Research on Forming Mechanism and Hot Working Diagram of TC4 Alloy in Multilayer Sheet Hot Stamping and Cold Die Process
摘要
The titanium alloy components are formed at lower than 0.01 s-1 during the isothermal forming process. It caused a poor production efficiency and serious costs. And it will cause significant wear and tear to the molds. We must seek a highly efficient and reliable process to solve this problem. In this paper, the hot working diagram and hot deformation behavior of TC4 titanium alloy were studied to analyze the formability and mechanism of TC4 under non-isothermal deformation conditions higher than 0.01 s-1. The hot deformation behavior and hot working diagram of TC4 alloy in the temperature range from 700 to 850 °C were studied at different strain rates. Numerical simulations were conducted for different hot stamping processes. It was found that in the hot stamping process of multilayer sheets, the temperature loss of the parts was the slowest, the Mises stress was the smallest, and the forming efficiency was the highest. By comparing the thickness distribution of the parts under different stamping speeds, it is found that the thickness distribution of the parts obtained under the condition of a stamping speed of 40 mm/s is the most uniform, and the thinning rate was the smallest, which was 20.5%. When forming at 850 °C, the Mises stress of the material was the smallest, approximately 204 MPa. The numerical simulation results show that the parts formed by multilayer hot stamping at high temperature and high stamping speed have the best quality. Due to the strain strengthening effect, the forming performance of TC4 will be improved at higher stamping speeds. Finally, the hot stamping experiment was carried out. The simulation results were compared with the experimental results, and the results were highly consistent. The influences of temperature and stamping speed on the internal structure of materials were revealed by electron backscatter diffraction (EBSD). After hot stamping, the size of grain is smaller and the grain orientation tends to be uniform. With the increase in temperature, the grain in sheet grows and the grain boundary diffused, which reduces the stress level in the material. Since the elevated temperature, the dynamic recrystallization (DRX) effect is enhanced, and the plasticity of the material is improved. At high temperature and high stamping speeds, the low-angle grain boundaries (LAGBs) of the large grains will rotate into high-angle grain boundaries (HAGBs) and then form grains with smaller size. Thus, the plastic deformation ability of TC4 alloy parts is enhanced.