Casting of Alnico VII with Improved Physical Properties

Alnico VII having the highest coercive force Hc = 1000 of the Alnico alloys has not come into extensive use because of its brittleness. Studies in the metallurgical laboratories show that highly purified Alnico VII castings melted under a hydrogen atmosphere are just as brittle as castings air melted with standard raw materials. On the other hand, silica shell molds which cut the initial average cooling rate of the castings to about 20°C per min afford a convenient way to cast sound crack-free Alnico VII magnets which can withstand the handling and thermal shock of the subsequent heat treating process. Satisfactory magnetic quality and chemical uniformity are obtained by air melting standard raw materials of commercial purity. The magnets are amenable to cutting and grinding and have an excellent surface. Though the molds are costly, and their fragile nature necessitates careful handling, the use of silica shell molds will produce an Alnico VII with good magnetic and physical properties.