Improved casting lubricant containing metal fluoroborate salt and improved direct chill casting process

摘要

An improved casting lubricant for direct chill casting of easily oxidizable metals comprises a lubricating oil and a metal fluoroborate salt which decomposes to form boron trifluoride gas when heated to a temperature at or above a melting point of the easily oxidizable metal. The casting lubricant is preferably used in the hot top method of direct chill casting in which ingots of an easily oxidizable metal are cast in an open-ended mold cavity. In this process, a controlled flow of the casting lubricant is pumped into the mold cavity to contact the metal being cast. The metal fluoroborate salt dispersed in the lubricant decomposes to produce boron trifluoride gas at the metal/mold interface, thereby forming a protective, oxidation-preventing shell on the outer surface of the metal. Preferably, the metal fluoroborate salt is an alkali or alkaline earth metal fluoroborate salt which is a solid at ambient temperatures. Most preferably, the lubricant comprises a 1.0 wt.% dispersion of potassium fluoroborate in castor oil, and is used in the direct chill casting of magnesium, magnesium alloys and aluminum alloys containing magnesium.