Research On The Evolution Of Microstructures And Recrystallization Behavior Of High Purity Aluminum Foils During Annealing Processing

High purity aluminum foils is the basis material used in the manufacture of high voltage electrolytic capacitor anode,the specific capacitance of high purity aluminum foils after corrosion depends on the microstructure and cube texture content.At a given condition of chemical composition and rolling deformation process,the annealing process plays a decisive role on microstructures and textures.The evolution law and mechanism of microstructure and texture on the high purity aluminum foils during the annealing process are still quite unclear.Therefore,this article attempts to investigate microstructure,texture and recrystallization behavior of high purity aluminum foils during the annealing process,and explore the microstructures,textures evolutions regularity and mechanism,which provides theoretical support and technical guidance for the fine controlling of microstructures in high purity aluminum foils during annealing.A systematic research was carried out on the microstructure character,dislocation configuration and texture evolution of high purity aluminum foils under different conditions of inter-annealing temperature,annealing heating rate and annealing hold time,and it also revealed the formation mechanism of recrystallization cube texture.The microstructure characterization techniques mainly include polarized optical microscope(POM),field emission scanning electron microscopy(FEI-SEM)equipped with the electron back scattering diffraction(EBSD)system,transmission electron microscope(TEM),and combined with X-ray diffraction(XRD).The main conclusions of this paper are presented as follows :(1)The effect of different annealing process parameters on recrystallization and dynamics was analyzed and discussed.Its kinetics found that compared with the inter-annealing temperature,the finished product annealing heating rate has a greater influence on the recrystallization of high purity aluminum foils.An increase in heating rate can lead to the rapid growth of recrystallization grains with annealing at 500?,which also generates the coarse-grain structures;Based on theories of recrystallization,grain boundary migration and JMAK transformation kinetics,it can be inferred that the cubic-orientation grains have priority with the nucleation and growth during annealing process.Besides,the volume fraction of recrystallization decreases and the grain size becomes smaller with an increasing heating rate.(2)The evolution law of microstructure and texture on high purity aluminum foils during finished product annealing with different heating rates was studied and the critical heating rate was determined to be about 50?/min.It can be inferred from calculation that the stored energy of 30 s fast annealing was 2.2 times as large as that of slow annealing(3?/min)at 320?,which provides the driving force for grain growth during subsequent heating and results in a significant coarsening of grains through fast annealing.In contrast,the intensity of cubic texture after slow annealing is significantly higher than that after fast annealing.A homogeneous microstructure and strong cubic texture can be obtained with the critical heating rate of 50?/min.(3)Based on the model of grain growth in thin film materials,the recrystallization grain growth mechanism of high purity aluminum foils during annealing was explored.The studies showed that Cube grains will grow up preferentially due to the orientation relationship of 40°<111> with S grains.The grain growth is mainly controlled by surface energy when grain size exceeds the thickness of aluminum foil.It can be verified that the criterion of abnormal growth in aluminum foils is ??sur>0.02?b.(4)The grain growth exponent,activation energy of recrystallization and rate constant in high purity aluminum foils were determined through calculation to be n=4,Q=129kJ/mol and K=1.28×10-8m4s-1,respectively.The grain growth rate is V=2.43?m/s when annealed at 500?,which suggested that the grain has a rapid growth rate at this temperature.Thus,it can effectively explain the reason for the abnormal grain growth of aluminum foils during fast annealing.(5)The effect of annealing process on dislocation configuration of high purity aluminum foils was clarified.The statistical results of pitting corrosion density in aluminum foils showed that the dislocations with moderate density distribute uniformly in the aluminum foil through inter-annealing and slow-annealing.(6)The micro-area grain orientation and the variation of cubic texture content in aluminum foils with different annealing processes were studied systematically,and the nucleation mechanism and evolution laws of cube orientation grains were also discussed.The results showed that the lower-temperature inter annealing can provide cubeorientation nucleation sites,leading to the enhancement of the cube texture in aluminum foils after final annealing.In the course of texture evolution,Cu-{112}<111> texture and S-{123}<634> texture from cold rolling contribute to the formation of Cube-{001}<100> texture in aluminum foils after annealing,which depends on the decomposition of Cu and S components and their transformation into Cube components.The nucleation and rapid growth of {001}<100> orientated grain are mainly attributed to its 40°<111> orientation relationship to {123}<634> texture.