Superalloys are used in the strategic sectors of aerospace, defence and nuclear in view of their high specific strength at elevated temperatures, good weldability and excellent oxidation resistance. Inconel 600 is a nickel-chromium alloy with good oxidation resistance at higher temperatures and resistance in carburizing and chloride containing environments. Tungsten inert gas (TIG) welding is the most versatile fabrication process used due to its efficiency and high level of process control. However, the depth of penetration obtained in the weld joint is quite low making multi-pass welding inevitable for thicksection welding. To improve depth of penetration of TIG welded joints, flux bounded tungsten inert gas (FBTIG) welding was developed, that uses thin layer of activating flux on top of the surface of the workpiece, by maintaining gap along the weld line. The present work focuses on the effect of nature of flux and flux gap on the weld bead geometry and mechanical properties of FBTIG welded Inconel 600 alloy. Optimization of weld and flux parameters was carried out through bead-onplate welding and butt weld joints of Inconel 600 was fabricated using the optimized parameters. Radiographically qualified aerospace quality welds were fabricated and depth of penetration has increased three times for FBTIG welds coated with SiO 2 flux using flux gap of 2.5 mm. The room-temperature tensile strength of FBTIG weldments is comparable to that of TIG welds and it was concluded that single-pass FBTIG welding can replace multi-pass TIG welding, to join thicker sections of Inconel 600.
Inconel 718 is widely used superalloy in the Indian space program for high temperature application. Some of the newer applications envisage use of this alloy in very critical high pressure oxygen carrying vessels. The alloy is frequently used in welded condition which requires extensive characterization of various types of welds viz Electron beam welding (EBW) and Gas tungsten arc welding (GTAW). In many cases the weldability of Inconel 718 is found to be limited by microfissuring phenomenon in the weld heat affected zone. Microfissures are fine intergranular cracks and their severity strongly depends on pre-weld solution treatment temperature (grain size), weld heat input and concentration of impurities (B, P and S) in the base metal. In the present work, a study was undertaken to compare the microfissuring tendency in EBW and GTAW processes using two pre-weld solution treatment temperatures. The samples were solution treated at 970°C and 1050°C to generate different grain sizes. Amount of heat input and cooling rate were calculated since they are known to affect the microfissuring and an effort was made to understand their role on the microfissuring. It was observed that microfissuring susceptibility is more at coarser grain size. Severity is more in EBW. The reasons for this phenomenon have been discussed in this paper correlating microfissuring with microstructures and other factors. Procedures to achieve minimal microfissuring during welding have also been brought out.
Electron Beam Welding (EBW) was performed on the highly corrosion resistance superalloy 686. The present research work investigates the metallurgical and mechanical properties of the weld joint fabricated by Electron Beam Welding technique, and the results are compared with the base metal. Optical and Scanning Electron Microscope (SEM) analysis were carried out to study the structural properties of the weld joint. The fine equiaxed dendritic structure was revealed in the Center Fusion Zone (CFZ). The columnar dendrite was noticed in the Transition Fusion Zone (TFZ). Energy-dispersive X-ray spectroscopy (EDS) analysis results show that segregation of Mo and W were noticed in the sub-grain boundary. X-ray diffraction analysis (XRD) confirmed the presence of Mo and W rich phases in the weldment. Tensile testing was carried out to evaluate the strength and ductility of the weld joint. The result revealed that the weld strength was equal to the base metal strength. The presence of Mo and W-rich intermetallic phase reduced the ductility and toughness of the weld joint compared to base metal. Bend test confirmed the defect-free weld joint that was achieved in the Electron Beam Welding technique. The corrosion rate of base metal and EBW weldment are calculated in the synthetic seawater environment with the help of Potentiodynamic polarization experiment, and corrosion rate is measured with Tafel's interpolation technique. The corrosion test result shows that the resistance of EBW weldment is lesser than base metal corrosion resistance because of the microsegregation of alloying elements in the interdendritic region.
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