Process And Principles Investigation On Electrochemical Micro-Machining Of NiTi Shape Memory Alloy Using Ethylene Glycol Electrolyte

NiTi shape memory alloys（SMAs）have good mechanical properties,biocompatibility,superelasticity and shape memory effect.These benefits make SMAs with wide application in industries and biology.However,because of the excellent mechanical properties and the superelasticity,it is hard to machine Ni Ti SMAs with traditional processes.It is even harder to do the machining in micro-scale.Electrochemical micromachining（EMM）is a machining process to remove material based on electrochemical corrosion.It is not restricted by the properties of the materials.There is great potential to machine Ni Ti shape memory alloys with EMM.However,the EMM of Ni Ti SMAs faces two challenges:The first is to develop the suitable electrode.The second is to improve the machining quality of the Ni Ti SMAs.Therefore,the following projects were investigated in this research.To reduce the sever dispersion corrosion when using traditional column electrode,this research brought up two machining process to make electrodes.One is to fabricate electrode with reverse conical shape tip and high aspect ratio.In the machining process,the tungsten rod is passed through the electrolyte liquid membrane and make a linear reciprocating motion.The anodic dissolution occurs only in the region where the rod and electrolyte are in contact.Comparing to the conventional method,this proposed method can improve the shape accuracy of micro-tools.The influence of parameters such as electrolyte concentration,feeding rate,the thickness of electrolyte and the position of the micro-tools on the machining the electrode was also investigated.The other is to fabricate a electrode which is insulated by a capillary glass tube.In this research,ethylene glycol based electrolyte was firstly applied in the EMM of NiTi SMAs.The influence of peak impulse voltage and machining speed on the EMM machining performance were investigated and a micro-groove was machined on the NiTi SMA with the best parameter.Ethanol was added to the electrolyte to suppress the formation of oxide layer which results in a bad machining quality.By comparing the machining quality with different ethanol addition,it was found that the best machining quality was achieved when 20 vol.%ethanol was added.It was proved that ethanol suppresses the formation of oxide layer after conducting CV in solution with different ethanol addition.XPS analyst was conducted on the machined surface and it was found that the oxide layer was mainly composed of TiO₂.The mechanism of how ethanol influences the EMM of NiTi SMAs was proposed.During the EMM of NiTi SMAs,a layer of TiCl₄ forms on the surface that was machined.The TiCl₄ layer covers to TiO₂ layer after reacting with water.The ethanol can adjust the formation of TiCl₄layer thus the formation of oxide film is limited.If ethanol was not added to the electrolyte,there will be excessive TiO₂ formed on the surface.But if excessive ethanol is added,the machining accuracy and surface quality will be affected because processed section is more comfortable to be dissolved.