Study on chip formation characteristics in turning NiTi shape memory alloys

NiTi shape memory alloys (SMAs) are widely applied as a smart material in many areas. Its unique properties, such as high ductility, corrosion resistance, shape memory effect, and superelasticity, attracted much attention to this material. However, the NiTi SMAs are unfavorable in machining operations. Chip formation characteristics provide a favorable factor in analyzing the machining mechanism and then providing support to improve the processing technology. Experiments on turning NiTi SMAs at different cutting speeds varied from 7.5 m/min to 125 m/min was performed in this paper, and the feed rate was 0.15 mm/r and the depth of cut was 0.2 mm. The shape and microstructure of the chip were analyzed to reveal the material flow behavior in the chip. Together with the change of micro-hardness and the differential scanning calorimetry(DSC) curves of NiTi SMAs, this paper presented that the martensitic phase transformation plays an important role in the material flow behavior, and thus influence the formation of the chip. When the cutting speed is lower, strain hardening is a primary driving force for the material flow in the chip. The thermal softening effect and degree of martensitic transformation increase with an increment in cutting speed and this coupled effect promoted the material softening. Furthermore, the critical cutting speed and the shear angle for the formation of the serrated chip were depicted from the experimental data, which resulted in the cutting speed of 40 m/min and the shear angle of 45 degrees.