Influence of Input Parameters on the Characteristics of the EDM ...

Strojniški vestnik - Journal <strong>of</strong> Mechanical Engineering 57(2011)9, 689-696Table 1. Mechanical and physical properties <strong>of</strong> AISI H13 [10]Temperature[°C]Density[kg/dm 3 ]Specific heat[J/(kg·K)]Electricalresistivity[Ω·mm 2 /m]Modulus <strong>of</strong>elasticity[N/mm 2 ]Thermalconductivity[W/m·K]20°C 7.80 460 0.52 215×10 3 24.30500°C 7.64 550 0.86 176×10 3 27.70600°C 7.60 590 0.96 165×10 3 27.50Liquidus temperature 1454 °C Solidus temperature 1315 °C[8], the optimum utilization <strong>of</strong> the EDM processrequires the selection <strong>of</strong> an appropriate set <strong>of</strong>machining parameters that would result in theminimum thickness <strong>of</strong> the recast layer and depth<strong>of</strong> heat affected zone [9].This paper aims to fill the gap in theexisting literature with respect to the processing<strong>of</strong> AISI H13 tool steel with EDM. In particular,EDM machining experiments were conductedon AISI H13 samples having a hardness <strong>of</strong> 52.7HRC using copper electrode to investigate thecorrelations between the EDM parameters (pulseon-time and current) and the EDM characteristics<strong>of</strong> such a workpiece. The output factorsinvestigated were the material removal rate, toolwear ratio, surface roughness, as well as thethickness <strong>of</strong> white layer and depth <strong>of</strong> heat affectedzone <strong>of</strong> EDMed workpiece. This experimentalstudy results in the selection <strong>of</strong> optimum processparameters to achieve the desired EDM efficiency,surface roughness, and surface integrity whenmachining such a workpiece material.1 EXPERIMENTAL SETUP AND PROCEDUREThe workpiece material used in this studywas AISI H13 tool steel. Prior to EDM processing,the workpiece was cut in a cylindrical shape witha length <strong>of</strong> 20 mm and a diameter <strong>of</strong> 20 mm. Themain mechanical and physical properties <strong>of</strong> suchworkpiece material at different temperatures aregiven in Table 1.The tool material was forged commercialpure copper with the main properties given inTable 2. The experiments were performed on a diesinking EDM machine (CHARMILLES ROBO-FORM200) which operates with an iso-pulsegenerator. Machining tests were carried out at fivepulse current settings, as well as four pulse ontimesettings. As a result, 20 experiments could bedesigned. Each machining test was performed for15 minutes. Table 3 presents the experimental testconditions.Table 2. Physical properties <strong>of</strong> copper electrode[11]Physical properties CopperThermal conductivity [W/m·K] 380.7Melting point [°C] 1083Boiling temperature [°C] 2595Specific heat [cal/g·°C] 0.092Specific gravity at 20°C [g/cm 3 ] 8.9Coefficient <strong>of</strong> thermal expansion[×10 -6 (1/°C)]17A digital balance (CP2245-Surtorius) witha resolution <strong>of</strong> 0.1 mg was used for weighingthe workpieces before and after the machiningprocess. The tool wear ratio is defined as thevolume <strong>of</strong> material removed from the tool (V E )divided by the volume <strong>of</strong> material removed fromthe workpiece (V W ). Eqs. (1) and (2) show thecalculations used for assessing the values <strong>of</strong> MRRand TWR.MRR = (M 1 – M 2 ) / ( ρ w·T) , (1)TWR = (V E / V W )·100% , (2)where M 1 and M 2 are the weight <strong>of</strong> workpiecebefore and after machining [g], respectively. ρ wis the density <strong>of</strong> workpiece [g/mm 3 ], and T is themachining time [min].According to Lee and Tai [12], theamount <strong>of</strong> white layer thickness (WT) has beenmeasured by measuring this layer’s thicknessat 30 different points by utilizing VEGA\\TESCAN scanning electron microscopy (SEM)and accounting for their average (Figs. 1 to3). Therefore, the machined specimens weresectioned transversely by a wire electrical690 Shabgard, M. – Seyedzavvar, M. – Oliaei, S.N.B.