Wettability of AlSi7Mg alloy on alumina, spinel, mullite and rutile and its influence on the aluminum melt filtration efficiency

Abstract Despite the application of aluminum melt filtration with ceramic foam filters since the 1960s the process is not fully understood. Outstanding issues are for example the influence of the wetting behavior and filter chemistry on the filtration efficiencies. Contact angle measurements by the sessile drop method coupled with non-contact heating and capillary purification technique were conducted at 730 °C with AlSi7Mg alloy on the substrates of four different oxides (Al 2 O 3 , MgAl 2 O 4 , 3Al 2 O 3 ·2SiO 2 and TiO 2 ). The highest contact angle was measured on 3Al 2 O 3 ·2SiO 2 , followed by MgAl 2 O 4 , Al 2 O 3 and TiO 2 . Experimental values of contact angles were correlated with the evaluated filtration efficiencies of filtration trials performed at Constellium. They show a good correlation for inclusions smaller than 110 μm - the larger the contact angle the higher is the filtration efficiency. The adhesion forces of alumina inclusions on the four filter materials were experimentally estimated with an AFM in water based model system. The highest adhesion forces were measured on the Al 2 O 3 substrate, followed by the 3Al 2 O 3 ·2SiO 2 , the MgAl 2 O 4 and the TiO 2 substrate. For inclusions >70 μm, both the measured filtration efficiencies for >70 μm and the adhesion forces increase by the following order of coatings: TiO 2 , 3Al 2 O 3 ·2SiO 2 , MgAl 2 O 4 , Al 2 O 3 .