Effect of heat treatment on properties of titanium alloy

At present, the annual output of titanium alloy processing materials in the world has reached more than 40,000 tons, with nearly 30 titanium alloy brands. Because titanium alloy has high strength and small density, good mechanical properties, toughness and corrosion resistance is very good, has been developed in the aviation industry, so that the titanium industry at an average annual growth rate of about 10%. In addition, titanium alloy process performance is poor, cutting difficult, in hot processing, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon. There are poor wear resistance, complex production process.

Industrial production of titanium began in 1948. In the application of various titanium alloy products, forgings are mostly used in gas turbine compressor disc and medical artificial bone, which require high strength, high toughness and high reliability occasions. Therefore, the forging not only requires high dimensional accuracy, but also requires the material to have excellent characteristics and high stability. Therefore, the characteristics of titanium alloy should be given full play in the manufacturing process of titanium forgings to obtain high quality forgings. Titanium alloy is difficult to forge, easy to crack. Therefore, the most important thing in the production of titanium alloy forging is to properly control the forging temperature and plastic deformation. In order to select a reasonable annealing process, we first observed the effects of heating temperature and cooling mode on the microstructure and mechanical properties of TC4 titanium alloy. In order to obtain the best comprehensive strength and plastic properties of TC4 titanium alloy, while having good creep resistance and fracture toughness, the annealing process of air cooling (or water cooling) can be adopted after holding at 950℃ for 1 hour. In order to facilitate the subsequent processing, when the metallurgical plant leaves the factory, TC4 titanium alloy is used in 700 ~ 800℃ insulation 1 small time and space cold process. For some large forgings, in order to ensure the uniformity of performance, sometimes furnace cooling process is used. The total deformation rate of TC4 titanium alloy rod hot rolled at 920℃ is about 80%, and the a+β/β phase transition point is 980 ~ 990℃. The samples were heated at 1000℃, 950℃, 930℃ and 830℃ for 1 hour, and then air cooling, water cooling and furnace cooling were carried out respectively. The microstructure and mechanical properties are affected by different annealing methods.

The cooling rate has great influence on the microstructure and mechanical properties of the four temperatures mentioned above. At 1000℃, 950℃ and 930℃, martensitic transformation takes place in β phase components at equilibrium, and β phase transforms into martensitic a 'needle. At 1000℃, the mechanical properties of Westenitic structures are similar to those of air cooling at 1000℃. At 950℃ and 930℃ with water cooling, the microstructure is similar to that of air cooling, but there is β+ martensite a 'needle between the a phase of equiaxed primary. At this time, the corresponding comprehensive performance is the highest, and has better creep resistance than the air-cooled structure. At 830℃, the equilibrium β-phase components did not touch the M-line, but after water cooling, very small acicular transition products were found in the intercrystalline β-phase, which could only be distinguished by electron microscopy. But the structure of the acicular product has not been determined. At this time, tensile strength and section shrinkage are very low. As for furnace cooling, due to the slow cooling rate of the sample, the long residence time at high temperature, the polymorphic transformation is fully carried out, and all the A-phases become coarse. After cooling at 1000℃, coarse a and interlamellar β phase are produced in the original β grain, and there is a thick net formed by strip A phase on the original β grain boundary, generally known as the net basket structure. After cooling at 950℃, 930℃ and 830℃, the A-phase tended to nucleate and grow at the interface of the original A-phase, and the microstructures were equal axial a and intergranular β phases. The tensile strength of furnace cooling at 1000℃ is lower than that of air cooling and water cooling, and the tensile plasticity is higher. The comprehensive performance of furnace cooling at other temperatures is also lower than that of water cooling and air cooling.