DEVELOPMENT AND INVESTIGATIONS ON PRESSURIZED FLOW ECDM PROCESS

Abstract

Miniaturized devices are rapidly increasing in microfluidic applications. Micro-hole is an essential feature of microfluidic devices. Glass is one of the commonly used materials in microfluidic applications owing to its superior properties such as transparency, thermal and chemical resistance etc. Electrochemical discharge machining (ECDM) has gained huge attention towards the fabrication of micro-holes on electrically non-conductive materials like glass and ceramics. The ECDM is a hybrid process of two non-conventional machining processes, namely electrochemical machining and electric discharge machining. Although some investigations have explored the ECDM process to machine micro-holes for microfluidic applications, however, these are mostly limited to a shallow machining depth (up to 1537μm). The lack of electrolyte and the accumulation of the machined by-products (e.g., sludge and debris) in the machining zone limit the machining depth. Therefore, the present research work aims to enhance the capability of the ECDM process to machine deeper micro-holes with higher productivity, repeatability, accuracy, and better quality of the machined surface. Initially, the ECDM experimental facility with the adaptive tool feeding system was designed and developed in-house. Subsequently, the parametric investigation with an adaptive tool electrode feeding system is conducted during the machining of micro-holes on borosilicate glass. Three process parameters, i.e., tool forward feed rate, working gap and dwell time, were selected for experiments. The depth and entrance diameter of the hole was selected as process outcomes to evaluate the performance of the ECDM process. A comparison of the process performance of ECDM with a solid and tubular tool electrode was performed. The experiments were performed by varying the applied voltage and pulse on time following the one-factor-at-a-time (OFAT) approach. The results indicate that the tubular tool electrode provided a more uniform distribution of electric field from both the edges (i.e., inner as well as outer edges) as compared to the solid tool. The performance of the ECDM process depends upon the characteristics of discharges (i.e., applied thermal loading), which are controlled by the applied voltage. Therefore, a comprehensive investigation of the effect of thermal loading on the machining behavior of glass was conducted. Further, the analytical model of thermal stresses produced in glass due to thermal loading was developed. Lower values of the applied voltage resulted in lower MRR, and higher values of applied voltage (beyond 52 V) resulted in high MRR with severe thermal damage on the work material. The applied voltage from 44 V to 52 V resulted in higher MRR with lower thermal damage. The localized temperature rises above the vaporization temperature of the water. In this regard, a systematic quantification of the vaporized quantity of the electrolyte was performed by developing an analytical model, which had been experimentally validation. The ECDM process provides a limited depth of micro-holes due to the underlying process mechanism. The availability of electrolyte reduces in the machining zone due to vaporization of electrolyte and remains the machined by-products. The replenishment of vaporized electrolyte by controlled delivery of fresh electrolyte can enhance the machining performance. Therefore, the pressurized flow ECDM (PF-ECDM) facility was developed. The experiments were conducted to investigate the effect of electrolyte flow rate (EFR) on process outcomes during the machining the micro-holes on borosilicate glass. The EFR was selected at 0 ml/hr for the ECDM process and 1-6 ml/hr for the PF-ECDM process. The depth and entrance diameter of the hole were selected as process outcomes of this experimentation. The mechanism of material removal during ECDM, PF-ECDM with adequate electrolyte flow and PF-ECDM with excess electrolyte flow were proposed. Additionally, multi-response optimization using the desirability approach was conducted. The optimal setting was utilized for machining the deep micro-hole at various machining times (i.e., 180, 210, 240, 270, 300 s). Eventually, a micro-hole with 2478 μm depth was achieved using the PF-ECDM process. The controlled delivery of electrolyte into the machining zone improves the depth of micro holes. But, the accumulation of bubbles at the entrance and the machined by-products in the machining still limited the depth of micro-hole. Thus, the two-phase flow (in intermittent mode) was injected into the machining zone. The performance evaluation of the ECDM process (without flow) and PF-ECDM process (with electrolyte flow and two-phase flow) was performed by comparing the process outcomes. Three modes (i.e., mode-I: without flow, mode-II: with electrolyte flow and mode-III: with two-phase flow (i.e., electrolyte + air)) were analyzed to discriminate the flow conditions during ECDM and PF-ECDM process. The depth and entrance diameter of the micro-hole were selected as process outcomes in this experimentation. The experimental results revealed that the mode-III (i.e., two-phase flow) during the PF-ECDM process results in higher depth and lower entrance diameter of the micro-holes as compare to mode-I and mode-II at all the parametric conditions.