WELDING OF NICKEL-BASE SUPERALLOY SINGLE-CRYSTALS

The welding and weldability characteristics of single crystal nickel b ase superalloy PWA 1480 were investigated over a range of welding cond itions and orientations using electron beam and laser welding processe s. Using differential thermal analysis, the freezing range for this al loy was established and the formation of the gamma/gamma' eutectic dur ing the last stages of solidification was identified. Both electron be am and laser welds showed extensive fusion zone cracking, and the crac ks were identified as solidification clacks, i.e. hot cracks. Clack fr ee welds could be made over a very narrow range of welding conditions with preheat. Most of the microstructural features observed in the wel d are similar to those previously observed in Fe-Ni-Cr alloy single cr ystal welds. The welds contain misoriented stray grains which play a c ritical role in the promotion of hot cracks in the welds. The origin o f these grains is explained in terms of constitutional supercooling an d the growth conditions. Detailed atom probe field ion microscopy anal ysis was carried out to determine the gamma/gamma' compositions and th e segregation at the gamma/gamma' interface. The results revealed no s olutal segregation at this interface, and the composition of gamma/gam ma' was close to that of the equilibrium composition. (C) 1997 The Ins titute of Materials.