Investigation of the effects of secondary rolling process and heat treatment on mechanical behavior and microstructure of pure aluminum and aluminum alloy sheets, after severe plastic deformation caused by constrained groove pressing
Abstract:
Constrained Groove Pressing (CGP) is one of the best processes of severe plastic deformation with a strengthening approach in metal sheets in terms of the high amount of strain it creates in and also because of the ease and cost of the process. However, due to the nature of the process, which is mainly caused by the effect of the sharp corners of the corrugated die, this process has limitations that have challenged its industrial application. Stress concentration, non-uniformity of material properties and residual stresses created in the sheet are among the inherent defects of this process. In order to improve mechanical properties and reduce stress concentration, methods have been proposed by researchers. In this research, with the aim of reducing the residual stress, the effect of applying secondary strain by the rolling process after the CGP of the sheet has been studied. With the aim of studying the best combination of tests to reduce the residual stress, the rolling process was repeated up to three stages. Also, in order to investigate the effect of the secondary strain direction, reverse rolling was also performed and its results were evaluated. With the aim of investigating the effect of alloy elements on the amount of residual stress, three types of aluminum sheets were used in the experiments; Pure aluminum sheet, 1020 aluminum sheet with low percentage of alloy elements and 6061 sheet with higher percentage of alloy elements. Finite element model of sheet deformation in CGP process and four modes for rolling process was developed. In each simulation step, the mechanical properties of the sheet are changed due to work hardening and residual stress. To study the effect of the next process, the deformed sheet should be transferred to the next step with the final shape and the resulting properties. X-ray diffraction analysis (XRD) has been used to measure residual stress in practice. Using this method, the residual stress can be measured at points close to the surface of the sheet, usually at a depth of several microns. Residual stress causes small changes in the crystal lattice spacing of a material that can be detected by very high sensitivity XRD. The results of the finite element model showed that the use of direct rolling process reduces the amount of residual stress. Repeating the process up to two passes caused a further reduction of the residual stress, but with the implementation of the third rolling step, the amount of residual stress increased. However, the use of reverse rolling after the first rolling pass caused a greater reduction in residual stress compared to two times of direct rolling. The results of XRD analysis also prooved the trend of changing the residual stress similar to the results of the finite elements, although the numbers obtained for the residual stress are different.