The Effect of Flood, High-Pressure Cooling, and CO₂-Assisted Cryogenic Machining on Microhardness, Microstructure, and X-ray Diffraction Patterns of NiTi Shape Memory Alloy

This study focuses on the effects of various cutting speeds and cutting conditions including dry, CO₂, HPC and flood on the surface integrity characteristics of the machined NiTi alloy. Machining-induced affected layer, microstructure, microhardness and XRD analysis are considered to assess the surface integrity characteristics of NiTi alloy. The findings from this current study reveal that as the cutting speed increased, the depth of the machining-induced layer decreased. While the microhardness value of the machined samples increased in all of the cutting conditions compared to the as-received hardness, the greatest increase was in the CO₂ condition, with 36%. The highest peak intensities of the B2 main austenite XRD peaks occurred at the cutting speed of 70 m/min. The full width at half maximum values of the XRD peaks increased in all of the cutting conditions, especially at the cutting speed of 20 m/min, and this situation supports the microhardness increase. The smallest crystallite size occurred under the CO₂ condition at the cutting speed of 20 m/min, while the highest dislocation density occurred under the HPC condition at the same cutting speed.