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Influence of Superplastic Forming and Heat Treatment on Microstructure Evolution of TC21 Titanium Alloy

TC21 titanium alloy is a high-strength, high-toughness and high damage tolerance titanium alloy with good strength and plastic fracture toughness and low crack growth rate. The microstructure of TC21 titanium alloy determines its use performance and mechanical properties. The microstructure can be effectively improved through thermomechanical heat treatment to achieve better performance. The superplastic forming technology of titanium alloy is a technology developed by using the excellent deformation performance of the material in the superplastic state. When the titanium alloy is in a superplastic state, it has good flow properties and is easy to fill, so it is easy to form complex qualified parts. Different from the ordinary forging microstructure, dynamic recrystallization obviously occurs in the deformation zone of the sample after superplastic deformation, and the microstructure is obviously coarsened, which affects its performance. The superplastic deformation of the TC21 titanium alloy is heat treated to study its structure evolution law, which provides a reference for the development of isothermal forging and superplastic precision forging of the alloy.

The superplastic tensile test adopts the electronic universal testing machine controlled by the SANS-CMT4104 microcomputer. The sample is a standard tensile sample of 5mm×15mm, which is heated and insulated by a resistance furnace. After TC21 titanium alloy is superplastically deformed under different conditions, it is subjected to double annealing heat treatment to study the effect of hot working process on the microstructure evolution of TC21 titanium alloy. The results show that when the deformation temperature is 890~960℃, the elongation of TC21 titanium alloy first increases and then decreases with the increase of deformation temperature. The best superplastic deformation temperature is 910℃; TC21 titanium alloy exceeds the α+β phase region. Plastic deformation, and then double annealing treatment in the α+β phase area to obtain a two-state structure; superplastic deformation in the quasi-β area and double annealing treatment in the α+β phase area to obtain a mesh basket structure.