Aluminum Melt Filtration with Carbon Bonded Alumina Filters

The wetting behavior was measured for Al₂O₃-C in contact with AlSi7Mg with a conventional sessile drop test (vacuum, 950 °C and 180 min) and a sessile drop test with a capillary purification unit (vacuum, 730 °C and 30 min). The conventional test yielded contact angles of around 92°, whereas the sessile drop measurement with capillary purification showed a strongly non-wetting behavior with a determined apparent contact angle of the rolling drop of 157°. Filtration tests, which were repeated twice, showed that the Al₂O₃-C filter possessed a better filtration behavior than the Al₂O₃ reference filter. For both filtration trials, the PoDFA (porous disc filtration analysis) index of the Al₂O₃-C filter sample was equal to half of the PoDFA index of the Al₂O₃ reference filter sample, indicating a significantly improved filtration performance when using Al₂O₃-C filter. Notable is the observation of a newly formed layer between the aluminum and the Al₂O₃-C coating. The layer possessed a thickness between 10 µm up to 50 µm and consisted of Al, C, and O, however, with different ratios than the original Al₂O₃-C coating. Thermodynamic calculations based on parameters of the wetting and filtration trials underline the possible formation of an Al₄O₄C-layer.