Expanding heat source model for thermal spalling of TiB₂ in electrical discharge machining

Abstract

A model is presented to explain the recently reported mechanism of thermal spalling for shaping high melting point ceramics by electrical discharge machining. Since previous models fail to explain the experimental observations completely, an expanding circular heat source created by growth of plasma is assumed to act on the surface. Erosion of materials by spalling is caused by thermally induced compressive stresses during heating-up periods and tensile stresses during cooling-down periods. This model explains material removal for anodic erosion in general (wire-cutting machines) and for cathodic erosion (die-sinking machines) whenever long pulse duration is used. Simulation of the model for TiB2 provides a local melt front that penetrates to a depth of submicrometer, then recedes as pulse duration increases. Spalling develops flakes with thickness correlated to pulse duration. The results were verified by the experimental observations which showed that large flakes having the predicted maximum thickness as well as few quenched spherical droplets containing titanium were obtained.

Fabrication of a hydroxyapatite-containing coating on Ti–Ta alloy by electrical discharge coating and hydrothermal treatment

“…The pulse duration shows the similar effect that is the MRR increased when the pulse duration was raised. However, over-extending the pulse duration resulted in a decreased current density because of the expansion of the plasma channel [15]. Furthermore, adding HA powder significantly reduced the MRR and EWR, which can be clarified by voltage and current curves measured during EDC, as shown in Fig.…”

A review on optimization of machining performances and recent developments in electro discharge machining

“…The model could predict flake thickness on surface. This model could be used for diesinking EDM and wire-cutting machine [124].…”

Electrical discharge machining of a NiAlFe ternary shape memory alloy

“…The MRR is also expected to increase with pulse duration, because its high-accumulated electro-discharge energy will rapidly melt and evaporate the material. Moreover, an extended pulse duration will expand the plasma channel [25]. The EDM covered area is widened and the average energy density is decreased, leading to a decrease in MRR.…”

“…High-discharge current affords the large amount of discharges to strike the workpiece surface more intensely, and the resulting worsened erosion effect brings about a deterioration of the surface roughness. Furthermore, the longer pulse duration gives greater discharge energy to melt and penetrate deeper into the material, which generates larger and deeper discharge craters, leading to an increased surface roughness on the workpiece [25]. Based on the above discussion, an excellent machined surface finish on NiAlFe SMA can be obtained by setting the machine parameters at a lower discharge current I P and a shorter pulse duration τ P , but this approach takes more time.…”