SECTION 12.6 MANUFACTURING TECHNOLOGIES OF RARE-EARTH-BASED MAGNETS 119
12.6. MANUFACTURING TECHNOLOGIES OF
RARE-EARTH-BASED MAGNETSThe simplest production route for permanent magnets is schematically rep
resented in Fig. 12.6.1. This is a well-established powder-metallurgical treatment that leads
to high-performance magnet bodies. The manufacturing process to prepare
permanent magnets is basically the same.
The main steps consist of alloy preparation, pre-milling, milling, control, and adjust
ment of the overall composition, particle alignment and pressing, sintering and heat
treatment. After this treatment, the sintered magnet bodies can be machined into the
shape desired, and are then magnetized. The consecutive steps shown in Fig 12.6.1 will be
discussed in more detail below.
The most common way of alloy preparation is vacuum melting of the components in
an induction furnace. First Fe and B are melted together in an alumina crucible
under purified argon gas. Subsequently, the reaction vessel is degassed under vacuum
and Nd metal is added to the melt after the latter has reached a temperature only slightly
above the Fe–B liquidus temperature. The casting is done in such a way so as to allow
rapid cooling of the melt in order to prevent oxidation as far as possible. The composition
of the alloy is generally chosen somewhat more Nd-rich than would correspond to the
formula composition In that case, the grains are surrounded by small
amounts of the Nd-rich eutectic present in the Nd–Fe–B phase diagram. The presence of
intergranular material of the eutectic composition is important in the liquid-phase sintering
process.
The chill-cast alloys are generally obtained in the form of large ingot lumps, too large
for direct milling. These lumps are therefore first crushed by means of hammer mills.
After a sufficiently small particle size has been reached, further size reduction is achieved