Effects of Wheel Surface Conditions on the Casting of Amorphous Metal Ribbons
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THE fabrication
of amorphous metal ribbon ~-3 requires continuous feeding of liquid metal onto a rapidly spinning wheel. The wetting of the liquid metal on the casting wheel surface is important in the process insofar as it influences the ribbon surface smoothness, heat transfer at metal-wheel interface, and the separation distance of ribbon from the wheel. In general, the liquid metal under study (Fe-B-Si alloys) tends to wet the casting wheel surface, particularly when it is ejected under pressure and confined to a small gap between the bottom surface of the crucible and the wheel surface. 4 While a smooth clean surface is readily wet, an air boundary layer associated with the casting wheel surface tends to restrict the wetting. Consequently, a "wetting pattern" is printed onto the wheel-side ribbon surface, which consists of areas of air pocketing and areas of direct contact between metal and wheel. 5 The wetting pattern should thus provide information about the metal-wheel interfacial characteristics. This work represents our initial attempts to study the wetting pattern and to use it as diagnosis for further processing improvement. Experimental results will be presented regarding the effects of the condition of the casting wheel surface on the wetting pattern and the related interfacial phenomena. In particular, the' mechanism of formation of the periodic ribbon surface defect known as "fish scale" and the elimination of this defect by changing the condition of the casting wheel surface will be discussed. EXPERIMENTAL PROCEDURE The castings were carried out in air on a watercooled, copper-1 pct chromium wheel. The 26 cm diam casting wheel was rotated at 1100 rpm, which gave a linear surface speed of 1500 cm/s. The quartz crucibles had slot openings of 0.043 to 0.048 cm by 1.27 cm. The gap between crucible bottom surface SHYH-CHIN HUANG and HOWARD C. FIEDLER are Staff Metallurgists, General Electric Company, Research and Development Center, Schenectady, NY 12345. Manuscript submitted July 7, 1980. METALLURGICAL TRANSACTIONS A
and wheel surface was typically 0.013 to 0.026 cm during casting. The ejection pressure was between 1.4 x 103 and 2.1 x l03 pascal. The small variations in the casting conditions stated above did not change the wetting pattern significantly. On the other hand, the preparation of the casting wheel surface affected the wetting pattern. A " s m o o t h " wheel surface was obtained by polishing the wheel prior to casting through 600 grit polishing paper and conditioning the wheel during casting with an ultra-fine Scotch-Brite wheel driven at 1100 rpm counter to the casting wheel rotation. A " m a t t e " wheel surface finish, which is slightly rough but uniform, was obtained by polishing the wheel before and during casting with a 15 cm diam steel brush wheel comprised of 100/~m diam wires, driven at 1100 rpm counter to the casting wheel rotation. The effectiveness of the steel brush in maintaining the matte wheel surface condition during casting depends on the steel bristle stiffness, the ca
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