Examination of the Strength of Oxide Skins on Aluminum Alloy Melts
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I.
INTRODUCTION
THE surface quality of cast products and the presence of skin inclusions depend on the strength of the oxide skin on the melt, Therefore, the adhesion strength of the skin is a quantity of interest in continuous casting and mold casting. At present no testing method exists for an unambiguous strength parameter of oxide skins on liquid metals. Experimental data are published only on the mold filling capacity, ~ the flow properties 3'4,5 and on surface tension obtained by the methods of sessile drops, 67 of maximum bubble pressure, 3'8-~ and of capillary depression. 8 ~2 In order to prevent oxide skin formation, most surface tension experiments were performed in inert gas atmospheres or under a vacuum of 10-3 bar. However, the results are still strongly influenced to an unknown extent by oxide skins as discussed by the authors. In this study, a new technique has been used. The strength of oxide skins on aluminum melts is characterized by the maximum force an oxide skin, extended between two moving dies, can tolerate before cracking occurs. Two different arrangements for the dies were tested, one with linear movement of two parallel plates and one with a rotating disc inside a ring. The second method yields data of a strength parameter with an accuracy high enough to determine the effects of temperature, oxidation time, and of alloy and impurity additions on the mechanical stability of the skin. The conceptions of the two test methods are discussed, and results obtained with the technique with rotational movement are presented for pure A1 with and without additions and for some commercial AI alloys. They demonstrate that reliable data for the strength of oxide skins can be determined with this relatively simple testing method.
I1.
EXPERIMENTAL
A. Apparatus with Linear Die Movement Figure 1 shows, schematically, the measuring principle. Two parallel plates (8 x 1 cm) are pressed against the skin W. KAHL. formerly Ph.D. Student at the Max-Planck-Insntut, is now with Lmchtmetall GesellschafI MBH D-4300 Essen-Borbeck, Germany E. FROMM ~s Research ScLentlst at the Max-Planck-InStltUt f~ir Metallforschung, Inst~tut f~ir Werkstoffw~ssenschaften, Seestrage 92, D 7000 Stuttgart 1, West Germany. Manuscript submitted February 27, 1984. METALLURGICAL TRANSACTIONS B
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Fig. 1 - - Apparatus with hnear d~e movement (schematic) (1) Moving &e, (2) die for force measurement, (3) tension wire, (4) fixed dies to prevent edge effects, and (5) distance sensor.
surface. For an indentation depth of 1 to 2 mm the aluminostatic pressure suffices to prevent sliding of die (1) on the skin surface during movement at constant rate. Die (2) is suspended from a tension wire (3), and forces arising from the stressed skin produce a small displacement of die (2). This displacement is monitored by a distance sensor (5) and yields a signal proportional to the force exerted on die (2). The outer parts (4) of die (2) prevent errors produced by stress inhomogeneities near the edge of the dies. With increasing distance, the fo
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