Direct Observation of Wetting and Spreading of Molten Aluminum on TiB 2 in the Presence of a Molten Flux from the Alumin
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THE wettability of TiB2 by molten aluminum has been investigated in a wide range of temperatures and in different environments (i.e., vacuum, under controlled atmosphere, under salt cover, etc.) It has been reported in most cases that the wettability of TiB2 by molten aluminum occurs well above the aluminum melting point.[1–5] For example, Rhee[2] obtained a contact angle of 57 deg after a 30-minute hold at 1107 K (834 C) under vacuum. Bardal et al.[5] obtained a contact angle of 10 to 20 deg after a 100-minute hold at 1263 K (990 C) under vacuum. However, it has been observed that the wettability of substrates by aluminum is influenced by the thin aluminum oxide skin on the aluminum surface as well as by the nature of the substrate surface.[6] Therefore, the wettability can be influenced by a treatment of the alumina skin and/or a treatment of the substrate surface. In fact, the fine alumina film needs to break before aluminum wets and spreads on the substrate surface. This break may occur by cracking the alumina skin; by evaporation of Al2O, which is a product of the reaction between molten aluminum and the aluminum oxide skin at low oxygen pressure and high temperature[6] or is the reaction product of compounds that can form a more stable oxide than aluminum (e.g., zirconium)[7]; or by dissolution of the oxide skin into a molten flux,[8–11] which is the case investigated in this work. However, changing CHRISTIAN T. MUTALE, Senior Staff Scientist, and DOUGLAS A. WEIRAUCH, JR., Fellow, are with the Alcoa Technical Center, Alcoa Center, PA 15069. Contact e-mail: [email protected]. WILLIAM J. KRAFICK, Consultant, is with Manpower, Alcoa Center, PA 15069. Manuscript submitted April 20, 2010. Article published online September 24, 2010. 1368—VOLUME 41B, DECEMBER 2010
the chemistry of the substrate surface also can influence its wettability by molten aluminum. This occurs, for example, when the oxide layer is cleaned off by a chemical treatment or by dissolution in a molten flux. Molten fluxes were used to overcome the barrier effects of the alumina skin. The solubility of alumina in molten halide fluxes is dependent on different parameters such as the chemistry of the flux, the temperature, the initial alumina concentration of the flux, and so on. Yet the wettability of the TiB2 substrate depends on, among other things, the nature of the surface, the grain size and orientation, the nature of impurities, and their locations and concentrations in the substrate (i.e., impurities in the grain boundaries (GB) can behave differently than those inside the grain). Therefore, different types of fluxes and substrates were tested in this study.
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EXPERIMENTAL
The sessile drop technique was used in this study to investigate the wettability of TiB2 substrates in the presence of different fluxes. For the experiments, 0.3 g of high-purity (Puratronic) aluminum (5N) supplied by Alfa Aeser (Ward Hill, MA) was cleaned by sequential 10-minute exposures to a NaOH solution, deionized water, and ethanol, respectively. The flux in the form of a
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