Experimental and Numerical Investigation of Reverse-Micro-EDM Fabricated Arrayed Micro Protrusions

Abstract

Among micromachining processes, μEDM and its variants have revolutionized the surface texturing phenomenon on conductive materials irrespective of hardness values. A potential application of surface texturing could be in the thermal management of high heat flux devices when used as arrayed protrusions (or micro pin-fins) heat sinks. Reverse-micro-electric-discharge machining (RμEDM) as one of the μEDM variants has evolved as a method for the fabrication of single/arrayed micro protrusions. These protrusions find applications in many technological fields, including optics, surface energetics, lubrication, bioengineering, component assembly and thermal management of high-performance microelectronics etc. For fabricating these protrusions by RμEDM, the tool used in the form of a plate consisting of micro holes is one of the key components. These holes are generally required with high accuracy and precision arranged in the form of an array of similar shapes and sizes of the arrayed protrusions. Tool plate fabrication technologies such as mechanical and EDM micro drilling are extensively used. Though, having certain limitations such as longer machining time for an array of micro holes, overcut etc. Laser beam micromachining (LBμM) is identified as an alternative for the fabrication of tool plate for RμEDM due to enhanced machining speed and dimensional accuracy. In this regard, a comprehensive feasibility study has been done on the fabrication of arrayed protrusions by RμEDM using LBμM fabricated tool plates. This combined process is termed as ‘integrated RμEDM-LBμM technology’. To establish the developed integrated process for the mentioned purpose, issues such as burr formation in the LBμM fabricated micro holes and hence, its replication as damages at the edges of RμEDM fabricated micro protrusions, need to be taken care of. Comprehensive studies have been performed for finding optimum process parameters using GA for both processes to ensure minimum errors in the arrayed micro holes, thereby in the micro protrusions. The different machining responses associated with both processes have been critically analyzed in these regards. Improvement in the responses, such as 𝑀𝑅𝑅 and surface characteristics through parametric optimization in RμEDM can be achieved up to a limit only. Due to the stochastic behavior of debris in the tiny discharge gap, the removal of that debris is cumbersome, especially in the case of an array fabrication. This has led to a difference in the process stability due to the heavy occurrence of abnormal discharges in the tiny gap affecting the material removal efficiency and life of the tool plate. Therefore, a suction-based high-pressure dielectric flushing mechanism is proposed for effective debris removal. This technology ensures a reduction in total machining time, which is highly desirable in machining high aspect ratio arrayed micro protrusions. Numerical and experimental thermal performance evaluations of densely arrayed protrusions (to act as pin-fin heat exchangers) in both inline and staggered arrangements is another key contribution of the thesis. A geometrical design-based optimization study using the Particle swarm multi-objective optimization (PSO) algorithm has been performed to quantify the optimal geometrical design of the cross-section of micro pin fins (MPFs) in an array. It is followed by the thermal performance evaluation of different arrayed MPFs cross-sectional profiles and arrangements through numerical simulation and experimental approach. A thermal performance index (TPI) parameter is defined in a laminar flow regime under constant heat loading conditions for evaluation. An in house testing facility has been designed and developed for evaluating the overall thermal performance of optimized arrayed MPFs experimentally. The thesis will lay the ground for the quality fabrication of arrayed protrusions (as MPFs) by enhancing the processes' stability through geometrical design optimization, thermal performance analysis, and experimental validation. This work is significantly important for the early mitigation and evaluation of fabricated micro protrusions for various applications.