Precipitates in coarse-grained heat-affected zone of Ni-based 718 superalloy produced by tungsten inert gas welding

Abstract

Wrought, as-cast, and homogenized sheets were welded using the tungsten inert gas (TIG) process. The precipitates in both the base metal and the coarse-grained heat-affected zone (CGHAZ) for each joint were analyzed by optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Statistical data regarding the precipitates were also obtained from metallographs. With the alloy element contents determined by EDS and the phase diagram obtained using ThermoCalc, the diffusion of Nb and the dissolution of carbides and the Laves phase were analyzed. The wrought base metal contained massive M23C6 particles and MC blocks. As the heat input increased, the amount of carbides in the CGHAZ decreased. No Laves phase was discovered in the base metal or CGHAZ. In the as-cast or homogenized base metal, MC blocks and the Laves phase were distributed in the interdendritic segregated region. As the heat input increased, the MC and Laves content did not change significantly. The prewelding heat treatment in the homogenization process reduced the amount of precipitates in the base metal. The CGHAZ of the joint of the homogenized metal lost more dendritic structure than did the CGHAZ of the joint of the as-cast metal.