Numerical Simulation And Experimental Investigation On Pure Aluminum Powder Consolidate During Equal Channel Angular Pressing And Torsion

With the rapid development of material science and the further improvement of processing technique, bulk ultrafine grained materials (UFG) have attracted enormous attention because of their potential mechanical and physical properties as compared with conventional materials. Generally, the strength of polycrystalline materials is related to the grain size and grain refinement is usually an effective method for the enhancement of the mechanical properties of materials. At present, severe plastic deformation (SPD) is considered to be the most attractive and promising process in obtaining bulk UFG materials without any contamination and residual porosities. Equal Channel Pressing and Torsion (ECAPT) is a newly developed technique of SPD, which combines the advantages of both ECAP and TE. In this method, two processes of ECAP and TE occur subsequently in a single die and a twist channel has been manufactured on the horizontal part of conventional ECAP die. Therefore, it is possible to carry out multiple passes in ECAPT without changing the cross section of the sample. In addition, it leads to a larger strain accumulation of the materials which finally accounts for the grain refinement and properties enhancement.Powder metallurgy (PM) material represents an important part of material family. However, due to its weak plastic forming ability, the mechanisms of densification and grain refinement are usually complicated. In conventional powder metallurgy, powder consolidation is always accomplished through sintering at high temperatures, which tends to coarsen the special microstructures of the particles and limits the development and application of PM materials as well. Recently, SPD methods have been widely used to fabricate bulk UFG materials by the consolidation of powder particles and it has been proved that the efficiency and quality of the consolidated materials are greatly improved.Therefore, in the present work efforts were made to understand the deformation and densification behaviors, the evolution of microstructures and mechanical properties as well as the grain refinement mechanisms of ECAPT consolidation of pure Al powder particles. Many kinds of professional knowledge were employed including finite element method, metal plastic forming technique, advanced technology of plastic forming, basic material science and so on. Additionally, some useful conclusions were also obtained. All this research will play a significant role in promoting the further investigation and industrial application of PM materials fabricated by SPD methods.Based on the compressible continuous medium theories and with the full consideration of deformation and temperature fields, the compressible rigid viscoplastic thermodynamic coupling finite element formulas were established. Then the deformation behavior of pure Al porous materials with powder in tubes during a single pas of ECAPT was obtained by FEM simulation. It was found that ECAPT had a significant effect on the strain accumulation and compact densification of PM materials. Extrusion load reached to the peak when the head of sample exited from the twist channel. During ECAPT process, both accumulated strain and hydrostatic pressure of the sample achieved maximum when it passed through the intersection part and twist channel of ECAPT die. After a single pass of ECAPT, the strain imposed on each sample was about1.4and the relative density reached to0.999. However, the deformation was not homogeneous in general. On the longitudinal plane of the sample, temperature slowly decreased from the tail to the top, which indicated that under present simulation conditions, exchange and radiation of the heat were much more than the thermal energy converted from the deformation energy.For comparison, FEM simulations of ECAP and TE were also conducted on the pure Al porous materials with the same simulation parameters. The results showed that ECAPT possessed many advantages over ECAP and TE in terms of increasing strain accumulation, promoting material densification and improving the deformation homogeneity. In specific, the imposed strain was increased by17.6%and9.3times as compared to ECAP and TE respectively. This was attributed to the repetitive shearing and back pressure provided by the twist channel.In order to investigate the deformation behavior of pure Al porous materials during multiple passes of ECAPT, continuous multi-pass ECAPT dies were designed for different routes. It is found that with the increasing number of ECAPT passes, the imposed strain was increased. Due to the occurrence of work hardening behaviors, the peak load was also increased. Moreover, sample deformation became more and more homogeneous. Route A and C were two optimal ECAPT routes because the sample could accumulate large strain without the loss of deformation homogeneity. As the hydrostatic pressure was increased under multiple passes, residual porosities in the PM materials were effectively shrunk and closed, which finally contributed to the improvement of homogeneity and density of the compacts.Pure Al particles were consolidated successfully into full dense bulk UFG materials at200℃using ECAPT and further deformed up to4passes. After1pass of ECAPT, relative density and microhardness of the compacts were greatly improved. During ECAPT process, twist channel played a role of back pressure and offered a significant advantage to PM materials such as increasing the hydrostatic pressure of the compacts and self-diffusion coefficient of Al atoms. As the deformation developed, grains were further refined and mechanical properties were largely enhanced. Combined compressive yield strength of123.3MPa and good ductility were observed after4passes of ECAPT. XRD results showed that the shape and intensity of diffraction peak on{111} plane changed during different ECAPT passes, which demonstrated grains were refined as well as rotated under ECAPT. The densification process of pure Al particles mainly occurred at the first pass of ECAPT based on the severe shearing and high hydrostatic pressure provided by ECAPT.After that, Electron backscattered diffraction (EBSD) was employed to characterized the microstructure and microtexture during ECAPT process. It is found that after the1pass of ECAPT, the microstructure consisted of many elongated grains and few equiaxed grains, but most of them were low angle boundaries with the average grain size of5.20μm. With the increasing number of ECAPT passes, deformation became more homogeneous, grains were further refined and the misorientation angle was increased as well. After4passes of ECAPT, PM materials contained fine grains of1.67μm in size and equiaxed in shape with boundaries of higher misorientation angles. The formation of texture was a dynamic process, which showed a fluctuation of<101>â†'<111>â†'scattering during ECAPT. This is because the external force acting on crystal class makes the segregation orientation change, which leads to the broken of aggregation state in the process of internal stress transmitted on the neighboring grain boundaries.In the last part of this investigation, microstructures of the processed materials were characterized using TEM for revealing grains of the order of1μm or smaller in size and subgrain structure. At the beginning of ECAPT process, the microstructure consisted of bands of subgrains and high dislocation density with the formation of dislocation tangles and dislocation cells, but most of the boundaries had low angles of misorientation. With the mumber of ECAPT passes, the samples accumulated larger strain and the dislocation density increased. After4passes of ECAPT, many of the subgrain boundaries evolved into high angle boundaries and there was a concomitant evolution of the arrays of well-defined cell or subgrain bands array into reasonably equiaxed ultrafine grains. This evolution was accompanied by the process of dynamic recovery and dynamic recrystallization. Thus, it can be inferred that the mechanism of grain refinement during ECAPT at200℃was the multiple effects of intensive shearing, large accumulated strains and dynamic recrystallization. Simple shearing and large strain were predominant factors initially while dynamic recrystallization became the leading point in the subsequent ECAPT passes.