Optimization of GTAW parameters for repair welding of aged FSX-414 cobalt-based superalloy: Microstructure and hardness characterization

This study investigated the repair welding of the FSX-414 cobalt-based superalloy using gas tungsten arc welding (GTAW) with FSX-414 filler material. The objective was to optimize GTAW parameters and assess the influence of the filler material on weld quality. To simulate realistic repair conditions, a used FSX-414 nozzle with 66,000 hours of high-temperature service was selected, sectioned into 6 mm thick sheets, and machined with 2 mm grooves. Microstructural analysis using scanning electron microscopy (SEM) and optical microscopy (OM) identified phases in the base metal, weld metal, and heat-affected zone (HAZ). The hardness profile resulting from the welding process was measured and analyzed as a function of welding current, which was varied at three levels: 50 A, 60 A, and 70 A. A current of 70 A caused hot cracking in the partially melted zone (PMZ), while 50 A led to insufficient penetration. Optimal, defect-free welds were achieved at 60 A, which provided an appropriate weld pool geometry. Differences in the cooling rate of the weld metal influenced its microstructure and hardness. Welding at 70 A produced a lower cooling rate compared to 50 A and 60 A. This slower cooling allowed more time for grain growth within the molten pool, resulting in a lower average hardness at the highest current level. Overall, this study offers valuable insights for selecting desirable parameters in the repair welding of cobalt-based superalloys, thereby helping to extend the service life of industrial components.