Effect of enhanced cooling on microstructure evolution of alloy 718 using the gas tungsten arc welding process

The microsegregation in alloy 718 fusion zone during gas tungsten arc welding affected the aging response and the mechanical properties. The average weld cooling rate was enhanced in gas tungsten arc (GTA) welding process by employing liquid nitrogen during the process. The combination of in-process and external cooling methods reduced the microsegregation of Nb in alloy 718 fusion zone. Detailed characterization of the weld microstructures in the as-welded and the direct aged conditions was conducted. The dendritic structure have been refined to a level of 10-20 μm dendrite arm spacing and Laves particle sizes ranging from 0.04 to 1.43 μm due to the enhanced weld cooling rate. Similarly, the average Laves volume fraction was reduced from 45 to 0.1 % when compared to the conventionally cooled GTA welding process. The microsegregation was quantified through the computed solidification time through the DSC analysis and the volume fraction of the eutectic phases. It was found that the solidification time was reduced significantly from 2.499 s to as low as 0.36 s with the liquid nitrogen-cooled GTA welding process. The microhardness survey revealed an improved aging response of the fusion zone. [Figure not available: see fulltext.]