MICRO ELECTRIC DISCHARGE MACHINING OF DIFFICULT-TO-MACHINE MATERIALS

Abstract

The application spectrum of Titanium alloy (Ti6Al4V) and fiber reinforced composites has increased at an unprecedented rate over the few years owing to their outstanding multifunctional properties including high specific strength and stiffness, low density, good damping properties, and good corrosion resistance as compared to the conventional materials. These materials are used for making of wide variety of sophisticated engineering components used in aircraft, aerospace, and automobile industries. Titanium, being a light-weight material, is widely used in engine and compressor parts of an aircraft. Also, the biocompatibility of the titanium makes it a favorable material to be used in biomedical applications. On the other hand, the CFRP has found its application to be used for structural components. Micro machining is the important technology in advancing the miniaturization, which is gaining more and more attention due to the rapidly growing demand for micro parts and components especially in the following fields: semiconductor, biomedical device, micro fluid devices, automotive, and aerospace. Micro parts and components made up of titanium alloy can be found in the products such as: medical implants, fuel injection nozzles, micro-pump, micro-engines, etc. The small components of CFRP has been used in micro robot flight components such as bending actuator and hinge locking components. Both Ti6Al4V and CFRP comes in the category of difficult-to-machine materials as the tool wear during machining of these materials is very high. Micro machining, further is a more challenging task and needs the identification of the machining processes which can be employed for the purpose. Conventional micro machining of these type of materials is not a feasible process, as the forces induced during the machining cause the failure of the micro tool. In the last few decades, researchers have tried to conceptualize and develop the unconventional or advanced routes of processing, with an objective of reducing the damage, and improving the aspect ratio of the hole drilled. Electric discharge machining, being a prominent unconventional machining process, offers process capability of material removal without any physical contact between the tool and the workpiece. EDM is capable to produce good quality micro holes in titanium alloys as well as in CFRP laminates. In the present research endeavor, the electric discharge drilling behavior of Ti6Al4V and CFRP laminates has been experimentally investigated. Micro holes have been drilled in Ti6Al4V using micro electric discharge machining using different values of input process parameters such as voltage, capacitance and tool speed. Influence of input process parameters has been investigated on the output characteristics such as material removal rate, tool wear rate and surface roughness. Multi-objective optimization has been performed to identify the best combination of process parameters for maximum material removal rate, minimum tool wear rate and minimum surface roughness. Micro electric discharge drilling behavior of Ti6Al4V has been investigated using tool electrodes having modified geometrical features. Four different tool geometries i.e. (i) solid cylindrical, (ii) electrode featured with longitudinal micro slot, (iii) electrode featured with inclined micro slot, and (iv) electrode featured with inclined micro holes have been fabricated and used to drill blind micro holes. The objective of the featured slot/hole into the tool electrode is to provide a path for the debris to escape easily from the machining zone using the rotation of the electrode. The mechanism of debris removal has been explained with the help of images captured using high speed camera. Influence of the electrode geometries and the tool speed on the material removal rate, tool wear rate, hole taper angle, hole corner radius and aspect ratio has been investigated. Electrode featured with inclined micro holes has been found to be the best electrode providing a hole depth of 9.8 mm with a taper angle of 0.38 radian. Micro electric discharge drilling behavior of CFRP laminates has been investigated using three different tool geometries i.e. (i) solid cylindrical, (ii) electrode featured with single notch, and (iii) electrode featured with double notch. Electric discharge drilling has been performed by focusing on the conductive carbon fiber layer only. The tungsten carbide micro electrodes have been fabricated using wire electric discharge grinding. Influence of input process parameters (voltage, capacitance and tool speed) has been investigated on the output characteristics (material removal rate, tool wear rate, surface quality and aspect ratio). A comparison of aspect ratio has been performed among the similar previous experimental studies and the current investigation. It has been found that the electrode featured with single notch provides the aspect ratio of 29.17 which is much higher than reported in the literature. The work being reported in the present research initiative will help in extending the understanding of micro electric discharge drilling behavior of both Ti6Al4V and CFRPs. Many new questions and potential areas for work in future have arisen during the investigation, which shows that this area of research requires exhaustive studies to further enhance the understanding and knowledge in this field.