Development of a pilot scale strip casting system to produce optimised alloys for neodymium-iron-boron magnets

Strip casting is a rapid processing technique used in the production of alloys for both sintered and HDDR bonded NdFeB magnets. During strip casting, molten alloys are gravity fed onto a rotating copper wheel. The rapid cooling rate produces NdFeB microstructures which have many advantages over conventionally cast NdFeB book-mould alloys such as less alpha iron, near stoichiometric alloy compositions and a finer grain structure. The wheel texture of a pilot scale strip caster was altered in order to optimize the microstructure and improve the consistency of strip cast flakes for the production of NdFeB sintered magnets. This made it possible to reduce the flake thickness distribution and alpha iron content, improve the grain size consistency and increase lamellar alignment. In NdFeB alloys for HDDR (Hydrogenation Disproportionation Desorption and Recombination) powders, which are used in bonded magnets, a large grained material is preferable with little or no alpha iron. By implementing a combination of surface texture and reduced wheel speed, the grain width was increased from ~5 to ~40 μm, whilst maintaining a low alpha iron content. The magnetic performance has been shown to be comparable to conventional book-mould alloys that have been heat treated for 10 hours at 1140°C