Modelling of single spark interactions during electrical discharge coating

Samer J Algodi,Adam T Clare,Paul D Brown

doi:10.1016/j.jmatprotec.2017.10.029

Samer J Algodi, Adam T Clare + Show 1 more

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https://doi.org/10.1016/j.jmatprotec.2017.10.029

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Abstract

Electrical discharge coating (EDC) methods may be used to enhance the surface functionality of electrical discharge machined components. However, industrial uptake of EDC has been restricted due to limited understanding of the fundamental interactions between energy source and workpiece material. The fraction of energy transferred to the workpiece, Fv, as a consequence of sparking, is an important parameter which affects directly crater geometry and the microstructural development of the near surface modified layer. In this paper, a 2D transient heat transfer model is presented using finite difference methods, validated against experimental observations, to estimate effective values for Fv as a function of processing conditions. Through this method we can predict coating layer thicknesses and microstructures through appropriate consideration of heat flow into the system. Estimates for crater depths compared well with experimentally determined values for coating layer thicknesses, which increased with the increasing fraction of energy transfer to the workpiece. Predictions for heat transfer and cooling of melt pools, arising from single spark events, compared well with experimental observations for the developed cermet microstructures. In particular, intermediate processing conditions were associated with the development of complex, banded, fine-grained microstructures, reflecting differences in localised cooling rates and the competing pathways for heat conduction into the substrate and convection within the dielectric fluid. Increased pulse-on times were associated with a propensity towards increasing grain size and columnar growth, reflecting the higher energies imparted into the coatings and slower cooling rates.

Highlights

Electrical discharge coating (EDC) is an adaptation of electrical discharge machining (EDM), which is able to deposit high melting point materials, such as hard-wearing, electrically conductive ceramics, onto a substrate, using a semi-sintered tool electrode
The fraction of total discharge energy transferred to the workpiece is an important factor determining temperature distribution within the modified layer; the modelling of which allows for prediction of the developed microstructure and correlation with layer mechanical properties (Algodi et al, 2016)
A theoretical model has been developed to predict the amount of energy transferred into the workpiece during the EDC processing of cermet coatings. 2D transient heat transfer principles solved by the finite difference method enabled coating thicknesses and microstructures to be predicted and validated against experimental observations

Summary

Introduction

Electrical discharge coating (EDC) is an adaptation of electrical discharge machining (EDM), which is able to deposit high melting point materials, such as hard-wearing, electrically conductive ceramics, onto a substrate, using a semi-sintered tool electrode. EDM is considered as a non-contact process that can be used to machine challenging electrically conductive materials, regardless of their mechanical properties of hardness, toughness or strength (Lin et al, 2008). The energy distribution into the workpiece material may be estimated with reference to the physical properties of the electrodes in question, by assuming either all the electrical energy is converted into heat (Singh and Shukla, 2012); or by using refined process models, using either analytical (Yeo et al, 2007) or numerical approaches (Das et al, 2003) based on underpinning electro-thermal and electromechanical mechanisms of material removal (Tan and Yeo, 2008). There is variability in the bases of these modelling approaches, for single spark events, which consider a variety of heat sources (e.g. point, cylindrical or Gaussian) and assume different levels of energy transfer, in order to calculate temperature distributions, crater geometries and the volume of material removed

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