CHARACTERIZATION OF ELECTRON-BEAM WELDING OF SUPERPLASTIC-8090 AL-LI ALLOYS

Systematic characterisation of the behaviour of electron beam welded t hin 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 mms(-1) respectively. The relationships of fusion zone dept h, width, depth/width ratio cross-sectional ar ea, and postweld tensil e properties versus voltage, current, velocity, and heat input were es tablished. It was found that the beam current exerted the most pronoun ced effect. Based on michrohardness values, election probe microanalys is, and transmission electron microscopy, the fusion zone was harder i n the as welded condition but weaker in the as welded + T6 tempered co ndition due to different distributions of delta', S', T-2, T-1, and S phases. The porosity was found to increase with heat input, most likel y as a result of the hydrogen enriched surface layer. The contribution from Li and Mg evaporation during welding is considered to be less im portant. Cracking was observed in superplastic 8090 under high heat in put conditions, but not in 5083 (Al-Mg) or binary Al-Li alloys, which can be explained in terms of insufficient backfill effect, high therma l stress, and coarse grain boundary particles. The optimum postweld me chanical strength for full penetration welds enabled a joint efficienc y of > 85% to be achieved, thus the combination of electron beam weldi ng and superplastic forming is thought to be a promising technique.