Electrochemical formation of a hierarchical porous surface layer on NiTi alloy for enhanced machinability

A B S T R A C T NiTi shape memory alloys remain challenging to machine because of their low thermal conductivity and the concurrent high strength and ductility, which often accelerate tool wear and deteriorate surface integrity. Although electrochemical modification-assisted machining has shown promise for difficult-to-machine metals, the highly stable passive film on NiTi often results in slow surface modification and non-uniform modified layers. In this work, a rapid electrochemical surface modification strategy was developed for NiTi alloy using a two-electrode constant-voltage process in a mixed electrolyte of 1 M HNO 3 and 0.3 M NaCl. A hierarchical porous modified layer with a thickness of approximately 5 μm was formed within 180 s. The modified layer exhibited an oxide-rich multiscale architecture composed of micron-scale primary pores and nanoscale secondary porosity. EDS and XPS analyses revealed pronounced Ni depletion, oxygen enrichment, and a TiO 2 -rich outermost surface. Compared with the untreated substrate, the modified surface showed a decrease in water contact angle from 73.54° to 8.09°. Scratch tests showed reduced material pile-up and a decrease in specific scratching energy from 4.02 to 1.53 J/mm 3 , indicating lower local material-removal resistance under the present scratching condition. In micro-milling, the modified specimens exhibited alleviated surface tearing and improved surface quality, with Sa reductions of 25.62%, 29.19%, and 37.18% at axial depths of cut of 5, 10, and 15 μm, respectively. These results demonstrate that electrochemical surface modification is an effective approach for tailoring the near-surface structure of NiTi alloy and improving its machined-surface quality under the tested conditions.