Numerical Simulation And Experiment Of High-pressure Torsion

Severe plastic deformation (SPD) is a new method of plastic working to obtain ultra-fine grained (UFG) materials. As a typical representative of severe plastic deformation method, high-pressure torsion (HPT) could significantly refine the grain and improve the mechanical properties of materials, it has become an attractive research field of materials science and engineering.In the process of HPT, the microstructure of the specimen has intimate relationship with field parameters, such as strain and temperature etc. Therefore, it is important to get the flowage information of the metal and the distribution of relative field parameters, that we can choose reasonable technical parameter, optimize die structure and realize initiative control the whole deformation processing. Based on the characters of nonrestrictive HPT, this paper used three-dimensional modeling software UG to establish three-dimensional geometric models and imported the models into DEFORM-3D which is a 3D finite element simulation software, then established a process finite element model to analyze the HPT process. Through analyzing the HPT process, the information of material flowage, deformation behavior and temperature distribution in the HPT process was obtained. The simulation result shows that HPT can severely deform the material. Temperature distribution characteristics and equivalent strain distribution characteristics in the specimen are consistent. Because of the existence of large relative sliding between the specimen and die, the actual torsion angle of specimen is distinctly different from the torsion angle of the bottom die.Based on the research, comprehensive numerical simulation of HPT was done under different conditions. Strain, relative torsion angle and torque was obtained and analyzed. The simulation result shows that the friction factor affects the equivalent strain distribution along radius of specimen. When the friction factor is 1, the equivalent strain along radius of specimen shows the distribution of "M"-type; equivalent strain and relative torsion angle increase significantly with the increasing of friction factor, torsion turns, pressure and temperature; aspect ratio has little effect on the relative torsion angle while deformation uniformity along axial direction of specimen is improved if aspect ratio is decreased.Based on FEM analysis, this paper designed die for experiment. 20CrMnTi specimen which had been annealed was processed by HPT utilizing 2000KN hydraulic press in laboratory. Microstructure and microhardness of specimen which were processed in different press and turns were researched. The experimental result shows that microstructure was severely refined in the place which is 5mm from the center along radius of specimen, while microhardness reach to peak value in the same place and shows the distribution of "M"-type too. The distribution of microhardness and equivalent strain has the same trend, so the FEM model is credible in the paper. As pressure and torsion turns are increased, the microstructure of meridian plane and end face turn to fine gained, pearlite turns to acerate or striate in the basal body of ferrite and microhardness is elevated evidently; Both low pressure many torsion turns and high pressure few torsion turns can obtain the similar microstructure after HPT. It shows that both increasing pressure and increasing torsion turns are very important for increasing the deformation of specimen and refining gain.Presently HPT was used in making ultra-fine grained and it was also expected be used in aerospace and medicine. With pure aluminum powder as the research object, this paper made use of HPT for making cone-shaped part prepared by pure Al powder sintered material. FEM simulation shows that HPT can effectively improve relative density of cone-shaped part; Based on simulation,cone-shaped part prepared by pure Al powder sintered material was made by experiment. metallographic observation, microhardness measurement and relative density measurement were carried on subsequently. Simulation and experiment show that the relative density of cone-shaped part reachs more than 0.98. The deformation of top part in cone-shaped part is different from that of edge in cone-shaped part. In top part, the deformation of top part is not severe, the relative density is improved somewhat after HPT, microhardness reach only to 41.6kg/mm2 because work hardening is inapparent. In edge, the deformation is severe, so microstructure has strong directional, the size inner pore is about 2μm, relative density is improved markedly, and microhardness reach to 61.12kg/mm2 because work hardening is distinct.