Study On Technology And Structure Homogeneity Of High Purity Aluminum And Its Alloy With Large Size Processed By Equal Channel Angular Pressing

Equal channel angular pressing?ECAP?is the most potential large plastic deformation technique that is capable of producing homogeneous bulk ultra-fine metal materials.ECAP is a research hotspot in the field of fine-grained metal material preparation owing to its simple equipment,material adaptability,unchanged shapes or sizes of semifinished products,and capability of large deformation via repeated extrusions.Relevant theories of ECAP have been increasingly improved after decade of efforts.Ultra-fine materials?sectional area<10×10 mm²?with the grain sizes less than several micrometers can be prepared under laboratory conditions.However,from the industrial perspective,mold-based scale production can be hardly achieved by using the existing processes.The fundamental reason is that the ECAP process itself has a nonuniform deformation,and there is a side wall friction effect during the deformation process.When the sample size is small,the full cross section of the sample may be within the influence of the side wall effect.The real refinement of the ECAP process is concealed due to local refinement caused by the side wall effect,resulting in the original process suitable for small size specimens can not be directly applied to industrial production.The refining efficiency of ECAP can be fully utilized only by enlarging the sample,which results from the decrease in the section proportion of the side wall effect.Based on these considerations,we targeted at high-purity Al and its alloys that are commonly used in theoretical research of ECAP,and designed large size ECAP molds with size up to 180.2×180.2×22.2mm³.Metallographic microscopy,scanning electron microscopy,electron backscatter diffraction and X-ray diffraction were used to study the structure homogeneity of high-purity Al and its alloys during large size ECAP.?1?Regarding the coarse grains and insufficiency of high angle grain boundary density,we compared the structure evolution between high-purity Al single cryatal and polycrystal materials during ECAP.It was found the original high angle grain boundaries contributed to the formation of nearly equiaxed grain with large misorientations during the process of ECAP.The reason was that the dislocation movement was significantly blocked at high angle grain boundaries,and consequently,local dislocation pile-up rapidly occurred under relatively small deformation,which contributed to the simultaneous start-up of multiple slip systems.However,in the single crystal status without original high angle grain boundaries,deformation occurred along the same slip system after the start-up of initial slip system,due to the small resistance of dislocation motion.Under the same deformation path and magnitude,no efficient local high-energy zone was formed,and the simultaneous start-up of multiple slip systems was complicated,which slowed down the formation of equiaxed grains.Stable uniform structures with large misorientations can be prepared from single crystal after16 passes,but polycrystal only need 4 passes.However,the final grain sizes in polycrystal are larger than those in single crystal.The main reason is that no crystal boundaries that blocked dislocation movement existed in the single-crystal materials,and owing to the uniform orientations,deformation could uniformly accumulate throughout the whole section,which prevented the formation of local high-energy zones.Thus,uniform refining was achieved under satisfactory conditions,though the accumulation of such deformation energy required more passes of extrusions.In comparison with polycrystal,the existence of original boundaries and the different orientations of grains both contributed to deformation concentration and the formation of local high-energy zones.As a result,recrystallization occurred earlier and only locally.Thus,large areas with insufficient refining were reserved in the structures,which led to the formation of relatively large grains in the final samples.?2?We studied how the forms and distribution of trace element neodymium in the high-purity Al alloys would affect the structure evolution during ECAP.It was found the solid solubility of rare earth element Nd was very small in high-purity Al,and Nd mainly existed in the eutectic grain form with Al and Al₁₁Nd₃.After the addition of low Nd?50 ppm?,mainly spherical eutectic grains with the diameter of 3?m were formed inside the crystal grains.With the increase of dosage,the diameter of the spherical eutectic grains were enlarged,and strip-shaped eutectic grains with thickness of 0.13 or 1.25?m were separated out at the crystal boundaries.During ECAP,the addition of Nd significantly promoted the refinement of alloy grains.On the one hand,the reason is the solid-dissolution of Nd in Al reduced the stacking fault energy of Al,and on the other hand the action of spherical and strip-shaped eutectic grains on arousing nucleation and pinning boundaries during the deformation,the eutectic grains accumulated at the crystal boundaries would drag down the dislocation movement and grain boundary migration,which blocked the grain boundary migration and refined the crystal grains.?3?We studied how the ECAP deformation route would affect the structure evolution and homogeneity of plate sample with large size.It was found the deformation route largely affected the strain distribution,refining efficiency and structure homogeneity of ECAP.By route C_x,due to the mutual offset of compression strain effects between adjacent passes,the boundaries were better refined compared with the middle part.By route C_y,due to the consistency of deformation directions among different passes,the strain was gradually accumulated and intensified,so the grain refining effects on the whole sections were satisfactory.The more homogeneous structures was improved by C_y compared with C_x.After 4 passes by C_y,the wall effect was restricted within 1.5 mm,while the end effect was effective at 20.5 mm of the head and 16 mm of the end,and the homogeneous deformations accounted for 57%of total volume.The columnar grains with the original width of 4 mm were refined into uniform near-equiaxed grains with the average size of 68.6?m,and the deformed materials appear thermally stable.Severe{100}<001>oblique cube structures were formed with generally consistent orientations.The strength and hardness of the materials were both doubled,without directionality.In all,we studied how the original high angle grain boundaries,existing forms and distribution of alloy elements would affect the refinement during ECAP,expounded the effects of deformation routes on the structure homogeneity during ECAP of plate sample with large size,and clarified the range size having homogeneous structure obtained by route C_y.This study has some significant implications for the technology improvement and industrial application of ECAP.