Surface Integrity Evolution in WEDM of Nickel-Based Superalloys: Formation and Implications of Recast Layer, Residual Stresses, and Microhardness—A Critical Review

Abstract

Wire Electrical Discharge Machining (WEDM) is widely applied for machining nickel-based superalloys used in aerospace and high-temperature systems; however, thermally induced surface alterations strongly affect structural reliability. This review synthesizes recast layer thickness (RLT), microhardness evolution, residual stress development, and elemental redistribution in precipitation- and solid-solution-strengthened alloys. Discharge energy—mainly governed by pulse-on time and peak current and influenced by gap voltage—controls melt-pool behavior, resolidification, chemical modification, and stress evolution. Reported RLT values vary from ~1 to 50 µm depending on machining regime, while low-energy multi-trim strategies can restrict thickness below ~5–6 µm for aerospace requirements. High discharge energy promotes Cu–Zn electrode transfer, oxygen incorporation, and tensile residual stresses (~200–800 MPa), increasing crack susceptibility. Microhardness response is alloy-dependent, often showing subsurface softening under aggressive conditions. This review establishes an energy–microstructure–performance framework for improving surface integrity in critical applications.