Laser beam weld microstructures in directionally solidified alloy IC 6

The microstructures of fusion zone (FZ) and heat affected zone (HAZ) of laser beam welded directionally solidified alloy IC 6 were studied. Electron diffraction and energy dispersive X-ray microanalysis by scanning transmission electron microscopy of secondary phases in thin foils prepared from the FZ showed that interdendritic eutectic-type microconstituents consisted of γ, γ′ and NiMo phases (Y). Extensive FZ cracking was observed to be closely associated with these secondary solidification products and their formation has been related to the modification of primary solidification path due to a reduced solute microsegregation during rapid laser weld cooling. In addition, HAZ microfissuring was observed in regions surrounding the FZ and was found to be associated with grain boundary liquation. Careful microstructural examination showed that intergranular liquation resulted from contribution from interfacial liquation of M6C-type carbide, (Mo2Ni)B2-type boride particles and eutectic-type transformation reaction of the major phase of the alloy, γ′ precipitate particles.