Effect of multi-stage heat treatment on the microstructure and mechanical properties of Ti-6Al-4V alloy deposited by high-power laser melting deposition

The utilization of high-power laser melting deposition (LMD) presents significant advantages in the production of large-scale titanium alloy components. Nonetheless, Compared with the Ti-6Al-4V forgings, LMD-ed titanium alloy tends to form a heightened strength but reduced plasticity. In this study, a three-stage heat treatment technology was employed to modify the strength and toughness of as-deposited Ti-6Al-4V alloy. The results show that as-deposited Ti-6Al-4V is made of coarse grain. There is a basket-weave structure mainly composed of acicular alpha with an aspect ratio of 14.87 in the grain. After three-stage heat treatment, the alpha phase becomes coarse with an aspect ratio of 3.27. Besides, fine and acicular secondary alpha precipitate from the residual beta phase. The average hardness value notably increased after heat treatment from 315.77 HV to 345.18 HV, and the yield strength and tensile strength experienced marginal changes from 870.94 MPa to 865.99 MPa and from 966.69 MPa to 956.96 MPa, respectively. Notably, the elongation increased from 8.80% to 14.29% following the application of multi-stage heat treatment. The coarse primary alpha phase can improve the deformation ability of the material and reduce the strength of the material, but the small secondary alpha phase precipitated increases the crack propagation resistance, resulting in a significant increase in the plasticity of the heat-treated sample under the condition that the strength of the sample does not decrease. Besides, the fatigue crack growth experiments revealed that the multi-scale alpha microstructure obtained after heat treatment exhibited a coordinated effect. The secondary lamellar alpha altered the crack growth path, generating secondary cracks, dissipating more energy, reducing the crack growth rate, and exhibiting superior plasticity and toughness compared to the as-deposited sample.