Potentiodynamic corrosion behavior and microstructural features of gas tungsten constricted arc (GTCA)-welded superalloy 718 joints

Abstract

Abstract The gas tungsten constricted arc welding (GTCAW) process was used to join thin Su-718 alloy sheets to minimize alloying segregation and Laves phase precipitation in the fusion zone (FZ). The potentiodynamic corrosion behavior of GTCAW Su-718 alloy joints was studied and correlated to the microstructural features of welds. The potentiodynamic corrosion test was done in a 3.56 wt.-% NaCl solution to determine the corrosion rate of Su-718 alloy joints. The optical microscopy (OM) technique was used to analyze the microstructure of corroded weldments. The scanning electron microscopy (SEM) technique was used to study the Laves phase development in FZ. The SEM X-ray energy dispersive spectroscopy (EDS) technique was used to for elemental mapping of FZ. The corrosion resistance of Su-718 joints is inversely proportional to the precipitation of Laves phase in FZ. The GTCA welded Su-718 alloy joints disclosed superior corrosion resistance for the joints with lower Laves phase precipitation. It is correlated to the refining of FZ microstructure, which aids in minimizing the Laves phase precipitation. The joints with higher Laves phase precipitation revealed inferior corrosion resistance. It is attributed to coarsening of FZ microstructure, which raises the alloying segregation and leads to depletion of alloying elements in FZ. The dendritic core regions showed severe corrosion compared to the interdendritic regions. The corrosion resistance of GTCA welded Su-718 joints is better than that of CC-GTAW and PC-GTGAW joints and comparable to that of EBW and LBW joints. It refers to the arc constriction and high frequency current pulsation.