Weldability of high-strength aluminium alloy EN AW-7475-T761 sheets for aerospace applications, using refill friction stir spot welding

Abstract

Refill friction stir spot welding is a solid-state welding process for joining lightweight sheets in the overlap configuration by means of frictional heat and mechanical work. The objective is to investigate refill friction stir spot welding of aluminium alloy EN AW-7475-T761, since fusion welding of such high-strength alloys is problematic due to solidification and liquation cracking. This alloy is used in highly stressed structural parts in aerospace applications, because of its low weight, superior strength, high corrosion resistance and corrosion fatigue behaviour. The process parameter optimization and the effect of the parameters on the weld characteristics were examined. The dwell time, the plunge depth and the rotational speed were varied according to a multi-level factorial design. An increase of the dwell time resulted in a larger stir zone area and smaller amount of defects. The width of the central coarse-grained band became smaller, containing a more refined grain size. An increase of the rotational speed lead to a smaller stir zone area, a discontinuous joint line remnant and a larger width of the central coarse-grained band. An increase of the plunge depth lead to a larger stir zone area. Welds produced with a high heat input exhibited a lower average hardness and a higher hardness drop, compared to welds with a low high heat input. Analysis of variance has shown that the dwell time, the plunge depth and their interaction have a statistically significant effect on the lap shear strength: a longer dwell time, a higher plunge depth and their interaction resulted into a higher lap shear strength.