Thermomechanical Treatment Process Of TC18 Titanium Alloy

Thermomechanical treatment (TMT) is one of the important technologies of the titanium alloy material processing, which can produce significant effect on microstructures and properties of titanium alloys, especially for α+β titanium alloy. Therefore, it is of great significance to research the TMT process of TC18 titanium alloy. According to the production situation of Pangang Group Jiangyou Changcheng Special Steel Co., Ltd. Titanium Industry Branch, on the basis of predecessors’ research, the effect of TMT on deformation behavior and microstructure of TC18 titanium alloy was studied, aims to provide the Factory-production-guidance of forging technology of titanium alloy rod products.The true stress-strain curves of TC18 alloy were obtained through isothermal compression tests on a Gleeble 3500 thermal mechanical simulator, and the effects of temperature and strain rate on the flow stress were investigated. Besides, the σp-T and σp-ln ε relationship curves were established, and relationship between peak stress and temperature as well as strain rate during deformation was also studied. Results show that:(1) Flow softening mechanism of hot deformation are dynamic recovery and dynamic recrystallization. Dynamic recovery can occur in various deformation parameters, while the dynamic recrystallization occurs only at high temperature and low strain rate (875 ℃, ε≤0.1s-1 and δ50 ℃, ε=0.01 s-1). (2) Peak stress decreases with the increase of temperature, and decreases faster in β phase ragion than in a+pphase ragion. At a certain temperature, peak stress increases with the increase of strain rate, and the peak stress is more sensitive to strain rate under a lower temperature.Samples underwent heat treatment after deformation, and optical microscope was used for microstructure observation, the effects of deformation parameters on microstructure were also analyzed. Results show that dynamic recrystallization is sufficient with the deformation at 875 ℃,ε≤ 0.1s-1, and fine homogeneous microstructures formed after heat treatment, while the dynamic recrystallization is insufficient with the strain rate of 0.01s-1. After heat treatment, alpha phase along the lamellar grain boundaries to grow, resulting in coarse microstructure. recrystallization is very difficult when deformed at 850℃, with the increase of strain rate, a density of dislocations and other sub-structures increasing, in the subsequent heat treatment process, a large number of nucleation formed and grow into fine microstructure. When deformed at 820 ℃, the microstructure is not sensitive to the change of strain rate, and all can get fine and uniform microstructures.In order to better simulate the actual forging process, double-hit hot compressions were carried out using a Gleeble 3500 thermal mechanical simulator. The hot compressions were performed at different temperature in two stages with the inter-stage delay time of 30 min. The effect of isothermal forging temperation on microstructure of TC18 titanium alloy was studied by microstructures observation.The result showed that the density of sub-structures insideaphease will increase with the increase of strain or decrease of temperature. During heat treatment, beta phase along the sub-structure boundary to grow, which results from the a phase be refined and fine uniform microstructures be acquired.The deformed alloys underwent the following heat treatment process:(830℃ ×2h, FC)+(750℃×2h, FC)+(570℃×4h, WC). Then, optical microscope was used for microstructure observation. The results show that supersaturated P solid solution would precipitate a phase in the process of heat treatment. With the decrease of temperature, the size of alpha phase decreases and the morphology characteristics change from a "block" or "flake" to "short rod" and "needle". By using SEM electron microscope to detect the microstructure, it was found that some particles dispersion distributed In the initial alpha phase, and carried out the conclusion that the particles may be Ti3Al.