Ahmed, Nuhaize
(2023)
Formation mechanisms and performance of electrical discharge coatings.
PhD thesis, University of Nottingham.
Abstract
Electrical discharge coatings (EDCs) are a variant of the electrical discharge machining (EDM) process, and can be applied on complex metal components to repair or act as a protective wear resistant layer. They can be used to make coatings up to several millimetres thickness on electrically conductive substrates using sacrificial electrodes. The technology can be classified under thin EDC, which is a surface modification process, and thick EDC, which is a surface coating process.
The aims of the thesis were to investigate the formation mechanism of thick EDCs, and to assess the functional properties to allow for process and coating improvements. This was carried out by first analysing EDM and EDC crater morphology and composition in order to understand the material removal mechanism and the effect of the current waveform. The differences between thin and thick EDCs were then defined in terms of the microstructure, composition, and residual stress level. Following this, the dry sliding wear and residual stress levels were compared to laser coatings in order to understand the benefits of the microstructure of thick EDCs.
Electrical discharge coatings were produced using an EDM machine with an EDC dedicated power supply, using sacrificial Co-Cr electrodes on stainless steel substrates. Individual thick EDC discharge craters were produced in order to understand the material transfer mechanism. A single thick EDC discharge crater was found to contain both melted and unmelted electrode material, indicating that both melting on the surfaces and within the gap, and direct transfer of electrode material, takes place within a single discharge. During a continuous coating process, material transfer was found to be dependent on the density of debris within the gap; at an ideal density of debris, uncontrolled and preferential sparking/arching, and the clustering of discharges, aided in material deposition and attachment.
The shape of the current waveform, which consists of a high and low current pulse section, was found to play a significant role in the formation of EDCs. Individual caters and their corresponding coatings were produced while varying the levels of the current pulses. The high current pulse section of the current waveform was found to contribute primarily to mechanical pull-out of electrode material by thermal shock induced fracture. The resulting high deposition efficiencies were found to decrease energy into the coating per volume of material, reducing the melting and re-melting of deposited material and resulting in thicker coatings with low tensile stresses up to 100 MPa, aided by interfacial sliding and edge relaxation.
The low current pulse section of the waveform was found to contribute primarily to increased melting, re-melting and agglomeration of material within the discharge gap, electrode and workpiece surfaces. The decrease in material transfer, due to the lower energy of the pulse, resulted in an increase in the energy into the coating for the melting and re-melting of material. By excluding the high current pulse, reduced material transfer and increased melting and quenching of material resulted in a more nano-structured or amorphous layer, producing thick coatings with compressive stresses up to -140 MPa due to amorphisation induced volume expansion.
By using a very low machining energy and increasing the gap voltage, thin EDCs could be formed. The significant reduction in electrode material transfer increased the energy into the coating per volume of coating material, resulting in complete melting and rapid quenching of material, producing highly tensile residual stresses of up to 490 MPa due to thermal expansion mismatch between the layer and substrate. These thin EDCs comprised of a mixture of a cobalt phase from the electrode, and an iron phase from the substrate, with the thickness being that of a standard EDM recast layer at around 10 µm, with highly tensile residual stresses.
By contributing to the fundamental understanding of the formation mechanism of thick EDCs and in the analysis of primary coating functional properties, greater control over the coating microstructure and resulting properties is possible. Through the present work, it is expected that industrial usage of thick EDCs in the repair and coating of parts may grow, and that the work may form the basis of further research on the production of improved thick EDCs composed of a variety of materials, and for a variety of applications.
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