| dc.description.abstract | It is known that the addition of grain refiner particles reduces the filtration efficiency of Ceramic Foam Filter (CFF) for aluminum melts under a high inclusion load. The main objective of this PhD work is to study the influence of grain refiners on melt filtration performance in terms of how they interact with CFF and inclusions. Such knowledge is important for an in-depth understanding of the filtration mechanism and further improvement of the filtration efficiency of aluminum melts by CFF.
In the first part, the interaction between grain refiner particles and alumina and its influence on the wettability between aluminum melt and the alumina substrate has been studied by a sessile drop wetting test. One of the major differences between inclusion particles and grain refiner particles is their wettability to the aluminum melt. Grain refiner particles have excellent wettability with the aluminum melt, while the wettability of oxides is poor. Since wettability could be one of the parameters that affect the filtration efficiency, sessile-drop wetting tests were carried out to study the influence of grain refiners on the wettability between aluminum melt and alumina substrate. It is found that the grain refiner particles reduce the wettability between the aluminum and alumina substrate. However, the effect is limited. Therefore, it is suggested that the reduction in filtration efficiency is not a result of the wettability change given by the grain refiner particles. For the present test, no chemical reactions were observed between the grain refiner particles and the alumina substrate. This suggested that the reduction of filtration efficiency and wettability is also not due to the chemical reaction caused by the grain refiner particle addition. Most importantly, it was found that the grain refiner particles tend to adhere to the oxide layer at the droplet surface, showing a strong interaction with the oxide films.
In the second part of the work, a settling test was conducted to further study the interactions between grain refiners and inclusions. A large amount of grain refiner particles, in form of Al-3Ti-1B master alloy, were introduced to the melt to study their agglomeration and settling behavior in the melt. In addition, by introducing inclusions to the melt, the interactions between grain refiner and inclusions are also revealed. By comparing the as-solidified samples, it is confirmed that the oxide films have a strong adherence with both grain refiner particles and oxide particles. These oxide films significantly reduced the settling of grain refiner particles, TiB2, in the aluminum melt. It is revealed that the grain refiner particles tend to adhere and agglomerate onto the oxide films in the melt. It is also shown that the presence of oxide films seems to slightly alleviate the fading effect of grain refiners.
In the third part, pilot-scale filtration tests conducted at Hydro Aluminum in Sunndalsøra, Norway, are discussed. These tests were conducted to further investigate the interactions between grain refiner, inclusions and filter grade, and their effects on the filtration performance of CFF. Nine tons of aluminum melt was filtrated through both 50 and 80 Pores Per Inch (PPi) CFF. Liquid Metal Cleanness Analyzer (LiMCA) II measurements and Porous Disc Filtration Apparatus (PoDFA) were used to examine the level and type of inclusions before and after filtration. By adjusting the addition level of grain refiners and inclusion load, the influences of grain refiners and inclusions have been studied quantitatively. The experimental results show that the filtration efficiency is not affected by a low addition level of grain refiners for both 50 PPi and 80 PPi filters, regardless of the inclusion load condition. Once the grain refiner addition level is increased to 2.0 kg/tonne, the detrimental effect starts to appear for the 50 PPi filter. Interestingly, the addition of a high level of grain refiners (2.0 kg/tonne) does not affect the filtration performance of CFF for aluminum melt with an ultra-high inclusion load with an N20 (number of inclusions larger than 20 μm in diameter) > 11 k/kg. However, it does affect the filter performance under a high inclusion load (N20: 3-11 k/kg). This indicates that the filtration efficiency is dependent on the filter pore size, inclusion load, and grain refiner addition level. Moreover, it is also observed that a higher inclusion load in the aluminum melt (N20 > 11 k/kg) leads to improved filtration efficiency and an increasing pressure drop with filtration time for the 80 PPi filters.
A detailed characterization of the spent filter with solidified aluminum shows that a thick cake layer composed of inclusions has formed on the top window of the 80 PPi filter when filtrating aluminum melt with an ultrahigh inclusion load. This has been confirmed by the increased pressure drop of the aluminum melt with increasing inclusion load and filtration time. It is suggested that the formation of the inclusion cake layer has the influence of improving the filtration efficiency. Therefore, the addition of grain refiners does not affect the filtration performance. On the other hand, due to the larger pore size, a similar cake layer is difficult to form on the top window of a 50 PPi filter during filtration. As an experimental evidence, the pressure drop for the 50 PPi filter remains stable even after the introduction of an ultra-high inclusion load.
In addition to the cake layer formation, the oxide films are also found to be attached to the filter wall, with one end floating in the liquid in the filter. This looks similar to seaweed attached to a rock and is thereby named “seaweed”-like oxide films in the present work. These seaweed-like oxide films increase the efficient capture area of the filter, and therefore enhance the depth filtration mode and filtration efficiency.
It is suggested that the reduction of filtration performance by addition of grain refiner particles is due to the strong adherence between grain refiner particles and oxide films, by which the TiB2 particles can be easily captured by oxide films during filtration. First, the oxide film inclusions adhered by TiB2 particles give a larger gravitational number (closer to zero) than other oxide films (having negative gravitational numbers), and therefore have less chance to be blocked by the filter. Secondly, the formation of composite inclusions composed of oxide films and agglomerated TiB2 particles, increase the number of inclusions with a larger size. Once such composite inclusions survive from the filtration process, they can be more easily detected by LiMCA, which has a detection limit of 15μm.
Usually, melt filtration is supposed to have no influence on the grain refinement efficiency of inoculated aluminum alloys. However, it has been shown in the present research work that a strong adherence exists between grain refiner particles and aluminum oxide films, which suggests that filtration can influence the grain refinement efficiency. Thus, in part 4 of the Ph.D. project, TP-1 type tests were conducted during the filtration process of AA6060 alloy melt with an ultra-high inclusion load with two different grain refiner addition levels, 0.5 kg/tonne and 2.0 kg/tonne, by using an 80 PPi filter. A systematic study on the grain structure and cooling curve for the TP-1 solidification samples of the aluminum alloy before and after filtration shows that the grain refinement efficiency is significantly reduced by the melt filtration. The reduction in grain refinement efficiency is more serious when the grain refiner addition level is high. By comparing the measured grain sizes to predicted grain sizes by a grain size prediction model, it is found that the grain refinement efficiency has been reduced by the ultra-high inclusions in the melt before the filtration. The filtration process has further reduced the grain refinement efficiency. | en_US |
