The physically-based internal state variable （ISV） models were used to describe the changes of dislocation density, grain size, and flow stress in the high temperature deformation of titanium alloys in this study. The constants of the present models could be identified based on experimental results, which were conducted at deformation temperatures ranging from 1093 K to 1303 K, height reductions ranging from 20% to 60%, and the strain rates of 0.001, 0.01, 0.1, 1.0, and 10.0 s-1. The physically-based internal state variable models were implemented into the commercial finite element （FE） code. Then, a three-dimensional （3D） FE simulation system coupling of deformation, heat transfer, and microstructure evolution was developed for the blade forging of Ti-6Al-4V alloy. FE analysis was carried out to simulate the microstructure evolution in the blade forging of Ti-6Al-4V alloy. Finally, the blade forging tests of Ti-6Al-4V alloy were performed to validate the results of FE simulation. According to the tensile tests, it is seen that the mechanical properties, such as tensile strength and elongation, satisfy the application requirements well. The maximum and minimum differences between the calculated and experimental grain size of primary α phase are 11.71% and 4.23%, respectively. Thus, the industrial trials show a good agreement with FE simulation of blade forging.
Isothermal compression of TC11 alloy at the deformation temperatures ranging from 1023 to 1323 K with an interval of 20 K, the strain rates of 0.001, 0.1, 1.0, and 10.0 s-1, and the height reductions of 50% and 70% was conducted on a Gleeble-1500D thermomechanical simulator. According to the experimental results, the isothermal compression and the processing maps of TC11 alloy at different strains were drawn by using the dynamic material model （DMM）. Based on the processing maps, the proper forging parameters, including a combination of defor-mation temperature and strain rate, vary with the strain in different phases of TC11 alloy.
Isothermal compression of the Ti-6Al-4V alloy at the deformation temperatures of 950 and 980℃,height reductions of 30% and 60%,and strain rates of 0.001,0.010,0.100 and 1.000 s-1 was conducted,wherein the variations of microstructure with strain rate were investigated.The experimental results showed that the variation of the microstructure with the strain rate under one condition was significantly different from that under another condition,which meaned that the interaction between the processing parameters was great.The optimization of the strain rate under one condition was not suitable for another condition.Therefore,selecting the forging equipment and optimizing the strain rate should be based on simultaneously considering the deformation temperature and height reduction.
The dynamic conditions for plastic deformation mechanism,surface source mechanism,interface source mechanism and creep mechanism in the diffusion bonding process are proposed.Based on these dynamic conditions,a model for void closure in the diffusion bonding process is derived.The effects of diffusion bonding parameters on the bonding mechanisms are analyzed.For the diffusion bonding process of TC4 alloy,at a low diffusion bonding temperature or pressure,or for a short time,the interface source mechanism plays a main role.But,the creep mechanism is the dominant mechanism and operates until the diffusion bonding is fulfilled.As the diffusion bonding time increases,the surface source and interface source mechanism enhance and then stop at a certain stage of diffusion bonding,while the creep mechanism enhances continuously.As the diffusion bonding temperature or pressure increases,the interface source mechanism weakens and the other mechanisms enhance.The maximum and average errors between the calculated and the experimental results in the diffusion bonding for TC4 alloy are 12.86% and 5.79%,respectively.
Isothermal compression of a Ti-5.6Al-4.8Sn-2.0Zr alloy was conducted on a Thermecmaster-Z simulator at the deformation temperatures ranging from 960 to 1060℃, the strain rates ranging from 0.001 to 10.0 s^-1, and the maximum height reduction of 70.0%. In the two-phase region of the Ti-5.6Al-4.8Sn-2.0Zr alloy, the volume fraction of α phase decreases with an increase in deformation temperature, but the grain size has a slight variation with deformation temperature. The strain rate affects both morphologies and grain size of the α phase in the isothermal compression of the Ti-5.6Al-4.8Sn-2.0Zr alloy. The optimal height reduction also contributes to the small and well-distributed α phase in the isothermal compression of Ti-5.6Al-4.8Sn-2.0Zr alloy.
LI Miaoquan,LUO Jiao,and PAN Hongsi School of Materials Science and Engineering,Northwestern Polytechnical University,Xi’an 710072,China
The thermomechanical coupling simulation of the isothermal equal channel angular pressing（ECAP） of Ti-6Al-4V alloy was conducted.The effect of processing parameters,ECAP pass number and the residual billet on the effective strain,stress and temperature distribution was investigated.Based on the coupling simulation results,it is found that the shear factor,ram speed,deformation temperature,channel intersection angle and residual billet significantly affect the ECAP deformation behaviors.Meanwhile,the experimental study of the isothermal ECAP process of Ti-6Al-4V alloy using route C,in which the repeated rotation angle around the longitudinal billet axis before reinsertion in the die was 180°,were conducted at a deformation temperature of 750°C,a ram speed of 0.3 mm·s-1,an outer arc of curvature of 60° and a channel intersection angle of 120°.Furthermore,a large amount of recrystallization occurs and some prior α phase grains grow in the post-ECAP process of Ti-6Al-4V alloy.The yield strength of post-ECAP Ti-6Al-4V alloy increases compared with that of as-received Ti-6Al-4V alloy.
Isothermal compression of Ti-6Al-4V alloy was conducted in the deformation temperature range of 1093-1303 K, the strain rates of 0.001, 0.01, 0.1, 1.0, and 10.0 s-1, and the height reductions of 20%-60% with an interval of 10%. After compression, the effect of the processing parameters including deformation temperature, strain rate, and height reduction on the flow stress and the microstructure was investigated. The grain size of primary a phase was measured using an OLYMPUS PMG3 microscope with the quantitative metallography SISC IAS V8.0 image analysis software. A model of grain size in isothermal compression of Ti-6A1-4V alloy was developed using fuzzy neural net- work （FNN） with back-propagation （BP） learning algorithm. The maximum difference and the average difference between the predicted and the experimental grain sizes of primary a phase are 13.31% and 7.62% for the sampled data, and 16.48% and 6.97% for the non-sampled data, respectively. It can be concluded that the present model with high prediction precision can be used to predict the grain size in isothermal compression of Ti-6Al-4V alloy.
