Stability of Friction Stir Welds at Superplastic Forming Temperatures

Abstract

Friction stir welding is a solid state joining process which enables most metals and alloys to be welded without fusion temperatures occurring. The centre of the weld is comprised of dynamically recrystallized material which is beneficial for superplastic deformation. The high temperatures involved with conventional fusion welding techniques disrupt the delicate microstructure of superplastic materials. Superplasticity is heavily reliant on a small grained equiaxed matrix structure pinned by a fine distribution of hard second phase particles to inhibit grain growth during forming of the material. During the superplastic forming operation, the heavily strained structure present in the parent material undergoes a transformation from a banded structure comprising of very long, thin grains to fine equiaxed grains through various static and dynamic recrystallization mechanisms. During forming of friction stir welded materials the stability of the weld region has been investigated. Grain growth is more apparent in the strain hardening of AA5083 due to the relatively small amounts of strengthening precipitates. This material statically recrystallizes during the preheat stage of the superplastic forming process, the grains then begin to grow during a dynamic recovery process becoming far too large to allow superplastic deformation. AA2004 is specially designed superplastic forming (SPF) alloy which contains a large amount of Zr for grain stability. This alloying element is preferentially distributed along the grain boundaries which prevents grain growth during SPF. During the forming process the AA2004 dynamically recrystallizes; disruption to the parent material structure causes discontinuous dynamic recrystallization that results in a heterogeneous structure and makes the material prone to abnormal grain growth. The weld regions of FSWs in AA5083 and AA4004 have been shown to exhibit AGG in weld nugget and shoulder influenced regions. The stability of the superplastic material is reliant on their strengthening precipitates. The Zr in the AA2004 is a much more effective precipitate for maintaining stability.