, the plasticity of the welded joint decreased, although the strength was improved after heat treatment.