Effect of extrusion press exit temperature and chromium on grain structure of EN AW 6082 alloy forgings

Extruded bar is the typical forging stock in the manufacture of EN AW 6082 forged suspension components. The frictional heating and high strains introduced during extrusion leads to fine equiaxed surface grains that inevitably grow during forging owing to a very small initial grain size and a strain-free state. Higher extrusion temperatures as well as Cr addition have a favourable impact on the depth of the coarse grain zone in 6082 forgings. The interior fibrous grains may recrystallise to produce small equiaxed grains during forging and suffer abnormal growth during the solution heat treatment. Sections with entirely coarse grains appear to be the standard after the T6 heat treatment when the 6082 alloy is Cr-free regardless of the press exit temperature of the extrusion step in the production cycle. However, coarse grains are confined to the surface regions in forgings produced from 6082 alloy with as much as 0.13 wt-% Cr regardless of the press exit temperature. High extrusion temperatures reduce the strain energy while Cr forms dispersoids that retard recrystallisation and grain growth during subsequent processing.