Ultra-short pulse simulation for characterising oxide layer formation on stainless steel during μECM

Electrochemical machining is a controlled metal shaping process based on anodic dissolution performed by the electrode reactions. When pulse signals, in the micro-second or less range, are combined with a small inter-electrode gap, the process is referred to as Micro-Electro-Chemical machining or μECM. The application of using nano-second/micro second pulses for μECM for certain passivation metals can be very limited by the formation of an oxide layer that prohibits the penetration of ultra-short pulse durations due to the capacitive effect of the oxide layer. This paper will present findings from the development of a μECM numerical model that incorporates an advanced hybrid time stepping approach for representing nano-second pulse interactions with the physical phenomena of the electrolytic environment associated with μECM, thereby enabling the characterisation of the capacitive model of the formed oxide barrier layer. Experimental work was conducted using nano and micro-second pulses on the machining of 18CrNi8 and copper using NaNO3 electrolyte. The μECM simulation model contributed to quantifying the lack of faradic machining of 18CrNi8 that was experimentally observed. A particular solution, believed to be effective in breaking through the oxide film, is to directly pre-polarize the layer by super imposing a DC signal prior to the ultra-short pulses, such that its capacity has been already loaded when the short pulse signal is applied. However, too long of a DC signal application time can lead to machining damage. Through the development of an ultra-short pulse simulation model, this work has established, that we can numerically determine an optimal DC signal duration necessary to load the oxide layer just enough for facilitating anodic dissolution during physical μECM process, thus minimising machining damage.