Electromagnetic refining of aluminum alloys by the CREM process: Part I. Working principle and metallurgical results
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I.
INTRODUCTION
T H E mechanical properties of aluminum alloys are connected with the size and shape of the grains. Consequently, the crystal structure of the as-cast ingot is of primary importance. It is an established fact that the production of a fine-grained equiaxed structure leads to a substantial improvement in the quality of the metal and allows an increase of the ingot drop rate and the reduction of cracking.fl-3J The addition of a nucleating agent is a commonly adopted method in dc casting of aluminum alloys, tl-31 and it is well-known that inoculation by small amounts of grain-refiner master alloys, such as A1-5 pct Ti-1 pct B (At5B), results in a significant reduction of the grain size. However, the addition of grain refiners may cause negative secondary effects, thus reducing the quality o f the metal. For instance, this technique can be detrimental to the quality of the finished product, due to the presence of conglomerates of TiB2 in the metal. Moreover, the refinement may become practically ineffectual, when the efficiency of the filtering system is sufficiently high to retain a large part of the nuclei of TiB2 before casting. Furthermore, high-strength aluminum alloys, especially those containing zirconium and lithium, are historically difficult to grain mime with conventional titanium-boronaluminum master alloys. ]4,5] An alternative way, consisting of several dynamic methods producing a vigorous forced convection in the melt during freezing, leads to substantial grain refinement. In these processes, heat and fluid flow are controlled by various externally applied forces. These methods primarily include the use of sonic, or ultrasonic, vibration and mechanical, or electromagnetic, stirring, t~,2"6-9! Among these techniques, the application of a time-varying magnetic field is of particular interest, because forced convection is generated in an easily controllable manner, while avoiding direct contact between the stirrer and the melt. The effects of the electromagnetic stirring have been
CHARLES VIVI~S, Professor, is with the Laboratoire de Magnrtohydrodynamique, Facult6 des Sciences, 33, rue Louis Pasteur, 84000 Avignon, France. Manuscript submitted March 28, 1988. METALLURGICAL TRANSACTIONS B
extensively studied on the solidification of steel, particularly in continuous casting, t7'8] However, the casting techniques of steel and aluminum have their own peculiarities. For steel, the depth of the sump is several meters, so it is relatively easy to arrange around the solidifying ingot several electromagnetic stirrers consisting, for instance, of a combination of linear and rotary induction motors. On the contrary, in the case of aluminum alloys, the sump depth is of the order of 15 centimeters, i.e., of the order of the ingot mold height; as a result, it is practically impossible to set such stirring systems on account both of their bulkiness and of the small gap which exists, in the case of the conventional casting equipment, between the ingot mold and the cooling device. Because of this, it see
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