The Flowing Stress Softening And Hot Working Parameters Optimization Of Powder Metallurgy Ti-22Al-25Nb Alloy

Powder metallurgy Ti-22Al-25 Nb alloy,which is focused as a lightweight high-temperature structural material,possesses not only excellent physical mechanics properties as a traditional Ti₂AlNb-based alloy,but also avoids the defects such as shrinkage during casting and process complexity of ingot.Hot-working deformation is an effective method to improve the performance of PM material.Investigation on flow stress behavior plays an important role on determination of thermal process parameters that greatly affect mechanical properties and microstructure of the workpiece.In the present paper,the flow stress was investigated by applying a series of isothermal constant strain-rate compression tests under the deformation temperature of 975-1075 oC with 20 oC intervals and the strain rate range of 0.001-1.0s-1,so as to characterize more accurate deformation behavior of Ti-22Al-25 Nb alloy during hot compression.For the sake of studying the hot workability of Ti-22Al-25 Nb alloy in different hot processing areas,processing map technology was utilized.Combining with the analysis of microstructure evolution of the studied alloy,hot working parameters were optimized.Based on above researches,secondary development of finite element simulations?FEM?was used to make distribution and evolution of the necessary parameters,power dissipation efficiency???and instability parameter???,visualization during hot processing.Flow stress and softening behavior of PM Ti-22Al-25 Nb alloy were investigated through analyzing the simulation results of hot compression.A novel constitutive model considering dynamic recrystallization?DRX?critical model and kinetics model was established.In order to evaluate the developed models,microstructure observation and error statistical analysis were carried out.The results show that DRX is the main softening mechanism and activation energy?Q?of the studied alloy is 410.17kJ/mol.Additionally,the linear fitting coefficient?R?equals to 0.992,through contrast of calculated values and experimental values.The veracity and applicability of complete model were verified well with the average error of 2.8MPa and the average relative error of 0.9%.The workability of Ti-22Al-25 Nb alloy under various processing parameters was studied by observing the deformed microstructure and the constructed processing mapsbased on dynamic material model and Prasad instability criterion.It turns out that ?values are large in the strain rate range of 0.01-0.1s-1.The values decline slightly and then go back to the maximum as the temperature rises.The samples under the area where instability does not occur and the ? values are large possesses fine and uniform grains.By comprehensive comparison,the area of 1050-1065oC/0.01-0.1s-1is deemed to be the optimal heat processing window.According to above results,the secondary development function of ABAQUS was used to simulate the hot compression process of Ti-22Al-25 Nb alloy.The deformation of specimens was divided into three characteristic zones.The results of network change and metallograph contrast also fit the three features.The distribution of ? values correspond with the three deformation areas and region ? at the center always has the maximum.Moreover,the values of ? increase first and then decrease with the increasing strain rates.The variations of instability parameter ? distributed situation can be described simply,that is the units of ? <0 exist steadily and expand only under the parameters located in the unsafe area.