Improving properties of single crystal to polycrystalline cast alloy welds through heat treatment

Joining between single crystal and polycrystalline cast materials is desirable for advanced steam cooled land based gas turbine airfoil components. Electron beam weld joints between single crystal Rene N5 and polycrystalline GTD-222 have been characterized in terms of microstructure, crystallographic orientation, low cycle fatigue behavior, and chemistry across the weld centerline. The weld centerline was shown to be the vulnerable area of the joint due to carbide segregation and crystallographic mismatch. In addition, the weld fusion zone hardness was significantly higher than that of either base material. The effect of several proposed heat treatments on weld hardness was examined. All of the heat treatments significantly reduced weld hardness as compared to a standard GTD-222 solution treatment and age cycle. Scanning electron microscopy analysis revealed the hardness drop was due to coarsening of gamma prime in the weld fusion zone.