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Electro-discharge machining (EDM) of thin deep features widens processing opportunities of difficult-to-cut materials, such as Ni-based alloys, towards attractive industrial applications. Conversely, the complex interaction between electrical, thermal, and chemical phenomena in EDM thwarts process modeling and prediction. As a matter of fact, recent experimental discoveries encourage setting forth a new theory of discharge ignition in the gap, based on the role of debris, which lays foundations for a recursive mathematical model showing a chaotic evolution. The paper reports on electro-discharge drilling of small deep holes in Inconel 718. Process performances are measured with varying electrode size and geometry, attesting a pivotal effect of gap pollution on productivity and on the onset of a secondary detrimental removal of material by intergranular corrosion. All findings support the emerging model for discharge ignition via debris bridges, and, most notably, debris chains are documented for the first time in the field of metals, second only to a previous case for a ceramic composite. On the whole, the paper provides conclusive validation of the role of debris in the ignition of discharges. [ABSTRACT FROM AUTHOR]

The long-accepted thermoelectric model for electro-discharge machining is being brought into question. Several experimental facts prompt the proposal of a new theory based on the effect of gap pollution on the ignition of discharges. The first experimental proof comes from the recently reported observation of debris chains and clusters. In this view, each step of the process depends on the previous ones through a deterministic relation, even if the overall evolution is unpredictable. The paper establishes mathematical grounds for the abovementioned intuitions by setting up and solving a recursive equation for the machining energy employed at each discharge event. By means of numerical and algebraic tools, the above equation is studied and shows a chaotic evolution similar to that of the logistic map. Results reconcile the apparent paradox between deterministic nature and stochastic localization of the discharges and introduce a description of the chaotic dynamics of electro-discharge processes. [ABSTRACT FROM AUTHOR]

Ceramic matrix composites (CMCs) can be attractive for structural applications, but their machining by conventional methods is expensive and often critical. Complex geometries on advanced ceramics require contactless processes, such as electrodischarge machining or drilling (EDD). These proved to be viable for CMCs with electrical resistivity below a critical value in the range of 1-3 Ω m. The condition is complied with by many CMCs: an example is alumina with titanium carbide. Material removal of ceramics by electrical discharges is a complex process involving different mechanisms, depending on the process setup. The paper describes an experimental study on EDD of 0.4-mm diameter holes with an aspect ratio of 20 in AlO-TiC, using copper electrodes. Peak current ( I), pulse-on time ( t), and pulse-off time ( t) are varied as independent variables. Four performance indicators are measured: material removal rate, electrode wear rate, overcut, and surface roughness ( R, S). Empirical models are proposed to describe the effect of process parameters on the output indicators. The analysis is supported by the observation of the surface and subsurface morphology, with the aim of investigating the material removal mechanisms and attaining a full comprehension of macroscopic results. It is found that removal mostly occurs by melting and evaporation and that surface morphology is determined by two phenomena ruled by pulse power. A process description is proposed, built around power as the ruling factor. [ABSTRACT FROM AUTHOR]

Cobalt-base alloys are generally adopted in applications that require wear, corrosion, and heat resistance. In particular, cobalt-chromium-molybdenum (Co-Cr-Mo) alloy is currently being used in the aerospace and medical field. Both applications require product personalization and take advantage by parts manufacturing through additive technologies, such as direct metal laser sintering (DMLS). This technology still has limitations for the manufacture of small cavities, such as holes with diameters below 0.6 mm and high aspect ratio, which require drilling of the additive manufactured part. This research aims at investigating the effects of electrodischarge drilling (EDD) on surface and subsurface features of CoCrMo samples built by DMLS. High aspect ratio holes are produced with diameter of 0.6 mm and depth 15 times higher. Due to the absence of physical contact between tool and workpiece in EDD, machining forces are negligible and microfeatures can be produced with a high accuracy irrespective of the material hardness and strength. In this study, peak current, pulse-on-time, and duty ratio are adopted as control factors using the Taguchi method. Material removal rate, Electrode wear rate (EW), and surface roughness are measured as responses. S/ N ratios and analysis of variance analysis lead to identify the significant process parameters and the optimal combination level of machining parameters. From the responses can be determined which ensures high productivity and smooth surface finish, even if tool wear is medium high. All the analyses are supported by the study of material removal mechanisms and surface morphology in the microscale, to account for the macroscopic trends. [ABSTRACT FROM AUTHOR]