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Weld Cracking in Directionally Solidified Inconel 738 Superalloy

Abstract The microstructure of a laser beam welded directionally solidified Inconel 738 (DS IN738) superalloy was studied. The alloy was welded along the longitudinal (the solidification direction) and the transverse (perpendicular to solidification) direction in two pre-weld heat treated conditions. The material suffered intergranular microfissuring in the heat affected zone (HAZ); however, the centerline fusion zone cracking observed in most other directionally solidified or single crystal alloys, was not observed in any of the welded samples. The extent of HAZ microfissuring was observed to be significantly smaller in samples welded along the transverse direction compared to samples welded along the longitudinal direction which can be attributed to a fewer number of grain boundaries intersecting the weld bead in the transverse direction samples. The results indicated that in contrast to most reports on weld HAZ cracking, an increase in grain size produced by directional solidification processing can cause a decrease in grain boundary liquation cracking in crack susceptible nickel base superalloys. Cracking in the present directionally solidified (DS) IN738 was further reduced by the use of a recently developed overageing pre-weld heat treatment.