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Research and Development of Molecular Dynamics Simulation Software for Micro EDM Discharge Characteristics

Author: HanXiao
Tutor: YangXiaoDong
School: Harbin Institute of Technology
Course: Mechanical Manufacturing and Automation
Keywords: micro-EDM molecular dynamics ablation mechanism hydrostaticpressure thermal stress residual stress
CLC: TG661
Type: Master's thesis
Year: 2013
Downloads: 52
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Abstract


Electrical Discharge Machining (EDM) is a thermal process, the essence of itsprocess is that applying an intense thermal effect generated by a single pulsedischarge between the tool electrode and the workpiece electrode, changing thetemperature distribution and material structure of the electrode and thenachieving material ablation. EDM process involves a combined effect of the fluidforce field, the magnetic field and thermal conduction and other complex effect,resulting in a characteristic of complex and random. And due to the limit of itsminimal time scale and spatial scale, it is difficult to define the physical nature ofEDM process whether by experimental observations or theoretical study. So untilnow there’s still not a complete and unified theory which has a universalsignificance to reveal the mechanism of EDM, resulting a greater limit on practicalapplication and further development of EDM.Molecular dynamics (MD) is an effective method to describe the microscopicbehavior of atoms. With the development of high-perfomance computer andparallel algorithms, the size of MD simulation is increasingly, meanwhile nanoscaleEDM has also been possible to achieve, thus making the MD simulation scale iscomparable to the actual machining scale of micro-EDM. In this paper, a moleculardynamics simulation software of micro-EDM based on MPI message passingmodel was developed, which was used to simulate and study the machining processand the removal mechanism of micro-EDM.In this paper, the formation process of discharge crater and the physicalphenomenas accompanied such as the temperature distribution and materialstructure of the electrode during a single pulse discharge EDM process wassimulated and studied. It was found that the material ablation occurs right after thebeginning of discharge rather than after the end of the discharge, and the ablationoccurs mostly during the discharge duration. In addition, the melting atoms andablation atoms statistics shown that not all of melting atoms were ablated, the othermelting atoms remained in the bottom of the crater as resolidified layer at last,some of those remained in the bottom of the crater, others occurred plastic flow,forming the bulges around the crater. Besides, the change of temperaturedistribution has also led to changes in the crystal structure of the heat surface of electrode material, resulting different degrees of dislocations, and there was stillresidual dislocations and plastic defoemation in the suface and internal of the finalcrater.Electrical discharge machining (EDM) is a thermal process. The temperaturerises instantaneously during the discharge, resulting a great temperature gradientinside the electrode which led to thermal stress. Thermal stress during the dischargehas a direct influence on ablation process and discharge crater formation. Theresidual stress is one of the key factors affecting the machined surface quality and itsfunctional performance. In this paper, the hydrostatic pressure distribution inmelting area was simulated. The analysis shows that the pressure generated insidethe melting area is even higher than on the surface during discharge duration. Theincreasing pressure gradient along the depth direction of melting area can beconsidered the main mechanism of material removal in EDM. In addition, theformation mechanism of the thermal stress and residual stress, and thierdistribution were simulated, It was found that there existed larger stress gradient inthe melting region and in the solid region in contact with the melting area. Theshear stress field is the important reason for the formation of the bulge of thedischarge crater. From the distribution of the residual stress, it was found that theresidual stress on the electrode surface is tensile while it is compressive inside theelectrode, indicating that cracks are easy to be generated in the re-solidified layer.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metal cutting and machine tools > Special machine tools and processing > Electric machines and their processing
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