Anne Kvithyld

Wetting of pure aluminium on graphite, SiC and Al2O3 in aluminium filtration

Abstract The wettability of pure aluminium on filter materials and on inclusions is believed to be an important factor affecting the filtration of aluminium. The contact angles of molten aluminium on alumina, SiC and graphite were measured under 10−8 bar high vacuum in the temperature range of 1000–1300 °C. To describe the wetting behaviour of the Al on ceramic at lower temperatures used in filtration and casting aluminium, a semi-empirical calculation was employed. The calculated contact angles at 700 °C were around 97° for alumina, 92° for vitreous graphite, 126° for single- and poly-crystal graphite, and 79° for single crystal SiC, respectively. This indicates that aluminium does not wet alumina or graphite (or Al4C3) around the casting temperature, but wets SiC at this temperature. Thus a priming height is required for aluminium to infiltrate an alumina filter. Increasing temperature can also improve the wettability of Al on ceramic.

Wetting behavior of aluminium and filtration with Al2O3 and SiC ceramic foam filters

The wetting behavior between liquid aluminium and substrates made from industrial Al2O3 and SiC based ceramic foam filters (CFF) was investigated. The same CFF filters were also tested in plant scale filtration experiments. The wetting experiment results show that the SiC based filter material is better wetted by liquid aluminium than the Al2O3 based filter material. This indicates that the improved wetting of aluminium on a filter material is an advantage for molten metal to infiltrate the filter during priming. Also, better wetting of Al-filter might increase the removal efficiency of inclusions during filtration due to better contact between filter and metal. Non-wetted inclusions are easier to be removed.

Quality Assesment of Recycled Aluminium

One of the serious problems during remelting of aluminium is the presence of surface oxide including coating. In this work, a wrought and a cast alloy were selected and subjected to remelting experiments. 3000 wrought alloy sheets with three surfaces; (i) before anodising, (ii) between anodising and coating and (iii) after coating, was investigated. The bifilm index was measured as a measure of metal quality; 3–point bending and tensile testing samples were collected for mechanical testing. A good correlation between the mechanical properties and the bifilm index was found. For the surface treated sheet skimming reduces the bifilm index. After skimming the melt in (i), (ii) and (iii) have the same quality, that is a comparable bifilm index and mechanical properties. However, the bifilm index of the cast alloy decreased after remelting three times, thus decreasing the quality.

QUALITY COMPARISON BETWEEN MOLTEN METAL FROM REMELTED SHEETS; MILL FINISH AND COATED

Deterioration of metal quality caused by contamination (e.g. coatings) is an issue in remelting of aluminium scrap. Therefore molten metal quality from remelting sheet material with and without coating is compared. In the experiments the crucibles are placed inside a resistance furnace to ensure that the charges are melted under the same conditions at the same time, measuring temperature and hydrogen. The melts was subjected to (i) settling overnight (ii) blowing air through a porous plug for 2.5 min to generate oxides (iii) settle overnight and adding turnings and (iv) adding carbon. The bifilm index is used as a measure of metal quality. The results show a statistically significant difference in the quality of the samples comparing coated and non-coated for all the melt treatments (i) – (iv), even if the charge material only contained approximately 0.4 % coat.

Effects of 2 ppm Beryllium on the Oxidation of a 5XXX Aluminum Alloy at Temperatures Between 500 and 750 °C

The oxidation of molten aluminum has been thoroughly studied, and it is established empirically that beryllium has an inhibiting effect on the oxidation behavior. The aim of this work was to increase the fundamental understanding of this inhibiting effect. Two 5XXX aluminum alloys (4.7% Mg), one with 2 ppm of beryllium and one without beryllium, were oxidized in a tube furnace under a dry air atmosphere. Samples were oxidized at temperatures from 500 to 750 °C for 10, 30 and 120 min. The composition and morphology of the oxide layers were examined with FIB and EDS, revealing the differences between the samples. The results showed up to a tenfold reduction in mass gain in samples containing beryllium. Significant differences in the oxide layer morphology were found for the two alloys at all times, including from 10 min. Thus, beryllium plays a significant role from the onset of high temperature oxidation.

