ABSTRACT
The present study performs an experimental investigation to identify the EDM
processing parameters which suppress the formation of surface cracks in the
machined surface of SKD11 tool steel specimens. The rapid heating and cooling
effects inherent in the electro-discharge machining (EDM) process result in the
formation of a brittle recast layer on the machined surface. This layer increases the
surface roughness of the machined component and is characterized by globules of
debris, shallow craters, pockmarks, voids and cracks.
In the EDM trials, the specimens are machined using pulse currents of 4A, 16A or
32A with pulse-on durations of either 4μs or 16μs. The various specimens are then
fatigue tested at loads ranging from 150~245 Kg in order to determine their
respective fatigue lives. A polished SKD11 specimen is also fatigue tested for
comparison purposes. The results show that the EDM processing parameters have
a significant effect on the surface cracking characteristics. Specifically, when the
EDM process is performed using a pulse current of 4A and a pulse-on duration of
16μs, the machined surface is found to have a high surface crack density and some
of the cracks penetrate as far as the parent material. With processing parameters of
16A /4μs and 32A/16μs, a recast layer is formed on the surface of the machined
specimen, but no surface cracks are observed. Hence, it can be inferred that
increasing the pulse current and reducing the pulse-on duration provides an
effective means of suppressing the surface cracking phenomenon. Higher values of
the pulse current and pulse-on duration (i.e. 32A and 16μs, respectively) are found
to increase the average thickness of the recast layer and to prompt a corresponding
reduction in the fatigue life of the machined specimen.