MACHINABILITY AND PHASE TRANSFORMATION TEMPERATURES OF MARTENSITIC AND AUSTENITIC NITI SHAPE MEMORY ALLOYS IN DRY AND CHILLED AIR MACHINING

This study investigates the effects of dry and chilled air milling on the machinability and phase transformation behaviour of martensitic and austenitic nickel-titanium (NiTi) alloys. NiTi alloys are widely used in biomedical and automotive industries for their shape memory effect and super-elasticity; however, their high ductility and temperature sensitivity pose machining challenges, leading to severe tool wear and poor surface quality. Key machinability aspects including cutting force, tool wear, burr formation, and surface roughness, were analysed alongside phase transformation characteristics using Differential Scanning Calorimetry (DSC). Results show that chilled air cutting significantly reduces cutting force for austenitic NiTi but has a limited effect on martensitic NiTi. It also lowers cutting temperatures to 40°C, enhances tool wear resistance, and minimises burr formation. Chilled air cutting also improved surface roughness by 50% and 20% for martensitic and austenitic NiTi, respectively. While neither cutting condition fully preserved NiTi transformation curves, chilled air cutting promoted the inhibition of multi-peak appearances in martensitic NiTi, potentially improving phase stability.