Study On Texture Evolution And Mechanisms Of Cross Shear Rolled High Voltage Electronic Aluminum Foils

The present work aimed at investigating the effects of the content of trace elements on the textures and electrical properties of aluminum (Al) foil used for the high voltage anode electrolytic capacitor. The effects of the trace elements on the texture composition, content and electrical properties were examined for the effective control of the type and content of the textures to obtain the cube texture with high volume fraction. The high purity Al foils with and without pre-annealing treatment before the finish rolling were cold rolled with tension and without tension at different speed ratios. The effects of cross shear rolling parameters on the cold rolling textures and recrystallization textures of the Al foils were examined. The influencing mechanism of the process parameters on the behavior of micro-orientation flow of the Al foil during cross shear rolling was also studied for providing a basis for obtaining the cube texture with high volume fraction. The transformation relationship between the cold rolling textures and recrystallization textures in the cross shear rolled Al foils with tension during annealing were systemically analyzed and the evolution rules of the cube texture in the foils was discussed in order to provide the theory basis for the process optimization in the production of the high purity Al foils.The results showed that the trace elements of Si, Fe, Cu, Pb, B, Mg, Ti, Mn, Zn, V, Cr and Ga were detected in the Al foils and the contents of Fe, Si, Cu and Pb are higher in comparison. The control of Fe and Si contents can improve the texture and electrical property of the foils, while the contents of Cu and Pb elements have no obvious effect on them. The enrichment of the trace elements within a certain range on the foil surface has no obvious effect on the formation of the favorable textures and the electrical property, while the average content and enrichment degree show direct effect on the texture composition and the final electrical properties of the Al foils. The remarkable variation of near-surface concentration gradient of the trace elements along the depth direction leads to a change of micro-mechanism of the trace elements during pit etching of the foils, which is harmful for the stablity growth of etching pits. The varation of near-surface concentration gradient of Fe and Cu along the depth direction is another significant infuencing factor for electrical property of the Al foils.The major components of the cold rolling textures of high purity Al foils are the S texture, C texture and B texture. The relative content of the textures in the foil after cross shear rolling is different from that after asymmetrical rolling. With increasing speed ratio, the S texture and C texture, as well as the cube texture and rotating cube texture near the fast and slow roller side exhibit the same change tendency in the foils by cross shear rolling without tension. While the Goss texture keeps lower in content with increasing speed ratio. On the contrary, as the speed ratio increases, the change of content of each texture is not obvious in the foils near the fast and slow roller side by cross shear rolling with tension. The content of cube texture is not high and the content of S and C textures show an increase with the increasing speed ratio.The maximum value of the content of texture is found in the soft Al foils by cross shear rolling at the speed ratio of 1.06. After the recrystallization annealing, the content of the cube texture can also reach the maximum value. The content of texture is higher in the hard Al foil by cross shear rolling at the speed ratio of 1.06 in comparison with that in the soft Al foil by the same rolling conditions. As a consequence, the content of cube texture in the hard Al foil is still higher than that of the soft Al foil after the same annealing treatment. For this reason, the use of hard Al foil for the final cross shear rolling is beneficial to obtain the cube texture with high volume fraction which will enhance the final electrical properties of the foil.The results showed that the pre-annealing before the final cross shear rolling of the Al foils promotes the transformation from the cold rolling texture to the cube texture during the final annealing, which is the necessary process to obtain the cube texture with higher volume fraction of the Al foils. In addition, the control of tension during the final cross shear rolling at the speed ratio of 1.06 provides the favorable conditions for the nucleation and development of the cube texture during the final annealing. Annealing at properly higher temperature is beneficial to obtain the Al foils with higher volume fraction and intensity of the cube textures. The proper temperature of the final annealing for the cross shears rolling the high purity Al foil in the present study is 500℃.During the final annealing of the Al foils by cross shear rolling, the cube texture has absolute advantage in the growth than the other crystallization textures. Cube orientation develops in plane strain mode, which makes most of the dislocations slip out of the deformed crystalline during the annealing treatment. And then the dislocation density of the substructure with cube orientation is significantly below that of the substructure with the other orientations, which makes the substructure with cube orientation is easy to form the recrystallization nucleus with low defect density and grow during the annealing.