The Effect of Electrical Discharge Machining Total Heat Generated on White Layer Thickness (WLT) and Fatigue Life for Die Steel

The present paper concerns with studying the high complexity nature of the EDM multiple discharge analysis transformed into a feasible solvable mathematical model for the die steel workpiece type AISI D2, the copper and graphite materials electrodes, and the kerosene dielectric by setting the Transient Thermal and the Multiphysics analyses domain loads models using the ANSYS 15.0 finite element analysis. Two load steps modeled the entering setting time analysis, six sub-periods setting time cycle, four heating, and two cooling periods, six transient temperature values, and four transient thermal convection models. The radius spark (discharge channel), the total number of discharges sparks, the total heat power generation, the absorbed heat flux fractions by the electrodes, the workpieces and kerosene fluid dielectric, the heat-affected zones (HAZ), the hard white recast layer thickness (WLT) and properties, the workpiece fatigue safety factor and life after EDM machining were determined and simulated. The thermal model errors compared with theoretical calculations and a modeled predicted equation were also deduced and verified. The experimental results evinced that the maximum total heat flux generated using the graphite material electrodes is (2.619E+009 W/m2) which is higher than when using copper material electrodes by (82.4%), while the minimum value of the white layer thickness (WLT) after EDM machining using graphite tool electrodes is (8.34 μm), which it gives an improvement comparing with using of copper tool electrodes by (40.0%). The macrographic and microstructure evaluation manifest that the discharge spark craters sizes when using graphite tool electrodes reached their sizes. The maximum fatigue stresses and fatigue safety factor when using copper tool electrodes are (240 MPa) and (0.89) which is higher by a value of (3.35%) and (3.45%) comparing with the using of graphite electrodes, respectively.

Investigation of crack density, white layer thickness, and material characterization of biocompatible material commercially pure titanium (grade-2) through a wire electric discharge matching process using a response surface methodology

Biocompatibility is usually observed as the absence of communication of a material with living tissue. Commercially pure titanium grade-2 has become the preferred biocompatible material for various devices mainly used in orthopedic and dental implants and it is also used in aviation and aircraft. Commercially pure titanium has good ductility, higher stiffness, and fatigue resistance. The novelty of the present research work was focused on studying the effect of wire electric discharge matching factors on surface roughness, material removal rate, crack density, and white layer thickness. After machining, the wire electro-discharge machined surface was analyzed through a scanning electron microscope. Further energy dispersive analysis and X-ray diffraction spectroscopy techniques were applied to investigate the material migration on a work surface and brass wire. Each output response has been modeled through analysis of variance to analyze the adequacy. It was observed that pulse on time, pulse off time, peak current, and spark gap voltage are the most significant factors. These factors have been significantly deteriorating the microstructure of machined samples remarkably deeper, leading to wider craters, globules of debris, and micro-cracks. A white layer thickness was also observed in a discontinuous and non-uniform pattern at the cross-section of the machined sample due to rapid heating and quenching phenomena in the wire electric discharge matching process. A multi-objective optimization “desirability” function was applied to obtain the optimal solutions by numerical and graphical methods. In essence, the wire electric discharge matching is a non-traditional process to achieve the accuracy of biocompatibility parts of commercially pure titanium and also finds the improvement in surface morphology.

Evolution of White Layer and Residual Stress in Electrical Discharge Machining

White layer and residual stress are the main reasons for the decrease in fatigue life of electrical discharge machined samples. Therefore, it is important to research the evolution of the white layer and residual stress in electrical discharge machining and explain the influence mechanism of machining parameters on them. In this study, the surface topography, white layer thickness, and residual stress of electrical discharge machined samples under different processing parameters were evaluated. The results indicated that surface roughness, white layer thickness, and residual stress increased as the discharge current (I) and pulse-on time (ton) increased. However, when the ton was short, the effect of I (≤ 9.8 A) on surface roughness is not very obvious. When the discharge energy is similar, surface roughness is high under high I conditions. When the discharge energy is similar and low, the average thickness of the white layer is thin under the low I. The effect of I on surface residual stress was greater than that of the ton. The I and ton affect the white layer and residual stress by affecting the amount of melting and removal of the materials. These results were demonstrated that the input process of discharge energy has an important influence on residual stress and the white layer. Therefore, under the premise of ensuring the processing requirements, they can be controlled by selecting the appropriate combination of the ton and I to improve the fatigue life of the workpiece.

A Brief Review of Experimental Investigations and Analytical Development of Powder Mixed Electric Discharge Machining (PMEDM)

Powder mixed electric discharge machining (PMEDM) is a newly developed technology in which EDM is performed by mixing electrically conductive micro or nanoparticles with dielectric fluid. The electrically conductive tiny particles when come at the gap of electrode and work piece, they will begin to create spark by the induction of electrode voltage which enhances the material removal and surface finish of the machined surface. In this paper a brief review has been done on different aspects of powder mixed electric discharge machining. It is observed that the researches are done in three main directions. Firstly, experimental studies are done to show the effect of several input process parameters on responses mainly material removal rate (MRR), surface roughness and tool wear rate. Secondly, the metallurgical characteristics of the machined surface are analyzed to measure the white layer thickness and amount of powder material inclusion onto the surface. The third one is the investigation of thermal characteristics of the tool and work pieces during the machining process. In these three sections of researches, the results of the investigations have been discussed in this review. Keywords: powder mixed electric discharge machining, metallurgical characteristics, nano particles, material removal rate, surface roughness, tool wear rate, white layer thickness, thermal characteristics

Cutting force based surface integrity soft-sensor when hard machining AISI 4140

Workpiece rim zone modifications during hard machining can be explained with the high thermo-mechanical loads induced by the cutting process. The formation of White Layers with a fine-grained microstructure by dynamic recrystallization (DRX) is one of those surface modifications that can negatively affect the functionality of a machined part by changing the residual stress state and facilitating crack initiation. As a consequence, the fatigue life of the machined parts is reduced. It is therefore of great interest to understand the thermo-mechanical conditions which induce White Layers formation in order to be able to control them by in-situ measurements if necessary. For this purpose, a cutting force based Soft-Sensor is developed in this study which enables the in-process estimation of White Layer thickness. Therefore, a cutting force based analytical model is used to estimate the resulting temperature fields and correlated with validated numerical chip formation simulations. In addition, the predictions of the White Layer thickness of the analytical model are then compared using light microscopy and the results of the numerical finite element model, in which a DRX model is additionally implemented.