Coupled Simulation Of Deformation-Heat Transfer-Microstructural Evolution For Forging Processes Of Titanium Alloys

With the application of recent achievements of fundamental science and technology to materials processing technology, the simulation on the metal forming processes, the microstructural evolution interacted with the deformation, and the prediction whether defects occur or not, can be carried out with the help of the computer technology, which will be beneficial to the transition of the hot working discipline from skill to science.During hot deformation, the microstructure in materials goes through a series of dynamic changes, which will influence the flow behavior of materials. The understanding of microstructural evolution during hot forming and the establishment of the flow stress model considered the effect of microstructural evolution are the basis to realize the coupled analysis of hot forming, heat transfer and microstructural evolution, which can provide scientific foundation for determining the process parameters and controlling the service quality of workpieces.The effect of process parameters, including deformation temperature, strain rate and strain, on microstructure variables, including the grain size and the volume fraction of the prior a phase, has been investigated by the quantitative metallography technology. Through integrating the advantages of the fuzzy set and the artificial neural network theory, a microstructural evolution model during hot deformation has been established according to the experimental results. Meanwhile, the prediction model is checked. The comparison of the prediction results with the experimental of the test samples shows that the prediction model is stable and reliable.Based on the characteristics of the stress-strain curves of the TC6 titanium alloy during isothermal deformation, a novel method has been put forward in order to establish the flow stress model of materials. According to the present method, a steady stress model of materials should be established first, and a function relating to microstructure variables will be introduced to revise the steady stress model. Thus it can realize to describe the flow behavior of materials during the whole stage of hot deformation. In the present model, effect of the microstructural evolution on the deformation has been taken into account. Furthermore, because the essential parameters only include the equivalent stress, the equivalent strain rate, the equivalentstrain and the deformation temperature, these models can be utilized conveniently and efficiently in practice.Integrating the flow stress model into the FE code, a coupled simulation of deformation with heat transfer and microstructural evolution has been carried out for the disk forging of the TC6 titanium alloy. The effect of process parameters, including the deformation temperature, the deformation velocity and the deformation degree, on the equivalent stress, the equivalent strain, the temperature raise and the microstructural changes during hot deformation has been studied. Finally, the difference of deformation and microstructural evolution between the isothermal forging and the hot forging has been given out.The disk forging of the TC6 titanium alloy has been carried out on the hydraulic press. After the isothermal forging of the TC6 titanium alloy, the microstructural variables, including the grain size and the volume fraction of the prior a phase, have been examined with the quantitative metallography technique. The comparison of the experimental results with the calculated results at the same deformation conditions has been investigated. The comparison results show that the maximum difference of the microstructural variables is less than 20%, the calculated results are always less than the experimental slightly, and both of them has the same distribution of microstructures in the disk. It validates that the microstructural evolution model and the flow stress model of the TC6 titanium alloy during hot deformation are reliable and stable.