A Study on Microstructure and Phase Transformation in the Weld Fusion Zone of TIG-Welded IN939 with IN625 and IN718 as Filler Metal

Microstructure and phase transformations were investigated in the weld fusion zone (WFZ) of tungsten inert gas (TIG)-welded IN939 nickel-based superalloy with IN718 and IN625 alloys used as filler metal. The optical and field-emission scanning electron microscopy (FESEM) and also energy dispersive spectroscopy (EDS) were utilized for the microstructural study of the WFZ. The microstructural examination showed that the Nb and Ti elements were microsegregated in the inter-dendrite region, causing a significant concentration gradient between the core and the inter-dendrite region within the WFZ. Observations indicated the formation of the (Nb, Ti) C carbide phases in the solidification process. With a thickness of about 0.2 μm, some plate-like δ phase was observed in the WFZ of both samples upon heat treatment at 750 °C. The results of X-ray diffraction analysis (XRD) confirmed the presence of the δ phase in the WFZ. However, the δ phase disappeared at temperatures beyond 1000 °C. The occurrence of phase transformation during the solidification process was identified by differential thermal analysis (DTA). The WFZ was seen to solidify in the temperature range of 1370°C to 1270°C.