Characterization of electrical discharge machining plasmas

Electrical Discharge Machining (EDM) is a well-known machining technique since more than fifty years. Its principle is to use the eroding effect on the electrodes of successive electric spark discharges created in a dielectric liquid. EDM is nowadays widely-used in a large number of industrial areas. Nevertheless, few studies have been done on the discharge itself and on the plasma created during this process. Further improvements of EDM, especially for micro-machining, require a better control and understanding of the discharge and of its interaction with the electrodes. In this work, the different phases of the EDM process and the properties of the EDM plasma have been systematically investigated with electrical measurements, with imaging and with time- and spatially-resolved optical emission spectroscopy. The pre-breakdown phase in water is characterized by the generation of numerous small hydrogen bubbles, created by electrolysis. Since streamers propagate more easily in a gaseous medium, these bubbles can facilitate the breakdown process. In oil, no bubbles are observed. Therefore, the breakdown mechanism in oil could be rather enhanced by particles present in the electrode gap. Fast pulses of current and light are simultaneously measured during the pre-breakdown. These pulses are characteristic of the propagation of streamers in the dielectric liquid. The pre-breakdown duration is not constant for given discharge parameters, but distributed following a Weibull distribution. This shows that the breakdown is of stochastic nature. After the breakdown, the plasma develops very rapidly (< 50 ns) and then remains stable. The plasma light is particularly intense during the first 500 ns after the breakdown and weaker during the rest of the discharge, depending on the current intensity. While the gap distance is estimated to be around 10–100 μm, the discharge excites a broad volume around the electrode gap, typically 200 μm in diameter. This volume grows slightly during the discharge. Vapor bubbles are generated in water and in oil by the heat released from the plasma. At the end of the discharge, the plasma implodes and disappears quickly. Light is still emitted after the discharge by incandescent metallic particles coming from the erosion of the workpiece. Their temperature is measured around 2'200 K, demonstrating that they are still in a liquid state in the beginning of the post-discharge. The spectroscopic analysis of the plasma light shows a strong Hα and continuum radiation, with many atomic metallic lines emitted by impurities coming from electrode and workpiece materials. The EDM plasma is thus composed of species coming from the cracking of the dielectric molecules (mainly hydrogen in the case of water and oil), with contamination from the electrodes. The contamination is slightly higher in the vicinity of each electrode, and the contamination from the workpiece increases during the discharge probably due to vaporization. The electron temperature, measured from copper line intensities with the two-line method, is found to be low. The temperature is around 0.7 eV (∼ 8'100 K) in the whole plasma, slightly higher in the beginning of the discharge. The electron density has been measured from Stark broadening and shift measurements of the Hα line. The density is extremely high, especially at the beginning of the discharge (> 2·1018 cm-3 during the first microsecond). Then it decreases with time, remaining nevertheless above 1016 cm-3 after 50 μs. During the whole discharge, the density is slightly higher in the plasma center. The EDM plasma has such a high density because it is formed from a liquid, and because it is constantly submitted to the pressure imposed by the surrounding liquid. This extreme density produces spectra with strongly-broadened spectral lines, especially the Hα line, and with an important continuum. During the first microsecond when the density is at its maximum, spectral lines are so broadened that they are all merged into a continuum. The low temperature and the high density of the EDM plasma make it weakly non-ideal. Its typical coupling parameter Γ is indeed around 0.3, reaching 0.45 during the first microsecond. In this plasma, the Coulomb interactions between the charged particles are thus of the same order as the mean thermal energy of the particles, which produces coupling phenomena. Spectroscopic results confirm the non-ideality of the EDM plasma. The strong broadening and shift of the Hα line and its asymmetric shape and complex structure, the absence of the Hβ line, and the merging of spectral lines are typical of nonideal plasmas. The EDM plasma has thus extreme physical properties, and the physics involved is astonishingly complex.