| Severe plastic deformation (SPD) is a new method to obtain ultra-fine grained (UFG) materials. As a typical method of SPD, the equal channel angular extrusion processing (ECAE) with great potential of industrial application can refine grain effectively and now has become an attractive research field in material science and engineering.Powder metallurgy material is an important part of material family. But its mechanical characters are often affected due to pore inside. Removing inner porosity, refining microstructure and improving its mechanical characters are important aims of powder plastic deformation technology. Currently, the main object of SPD technology is full dense material, while the related study on sintered powder material is just beginning. The workability of powder material is weaker than that of full dense material, and its plastic deformation, densification and refinement mechanism are complicated. Lack of theoretical study on this kind material during deformation confines its development and application. Therefore, this paper combined finite element numerical simulation with experiment method, deeply investigated the deformation, densification behavior and grain refinement regulation of powder material during ECAE processing, and finally provided essential theoretical foundation for manufacturing the same material with SPD.During equal channel angular extrusion processing, the microstructure of metal has intimate relationship with field variables, such as strain and temperature etc. Therefore, it is important to get the flowage information of the metal and the distribution of related field variables for choosing reasonable technical parameters, optimizing die structure and realizing initiative control of the whole deformation processing. Based on the characters of porous powder material and the compressible continuous medium theories, this paper deduced the compressible rigid viscoplastic thermodynamic coupling finite element formula, built the physical property parameters relationship of porous material and full dense material, thereby solved the key problem of simulating powder material plastic deformation processing with thermodynamic coupling method from theory aspects. Thermo-coupling FE model for the simulation analysis of pure aluminum ECAE powder material processing was built and the flowage information of the material, the deformation behavior and temperature of material in ECAE were also obtained. The simulation results show that ECAE has excellent densification effect for powder material, and it can effectively remove inner pore, and the sample's density distribution is basically concord with its strain distribution, which shows the shear deformation character offered by ECAE and is beneficial for closing pores.Based on the above research, comprehensive numerical simulation was done for pure aluminum extrusion under different conditions, and the effect rule of die geometry shape and relative technical parameter to the needed press load which make metal flowage, deformation and densification was concluded. The results show the key parameter is die channel angle, and a small channel angle is good for improving the metal flowage uniformity and getting a large strain value and high density, meanwhile an excessive small channel angle would cause flowage dead zone in outer angler of the die, which is not good for deformation. The outer round angle has an effect on metal flowage which is mainly acted on bottom metal flowage, and its effect grows as the channel angle decrease. Especially when channel angle is acute angle, it will produce extinguish effect on the whole sample densification and homogeneous deformation. Based on the investigation of this paper, under workability permission of the material, die geometry should choose as small channel angle as possible, match with proper out round angle to improve metal flowage in outer angle. Finally satisfied deformation and density can be achieved. The result of simulation under different contact friction shows a certain number of contact friction is beneficial for obtaining large strain value, homogeneous longitudinal deformation and dense material. As to powder material, multiple passes extrusion was simulated through different route, and the multiple passes extrusion results were given. The results show, as extrusion passes and accumulated strain increase, the density of sample is improved; through route A, multiple passes can diminish small sheared area at the head and tail of sample, but as extrusion passes increases, the main deformation zone distribution will become complicated and inhomogeneous; compared with route A, route C under even number passes extrusion, homogeneous and symmetrical deformation zone distribution will be acquired.To get the relative flowage information at the cross section of sample, this paper built 3D Thermo-coupling FE model for square section sample. 3D FE simulation results show, under contact friction influence, strain distribution at cross section is not homogeneous, and strain value is small at the core of the sample, while near the die surface has bigger strain with comparatively weak influence on longitudinal strain. Therefore, under the condition of small friction, 2D FEM still could have high precision. Multiple passes extrusion was simulated through extrusion route B_A and B_C, and influence of route B_A and B_C on deformation were achieved. The results show, after two passes through route B_A and B_C, the sample strain distribution turns to be inhomogeneous, and shear deformation mainly concentrates on cross position of two passes shear surface. Through route B_A, as extrusion passes increase, the deformation of sample longitudinal and cross section become inhomogeneous; while through route B_C, all surfaces are under shear, as extrusion passes increase, deformation of cross section becomes even step by step. Especially when four passes extrusion is done, with an extrusion cycle period is finished, which results in a more homogeneous strain distribution.Based on FEM analysis, this paper designed the die for ECAE experiment, the specialized extrusion equipment and heating device, fabricated extrusion roughcast with vacuum sintered method, conducted extrusion experiment for pure aluminum powder sintered material under different conditions, and successfully made bulk material with full density and ultra-fine structure. Single pass extrusion experiment results show, ECAE has powerful densification effect; grain has obvious shear deformation character; grain deformation and pore distribution at different zone are accord well with FEM simulation results, thereby testifying the reliability of FEM model built in this paper.Through optical telescope, scan electronic telescope and transmission electronic telescope, observation and analysis were done for microstructure of deformation material under different deforming conditions, and evolvement of pore and microstructure during ECAE process were studied. The results show, in single ECAE process closing effect of pore depends on shear deformation character and stress state of the sample, which is the key factors to obtain a high density material under large shear plastic deformation and high hydrostatic stress state. According to accumulated deformation and different shear deformation character multiple passes rearrange the powder, change pore shape, compact powder material further. Through TEM observation of deformation sample, pointing out deformation inducement mechanism is the main mechanism to refine powder material, and the grain refinement effect depends on some key factors such as hydrostatic stress, strain value and shear deformation character etc.Finally, based on numerical simulation and experiment analysis, aiming at problems in traditional ECAE technique of powder material, this paper proposed two improved techniques: powder in tubes-equal channel angular extrusion, PITS-ECAE, equal channel angular extrusion with back pressure, BP-ECAE, and systematical theoretical analysis was done by numerical simulation experiment method on extrusion effect. The results show, improved ECAE can effectively improve deformation homogenization and consolidation effects, meanwhile reduce the possibility of material demolishment. Especially, BP-ECAE can realize severe plastic deformation for low ductile material under relative low temperature, and consequently improve its mechanical properties and microstructure effectively.
|
PDF Full Text Request