Laser Surface Remelting in Single-Crystal Nickel-based Superalloy using a Continuous Wave Fiber Laser

Abstract

Abstract Single-crystal (SC) nickel-based superalloy castings offer high-temperature microstructural stability and superior creep resistance, due to which they are exclusively used in the hot sections of gas turbine engines. However, SC nickel-base superalloy components are difficult-to-cut while manufacturing. Worldwide research shows an interest in improving the machinability of superalloys. The present work is investigating the controlled surface damage on CMSX-4 superalloy through laser surface remelting technique towards the improvement of machinability. The specimens were laser-treated using a constant laser power and scan speed and varying the positive focal position to get a range of energy densities. The process, structure, and property were systematically studied in the fusion zone (FZ). The FZ shape changed from keyhole to conduction mode with the increasing focal position. The FZ showed a finer assorted dendritic structure and lesser elemental segregation than the base metal (BM). In keyhole mode penetration, large pores, and multi-directional cracks were observed in the base region. On the other hand, the conductive mode showed only vertical centreline cracks and no significant porosity. The cracks are attributed to thermal stresses and elemental segregation produced during solidification. Microcracking was also observed near the fusion boundary and is attributed to the presence of low melting Mo and Ti-rich eutectics. The FZ away from the cracks showed 10% lower hardness than the BM, which is attributed to the dissolution of 𝛾' phase. Overall, the laser processing under the given range of energy densities produced wide variants of surface defects in the FZ.