Performance Assessment of Rapid Tool Electrodes During Electrical Discharge Machining of Titanium and Its Alloy

Abstract

Electrical discharge machining (EDM) is a non-conventional machining process, which is widely used these days, for machining of difficult-to-machine materials or where other conventional machining processes are unable to machine the materials. In this dissertation work, possibility of potential application of tool electrodes made of copper, tungsten and boron carbide manufactured by conventional powder metallurgy (PM), microwave sintering (MWS) and spark plasma sintering (SPS) route has been explored during electrical discharge machining of titanium alloy (Ti6Al4V) work piece. In addition to usual performance measures like material removal rate and tool wear rate, surface integrity of the machined surface in terms of surface roughness, surface crack density, white layer thickness and micro-hardness on white layer has been emphasized. Effect of change of weight percentage of tungsten and boron carbide in the tool on the listed performance measures is evaluated with respect to a traditionally used solid copper tool. The experimental plan is made based on Taguchi’s orthogonal array for different tool electrode and work piece combinations so that maximum process related information can be gathered with limited number of experiments. Analysis of data gathered through extensive experimentation reveals that the tools can be made as good as normally used copper tool electrode provided that proper ratio of weight percentage of different powders is maintained in case of preparing the tool electrodes by conventional powder metallurgy route. However, compaction of the powders and sintering of the green sample during preparation of the tool are still major concerns of the tool makers. Densification of the tool plays a major role in delivering adequate performance in machining otherwise material deposition on the work piece occurs rather than machining. In case of microwave sintering, it is observed that increase in weight percentage of tungsten and boron carbide makes the tool less dense because bonding of powder elements such as tungsten and boron carbide with copper becomes difficult. Therefore, increase in weight percentage of tungsten and boron carbide in the microwave sintered tools exhibit excessive tool wear and increased surface roughness, white layer thickness and micro-hardness on the machined surface with reduced material removal rate. The microwave sintered tool is no way superior to conventional solid copper tool insofar as the performance measures considered in this dissertation work. The tool electrodes produced by spark plasma sintering process exhibit low tool wear as compared to conventional PM tool electrodes and microwave sintered tool electrodes at higher percentage of tungsten and boron carbide in tool electrodes (weight percentage of fifteen each). As far as cracks on the machined surface are concerned, more surface cracks are found with the use of solid copper electrode in comparison to composite tool electrodes produced by spark plasma sintering process with any weight percentage of tungsten and boron carbide. Therefore, it can be concluded that spark plasma sintered tool electrodes perform in a superior manner when integrity of the machined surface is emphasized. The possibility of tool electrodes composed of aluminium, silicon and manganese powders (AlSi10Mg) produced through additive manufacturing route (selective laser sintering) has been explored for potential application in electrical discharge machining of titanium and titanium alloy (Ti6Al4V). The performance of the tool produced by selective laser sintering (SLS) process is compared with conventional copper and graphite tool electrodes. It is found that higher material removal rate can be achieved with the use of graphite electrode followed by copper and AlSi10Mg SLS electrodes. Similarly, lower tool wear can be achieved by the usage of graphite electrode followed by copper and AlSi10Mg SLS electrodes. It is concluded that AlSi10Mg SLS electrode can be conveniently used if superior surface integrity (low value of surface roughness, surface crack density, white layer thickness, and micro-hardness) is desired. In this regard, AlSi10Mg SLS electrode performs well followed by copper and graphite electrode. EDX analysis of machined work surface reveals that transfer of electrode material onto the machined work piece surface occurs during machining. Presence of silicon and magnesium on the machined work surface with increased percentage of aluminium, oxygen and carbon is found in case of usage of AlSi10Mg SLS electrode. EDX analysis of machined electrode surface reveals the transfer of work piece materials like titanium, vanadium and aluminium onto the electrode surface during the machining of titanium alloy. Optimization of the process parameters is made with the use of well tested multi-objective optimization techniques such as grey relational analysis (GRA), technique for order preference through similarity to ideal solution (TOPSIS), Grey-TOPSIS, grey-TOPSIS combined with quantum behaved particle swarm optimization (QPSO), desirability based grey relational analysis combined with firefly algorithm (FA). This is to be noted that tool fabrication cost and time can be substantially reduced if the tools with complex shape is made by powder metallurgy or additive manufacturing route rather than making the tool by conventional machining processes.