Performance Evaluation of Electrical Discharge Machining of Titanium Alloy

Abstract

The EDM process uses electric discharges or sparks to machine material. These discharges are set between the workpiece and the tool electrode. A dielectric fluid is used in which both of these components, viz. workpiece and the electrode, are such that these two do not make a physical contact with each other. A voltage is applied across the tool electrode and the workpiece used. This voltage is increased linearly upto a point where the intensity of the electric field around the tool electrode and the workpiece used. This voltage is increased linearly upto a point where the intensity of the electric field around the tool electrode submerged in the dielectric becomes greater than the dielectric strength of the dielectric used. Due to this, the voltage breaks the barrier and finally electric discharges are produced emerging from the tool electrode and impacting on the surface of the workpiece. In this way, the discharges strike the surface and remove the material by increasing the localized temperature. These discharges are not continuous, but rather repetitive. Mhatre et al. (2014), did their work on the grey relational parameters machining of titanium alloys by non-conventional method such as EDM, of alloys like Ti-6Al-4V. TWR, MRR, and surface roughness are some of the parameters that were optimized. Results revealed that the copper electrode that is used gives optimum performance in the terms of lower tool wear rate and high material removal rate, and surface roughness of the workpiece samples used. The experiment that was performed on the Ti-6Al-4V to study out the performance characteristics by using the Taguchi method which was based on the investigation of both the significant parameters on MRR was found to be the peak current and the pulse on time. The least effective factor was pulse-off time (T_off) and servo voltage. Aspinwall et al. (2008), has studied the microhardness of the Ti-6Al-4V alloy which is machined with the help of EDM process. He stated that the change of machined surface has not produced due to the repetition of passes. He also concluded that the high voltage causes the increase in the microhardness upto a certain depth. Chen et al. (2007), observed that the hardness was increased from a value of 200 HV to 750 HV. He attributed these results to the formation of oxide layers TiO₂, TiNiO₃ and the debris that was found in the recast layers of the samples. Chen et al. (2008) investigated the machining characteristics of NiAlFe alloys using EDM process. The microstructure, composition, roughness and hardness of machined surfaces are discussed in this study. The Ti_35.5Ni_49.5Zr₁₅ high-temperature SMA is used as a comparative material. Sabouni et al. (2012) made research on EDM process parameter for NiTi SMA using graphite tool and they concluded that with increase in current MRR increases and when pulse on time increases MRR does not increase in this case as graphite is used as a tool material.MRR declines when pulse off time increases. Jatti and Singh (2014) studied the effects of cryogenic treatments on the machiniability of NiTi shape memory alloys workpieces in electro discharge machining. Due to cryo-treatment the electrical conductivity of workpiece exceptionally improved. Experimental results showed about 19% increase in material removal rate of cryogenic treated workpieces. Variations in tool wear rate were found to be marginal. Walkar et al. (2014) investigated the effect of magnetic field on the material removal rate and surface roughness, in conjunction with the variation of electrical parameters like pulse on-off times and gap current, while keeping other electrical parameters and work piece/ tool material constant. An experimental result showed that the magnetic field assisted EDM improves the process stability. Sadasiva Rao et al. (2012) investigated the effect of process parameters such as speed, feed and depth of cut and approach angle of the cutter on cutting force, tool life and surface roughness in face milling of Inconel 718. The experiments were carried out based on L₉ orthogonal array under dry conditions. Grey relational analysis was implemented to optimize the multi performance characteristics to minimize the cutting force and surface roughness and maximize the tool life criteria. Refaie et al. (2010) used Taguchi method grey analysis to determine the optimal combination of control parameters in milling. The measures of machining performance were material removal rate and surface roughness. Gupta et al. (2013) studied the optimization of unidirectional glass fibre reinforced plastic composite by using Taguchi method and grey relational analysis. Mixed L₁₈ orthogonal array is use during the study for conduction of experiments. Krishna et al. (2013) uses the Taguchi method for  the friction stir welding of dissimilar metal for tensile properties. In order to improve product quality and weld optimized welding conditions are evaluated for maximum tensile strength. Singh et al. (2012) studied the effect of continuous and discontinuous vibration on work piece. In this study experiments were carried out using Taguchi’s L₁₈ orthogonal array. For experimentation high chromium high carbon steel is being used as work piece and copper is being used as tool material. On the basis of experiment they concluded that discontinuous vibrations give more MRR and TWR. Also the intensity of development of cracks is more in case of discontinuous vibration as compared with continuous vibration. Sahoo et al. (2013) used response surface methodology and desirability approach to investigate cutting temperature during hard turning of EN24 steel. Optimization of processes parameter is obtained using desirability approach and prediction model was developed by response surface methodology. This observed value of hardness was found to be varying from the outermost surface of the machined sample to the innermost part of the sample.   

This study aims at Evaluation of mechanical properties and surface parameters of titanium alloy samples after electric discharge machining