Characterisation of electron beam welding of superplastic 8090 AI–Li alloys

Abstract

Systematic characterisation of the behaviour of electron beam welded thin superplastic 8090 Al sheet has been carried out. The beam voltage, current, and travel speed were varied within 52·5–60·0 kV, 4·5–6·0 mA, and 50–80 mm S^−l respectively. The relationships of fusion zone depth, width, depth/width ratio, cross-sectional area, and postweld tensile properties versus voltage, current, velocity, and heat input were established. It was found that the beam current exerted the most pronounced effect. Based on microhardness values, electron probe microanalysis, and transmission electron microscopy, the fusion zone was harder in the as welded condition but weaker in the as welded + T6 tempered condition due to different distributions of δ′, S′, T₂, T₁, and S phases. The porosity wasfound to increase with heat input, most likely as a result of the hydrogen enriched surface layer. The contribution from Li and Mg evaporation during welding is considered to be less important. Cracking was observed in superplastic 8090 under high heat input conditions, but not in 5083 (Al–Mg) or binary Al–Li alloys, which can be explained in terms of insufficient backfill effect, high thermal stress, and coarse grain boundary particles. The optimum postweld mechanical strength for full penetration welds enabled a joint efficiency of >85% to be achieved, thus the combination of electron beam welding and superplasticforming is thought to be a promising technique.

MST/1973