516 Time-resolved observation of the change of electrode-shape in single discharge of a thin electrode

Abstract

A very thin, needle-like electrode can be formed at the tip of thin tungsten electrode by a single discharge with rather high discharge current. We have demonstrated some practical applicability of such very thin electrode. Understanding the formation mechanism of such electrodes is an important issue to utilize the phenomenon to practical applications. We should see what happens on the electrode during the discharge. In this study, the change of electrode shape at tip part of a thin metal electrode induced by single discharge with a large current was observed dynamically by a stroboscopic imaging system with high time-resolution. In the imaging system, the second harmonics of a Q-switched Nd : YAG laser is used for lighting of CCD camera and it can take a photo with time-resolution about 10ns. Electrode materials were tungsten (W) and copper (Cu) of 100 μm diameter and the discharge current was 30 A with 400 μs duration. Both the positive and negative polarities of the electrodes in machining oil as well as in air were examined using steel work-peace. Rather drastic move of the melt formed by the discharge occurred after discharge in all cases. Observed images reveal that the electrode tip part after single discharge is in a molten state and the melt part moves upwards after the end of the discharge, and final electrode shape is formed at about 1 ms after the discharge. The thin, needle-like form of tungsten electrode is found to be the residue of un-melted portion of the electrode left after the move of melted part. Tungsten electrode becomes the needle-like shape when the thin electrode has negative polarity. When the polarity is reversed, or copper electrodes are used the tips of the electrodes become spherical forms. For copper electrode with negative polarity in air and at some specific time, droplets of molten metal are suspend in the space between the electrode and the work. The amount of melted electrodes is larger in air than in oil for both tungsten and copper.