Microstructural characterization of liquid nitrogen cooled Alloy 718 fusion zone

The interdendritic Laves phase and the microsegregation have been investigated in Alloy 718 fusion zone cooled with liquid nitrogen during welding. Conventional GTA welding process was employed with modified waveform and two types of shielding gas and filler metal (solid solution and age hardenable). The weld cooling rate was enhanced using liquid nitrogen cooling during Gas Tungsten Arc welding process. The resultant fusion zone microstructures were characterized using the metallurgical tools. Dendrite remelting phenomenon was observed from the optical micrographs. It was found that the enhanced cooling rate with liquid nitrogen reduced the interdendritic phases which were confirmed in both the electron microscopic and the X-ray diffraction analysis. The elemental mapping in scanning electron microscope-energy dispersive spectral analysis also confirmed the reduced microsegregation. The dendrite arm spacing was reduced from the range of 15-54 μm (CCPHE-CCAR, conventional) to 3-17 μm (CCPHE-CCAR, liquid nitrogen cooled) for the employed process variables. The computed weld cooling rate was found to be enhanced from seven to fifteen times than the conventional welding process.