Recrystallization and Texture Development in Hot Rolled 1050 Aluminum

The evolution of texture as a function of recrystallization has been characterized for hotrolled AA1050. Samples prepared from hot rolled sheet were annealed isothermally for sufficient time to allow complete recrystallization. The microstructural variation and texture evolution in the samples was observed by automatic indexing of Electron Back Scatter Diffraction (EBSD) patterns in a Scanning Electron Microscope (SEM). The spatial orientation variation within the deformed microstructure of nucleation, growth and orientations of recrystallized grains was examined. The orientation spread within grains was found to be a useful quantity for partitioning recrystallized and unrecrystallized regions. Also the effect of deformation texture on the evolution and growth of various recrystallization texture components was analyzed. The analysis is aimed at obtaining a correlation between the deformation microstructure, texture development and recrystallization kinetics in the hot-rolled condition. Preliminary results suggest only a weak correlation between the rate of recrystallization and the deformation texture component. Introduction The microstructural and texture evolution during hot rolling of commercial purity aluminum alloy is an important influence on the final properties of cold rolled sheets and finished products. During recent years, a significant number of studies have been carried out to study recrystallization kinetics and texture evolution during hot rolling [1, 2]. The understanding of recrystallization kinetics and texture evolution during hot rolling can be used for reduction of annealing times of hot rolled sheets, leading to energy and economic savings. The advent of Orientation Imaging Microscopy has led to important insights into the characteristics of deformed and recrystallized microstructures. An important property of deformed and recrystallized grains is the spread in average grain misorientation. The present analysis uses this property of microstructure in identifying deformed and recrystallized regions in partially recrystallized ssamples. An important consequence of this partitioning step is individual analysis of deformed and recrystallized regions for microstructure and texture evolution. The grain identification also helps in obtaining the information regarding the surroundings of a growing nucleus and the texture components into which it grows. The main texture components in the hot rolled material are S and Brass component, and the Cube component dominates recrystallized state. The main aim of the present study is to apply the concept of grain orientation spread and analyze the kinetics of recrystallization and texture evolution in hot rolled sheet of commercial purity aluminum alloy. The variation in different texture components in deformed and recrystallized regions is studied and the effects that various deformation components have on the growth of cube recrystallized grains is analyzed. The study is aimed at obtaining correlations between the deformed and recrystallized grains during different stages of recrystallization. 2 Title of Publication (to be inserted by the publisher) Sample Preparation Microstructural characterization and texture evolution was carried out on samples obtained from a hot rolled sheet of aluminum alloy 1050. The chemical composition of hot rolled sheet is given in Table 1. The as received sheet was hot rolled at 325C and allowed to cool in air. As a consequence of slow cooling at room temperatures, the material showed a significant amount of recovery in asdeformed structure. Samples of size mm mm mm 6 10 20 × × were obtained from the hot rolled sheet and were annealed at following temperature of 325 C, 350 C, 375 C and 400 C in a salt bath for time intervals ranging from 30s to 3600s for complete recrystallization of deformed samples. The annealed samples were ground and electropolished with perchloric acid solution for characterization in a Philips FEI XL40 Field Emission Gun scanning electron microscope using TSLTM EBSD software. A typical scan area was 800μm×800μm was selected for a scan and on an average; three scans were obtained from each sample surface. A step size of 1μm was used for deformed samples and 2μm for partially recrystallized and fully recrystallized samples. Table 1: Chemical composition for AA1050 (mass % element) Element Si Fe Cu Mn Mg Zn Ti Al Mass (%) 0.08 0.31 0.003 0.036 0.004 0.009 0.008 99.54 Recrystallization Kinetics The kinetics of recrystallization during annealing was obtained by partitioning the regions in Orientation Imaging Microscopy (OIM) maps between deformed and recrystallized regions. Several approaches have adopted in the past for this purpose [3, 4]. The basis for all approaches has been the different characteristics of deformed and recrystallized region leading to a variation in the parameters obtained during the scans. The two main methods of analysis in this regard have been image quality & confidence index variation and the variations in the orientations of individual pixels. An advantage offered by the method based on individual orientations is that a fixed value can be used for deformed and recrystallized regions and this value can be used for all scans of the material. However, the image quality and confidence depend on the scan settings and cut-off values need to be determined for individual scans. The partitioning of deformed and recrystallized regions in present analysis is based on orientation spreads of individual grains. A Grain Orientation Spread (GOS) parameter, defined as the average misorientation between all pixels within a grain is used for differentiating between the two types of regions. A high value of orientation spread indicates a high geometrically necessary dislocation (GND) content and more deformation in the sample whereas the recrystallized samples are characterized by low dislocation content and a corresponding lower value of orientation spread as measured by GOS. The critical GOS value in the present analysis is obtained from the variations in orientation spreads for completely recrystallized samples. A threshold GOS value for distinguishing a recrystallized grain from an unrecrystallized one was chosen such that only 5% of the points in these fully recrystallized samples are counted as unrecrystallized. Based on this criterion, a threshold value of 3° was chosen for the present analysis. Grains with GOS>3° are considered unrecrystallized whereas grains with GOS <3° are deemed recrystallized. Another important parameter in the study of deformed and recrystallized samples with GOS criteria is the step size used in the scan. Although a larger step size can be used for fully recrystallized samples, a lower value had to be used for deformed samples to include more points in the recrystallized nuclei in deformed and partially recrystallized samples [3]. A step size of 2μm was used for partially recrystallized and fully recrystallized samples while a value of 1μm was used for deformed samples. Figure 1 show a scan partitioned between deformed and recrystallized regions for a partially recrystallized sample with the corresponding Orientation Distribution Function (ODF) maps. Journal Title and Volume Number (to be inserted by the publisher) 3 The important parameters affecting the partitioning of deformed and recrystallized regions and eventually for selection of a nucleus among all the subgrains present are the minimum grain size used for defining a recrystallized grain and the presence of a high angle boundary surrounding a nucleus. The presence of a 15 boundary misorientation and a grain size minimum of 5μm are used as secondary parameters for determining a potential nucleus. The kinetics of recrystallization as obtained from GOS analysis compares well with those obtained from microhardness variation as discussed previously [5]. The fraction recrystallized is obtained from the number fraction of pixels in grains that have a GOS value less than 3°.