Heterogeneous solid-state phase transition behavior during different annealing treatments of the hybrid additive manufactured gradient dual-phase titanium alloy

Abstract

Hybrid manufacturing technique that combines the additive manufacturing technologies and conventional manufacturing technologies has a great potential to form large and complex titanium alloy parts with high efficiency and low cost. The post-process heat treatment is indispensable for the hybrid manufactured titanium alloys due to the inhomogeneous microstructures and unsatisfied properties. In this study, hybrid manufactured components were made by depositing TC11 alloy on the rolled TC11 plates and three typical annealing heat treatments were designed and performed to tune the gradient microstructures. Results showed that the as-deposited sample mainly included three obvious zones which were the laser deposition zone (LDZ), heat affected zone (HAZ) and substrate zone (SZ). During the subsequent heat treatment, the abnormal solid-state phase transition behavior occurred. Specifically, after 990 °C annealing treatment, the β grains morphology remained unchanged and a special bimodal structure containing the α_p lath and ultrafine lamellar secondary α (α_s) was formed in the LDZ. While the microstructure in the SZ changed significantly, it transformed into the traditional bimodal structure. After 1013 °C annealing treatment, the SZ and HAZ showed the equiaxed grain structure and these grains grown across the fusion line and swallowed the part of columnar gains at the bottom of the LDZ. When the annealing temperature raised to the 1035 °C, the microstructure in the three zones tended to be lamellar structure within the equiaxed grains. After different heat treatments, the strengths of the bonding zone samples were all increased with different degree. Especially, after 990 °C annealing heat treatment, the excellent strength (UTS = 1062 MPa, YS = 921 MPa) and plasticity (EL = 10.6 %, RA = 30 %) of the sample were achieved. Moreover, during the tensile process, the fracture positions were located in the LDZ of the bonding zone samples at different states indicated that the LDZ and SZ had the well binding. This work can provide experimental basis and theoretical guidance of the post heat treatment process for the gradient materials fabricated by the hybrid manufacturing technique, laser cladding, welding and other methods.