| Products fabricated by hot isostatic pressing(HIP)exhibit high density,good uniformity and excellent property.Therefore,this technology offers an advantage in manufacturing high-performance,complex-shaped alloy parts and medical implants,and especially preparing difficult-to-machine materials parts of titanium and titanium alloys.In the case of HIP,physicochemical properties and geometric shapes of powder particles greatly affect their densification behaviors and furthermore microstructure and mechanical property of sintered bulk metal parts.On this account,it is significant to predict HIP densification behavior of powder particles with the same composition and different physical properties by using the numerical simulation method.Especially,revealing HIP densification behavior of non-axisymmetric complex parts is of significance in extending engineering application of this technology.Two kinds of Ti6Al4 V powders with the same composition and different physical properties prepared by atomization and mechanical alloying respectively were selected as research objects in this dissertation.Their HIP densification behaviors were simulated under different conditions.Correspondingly,their HIP densification mechanisms were revealed by analyzing the changes in powder flow,density distribution and stress distribution during forming process.Moreover,HIP simulation of non-axisymmetric complex parts is accomplished by utilizing femoral stem as the typical part,which can provides theoretical basis for process optimization and structure design.Based on the plastic forming theory,the yield criteria of the two kinds of powder particles and corresponding cans were described by the modified Shima model and the Von Mises model.Meanwhile,the finite element algorithm of thermo-mechanical coupling is analyzed and the treatment methods of contact and friction are determined.The irregular Ti6Al4 V powders were prepared by mechanical alloying.For the two kinds of powder particles,a unified HIP process was designed;mechanical properties and thermal physical parameters used for simulation were measured by related experiments.HIP densification behavior of cylindrical compacts for the two kinds of powder particles were simulated,by considering the three factor of can size,powder properties and initial compacts density.Especially,corresponding HIP densification mechanism was clarified by analyzing powder flow trend and compacts density distribution at various typical time nodes.Furthermore,the simulated HIP densification behavior was verified by HIP forming experiment and performance tests,including density,Rockwell hardness,microstructure,and compressive mechanical properties,etc.The results show that under the same HIP conditions,the smaller compact size,higher initial relative density and powder particle kind can greatly facilitate densification behavior of powder particles.HIP densification behavior of the two kinds of powder particles follow the law that relative density decreases first and then increases,and the side corner effect always works in the can.Meanwhile,compact density distribution decreases gradually from the outside to the inside and from the two ends close to the right angle of the can to the middle.In addition,lamellar microstructure in HIPed parts contributes greatly to the plasticity improvement,while equiaxed microstructure is the main source of strength improvement.Based on the aforementioned results for the axisymmetric cylindrical parts,the femoral stem was selected to simulate HIP densification behavior of the typical non-axisymmetric complex parts.Contrastively,the relative density distribution of the two kinds of powder particles in each loading stages and the Von Mises stress distribution of the can are analyzed.Simulation results show that the stress distribution is well consistent with the relative density distribution.Especially,the similar HIP densification behavior to the cylindrical parts is validated in the typical non-axisymmetric femoral stem. |
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