Molten aluminium-silicon alloys treated with sodium - by tablets contg. sodium fluoride and maintaining sodium content of melt during long casting periods

摘要

The alloy is first treated with a conventional agent (a) providing sodium; and is then treated with tablets (b) contg. a mixt. of alkali halides and free carbon, where the mixt. contains sufficient NaF so its m.pt. is above the temp. of the molten alloy. Tablets (b) maintain the % Na in the melt at a desired level over long periods of time. Mixt. (b) pref. contains by wt. 40-60% NaF, esp. ca. 50%, and 3-20%, esp. ca. 12% free carbon. The tablets (b) are pref. added after agent (a) has been consumed, or are immediately placed on the surface of the alloy and maintained there until the end of casting. Sodium is added to prevent the pptn. of large Si crystals, but agents (a) are only effective for max. half hour. In gravity die casting, the molten alloy needs to be held for a much longer time in a furnace; and the invention (b) maintains the % Na during long times.