Analysis of laser-induced surface damage of single-crystal Ni-based superalloy towards improving machinability

Single-crystal (SC) nickel-based superalloys provide improved high-temperature strength, and resistance to creep, fatigue and oxidation, compared to equiaxed and columnar-grained components due to their higher solid solution strengthening, precipitation hardening and the absence of grain boundaries. However, the SC nickel-based superalloys are ‘difficult-to-cut’. To enhance the machinability, a concept of laser-induced surface damage (LISD)-assisted machining is being studied. Laser surface modification (LSM) experiments were been performed by employing the design of experiments (DoE) strategy L18, OA (orthogonal array). The LSMed geometries were characterized to identify various laser-induced surface defects including solidification cracks, microcracks, micropores, microstructural changes, phase changes, recrystallization, and deformation. Further, parametric interactions and optimization of the process parameters were performed to manufacture a larger LISD layer. The parameter-structure-property relationship was derived systematically and the mechanisms of LISD on the SC nickel-based superalloy were described in details. The results indicate that the LISD increase with an increase in laser power followed by a decrease in scan speed and beam diameter. LISD geometries possess 1 % to 4 % of crack density and lower hardness of around 27 % than the base metal. Slot milling experiments were then conducted on the LISD specimens, which showed up to a 40 % reduction in cutting forces when compared to untreated specimens. It is evident that this methodology can be adopted to further improve the machinability of various difficult-to-cut materials.