Study On The Stress Relaxation Behavior And Mechanism Of TA7/TA15 Near-? Titanium Alloy

TA7 and TA15 titanium alloys are widely used in aerospace structures due to their high specific strength,high temperature performance and good weldability.The occurrence of stress relaxation will lead to the delay of internal preload with time,and then lead to the leakage or loosening of the seal.However,there are few studies on the stress relaxation behavior of near ? alloy at home and abroad.In addition,there is a lack of research on the relationship between the relaxation resistance and temperature and initial stress.In this paper,the effects of temperature and initial stress on the stress relaxation behavior of near ? titanium alloy were studied by stress relaxation tests at different temperatures(500?,550?,600?,650?)and initial stress(100MPa,200 MPa,300MPa).The experimental results show that the stress relaxation curve of near ? alloy presents the characteristic of exponential decay stress relaxation,and the temperature has a great influence on the stress relaxation.In addition,Norton equation was used to calculate the stress index n,and the relaxation mechanism was preliminarily determined.The relaxation mechanism was analyzed by OM,EBSD,SEM,TEM and microstructure.The main research contents and results are as follows(1)The stress relaxation experiments of TA7 titanium alloy were carried out at 550 ?,600 ?and 650 ?.The stress exponent of TA7 titanium alloy under different conditions is 2-8,and the stress exponent becomes smaller and smaller with the increase of temperature,which indicates that the behavior controlled by atomic diffusion is more likely to occur with the increase of temperature.Combined with EBSD,it can be seen that the small angle grain boundary decreases with the increase of temperature,and the substructure gradually transforms to sub grain boundary and grain boundary It is accompanied by grain refinement and growth.In addition,through the change of Schmidt factor,it can be seen that there is a certain grain rotation phenomenon in the process of stress relaxation.(2)The stress relaxation experiments of TA15 titanium alloy were carried out at 500 ?,550 ? and 600 ?.The stress exponent of TA15 Titanium Alloy under different conditions is also 2-8.The main stress relaxation mechanism is also the transformation from dislocation slip to dislocation slip,dislocation climbing,grain boundary slip and grain rotation.Different from TA7,the small angle grain boundary increases with the increase of temperature.The grain refinement caused by stress relaxation is due to the effect of ? phase.(3)According to TA7 and TA15,the stress relaxation model is established and substituted into the finite element software to simulate the stress change during heat treatment,which has a significant correction effect on the simulation of stress relief annealing.