The Process Fundamentals and Parameters of Electro-Spark Deposition
Tang, Siu Kei
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Electrospark Deposition (ESD) is a micro-bonding process that is capable of depositing wear and corrosion resistance coating to repair, to improve and to extend the service life of the components and tools. During the coating process, short duration of electrical pulses ranging from a few microseconds to milliseconds are used to deposit the electrode material to the component’s surface producing a protective layer. The low net heat input and the ability to form metallurgical bonding of coating to substrate are some of the noticeable advantages of ESD coating process. However, the influences of the controlling process parameters on the resulting coating are not well understood and documented. As a result, cracking and delaminating between the coating and substrate often occur. The objectives of this study were to enhance the current understanding of the ESD process, of the material transfer mechanism, of the influence of major process parameters on the resulting coating and of the bond between the electrode and the substrate. To accomplish these tasks, the ESD process was set up to produce one deposition each time. In the study, sintered Titanium Carbide particles/Nickel (TiCp/Ni) metal matrix composite (MMC) was used as the electrode to coat copper (Cu) substrate. The movement of the depositing TiCp/Ni electrode was strictly controlled in static mode experiments. Meanwhile, in dynamic mode experiments, the electrode movement was governed by a spring mechanism. In addition, Nickel was also used as both coating electrode and receiving substrate to gain insight into the bonding mechanism. The current, voltage and the electrode displacement were measured by a PC computer-based data acquisition system. Based on direct observations of the experiments, a phenomenological model was developed to detail the events taking place during a single deposition in both static and dynamic modes. The process began with the ESD power supply switching to the discharging mode. A spark was initiated as the electrode came into contact with the substrate. This initial spark partially melted both the substrate and electrode. The spark also expelled any molten substrate outward to form a crater. The expelled molten substrate re-solidified on the edge of the newly formed crater. The electrical power stored inside the capacitors of the ESD power supply was only partially discharged due to the formation of a narrow gap between the electrode and copper substrate. At this stage, no material was transferred from the electrode to copper substrate. The continuous forward motion brought the electrode into contact with the substrate again. This facilitated the material transfer from the electrode to the substrate. The experimental results indicate that the material transferred between the coating electrode and the receiving substrate is primarily through direct molten metal – molten metal contact. At this second contact, the ESD power supply completely discharged the remaining electrical power. The sparking and the molten metal expulsion are responsible for removing contaminated materials from the contacting surfaces. This would result in a defect-free bonding interface. The set voltage is a major controlling parameter of the ESD process. The effects of voltage on the ESD coating were studied using 25V, 35V, 45V and 65V. A high voltage provided higher heat input and better cleaning action since more of the receiving substrate was melted and expelled further away from the depositing location. As a result, the high voltage reduced the overall number of cracks and shortened the crack lengths that were typically found at the coating and substrate interface. This would improve the bonding strength between the coating and the substrate. Although high voltage eroded and expelled more of the receiving substrate, it also increased the amount of electrode material depositing onto the substrate. To gain insight into the bonding mechanism between TiCp/Ni metal matrix composite and copper substrate, three different coating electrode – substrate combinations were used. They were TiCp/Ni electrode and Cu substrate, TiCp/Ni electrode and Ni substrate and Ni electrode and Cu substrate. In the Ni-Cu combination, a metallurgical bond was formed with the existence of an intermixing layer as predicted by the nickel and copper phase diagram. In the TiCp/Ni and Ni combination, the nickel diffused from the substrate into the TiCp/Ni coating. There was no intermetallic phase at the bonding interface between TiCp/Ni electrode and Cu substrate. The experimental results suggest that Ni in the TiCp/Ni metal matrix composite has low mobility since nickel is used primarily as binding agent. The bond is formed by the diffusion of the copper into the metal matrix composite without any intermetallic formation.