A Comprehensive Review of Machinability of Difficult-to-Machine Alloys with Advanced Lubricating and Cooling Techniques

Superalloys find widespread application in demanding environments owing to their robust strength and high resistance to heat and corrosion. Nonetheless, these very attributes render them difficult-to-cut materials. Enhancing machinability necessitates effective lubrication and cooling techniques, the efficacy of which varies depending on the specific alloy, manufacturing methodology, and machining conditions. This exhaustive review presents a fresh analysis elucidating the impact of diverse cooling/lubrication mothods—dry, flood, minimum quantity lubrication (MQL), cryogenic, and high-pressure cooling—on the machining of titanium, nickel, and steel-based superalloys fabricated through conventional and additive manufacturing processes. Key machining operations, including turning, milling, drilling, and grinding, are scrutinized. The ramifications of each cooling approach on critical machinability indicators such as surface roughness, cutting forces, tool wear, temperature, and environmental footprint are meticulously assessed through an extensive literature survey. Both conventionally produced and additively manufactured alloys are scrutinized to discern prevailing trends. The findings underscore the absence of a universally optimal technique across all scenarios. MQL and cryogenic methods exhibit notable efficacy in refining surface finish during titanium alloy machining. High-pressure cooling augments chip breakability and prolongs tool life in titanium machining, albeit yielding disparate outcomes in nickel alloy machining contingent upon wear mode. Additively manufactured alloys generally exhibit superior machinability compared to their wrought counterparts, although warranting further investigation. This holistic analysis furnishes fresh insights into aligning cooling strategies with alloy-process amalgamations to optimize machinability. It identifies extant challenges and avenues for advancing sustainable and efficient machining of difficult-to-cut materials.