Friction Stir Welding of Aluminium-Silicon-Coated Steel

Friction Stir Welding (FSW) is already used industrially for many so-called light-alloys in particular. For other alloys, including steels, the use of FSW is not so straightforward, mainly linked to the fact that their high strength and softening temperature lead to excessive tool wear. Although FSW is therefore not yet a serious contender to replace traditional welding processes for steels, there may be specific applications where it can be of benefit. One such example is the use of FSW for coated steels. For example, enrichment of the steel weld metal with aluminium from an aluminium-based coating may lead to the formation of brittle intermetallics, or to excessive grain growth. The fact that FSW is a solid-state process may help to overcome such issues. In the work reported here, FSW was investigated for welding of aluminium-silicon-coated steel. It included welding procedure development on sheets in the thickness range of 1.5 to 2 mm, and extensive joint evaluation through cross-sectioning, metallography, SEM and EPMA evaluation, as well as mechanical testing after heat treatment appropriate to the boron-alloyed quenching steel. It showed that although welds could be made, Al-Si from the coating tended to remain macroscopically present, rather than being finely dispersed in the weld, leading to weakening of the joint.