Process developments in electrochemical arc machining

Electrochemical arc machining (ECAM) utilises pulsed power across a cathodetool and an anode-workpiece, separated by a gap filled with electrolyte, in order to achieve both electrochemical dissolution (ECD) and electrodischarge erosion (EDE) of the workpiece. This thesis describes some further developments made in the ECAM process. The feasibility of using ECAM to drill holes in several alloys of industrial interest, which are difficult to machine by conventional methods, was investigated. The effects of the machining variables on the process parameters were evaluated and the optimum machining conditions established for each of the five materials drilled: chrome steel, cobalt alloy, nickel alloy, titanium and low alloy steel. The surface effects on the alloys drilled by ECAM were analysed by means of optical and scanning electron microscopy and microhardness testing. These revealed that for most of the alloys, the heat induced damage due to EDE phase can be eliminated by the ECD phase, leaving smooth, damage free surfaces. However, the last few millimeters of the hole exhibited EDE induced damage due to lack of electrolyte at the exit, which prevented the ECD action. One exception was titanium, which showed typical EDE damage consistently along the hole. This is because of the oxide film which develops in water based, salt electrolytes, inhibiting the the ECD action. Occasionally, some localised areas were found to exhibit extensive surface damage in the form of large craters, grain boundary cracking and microcracking. This type of damage was attributed to abnormal discharges. These are the prolonged discharges which occur in the same location causing severe damage to both the workpiece and the tool. In order to prevent these undesirable abnormal discharges an analysis into the gap phenomena in ECAM was undertaken. The radio frequency emission from the gap was used to differentiate between the normal and the abnormal discharges, since the RF level was found to be considerably higher for normal discharges than for abnormal ones. The monitoring of the RF signal together with the current or voltage can give a clear indication of the gap situation ; whether it is electrochemical action, a normal discharge, an abnormal arc or a short circuit. A theoretical model has been developed for the RF emission from the gap, assuming the discharge to be a small Hertzian dipole antenna. Finally, as an industrial application of the ECAM process, a portable ECAM drilling apparatus was designed and built. This can be used successfully to drill holes of high depth/diameter ratio. in large steel structures. One specific application for the Electricity Generating Industry is the drilling of holes in boilers to insert thermocouples