Effect of titanium alloy thermal process on microstructure and properties of thick plates

Abstract:

The effects of two hot working processes on the microstructure and mechanical properties of high temperature titanium alloy thick plate BTI-6431S were studied. The results show that the microstructure of (+) rolled thick plate is similar to that of isoaxial two-phase zone. The microstructure of  rolled thick alloy plate is typical fine sheet Weidler microstructure. The strength at room temperature and high temperature of the thick alloy plate is obviously higher than that of + rolled thick plate, and the plasticity is slightly lower. Comprehensive analysis shows that the alloy thick plate at 650℃ has high instantaneous strength at high temperature, which can be used in the manufacture of 650℃ short-time structural parts in aerospace industry. The nominal composition of BTI-6431S titanium alloy is Ti-6.5Al-3Sn-3Zr-3Nb-3Mo-1W-0.2Si. It is a kind of short-time high temperature titanium alloy with operating temperature of 650C~700C independently developed in China. The main strengthening mechanism is through solid solution strengthening of stable element Al and β phase. Neutral elements Zr and Sn and β phase stable elements Nb, Mo and W were added to improve the process performance. The nominal Al equivalent of the alloy is 8.9, Mo equivalent is 4.6, the alloy type is high aluminum equivalent martensite two-phase titanium alloy, has good thermal strength and weldability, and has good processing plasticity and process plasticity. Compared with BT18Y titanium alloy, THE Al equivalent of BTI-6431S titanium alloy is unchanged, which increases the content of Mo and Nb of high melting point elements. Meanwhile, it also increases the CONTENT of W by 1%, and increases the Mo equivalent by 3.6%. By adding high melting point elements, the Mo equivalent is improved, therefore, the high temperature strength of the alloy is improved. At the same time, the processing plasticity and process plasticity of the alloy are improved. In this paper, the effect of hot working process on the microstructure and properties of thick bTI-6431S titanium alloy plate was discussed. The properties and microstructure characteristics of thick plate at room temperature and high temperature (650℃) under different hot working process were analyzed and compared, which provided a certain theoretical basis and technological reference for the mass production of the alloy. In this experiment, BTI-6431S titanium alloy ф 110mm ingot was used after two times of vacuum consumable electric arc melting. Besides Zr element was added with pure metal, high melting point elements W, Mo, Nb and low melting point elements were added with intermediate alloy form, which ensured the high quality of the ingot. The phase transition temperature of the alloy is 990℃~1000℃. The ingot was opened and forged at 1180℃, and finally the slab was prepared in the α+β two-phase region. The slab is rolled into 20mm thick sheet by β and α+β heating temperature respectively. In order to compare the effect of hot working process on microstructure and properties, samples of R-state sheet under the two processes were taken to test its tensile properties at room temperature, tensile properties at high temperature, durability and creep properties at 650℃. The metallographic samples were cut from the r-state plate, and the corrosion solution was a mixture of HNO3, HF and H2O. The microstructure was observed by Axiovert200MAT optical microscope. 2.1 Microstructure of Thick Plate under two Rolling processes The Mo equivalent of BTI-6431S titanium alloy is 4.6, which is a martensite two-phase titanium alloy with high aluminum equivalent. The microstructure of thick plate under two rolling processes is shown in Figure 1. It can be seen from FIG. 1 that the microstructure of the sheet after rolling at (α+β) phase zone temperature is typical α+β transformation microstructure, primary  is strip and equiaxed morphology. In β rolling process, the microstructure of sheet is fine weidler microstructure with discontinuous grain boundary α, which is typical β processing microstructure. . . 3. Conclusion 1) For bTI-6431S thick titanium alloy plate, the microstructure of the finished product at (α+β) phase zone temperature is typical α+β transition microstructure. Under β rolling process, the microstructure of the plate is fine weidenberg microstructure, which has high strength and high temperature plasticity at room temperature and high temperature (650℃). 2) BTI-6431S titanium alloy has good comprehensive properties, especially at 650℃, has high high temperature strength and short-term creep properties, is a new type of short-term high temperature titanium alloy for aerospace applications.