Equal channel angular pressing (ECAP) of AA6082 : mechanical properties, texture and microstructural development
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This thesis deals with the concept of ECAP applied to a commercial Al Mg-Si alloy (AA6082.50). A detailed analysis of the strains introduced by ECAP in a single passage through the die has been made, based on direct measurements on partially pressed samples. Further, the mechanical properties of ECAP’ed samples have been investigated. The effect of initial material temper and post-ECAP artificial aging was investigated in addition to the effect of strain accumulation and processing route. Finally, a detailed study of the textural and microstructural development was made. The continuous evolution of texture and microstructure was followed through the ECAP deformation zone up to an accumulated strain of 2 (2 passes) by route A, and linked to strain measurements from the same zone. Strain measurements on sectioned samples have validated the plane strain assumption for ECAP. The shear angle has been measured and some typical features of ECAP have been corroborated, i.e. friction and material temper affect the strain distribution, the strain homogeneity and the workpiece corner angle, friction being the most significant here. Also, new conclusions have been drawn. The analysis of material element deformation histories along path lines reveals that ECAP can be interpreted as the combination of shearing and stretching (i.e. tension and/or compression). Furthermore, the final shear strain angle obtained in ECAP appears to be friction and material temper independent in the zone of homogeneous deformation. The 6082 alloy has been successfully processed by ECAP at room temperature to strains ε =6 to ε =8. The alloy has been pressed in the solutionized, T4, homogenized and soft annealed states. The highest tensile strength was obtained from the solutionized material, followed by T4, homogenized and soft annealed. This behaviour is linked to the solid solution content prior to ECAP and the potential for dynamic precipitation during ECAP processing. The tensile elongation to failure drops to a constant level between 4% and 9% after ECAP and is highest for the soft annealed and lowest for the solutionized material. The ductility in the solutionized material can, however, recover to ~18% elongation to failure (i.e. an increase by a factor 2-3) by low temperature heat treatment with only a small drop in tensile strength. Soft annealed and ECAP’ed material has been compared to cold rolling to similar strains. The tensile strength response to accumulated strain is similar, but the ductility and post uniform deformation is different. However, the ECAP’ed material can be processed to higher strains and, thus, achieving higher strength. The tensile yield strength behaviour of the homogenized and ECAP’ed material can be described by a simple relation to the grain size and the fraction high and low angle boundaries. The typical texture components related to ECAP of aluminium, pressed by route A, have been identified. The typical ECAP texture starts to develop already at ~25% strain and increases in intensity during the first pass. In the second pass, two of the stable texture components continue to increase in intensity, while the other texture components decrease. The microstructural development during the first pass is dominated by deformation banding leading to grain-subdivision. The average linear intercept distance (grain size) is reduced from ~80μm to ~4μm for high angle boundaries and from ~10μm to ~0.7μm for low angle boundaries. During the second pass, the linear intercept distance is further reduced to ~1.8μm for high angle and ~0.3μm for low angle boundaries. Deformation twins are observed during the second pass and are believed to play an important role in severe plastic deformation when the grains reach the sub-micron or nano-metre scale. The deformation banding have been explained in terms of the low energy dislocation structure (LEDS) theory, and has been shown to be an important mechanism in the early stages of grain subdivision, and is further believed to be the main source of high angle grain boundary formation by grain subdivision down to a grain size of approximately ~0 6μm, when other deformation mechanisms may be energetically more favourable.