Techniques to Achieve Texture in Permanent Magnet Alloy Systems

Abstract

The formation of columnar crystals in the anisotropic Alnico alloys has been known to enhance the magnetic properties, particularly the energy product. The requirements for columnar crystal growth are fairly steep temperature gradients in the liquid metal, coupled with low solidification rates. The first method is based on casting in a hot multicavity mold having a bottom chill plate so that directional solidification occurs. The mold has a great number of cavities with the thickness of walls between cavities less than 2 mm. It must be preheated to a temperature not lower than 1000°C before pouring. The second method is based on zone melting plus rapid directional cooling. This method is convenient to produce columnar crystals in magnets smaller than 20 mm in diam. The best magnetic properties of magnets 10 mm in diam can be obtained by lowering the fusion zone at the rate of about 25 mm/min. The magnetic energy product obtainable from both methods is almost the same in value, that is, 7.0–8.0 MGOe in commercial production. Addition of silicon and sulphur to Alnico alloys helps to produce columnar crystals, but titanium is unfavorable. Addition of sulphur helps to promote columnar crystals in Alnico alloys containing titanium. The oxide and nitride in the molten metal decrease with addition of sulphur and production of columnar crystals becomes easier. Recently, it was reported that large crystals had been produced by secondary recrystallization in some experiments in a laboratory. However, it is difficult to control the orientation of enlarged grains. The texture of rolled Alnico 8 alloy was found to contain three orientation components; (100) [011], (112) [11̄0], and (111) [112̄].