Laser surface melting and heat treatment of Tomilloy nickel-based superalloy as gas turbine transition part after service

In this study, a hot gas transition part made of Tomilloy nickel-based superalloy, which has been in service at 1000 °C for 32,000 h, was investigated. In this research, to recover the properties, the alloy underwent rejuvenation treatments. Initially, the microstructure of the alloy is examined, and then rejuvenation is performed by laser surface remelting and heat treatment. Laser surface melting was performed using a continuous-wave fiber laser at various speeds of 10, 12, 14, 20, 70, and 80 mm/s. The heat treatment was performed at 1100 °C for 2.5 h. Stereography, optical microscopy, and scanning electron microscopy were used to examine the microstructure and phases. In addition, microhardness testing was used to investigate the mechanical properties. The measured penetration depth at higher speeds, namely 70, and 80 mm/s, was low, and the passes had inadequate overlap. Surface melting at speeds of 10, 12, 14, and 20 mm/s showed proper overlap and greater penetration depth. The speed of 10 mm/s resulted in the maximum penetration depth of 378 ± 6 μm. In the microstructure analysis of the fusion zone, planar, cellular, columnar dendritic, and equiaxed dendritic solidification modes were observed. In the microstructure of the heat-treated samples, it was observed that the dendrites are disappeared and the sub-grain boundaries are no longer exist. The precipitates were distributed throughout the fusion zone and the structure became homogeneous. In the hardness test, the surface melted sample had the highest hardness of 292 ± 20 HV whereas the annealed sample exhibited a hardness of 232 ± 11 HV. Lower laser speeds of 10, 12, 14, and 20 mm/s produced samples with adequate penetration depths and no crack formation. The maximum penetration depth of 378 ± 58 μm is achieved at the laser speed of 10 mm/s, and accordingly this laser speed is suggested for the rejuvenating Tomilloy alloy.