Influence of Grain Refiners on the Wettability of Al 2 O 3 Substrate by Aluminum Melt
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ceramic foam filter (CFF) is an efficient, cheap, and popular way to remove inclusions from aluminum melt in order to increase metal quality. Filtration mechanism has been investigated by many researchers.[1–13] Massive progress has been made in understanding the influences of flow rate,[2] filter position,[14] filtration time,[15] filter types and filter surface treatment[5,8,15–19] on the filtration behavior. Bao et al.[20] studied the effect of the wetting properties between filter-melt and melt-inclusion on filtration behavior. It is proposed that a good wetting between the melt and filter as well as a poor wetting between melt and inclusions would increase the possibility that inclusions hit the inner wall of CFF and be captured.[3] However, Voigt et al.[21] pointed out that the higher the
JIAWEI YANG and YANJUN LI are with the Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: [email protected] SARINA BAO is with the SINTEF Industry, 7465 Trondheim, Norway. SHAHID AKHTAR is with the Research and Development Karmøy, Norsk Hydro, 4265 Ha˚vik, Norway. PING SHEN is with the School of Materials Science and Engineering, Jilin University, Changchun 130022, P.R. China. Manuscript submitted May 17, 2020; accepted September 23, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
contact angle between filter and melt, the higher filtration efficiency can be achieved. They also concluded that the rough surface of the filter wall improves the filtration efficiency. The rough surface inside the filter increases the melt flow resistance, where inclusions have a higher chance of hitting the porous wall. Therefore the chance for them to be caught and remain on the wall increases.[22] Nowadays, grain refiners, for example Al-Ti-B and Al-Ti-C master alloys, are often added into aluminum melt to refine the grain structure by increasing the number of nucleation sites during solidification. It is expected that the casting quality can be further improved with the combination of inoculation by grain refiners and CFF filtration. However, the addition of grain refiners was found to significantly reduce the filtration efficiency under high inclusion load.[13,23–26] One of the hypotheses suggested that the reduced filtration efficiency was due to the prevention or destruction of the inclusion bridge inside the filter by grain refiner particles. Unfortunately, no convincing experimental evidence has been revealed to show how and why the grain refiners could damage or prevent the formation of bridge. Another possible reason for the reduction of filtration efficiency could be the change of wettability between Al melt and CFF by addition of grain refiner master alloys.
Al-Ti-B and Al-Ti-C master alloys contain TiB2 or TiC particles and Al3Ti intermetallic phase. Experimental results have shown that Al3Ti alone in aluminum melt does not affect the filtration efficiency.[24] It is well known that TiB2 wets well with the molten Al. Weirauch et al.[27] showed that the contact angle between a pure polished TiB2 and Al melt can be as low as 0 deg after 17
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