Laser deposition manufacturing (LDM) technology is a digital advanced manufacturing technology emerged in recent years. Titanium alloy materials prepared by this technology are widely used in various fields. In the process of laser deposition manufacturing, problems such as deformation and cracking of the deposited sample may occur. Heat treatment is an effective means to control the microstructure and has a significant effect on improving the mechanical properties of the sample. This paper focused on the effects of nine heat treatment processes on the microstructure and mechanical properties of TA15/Ti2AlNb direct transition titanium alloy samples, and the experimental results were compared and analyzed. The laser deposition manufacturing system used in this article consisted of semiconductor laser device, synchronous powder feeding system, four-way coaxial powder feeding nozzle, oxygen analyzer and focusing system and so on. The whole depositing process was carried out under an argon atmosphere. The vacuum-dried TA15 and Ti2AlNb titanium alloy powders were used and TA15 plates were used as the substrates. First, TA15 powders were deposited on the substrate along the z-axis direction, and then Ti2AlNb powders were deposited. After the depositions were completed, heat treatments were performed and corresponding specimens were prepared. Optical microscope (OM) was used to observe the microstructure of the samples after different heat treatments, the electronic universal testing machine was used to stretch the tensile specimens at room temperature, scanning electron microscope (SEM) was used to observe the fracture morphology and the microhardness tester was used to test the microhardness of samples after different heat treatments. After the stress relief annealing, α phase gradually increased and cut off each other on the TA15 side, producing short rod-shaped α phases. As the annealing temperature increased, the length-width ratio of α phase increased, and α phases in different directions crossed and cut off each other, showing basket-like microstructure. When the annealing temperature reached 900 ℃, the microstructure of TA15 side was relatively uniform. After solution aging treatment, a part of coarse α phases and slender α phases appeared in the microstructure of TA15 side. After the stress relief annealing, the light gray B2 phases were heated and the α2 phases were precipitated along the grain boundary on Ti2AlNb side. with the increasing of the annealing temperature, O phases gradually transformed into B2 phases and α2 phases. After the solution aging treatment, O phases precipitated again and on Ti2AlNb side O phases and α2 phases were uniformly distributed in the matrix B2 phases. The transition interface was very obvious in the as-deposited state. As the annealing temperature increased, the originally clear transition interface became more and more insignificant, and the transition interface gradually disappeared after solution aging treatment. The tensile fractures were analyzed by energy spectrometer, and the fracture position of the TA15/Ti2AlNb samples in the as-deposited, annealed, and solution-aged states were all near the transition interface. Compared with the as-deposited state, the tensile strength of TA15/Ti2AlNb tended to decrease after annealing or solution aging heat treatment, but the plasticity tended to increase. The as-deposited fractures had river-like patterns and cleavage steps, so they were cleavage fractures. The annealed and solution-aged tensile fractures had both dimples and cleavage steps, so the fracture mechanisms belonged to semi-cleavage and semi-ductile fractures. The microhardness of TA15 side was higher than that of Ti2AlNb side. After 950 ℃×1 h air cooling and 800 ℃×4 h air cooling treatment, the difference of microhardness between the two sides was the smallest and the microstructure of the transition zone was also relatively more uniform. The heat treatment could effectively improve the microstructure of the transition interface of the TA15/Ti2AlNb deposited samples, making the structures of both sides more uniform and increasing the plasticity without significantly reducing the strength of the samples. So, by proper heat treatment the TA15/Ti2AlNb deposited samples could get a better comprehensive performance. The fracture position of each tensile sample was near the transition interface, resulted in the great property difference of these two alloys of TA15 and Ti2AlNb. In the future, much effort should be done on the optimization of laser manufacturing process parameters and the design of gradient transition layers of TA15/Ti2AlNb composite structure, in order to obtain better properties. © 2022, Youke Publishing Co., Ltd. All right reserved.
