Wire Electric Discharge Machining of P91 Steel: Microstructure and Thermal Modeling Studies
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ADVANCED machining processes, such as laser beam, electron beam, and electric discharge machining (EDM), are used for fabricating components with intricate geometry in high strength steels.[1–3] The wire EDM (WEDM) process is more advanced than the die sinking EDM process as it enables intricate machining with a high degree of dimensional accuracy and surface finish, without the need of special shaped electrodes.[3] The tool electrode (wire) and the workpiece are not in direct physical contact; thus, machining forces and the effect of mechanical vibrations are negligibly small. The gap between the wire and the workpiece is maintained within 20 to 40 lm. The applied voltage causes ionization of the dielectric, and a spark is generated. The highly intense energy deposited on the workpiece through the arc leads to ablation of the material; thus, the workpiece is machined. WEDM is a thermal process in which intense heat is deposited by the electric discharge in a very thin layer of the material for a very short period of time (a few tens of
S. BHATTACHARYA is with the Homi Bhabha National Institute, Mumbai 400094, India and also with the Bhabha Atomic Research Centre, Mumbai 400085, India. Contact email: [email protected] A. MISHRA, N. KESHKAR, S. KUMAR, V. KAIN, and G.K. DEY are with the Bhabha Atomic Research Centre. Manuscript submitted December 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
microseconds of arc on time). The material is removed by melting and vaporization of the workpiece material through the heat of the plasma channel. The molten material is flushed out from the melting site in the form of debris due to the collapse of the plasma channel during the off cycle.[4] Rapid extraction of the heat by the cold workpiece by conduction and quenching of heat by surrounding fluid leads to rapid cooling of a thin layer of material, close to the cutting face. This rapid cooling leads to resolidification and quenching of the molten material, and it forms a thin layer on the machined surface called the recast layer.[5] The morphology of the machined surface and microstructural characteristics of the recast layer depend on the WEDM process parameters and thermophysical properties and physical metallurgy of the material. The recast layer may contain tensile residual stresses and cracks, which are detrimental for the fatigue life of the manufactured parts.[6] WEDM is widely used in the machining of hard materials and high precision parts; therefore, it has always been a subject of interest for researchers. It was reported that the average recast layer thickness and its standard deviation increase with the increase in spark energy.[7] During the off cycle, the implosion of the plasma channel and the flushing jet pressure remove the molten material at a constant rate; therefore, the increase in energy of the spark will only increase the molten pool volume, though the amount of flushed material is the same. This will increase the thickness of
the solidified or recast layer. Below the recast layer is a heat-affecte
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