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$\alpha$–$\beta$ titanium alloy (Ti-6Al-4V) is being used in diverse applications, such as aircraft, chemical, automotive, and biomedical industries due to its excellent attractive properties. But machining of titanium and its alloy is a tedious task through conventional material removal processes. Removing the material at the micro level to create micro-sized features like a hole, slot and intricate projection in the titanium alloy is more complicated. In this paper, micro-sized through-hole machining on the $\alpha$–$\beta$ titanium alloy has been performed using electrochemical micro-machining (EC$\mu$M) with nonaqueous ethylene glycol (CH₂OH)₂–sodium bromide (NaBr) electrolyte and the influence of machining parameters, such as electrolyte concentration (EC), machining voltage (MV), tool feed rate (TFR) and duty cycle (DC) during micro-hole machining on the $\alpha$–$\beta$ titanium alloy is analyzed. The material removal rate (MRR), radial overcut (ROC), conicity ($C$), and surface roughness (SR) of the micro-sized through hole have been ascertained and analyzed. The selected machining parameters are optimized using Taguchi based Grey relation analysis and DEAR methods.

This research work explores the feasibility of drilling micro-holes on Ti6Al4V by applying a sequential electro-micro-machining process. A novel sequential combination of micro-electrochemical discharge machining ($\mu$ECDM) and micro-electro-chemical machining ($\mu$ECM) is applied for drilling micro-hole in titanium alloy (Ti6Al4V). The machining of titanium alloy by a $\mu$ECDM process is a challenging task and not reported yet. Therefore, the feasibility of machining of Ti6Al4V by $\mu$ECDM using various combinations of electrolytes has been studied. The best-suited electrolyte for drilling by $\mu$ECDM was selected by analyzing the hole depth, radial overcut, hole taper angle and the minimum time required to drill through holes in a 400$\mu$m thick sheet of titanium alloy. The $\mu$ECDM process produces micro-holes rapidly but it consists of recast layer, micro-cracks, and heat affected zone, etc. The surface characteristics of a $\mu$ECDMed hole were improved by applying $\mu$ECM process subsequent to the $\mu$ECDM process. The sequential combination of $\mu$ECDM shaping and $\mu$ECM finishing results in improved dimensional accuracy, machining depth, taper angle, and surface quality of the hole produced by sequential micro-machining process.

In the paper micro-holes are machined in electrochemical machining (ECM) with the passivating electrolyte disturbed by ultrasound. Based on the Faraday's law, theoretical model of the inter-electrode gap and the mechanism of ultrasonic effect are investigated. Moreover experiments on comparison of ECM micro-holes and ultrasonic assisted ECM micro-holes are carried out, and results show that the distribution of the inter-electrode gap and the localization of anodic dissolution are improved with the passivating electrolyte disturbed by ultrasound. Finally precision micro-holes have been obtained successfully.