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The advantages of copper-graphite hybrid electrodes

In the first step of the research project, the graphite residues are sorted into three different particle groups so that graphite dust with different average particle sizes d₅₀ is available. Particles that are too large are further comminuted with variable grinding time t_m. Copper powder is then added to the graphite dust in different proportions. This increases the thermal conductivity λ and electrical conductivity _K compared to pure graphite. Another advantage of combining copper and graphite is that the removal rate V̇_W can be increased by up to 30 percent and at the same time the relative wear ϑ is reduced by up to 10 percent [MUT09]. To ensure this, however, the porosity Φ of the electrode must be less than 5 percent, because otherwise the electrical conductivity _K decreases and the relative wear ϑ increases.

The copper-graphite mixture is then pressed at a pressure of p = 300 MPa and ultimately sintered. The sintering temperature ϑ _s and the sintering time t_s are also treated as variable parameters. The electrodes created by varying the particle size d_p, mass fractions of copper W_m and sintering parameters are then investigated experimentally. The electrical resistivity ρ_R, the density ρ, the porosity Φ and the compressive strength R_S are determined.

In the last step, finishing is carried out by means of milling, whereby the burr length l_g ≤ 6 µm for roughing technologies and l_g ≤ 2 µm for finishing technologies should not be exceeded (Fig. 3). To determine the optimum particle size d_p and the optimum mass fraction of copper W_m, experimental sink erosion tests are carried out, whereby the erosion duration t_ero, the relative wear ϑ and the surface roughness characteristics are determined.

Simulations are verified in practice by suitable erosion tests

The inclusion of internal flushing channels is to be combined with the sintering process, because this also allows for more complex configurations. Due to this, a total of n = 125 fluid mechanical simulations will be carried out, in which models with different flushing channel configurations will be examined. The number of flushing channels n_i, the radial distance between the channels a_i and the diameter of the flushing channels d_i are varied within the simulation model. In order to determine which flushing channel configuration is particularly suitable for the rapid removal of particles, the resulting flow velocity in the working gap c_s of the different models is considered. In order to ensure realistic comparisons of these simulations with the target process variables of electrical discharge machining, experimental sink erosion tests are also carried out in parallel (Fig. 4). Through these experiments, the removal rate V̇_W, the erosion duration t_ero as well as the wear rate V̇_E can be related to the simulated flushing configurations.

The aim of the project partners is to reduce the erosion time t_ero and the relative wear ϑ by 30 percent through a significantly improved flushing via the inserted internal channels compared to classical die-sinking erosion.

Use of electrodes made from recycled graphite dust without large additional costs

After the completion of the cooperation project, application-oriented knowledge, innovative material compositions for tool electrodes as well as technologies for their application are now available, which can lead to productivity increases and resource conservation in a large number of manufacturing companies in Germany. The project results are directly applicable without causing major investments. Even manufacturing companies with highly modern machine equipment can therefore exploit the potential for increasing sustainability and productivity with the innovative tool electrodes with internal rinsing channels based on recycled graphite dust.

Sources

[GAB22] GAB Neumann GmbH: Verfahren zur Herstellung, URL: https://www.gab-neumann.com/Verfahren-zur-Herstellung-von-impr%C3%A4gniertem-Graphit (Zugriff: 2022-07-21)

[GIS22] GisChem: Gefahrstoffinformationssystem Chemikalien der Berufsgenossenschaft Rohstoffe und chemische Industrie und der Berufsgenossenschaft Holz und Metall, URL: https://www.gischem.de/ (Zugriff: 2022-07-20)

[GRA22] Graphitprodukte - Herstellungsprozess, URL: https://www.graphitprodukte.de/produkte/funkenerosion/graphit-herstellung.html (Zugriff: 2022-07-21)

[MER22] Mersen: ELLOR Graphit für die Funkenerosion, URL: https://www.ellor.de/material.html (Zugriff: 2022-07-20)

[MUT09] Muttamara A.; Fukuzawa Y.; Mohri N.; Tani T.: Effect of electrode material on electrical discharge machining of alumina. Journal of materials processing technology 209 (2009), S. 2.545 – 2.552.

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