Temperature and speed effects in hot extrusion of aluminium alloys

Abstract

Extrusion is shown to be a thermally activated process in which mass transfer is important. Speed effects may be recognized in the mean equivalent strain rate and then combined with temperature effects in a temperature-compensated strain-rate term Z which leads to an equation of state for the extrusion process. This equation of state includes the high-temperature flow stress, and hence it is concluded that all effects in the extrusion process are temperature and speed controlled. It is shown that the mechanism of deformation is either dynamic recovery or dynamic recrystallization, according to whether the alloy is of high or low stacking-fault energy. This leads to extrudate structures which either contain subgrains, ideally with no static recrystallization occurring, or grains formed by successive dynamic, metadynamic, and static recrystallization. It is shown that either type of structure is related to, and can be controlled by, the prevailing Zener-Hollomon parameter Z. There is thus also a relationship between properties and Z. Some evidence is presented to show that the temperature and speed conditions may also be important to any subsequent heat treatment. It is further shown that surface quality and breakthrough pressure can be predicted if the Z parameter is accurately defined. It is concluded that control for structure and properties is possible and should be common practice.