The preparation of TiAl alloy wear-resistant plate has been overcome
The proportion of TiAl alloy is less than half of that of nickel-based alloy, and the use temperature can reach 700~900°C. Tial alloy has the advantages of light weight, high strength, corrosion resistance, wear resistance, high temperature resistance, etc., and has become an important alternative material in aviation and aerospace fields. TiAl alloy wear-resistant plate is not only expected to be used as structural material directly, but also as a pre-formed material for superplastic forming. The application of TiAl alloy wear-resistant plate in thermal structure and thermal protection system has been included in the European air transport research program, ready to be used in high-speed civil transport aircraft and reusable single-stage spacecraft. However, TiAl alloy is a refractory alloy and is not suitable for high temperature rolling. So far, the problems existing in the preparation technology of TiAl alloy have not been well solved, especially the hot rolling process is easy to crack, which makes the production of large size and high quality sheet is very difficult. Therefore, the preparation of TiAl alloy wear-resistant plate has become one of the important research topics in the field of TiAl alloy.
According to the existing research results, most of the following methods are used to improve the forming difficulties of TiAl alloy wear-resistant plate in rolling process:
1, the material quality control before rolling is stricter;
2, the use of special envelope rolling technology for rolling, strict design of the envelope process;
3, in the α+γ two-phase zone for rolling, between passes to return to the furnace heating;
4, carefully select rolling speed and pass deformation;
5, avoid the oxidation of the plate during the rolling process. But the actual results show that it is difficult to produce large size TiAl alloy wear-resistant plate by rolling even if the process parameters are carefully controlled.
Researchers have successfully prepared TiAl alloy wear-resistant plate with a size of 700mm*200mm*2mm by near isothermal clad rolling technology, and achieved good results.
1. Composition optimization design
It is found that adding a large amount of specific β phase stabilizing elements (such as V, Cr, Mn, Mo) and a small amount of grain refining elements (such as Y, B, C) to TiAl alloy can make its thermal processing performance significantly better than traditional TiAl alloy. After such composition optimization, the flow stress at high temperature of the alloy is reduced, the deformation capacity at high temperature is improved, and the defects such as cracks are not easy to occur in the process of hot working. In terms of phase composition, the alloy mainly contains β phase and γ phase. After heat treatment, the microstructure and phase composition of TiAl alloy can be controlled, which is conducive to the regulation of mechanical properties at room temperature and high temperature. The nominal composition of the alloy used by Harbin Institute of Technology is Ti-43Al-9V-Y (atomic percent). The raw material is sponge TiAl and high purity Al, and other added elements are intermediate alloys of added metal and Al.
Second, improve the process route
The near isothermal clad rolling process is adopted. The ingot is melted in a vacuum consumable electrode arc melting furnace after the ingredients are mixed according to the design. The ingot was homogenized annealing (900°C/48h) and hot isostatic pressing. The hot isostatic pressure adopts Ar gas as the protection atmosphere, and fills Ar gas while heating up to 170MPa. After being kept at 1250°C for 4h, it is cooled slowly with the furnace. The first forging temperature is 1200°C, and the total deformation is greater than 85%. After forging, the material is annealed at 900°C for 48h; It is then coated with stainless steel and rolled into plates at near isothermal temperature on a hot mill. The main process parameters of clad rolling are as follows: opening temperature 1200°C, rolling speed <0.5m/s, pass deformation of about 10%, pass return time 5~10min, total deformation of >70%. After rolling, the furnace is cooled to 400~500°C and air cooled to room temperature.
The results show that the prepared TiAl alloy wear-resistant plate has uniform deformation, no cracks and other quality defects, indicating that it has good hot-working performance.
