Study On Multi-step Superplastic Forming Technology Of TC4-DT Titanium Alloy

TC4-DT titanium alloy is a new type of alpha & beta titanium alloy with medium strength and high damage tolerance,which has been developed independently in China.It has been applied to aerospace applications as an important structural material.In this paper,TC4-DT titanium alloy without any special refinement was investigated,and the experiments were carried out under different parameters through multi-step superplastic tensile experiments and multi-step superplastic isothermal forging experiments.The thermal deformation behavior and microstructure evolution under different parameters were studied,and the triple heat treatment was carried out on isothermal forging,and the room-temperature mechanical property testing was conducted after the heat treatment.,in order to provide relevant technical parameters and experimental basis for isothermal forging forming of large complex structure.The multi-step superplastic stretch forming method of TC4-DT alloy is a method by which the different temperature conditions of the conventional constant strain rate superplastic forming is divided into two steps.At the first step,when the superplastic stretch will reach a certain pre-tension elongations,it can be waited in thermal environment for a clearance time.And then the second step of superplastic forming could be carried on.The results indicated that the TC4-DT alloy exhibited good superplasticity?305.93%⁵06.67%?when all deformation temperature ranged from 860 ? to 950 ?,strain rate ranged from 3.3×10-4 s^-1 to 1.0×10-2 s^-1,pre-tensile elongation ranged from 20% to 80% and clearance time ranged from 5 min to 30 min.TC4-DT alloy obtained best superplasticity with an elongation of 506.67% corresponding to the temperature of 890?,the pre-tensile elongation of 50%,the clearance time of 10 min,and the both steps strain rates of 3.3×10-4 s^-1.The true stress-strain curves showed that the stress at the second step was smaller obviously than the stress at the end of the first step,the deformation in first step softened the alloy in some degree.The microstructure showed that dynamic recrystallization accompanied the whole process of the multi-step superplasticity deformation,and still recrystallization occurred in the clearance time of two steps.The occurrence of recrystallization behavior provided fine microstructure for the superplastic deformation,which was beneficial to the improvement of the superplasticity of the alloy.Through superplastic isothermal forging experiments in the two-phase region on TC4-DT titanium alloy,the triple heat treatment on isothermal forging products and researches on microstructure and properties was studied,the multi-step superplastic isothermal forging method was put forward.At the first step,when the superplastic isothermal forge will reach a certain pre-compression,it can be waited in thermal environment for a clearance time.And then the second step of superplastic forming could be carried on.The results indicated that,at the same temperature,the isothermal forging process of each step in multi-step superplastic isothermal forging at the end of the cylinder pressure is less than one-step superplastic isothermal forging at the end of the cylinder pressure,the multi-step superplastic isothermal forging can effectively reduce forging pressure.And with the same deformation temperature,the content of the alpha phase of the multi-step superplastic isothermal forging was lower than that of one-step superplastic isothermal forging,and the higher of the content of the strip alpha phase than that.The higher the temperature was,and the more obvious the trend will be.The superplastic isothermal forging products after triple heat treatment of 960 ? /1h WQ+920 ? /1h WQ+820? /1.5h AC,which can get basket weave with different degree,and the better the weave degree was,the higher the strength will be.The products of two-step superplastic isothermal forging at 890 ? and one-step superplastic isothermal forging at 920 ?,after triple heat treatment of 960 ? /1h WQ+920 ? /1h WQ+820? /1.5h AC had good strength plastic fracture toughness comprehensive matching.