Melting And Refining Practices For Magnesium

THIS paper will outline briefly the practices commonly followed in this country for the melting and refining of magnesium and its alloys. The processes used for the various forms of primary magnesium, as jar as there are differences in the physical shape or behavior, will be discussed. The refining of general fine scrap or secondary magnesium was presented in an earlier paper and will be discussed briefly.1 Inasmuch as the use of fluxes is an essential part of all the melting and refining processes, this paper will deal with these in sufficient detail to point out their unique characteristics, in order to make their use more effective. The outline of this discussion will closely follow that of an earlier paper,2 with such changes and additions as seem necessary to bring it up to date. TYPES OF MELTING PRACTICE All melting and refining processes for magnesium and its alloys require the use of fluxes. These fluxes have a magnesium chloride base and other halide salts or oxides are added to give a density or behavior exactly suited to the particular melting practice. The successful handling of magnesium depends upon the proper use of the correct fluxes. There are four general methods of melting, summarized in the following paragraphs and treated in more detail in the section on Melting and Refining. Open-pot Method The open-pot method makes use of No. 230 flux, which provides protection during melting and a molten pool of flux into which the solid magnesium melts. The flux is stirred through the molten metal bath and agglomerates oxide or similar foreign bodies;, on quiet standing, it separates away, leaving the refined metal ball floating in an encircling layer of molten flux. It forms only a thin fluid film over the surface of the molten metal, which may be parted for hand-ladling processes and tends to cover the metal again after the ladle is removed. A very light dusting of the pot surface with fresh flux immediately after the ladle is removed is desirable. The open-pot method is used generally in the following processes: (I) alloying and remelting in the production of ingot, (2) in sand foundries for premelting and to a smaller extent for the production of small castings, (3) in permanent-mold founding for premelting and also direct ladling to castings, (4) for continuous methods of preparing metal in the production of billets or ingots from which wrought products are fabricated, (5) in general scrap recovery. Crucible Process The crucible process makes use of No. 310 flux, which has the property of being thinly fluid at the start, to provide protection and refining qualities, and then drys out or thickens after a time, leaving a protecting crust on the pot surface until the time of casting. At that time it can be readily skimmed off or held back, thus allowing the contents of the crucible to