Cryogenic and Dry Machining of NiTi Shape Memory Alloys

NiTi shape memory alloys are widely used in biomedical and aerospace industries. While the basic thermo-mechanical and shape memory properties have been determined in detail, the effect of final manufacturing processes on microstructural and shape memory properties of NiTi have not been as rigorously investigated. Machining techniques, especially those associated with very high strain-rate, strain, and temperature processes, have the potential to alter physical, mechanical, and shape memory properties of NiTi alloys.  This experimental work focuses on investigating the cryogenic and dry machining performance and the effects of machining on microstructural and shape memory properties of a NiTi alloy. Considering the temperature dependent microstructure of shape memory alloys, both dry and pre-cooled cryogenic machining were performed on a slightly Ni-rich binary NiTi alloy to determine the effect of machining temperature on the extent and type of subsurface machining-induced layer. The obtained results indicate that during the machining process, the machined phase of NiTi SMAs is a key factor for improving the machining performance and tailoring the machined surface and its shape memory properties. It has been found that a higher density of twin bands and dislocations were observed in the cryogenically machined specimens and the affected layer was at least twice as large and resulted in higher hardness compared with dry-machined specimens.