Influence of Machining parameters and crystallographic orientation on surface integrity and subsurface deformation in a Ni-Based Single-Crystal superalloy (CMSX-4)

This work presents a comprehensive investigation of the combined effects of machining parameters and crystallographic orientation on the surface integrity of the nickel-based single-crystal superalloy, CMSX-4, during milling. The effects of cutting speed, feed rate, and depth of cut, along with crystallographic orientations [001] and [103], were systematically evaluated. The results show that the surface roughness increases with feed rate (f) and depth of cut (d), whereas an increase in cutting speed (Vc) leads to improved surface finish. The microstructural analysis reveals the formation of a plastically deformed subsurface layer, accompanied by surface hardening, with an increase in microhardness from 366.2 HV5 to values ranging from 413.4 to 463.9 HV5 after milling, depending on the machining parameters. The milling deformation layer (MDL) thickness exhibits a strong dependence on feed rate and cutting speed. The SEM micrograph of the subsurface region after milling reveals severe plastic deformation induced by thermo-mechanical loading, characterized by distortion of the γ′ precipitates and their elongation along the shear direction. The lattice rotation remains minimal, indicating localized deformation without significant crystallographic misorientation. The surface integrity also shows a dependence on crystallographic orientation. Milling along the [103]y direction yields the best surface finish and the lowest microhardness due to favourable alignment with the {1 1 1}〈110〉 slip systems, enabling efficient shear-dominated material removal. In contrast, unfavourable orientations, [103] x and [001], promote restricted slip and enhanced ploughing, resulting in increased surface roughness and deeper MDL. This work establishes a direct mechanistic correlation between machining parameters, crystallographic orientation, and surface integrity, providing a framework for optimizing machining strategies for single-crystal superalloys in high-performance aerospace applications.