Plant Scale Investigation of Liquid Aluminum Filtration by Al2O3 and SiC Ceramic Foam Filters

Plant scale filtration experimentsof 10〝 × 10〝 × 2〝, 30PPi Al2O3 and SiC industrial filters were carried out. Wetting experiments show that the SiC filter wets better with molten aluminium than Al2O3. The assessments by LiMCA II and laser were employed to study the behaviour of the two filters. The Al2O3 filter shows improved time dependent behaviour, increasing filtration efficiency, during one hour filtration. This is not the case for the SiC filter. It decays faster than the Al2O3 one. The SiC filter requires less pressure drop to infiltrate the metal. The result suggests that the SiC can be a new filter choice in the aluminium industry.

Influence of Coating and De-Coating on the Coalescence of Aluminium Drops in Salt

In a rotary furnace for aluminium recycling and dross treatment, a salt flux is added which protects against oxidation and captures non-metallic impurities. Furthermore, the salt has to promote the coalescence of the metal drops in the dross. This work investigates the coalescence of molten aluminium for different types of scrap. One hundred discs were stamped from aluminium alloy sheets with and without coating. They were melted, covered in NaCl–KCl–Na3AlF6 molten salt, in an induction furnace at 790 °C. The solidified aluminum droplets were extracted by leaching the salt with water. The fraction of coalesced drops and the average diameter were determined to evaluate the coalescence efficiency. The effect of various de-coating temperatures was studied. The results show that the coalescence is negatively affected by coating. Long holding times has no effect. Complete coalescence off all discs are achieved with uncoated scrap. The drops coalesce if the temperature of the combustion reaction for the coating is attained.

Oxidation of Manganese-Containing Aluminum Alloys Studied by SEM

During remelting of aluminum manganese-containing alloys, some industrial experience has indicateda tendency toward increased oxidation losses when compared to unalloyed Al. A goal of thiswork is to increase current knowledge of oxidation of Mn-containing aluminum. An oxidation studywas performed on 99.99% Al, Al-1%Mn and Al-5%Mn materials in a muffle furnace in air. Theseoxidized materials were analyzed in the Scanning Electron Microscope using Energy Dispersive xraySpectroscopy (EDS) both for chemical composition and to analyze the structure of the oxide thatwas formed. The main findings from the SEM analysis of the oxidized 99.99% Al, Al1%Mn andAl5%Mn materials are: 1) The oxides formed on the Al1Mn and Al5Mn materials contained onlyminute amounts of Mn. However, small clusters of Mn metal were found among the Al2O3 particleson the surface of the Al1Mn and Al5Mn samples. 2) Dissolved Mn increased from the surface to theinterior, and Mn-rich particles were present in the interior of the Al1Mn and Al5Mn samples. 3) Theoxide layer thickness increases with Mn content, and the oxide film is more fragmented.

Tensile Properties and Hot-Tearing Tendencies of 3xxx Alloys

A set-up for tensile testing in the mushy zone allowing for studies of semi-solid mechanical behavior is available at SINTEF. A hot-tearing experimental set-up has recently been developed allowing for investigation of the hot-tearing susceptibility of industrial aluminium alloys and effects of e.g. alloying composition and grain-refiner. Load and temperature are registered during constrained solidification giving information on the mechanical behavior of the alloy during solidification. Two crack-prone alloys in the 3xxx-series (A and B) have been investigated using both techniques and the results analyzed using information about solidification path from a thermo-physical model. Alloy B is found to be mechanically weaker in the interval most susceptible to hot-tearing in agreement with cast-house experience. This study shows that the experimental techniques combined with thermo-physical modeling and characterization allow for a better understanding of the hot-tearing sensitivity of the alloys.

Oxidation of Manganese-Containing Aluminum Alloys

During remelting of aluminum alloys, industrial experience has shown a tendency toward increased oxidation losses with manganese-containing alloys. A goal of this work is to increase current knowledge of oxidation of Mn-containing aluminum alloys and relate this knowledge to reducing oxidation losses.