Lap shear and T peel strength of overlap joints of superplastic 8090 AI-Li sheets bonded by electron beam and laser beam welding

The overlap joint characteristics of superplastic 8090 aluminium alloy sheets made by electron beam welding (EBW) under vacuum and laser beam welding (LBW) using an argon gas protective atmosphere have been studied. The heat input applied was controlled between 6·7 and 13·3 kJ m^-1 for EBW and 36·9 and 109·1 kJ m^-1 for LBW, respectively. Grain structure and porosity dispersion were different for each joint, depending on welding method, welding parameters, and the position of the bond line interface within the fusion zone. Lap shear strengths of as welded (AW) and AW+T6 specimens were scattered within 110-180 MN m^-2 and 170-220 MN m^-2, respectively, with a joint efficiency of 70-85%. T peel strengths were 30-55 N mm^-1 for the AW samples and 20-75 N mm^-1 for the AW + T6 joints. These values are similar to those reported for diffusion bonded joints. In most cases, EBW under vacuum yielded better results in terms of fewer and smaller pores with higher depth/width ratios of the fusion zone. The dependence of shear and peel strengths as a function of welding depth, bonding width, heat input, and porosity area fraction (at the bond line interface) has been established and discussed. It appears that the porosity area fraction and spatial distribution of porosity are the controlling factors. The lower joint efficiencies of laser welded specimens is mainly caused by the much larger pores located at the central line, which are considered to be a result of the collapsed keyhole through incomplete closure during rapid solidification.