Weld metal cracking in laser beam welded single crystal nickel base superalloys

The weldability of two single crystal nickel base superalloys CMSX-4 and CMSX-486 were investigated by autogenous bead-on-plate laser beam welding. The analysis of microsegregation that occurred during solidification in the fusion zone indicated that while W and Re segregated into the gamma dendrites, gamma' forming elements Ti, Ta, and Al as well as Hf were rejected into the interdendritic liquid. Fusion zone cracking was observed in both the materials and was observed to have initiated and propagated along the high angle stray grain boundaries in the weld metal of the two alloys. Increased formation of stray grains and increased extent of fusion zone cracking were, however, observed in the CMSX-486 alloy. Extension of solidification temperature range mainly by microsegregation of grain boundary strengthening elements B, Hf, and Zr in CMSX-486 was suggested to be responsible for the increased susceptibility of the alloy to stray grain formation and weld metal cracking compared to CMSX-4 superalloy.