Power beam welding of thick section nickel base superalloys

Abstract

A study is presented of the power beam welding of two precipitation strengthened nickel base superalloys, Waspaloy and Udimet 720Li, which are widely used in the gas turbine industry. Processing was carried out on 4·3 mm thickness rectilinear sections using (i) an electron beam welding apparatus operating at 150 kV and 12 mA, and (ii) a Nd-YAG laser capable of delivering 3·5 kW at the workpiece. The laser beam welds yielded 'hourglass shaped' fusion profiles, due to the blooming of the laser beam; conversely, the profiles of the electron beam welds were very much more uniform. Radiography indicated considerable porosity in the laser welds, which was attributed to the entrapment of gases due to keyhole collapse. In Waspaloy, the fusion zone was about 40% softer than the baseplate following welding; in contrast, the Udimet 720Li fusion zone was about 15% harder. This was attributed to the different equilibrium fractions of the γ' phase, and the lack of any detectable in weld γ' precipitation in Waspaloy compared with very fine precipitation in Udimet 720Li. Under any given welding conditions, Udimet 720Li exhibited a higher degree of distortion, due to the greater shrinkage stresses resulting from its superior elevated temperature properties.