Evolution of surface recrystallization during indirect extrusion of 6xxx aluminum alloys

The fundamentals of coarse grain surface recrystallized structure formation in extrusion of 6xxx aluminum alloys are not yet completely understood. The objective of this article is to understand the metallurgical origins and mechanisms of the formation of the peripheral coarse grain (PCG) structure as the first step to understanding surface behavior of extruded aluminum alloys. Small-scale indirect extrusion tests were performed in which deformation parameters of strain, strain rate, and temperature were closely controlled. The deformed material was characterized via traditional metallography and orientation imaging microscopy (OIM) in order to understand the influence of processing conditions and alloy chemistry on surface grain formation. It was found that decreasing recrystallization-inhibiting elements such as Cr as well as increasing the starting extrusion temperature, extrusion ratio, and ram speed all increased the depth of the PCG. Additionally, a mechanism for favorable coarse grain formation at the surface of the extrudate is proposed based on microstructure development during extrusion.