Formation of thick electrical discharge coatings

Thick electrical discharge coatings, also known by the commercial name “MSCoating”, can be applied on complex shapes and cavities to repair components or act as protective coatings. A variant of the EDM process, it can be used to make coatings up to several mm thickness on electrically conductive substrates. In this paper, an insight into the microstructure and formation mechanism is made through the production of experimental coatings using sacrificial Stellite 31 electrodes on stainless steel substrates. The coatings comprised mostly of ‘splats’ of CoCr. In addition, a significant amount of directly deposited un-melted electrode material was present, along with regions of oxide splats and substantial porosity. Using single discharge analysis, individual deposits contained both melted and un-melted electrode material. Through analysis of the debris, it was found that electrode material either formed spherical particles through melting and resolidification within the discharge gap, or in the form of original unmelted electrode material. It is thought that the low peak pulse section of the current waveform results in increased melting and solidification of material within the discharge gap and on the workpiece. The high peak pulse section of the waveform results in increased mechanical pull-out from the electrode, a proportion of which attaches to the workpiece. The ratio between the peak pulses was found to affect the amount of energy per unit volume of coating material for re-melting. The process is then followed by several discharges preferentially located in the spark region resulting in coating consolidation and build-up through further deposition and re-melting of material. Clustering of discharges was found to be critical in coating formation, where such areas exhibited increased deposition of electrode material. Under the conditions used in the study, the threshold for electrode particle size was found to be between 45 and 70 μm, suggesting that powder diameter is fundamental to thick coating formation. The density of debris within the discharge gap, dependant on the Duty Factor, is thought to correlate with the rate of material deposition from electrode to substrate, where an ideal density of debris produces a large amount of uncontrolled and preferential sparking/arcing, enhancing depositon and attachment.