Machining-induced machinability and phase transformation behaviour of nickel–titanium shape memory alloys under graphene NanoMQL and chilled air assisted milling

Nickel–Titanium (NiTi) shape memory alloys are widely used in medical and industrial applications due to their unique superelastic and shape memory properties. However, their machining remains challenging because of temperature-sensitive phase transformations, high ductility, and severe work hardening, often resulting in excessive tool wear, poor surface finish, and burr formation. This study investigates the influence of graphene-enhanced vegetable oil nanoMQL and chilled air cooling on the milling performance of austenitic NiTi alloys. Thermophysical characterization confirmed that a 1.0% graphene concentration in the base oil significantly improves thermal conductivity, lubrication, and wear resistance, providing optimal conditions for machining. Milling experiments under six different cutting environments revealed that the combined chilled air and nanoMQL strategy substantially reduces tool wear (86%), burr formation (54%), surface roughness (58%), and cutting forces (73%), while lowering energy consumption and promoting compressive residual stress. X-ray diffraction and differential scanning calorimetry analyses demonstrated that this hybrid approach preserves the cubic crystal structure and maintains phase transformation temperatures, thereby sustaining the functional shape memory behaviour of NiTi. In contrast, dry and flood machining generated excessive heat, induced undesirable phase transformations, and required post-machining annealing to restore functional properties. These findings indicate that effective machining of NiTi alloys depends not only on reducing conventional wear and force metrics but also on controlling the coupled thermo-mechanical and phase transformation behaviour. The integration of graphene nanoMQL with chilled air provides a sustainable, energy-efficient, and industrially viable approach that enhances machinability while preserving the unique functional properties of thermally sensitive NiTi alloys.