Electrochemical Micromachining of NiTi Shape Memory Alloys with Ultrashort Voltage Pulses

Abstract

Electrochemical micromachining (ECMM) with ultrashort voltage pulses has been used to fabricate microstructures on a NiTi shape memory alloy (SMA). Because of its unique properties, NiTi is a desirable material for use in various applications including medical devices and actuators. ECMM is a heat-free, strain-free, and mask-free method for microfabrication, and therefore well suited for use with shape memory alloys. Microstructures were machined to a depth of 3 μm on NiTi surfaces, and the lateral resolution of the machining was found to be dependent on the duration of the voltage pulses used.Nickel-titanium (NiTi) is part of the group of materials known as shape memory alloys or SMAs. It is also commonly known as NITINOL, an acronym for Nickel Titanium Naval Ordnance Laboratory, based on the institute where the NiTi SMA was discovered in the 1960s. 1 Alloys with a composition of Ni x Ti 1−x , where 0.47 ≤ x ≤ 0.51, are typically made by annealing the two metals at high temperature. 2, 3 If the alloy is deformed at room temperature, it can be restored to its original shape with modest heating. The shape memory is the result of a solid-state martensitic phase transformation. 2, 3 The ratio of nickel to titanium sets the temperature at which the high temperature austenite phase and the low temperature martensite phase interconvert. 3 NiTi SMAs also exhibit pseudoelastic or superelastic behavior associated with this martensitic phase transformation. Under stress the NiTi can convert from the austenite to the martensite structure. Upon release of the stress, the martensite will transform back into austenite, and the original shape will be restored. 3 Because of their mechanical properties and biocompatibility, NiTi SMAs are frequently used in medical devices such as dental appliances and coronary stents. 4 NiTi is generally unreactive and corrosion resistant inside the human body because of a titanium-rich oxide layer on the surface of the NiTi. 2, 5, 6 The shape memory characteristics of the NiTi SMAs also make them desirable materials for use in actuators. [7][8][9][10][11] Devices such as stents and actuators often require the fabrication of microscale features on the NiTi SMAs. Several methods have been used to fabricate microfeatures on NiTi SMAs, including laser cutting, 12, 13 electro-discharge machining (EDM), 14, 15 mechanical drilling, 16, 17 masked chemical etching, 7-9 through-mask electrochemical machining, 10, 11 and a confined etchant layer technique. 18 These methods offer advantages and disadvantages. Laser cutting and electro-discharge machining can fabricate microstructures with minimal surface preparation, but create a heat-affected zone which can change the near-surface properties of the NiTi. Mechanical removal methods are an even simpler maskless method, but shape memory properties of NiTi make mechanical removal processes difficult, and mechanical methods often damage the workpiece. Masked chemical and electrochemical techniques offer a method for microfabrication with no heat effects, but re...

Shape‐Memory Topographies on Nickel–Titanium Alloys Trained by Embossing and Pulse Electrochemical Machining

“…This method facilitates the preparation technique for large areas and could enable applications of switchable surface arrays in tribology, switchable adhesion, or biomedicine Pulsed Electrochemical Machining plays in important role in NiTi processing due the oxidation and deformation free formation of complex microstructures without thermal damage or distortion . In the present work, we revealed that a formation of different protruding structures on a NiTi surface was possible via PECM using a tool of holes and grooves. We induced a TWSME by compression of surface structures.…”

Electrochemical micro-machining based on double feedback circuits

“…Shin optimized pulse signal parameters such as the voltage peak, pulse duration, and its period, to improve machining accuracy and stability 8 . Maurer fabricated microstructures on NiTi shape memory alloys using the ECMM technique based on ultrashort pulses 9 . Huang developed a nanosecond pulsed electrochemical micromachining system, conducted a series of experiments, studied the effects of pulse signal parameters on machining accuracy, and successfully prepared complex microstructures on nickel and superalloy plates 10 .…”

Real-time monitoring of process current and its correlation with micro-feature accuracy and surface topography in electrochemical micromachining of nitinol

“…There are both conventional and unconventional techniques to produce the microscale features that these devices frequently require on NiTi SMAs [9]. Nevertheless, the efficacy of ECMM as a non-traditional technique for micro-feature machining on Nitinol has already been demonstrated by several authors [10][11][12]. The unique combination properties of ECMM like its non-thermal nature, high precision, customization capabilities, and non-contact operation make it the ideal choice for microfabrication on Nitinol, opening up new possibilities for advanced microscale applications [13,14].…”