Effect Of Two Phase Region Multistage Heat Treatment On Microstructure And Properties Of TC21 Alloy

Titanium alloys are widely used in aerospace and other fields because of their high specific strength and excellent corrosion resistance.As one of our country independently developed biphase titanium alloys,TC21 alloy has high toughness and damage tolerance and its Strength level is up to 1100 MPa.Because the heat treatment parameters have a big impact on the microstructure and mechanical properties of TC21 titanium alloy,the research on the heat treatment process under different conditions provides a scientific basis for the thermal processing and forming process of TC21 titanium alloys.In this paper,the three stage heat treatment of TC21 titanium alloy is carried out in the two-phase region.It systematically studies the influence of heat treatment on three typical morphologies and content control methods of alpha phase and the effects of different microstructure on mechanical properties of TC21 titanium alloy.Experimental results show: The first stage heat treatment controls equiaxial primary ? phase content and morphology;the second stage heat treatment controls lamella secondary ? phase content and morphology;The third stage heat treatment can precipitate the small dispersion ? phase in residual ? phase after the two stage heat treatment.With the increase of first stage heat treatment temperature,the content of primary ? phase is gradually reduced;with the rise of second stage heat treatment temperature,the content of secondary ? phase is gradually reduced,and thickness is gradually increasing.At the same time,the content of small dispersion ? phase in residual ? phase is increasing with the increase of the second heat treatment temperature.The result of the room temperature tensile test of TC21 titanium alloy after different heat treatment shows: The first stage heat treatment samples have the lowest yield strength,tensile strength and the highest elongation;the yield strength of second stage heat treatment is higher than that of the first stage treatment samples,and the tensile strength is changed little,the elongation is decreased slightly;The three third stage heat treatment samples have the highest yield strength,tensile strength and the lowest elongation.At the same time,the mechanical properties can be adjusted by changing the first and second heat treatment temperature.With the increase of first stage heat treatment temperature,the strength and plasticity do not change much;with the increase of second stage heat treatment temperature,the strength of alloy increases while the plasticity decreases.The above results show: The high first stage heat treatment temperature should be avoided to obtain more equiaxed primary ? phase and to ensure the strength of alloy;the high second stage heat treatment temperature should be avoided to control the growth of lamella secondary alpha phase.The BP artificial neural network method is used to establish the prediction model of heat treatment parameters and room temperature tensile properties of TC21 titanium alloy.The input parameters include the first and second heat treatment temperatures;the output parameters include yield strength,tensile strength and elongation.Through simulation and test model prediction accuracy,the results meet the requirements of the precision of the model.The establishment of different heat treatment process parameters and performance prediction model simply and effectively predicts the relationship between process and performance.