Methods to inhibit localised recrystallisation in AA7020 alloy extrusions

摘要

Recrystallisation is a dynamic or static process driven by the stored energy of deformation. During this process, deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been ' entirely consumed. Low-strength aluminium alloys such as AA6060 and AA6063 in the AA6xxx series readily undergo recrystallisation and thus fully recrystallised grain structures are often observed in these alloys after hot extrusion. However, medium- and high-strength aluminium ' alloys such as 6082, 6061 and those in the AA2xxx and AA7xxx series often exhibit only partially recrystallised grain structures after hot extrusion. Localised recrystallisation and grain growth result in coarse grains at or near the periphery of the extrudate - a structure that is commonly termed as peripheral coarse grain (PCG) structure. It is a well-known defect occurring to extruded medium- and high-strength aluminium alloys. The defect is detected during product quality control at extrusion plants, meaning losses in material recovery and press time, or during failure analysis at customer's sites, leading to customer dissatisfaction and even liability issues, as the defect may seriously degrade extruded products in mechanical properties. The occurrence of localised recrystallisation is associated with a combination of factors including alloy chemistry, microstructure prior to extrusion, extrusion condition, die geometry and die surface condition in a complex manner. In this communication, the effects of alloying elements and homog-enisation practice on recrystallisation and grain growth after extrusion are discussed with the AA7020 alloy taken as an example of medium-strength aluminium alloys. Methods that may be used to inhibit recrystallisation are put forward.