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PurposeThis paper aims to study the breakdown, oscillation and vanishing of the discharge channel and its influence on crater formation with simulation and experimental methods. The experiment results verified the effect of the oscillating characteristics of the discharge channel on the shape of the crater.Design/methodology/approachA mathematical model that considers the magnetohydrodynamics (MHD) and the discharge channel oscillation was established. The micro process of discharging based on magnetic-fluid coupling during electrical discharge machining (EDM) was simulated. The breakdown, oscillation and vanishing stage of the discharge channel were analyzed, and the crater after machining was obtained. Finally, a single-pulse discharge experiment during EDM was conducted to verify the simulation model.FindingsDuring the breakdown of the discharge channel, the electrons move towards the center of the discharge channel. The electrons at the end diverge due to the action of water resistance, making the discharge channel appear wide at both ends and narrow in the middle, showing the pinch effect. Due to the mutual attraction of electrons and positive ions in the channel, the transverse oscillation of the discharge channel is shown on the micro level. Therefore, the position of the discharge point on the workpiece changes. The longitudinal oscillation in the discharge channel causes the molten pool on the workpiece to be ejected due to the changing pressure. The experimental results show that the shape of the crater is similar to that in the simulation, which verifies the correctness of the simulation results and also proves that the crater generated by the single pulse discharge is essentially the result of the interaction between transverse wave and longitudinal wave.Originality/valueIn this paper, the simulation of the discharge breakdown process in EDM was carried out, and a new mathematical model that considers the MHD and the discharge channel oscillation was established. Based on the MHD module, the discharge breakdown, oscillation and vanishing stages were simulated, and the velocity field and pressure field in the discharge area were obtained.

To study the change of residual stress during heating and solidification of SiCp/Al composites, a one-way FSI (Fluid Structure Interaction) model for the solidification process of the molten material is presented. The model used process parameters to obtain the temperature distribution, liquid and solid-state material transformation, and residual stress. The crack initiated by the thermal stress in the recast layer was investigated, and a mathematical model of crack tip stress was proposed. The results showed a wide range of residual stresses from 44 MPa to 404 MPa. The model is validated using experimental data with three points on the surface layer.

Worn cemented carbide tool bits are often discarded because of the difficulty of their repair, resulting in a great deal of waste. Surface strengthening technology often extends the service life of worn tools. Electro-spark deposition (ESD) coating and matrix materials are metallurgically and closely bonded, and the approach has the characteristics of small heat input, a small heat-affected zone, and low repair cost, so it is suitable for strengthening the surface of cemented carbide tools. As the surface of cemented carbide tools is often not flat, which affects the uniformity of the deposited layer, the surface needs to be polished before ESD. Therefore, this paper proposed a method involving the electro-spark additive and subtractive repair of worn cemented carbide. Experiments involving the ultrasonic-assisted EDM grinding (UEDG) of cemented carbide were carried out. The effect of brass, 45 steel, and tungsten electrode materials on the removal rate, tool wear, and surface roughness were investigated. The results showed that the material removal rate of the tungsten electrode could reach 3.27 mm3/min, while the electrode loss was only 8.16%, and the average surface roughness was only 2.465 μm, which was better than the other two electrodes. Thus, the tungsten electrode exhibited a high material removal rate, low electrode loss, and good surface quality. The effects of the TiC, TiN, and TC4 electrodes on cemented carbide ESD were studied using optical 3D surface topography and other instruments, and the surface roughness, thickness, and hardness of the deposited layer were compared. The results showed that the surface roughness of the TC4 material reached 52.726 μm, which was better than that of the TiN and TiC materials. The thickness of the TiC deposition layer was 172.409 μm and the hardness value was 2231.9 HV; thus, the thickness and hardness of the TiC material’s sedimentary layer were better than those of the TiN and TC4 materials.