The microstructure and properties of large-sized TA15 alloy trial production bars

The nominal composition of TA15 alloy is Ti-6.5Al-2Zr-1Mo-1V. Its main strengthening mechanism is through the solid solution strengthening of the α-stabilizing element Al. Neutral elements Zr and β-stabilizing elements Mo and V are added to improve the process performance. The Al equivalent of this alloy is 6.58%, and the Mo equivalent is 2.46%. It belongs to a high-Al-equivalent near-α type titanium alloy. Therefore, it has both the good thermal strength and weldability of α-type titanium alloys, as well as the process plasticity of (α+β) type titanium alloys.

The properties of the material are determined by its microstructure, and the microstructure largely depends on the material's processing technology. Therefore, studying the influence of the heat deformation process of TA15 alloy on its microstructure and properties is of great significance. Researchers will focus on studying the microstructure and properties of TA15 alloy test bars produced under different forging processes, in order to provide technical support for actual production and further enrich this research content.

TA15 alloy was obtained as a finished ingot with a diameter of Φ750mm through three vacuum self-dissolution arc melting processes. The bars were respectively fabricated using three forging processes. Process A: The rough forging was carried out in the β phase region at high temperature for 3 heating cycles of elongation, and the finished forging was carried out at low temperature in the β phase region for 3 heating cycles of elongation, and then the bar was rolled round. Process B: The rough forging was the same as Process A, and the first heating cycle of the finished forging was carried out at low temperature in the β phase region once, then it was carried out for 3 heating cycles of elongation in the two-phase region close to the phase transformation temperature, and then the bar was rolled round. Process C: The rough forging was first carried out in the β phase region at high temperature for 1 heating cycle of elongation, and then it was carried out in the same temperature for 2 heating cycles of extrusion; the finished forging was carried out at low temperature in the β phase region for 1 heating cycle of elongation, and then it was carried out for 3 heating cycles of elongation in the two-phase region at a lower temperature, and then the bar was rolled round. The bars were all air-cooled after forging, and then annealed at 800℃ for 1 hour, and then cross-section samples were taken and processed into national standard specimens for microstructure and property testing. The test results are as follows:

(1) The mechanical property test results of the bars under different forging process conditions show that from Process A to Process C, the plasticity indicators of the bars have been continuously improved, while the strength indicators have first increased and then decreased; in summary, Process C has better mechanical properties.

(2) The forged bars of Process A have a high forging temperature and insufficient deformation, resulting in coarse grains throughout the cross-section and poor uniformity of the microstructure. The original β grain boundaries were not fully broken, causing the mechanical properties of the forged bars of Process A, especially the plasticity, to be poor. The mechanical properties of Process B are improved compared to Process A. The rough forging of Process C in the β region was carried out with two large deformation amounts of extrusion, and the finished forging in the two-phase region further reduced the temperature and ensured a certain deformation amount, so that the microstructure of the forged bars was completely and fully broken, with fine grains, and had typical dual-phase characteristics. Therefore, the comprehensive performance of the forged bars of Process C is the best.

(3) From the tensile fracture of the bars at room temperature, it can be seen that from Process A to Process C, the crack pits of the bars gradually increase in size and the depth also increases. The better the plasticity of the material, the larger and deeper the crack pit. It can be seen from the fracture characteristics that the forged bars of Process C have better plasticity.