The nominal composition of TC18 titanium alloy is Ti-5Al-5Mo-5V-1Cr-1Fe, which is a high-strength and high-toughness titanium alloy. The outstanding advantage of the alloy is that it has a very high strength level (1080MPa) in the annealed state, and the strength can reach 1300MPa after intensified heat treatment. And the hardenability is excellent, the cross-section hardened thickness can reach 250mm, because the cross-section thickness is not limited by the hardenability, it is particularly suitable for manufacturing large-scale load-bearing members of the aircraft. This is also the most prominent advantage of the TC18 titanium alloy, which can use ordinary low cost Die forging (performed on a die forging hammer), hot forging, and isothermal die forging produce a forging that is close to the final shape. The alloy has been widely used in aircraft such as the Il-76.
Researchers made TC18 titanium alloy ingots through β-zone blooming, then used 3 different forging processes to obtain Ф400mm bars, and compared the obtained microstructures and mechanical properties after the same bars were heat treated. Through analysis and research, the ideal forging process of TC18 titanium alloy large-size bar is obtained.
The test uses three Φ804mm smelted ingots with a vacuum consumable arc furnace and an ingot weight of 4000 kg. The (α + β) / β phase transition temperature is 860 to 870 ° C.
The ingot is heated in the β zone, and the hydraulic press is used to open the forging, and then different forging processes are used. After multiple fires, the forging is heated and forged in the single-phase zone or the α + β zone, and finally forged into a Ф400mm bar. The obtained rods were sampled after the same heat treatment for microstructure and performance testing. The detailed process plan is as follows:
Process 1: Opening + high-low-high + 30% two-phase zone deformation;
Technology 2: Opening + high-low-high + 70% ~ 85% deformation in the two-phase area;
Process three: two-phase zone deformation of open billet + high and low, water quench + 70% to 85%.
(1) Compared with the other two process schemes, the microstructure of the Ф400mm bar obtained by the third process (opening + high-low-high, water quenching + 70% to 85% two-phase zone deformation forging process) is compared with the other two process schemes, and the microstructure is uniform and fine. The degree of transformation is better than other schemes.
(2) The strength index obtained by using process three (opening + high-low-high, water-quenching + 70% to 85% two-phase zone deformation forging process) of the Ф400mm bar due to the influence of the fine grain strengthening mechanism, while ensuring plasticity highest. In addition, due to the fine and uniform grains obtained by the third process, the crack propagation path increases, which makes the fracture toughness index obtained by this process significantly increase, and the comprehensive performance is the best.