Formation of Peripheral Coarse Grain in Thin-Walled Al–Mg–Si Extrusion Profiles

Abstract

AbstractInvestigations on microstructure and texture development during processing of Al–Mg–Si sheet and extrusion profiles focused on describing and modelling the changes in the bulk. However, understanding the evolution of microstructure in the sub-surface region is highly relevant, as this governs material performance under corrosion and bending deformation, which are crucial applications in mobility and transportation. The aim of this study was to correlate the effect of alloy composition and extrusion parameters to the formation of texture and peripheral coarse grain (PCG) layer. For this, five alloys of the AA6xxx class with varying content of dispersoid forming elements were extruded at increasing extrusion speeds into thin-walled hollow profiles. The microstructure at a plane section was investigated by means of EBSD and characterized in terms of distinct texture component distribution. It was shown that with increasing grain boundary mobility, through adapting dispersoid content and extrusion speed, the PCG layer grows into a fine-grained bulk. This growth of the PCG layer is only counter-acted upon, when the grain boundary mobility in the bulk is high enough, so that extensive growth of Cube oriented grains occurs. The PCG layer shows distinct orientations related to $$\langle 101\rangle ||\rm{TD}$$ ⟨ 101 ⟩ | | TD , so that a possible mechanism for the orientation selection during grain growth is proposed.