Bridges of debris in the EDD process: Going beyond the thermo-electrical model

Electro-discharge (ED) processes depend on the contemporaneous effect of many factors, which complicates process control/predictability and induced many authors in the last 60 years to work on explicative models. Studies split into two main approaches: theoretical and empirical. Theoretical works are based on the thermo-electrical theory and try to describe process phenomena by a physical model, with unavoidable assumptions and simplifications that cause a move away from veracity. On the contrary, experimentalists establish empirical models based on statistical analysis of results and optimization methods, but the findings are limited to domain-specific applications. In addition, numerous papers focus on single-spark analysis, failing in considering the interaction between successive discharges, or of transient phenomena as the presence of bubbles and debris in the gap. At present the scientific debate involves the ignition model, with two different points of view regarding the discharge-driving phenomenon: the debris bridge effect (pollutants in the dielectric drive the performances), and the dielectric strength effect (properties of the dielectric drive the performances). The paper addresses this dispute by investigating the debris formed during small-hole ED drilling of a 72 wt% Al2O3–28 wt% TiC composite. Particles are found to hollow out and pack within the gap, joining by necks. The first experimental evidence is given of the presence of chains and clusters of debris, towards a new model for electro-discharge processes that goes beyond the thermoelectric theory.