Machinability Enhancement of Inconel 718: A Review of Advanced Machining Processes and Sustainable Manufacturing Approaches

Inconel 718 is a precipitation-hardened nickel-based superalloy extensively used in aerospace, power generation, and high-temperature engineering applications due to its exceptional mechanical strength, corrosion resistance, and stability at elevated temperatures. Despite these superior properties, the alloy is widely recognized as one of the most difficult-to-machine materials. Its low thermal conductivity, high work-hardening tendency, strong chemical affinity with cutting tools, and ability to retain high strength at elevated temperatures collectively lead to rapid tool wear, high cutting forces, and poor surface integrity during machining operations. Consequently, improving the machinability of Inconel 718 has become a significant research focus in modern manufacturing science. This review paper presents a comprehensive overview of the machinability characteristics of Inconel 718 by systematically analyzing the fundamental challenges, tool wear mechanisms, machining performance indicators, and recent technological advancements reported in the literature. The review first discusses the intrinsic material properties of Inconel 718 that influence its machining behavior, including microstructural characteristics, strain hardening behavior, and thermal properties. Particular attention is given to the role of these properties in influencing cutting temperature, chip morphology, and tool–workpiece interaction. Subsequently, the study examines the performance of different cutting tool materials such as coated carbides, ceramics, cubic boron nitride (CBN), and polycrystalline diamond (PCD), highlighting their advantages and limitations in high-temperature machining environments. Various tool wear mechanisms including abrasion, adhesion, diffusion, oxidation, and notch wear are critically analyzed to understand the degradation of tool life during machining processes. Furthermore, the paper reviews the influence of key machining parameters such as cutting speed, feed rate, and depth of cut on machinability indicators including cutting forces, surface roughness, tool life, and chip formation. Advanced cooling and lubrication techniques such as cryogenic cooling, minimum quantity lubrication (MQL), and hybrid cooling approaches are also discussed, emphasizing their role in reducing thermal loads and enhancing machining efficiency. In addition, the application of advanced machining processes including electrical discharge machining (EDM), wire EDM, laser-assisted machining, and hybrid machining techniques is explored as alternative approaches for improving the machinability of Inconel 718. The review also highlights recent developments in process optimization, modeling, and simulation techniques that have been employed to predict machining performance and optimize cutting conditions. These include numerical simulations, finite element modeling, and data-driven approaches such as artificial intelligence and machine learning for intelligent manufacturing. Finally, the study identifies critical research gaps and proposes future directions for enhancing machinability through innovative tool design, sustainable cooling strategies, and advanced manufacturing technologies. Overall, this review provides a consolidated understanding of the machining behavior of Inconel 718 and offers valuable insights for researchers and industry professionals seeking to improve machining performance and productivity in high-temperature alloy manufacturing.