Expanding heat source model for thermal spalling of TiB 2 in electrical discharge machining
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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 TiB 2 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.
I. INTRODUCTION Electrical discharge machining (EDM) is a nontraditional technique which has recently been applied to the intricate shaping of hard brittle ceramics with high melting points. While in traditional techniques solid-to-solid contact between a hard abrasive tool and the workpiece is required, in EDM no physical force is exerted and the tool and workpiece are separated by a dielectric fluid. Moreover, intricate shapes having contours that are difficult to reach by normal techniques are expected to be easily machined with high accuracy using EDM. Unlike traditional techniques, EDM does not depend on material hardness but requires that the resistivity of the workpiece be lower than 100 fl-cm. 1 The fundamental principle of EDM is the erosion of materials as a result of a spark discharge between the tool and the workpiece which are connected to two electrodes. The type of EDM machines used, operating conditions, and surface quality were recently reported by Petrofes and Gadalla.1 There are two kinds of electrical discharge machines: die-sinking and wire-cutting. The die-sinking type uses a die to fabricate the workpiece which is generally connected to the cathode, and the wire-cutting EDM uses a metallic wire to cut the workpiece, which is connected to the anode. During EDM, controlled electrical sparks which create plasma are the main source of energy supplied to material surfaces. Transfer of intensified high thermal energy from the plasma to the material surface causes extremely high internal temperature gradients in suba)
Author to whom correspondence should be addressed. J. Mater. Res., Vol. 7, No. 10, Oct 1992
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layers of surface material. If melting and evaporation are the operating mechanisms, the erosion mechanism and rate of material removal are given by a series of publications2'3 for single phase isotropic materials. The melting mechanism has been reported for m
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