Electrolytic-dielectrics: A route to zero recast electrical discharge machining

Electrical discharge machining (EDM) is a widely used manufacturing process for machining hard or high melting point metals. A characteristic feature of this process is a brittle, porous and rough recast layer on machined surfaces, which undermines integrity and limits its applications. Previous attempts to remove the recast have increased process steps and complexity, and in the case of EDM ECM hybrid processes have failed to understand or explain process mechanisms and have not proposed methods to control removal mechanisms to produce tailored surfaces. Here, we introduce tailored electrolytic-dielectrics that eliminate the defective recast layer, while maintaining machining rate. By adding selected electrolytes to a conventional deionised water dielectric in a standard EDM machine, material removal mechanisms can be altered and controlled in a simultaneous electrical discharge and electrochemical process, producing discrete surface morphologies through passive oxidation, aggressive pitting, and electro-polishing. These bespoke dielectrics alter fundamental machining behaviour to alternate between conventional EDM discharging and electrochemical dissolution. For the first time, it is shown that machining rate can be maintained while reducing, and eliminating, the defective EDM recast layer, which would otherwise need removal, in a single step process that combines the accuracy of EDM with the surface finish of ECM with the potential to produce high integrity surfaces with high throughput. The mechanisms behind this unique machining process are described through fundamental waveform and discharge analysis, which revealed discrete pulse types and their significance to the process, the importance of pulse ratio balance, the time-varying nature of the process, and the significance of high-voltage regions to electrochemical removal. Key process parameters are determined, such as EDM: ECM pulse ratio balance, electrolytic-dielectric conductivity, salt type, machining/exposure time, and electrode/servo advance, which are crucial to producing tailored surfaces. A model is devised to describe the overall balance between discharge based and dissolution based removal. Through this model, the point during machining at which complete recast layer removal occurs is predicted and validated through experimental analysis. This adapted EDM process has significant potential to be used for producing recast-free features by using an easily modified dielectric.