Method of forming thin curved rare earth-transition metal magnets from lightly compacted powder preforms

A method of forming rare earth-transition metal powder directly into a thin, curved, precision shaped permanent magnet bonded to a ferromagnetic layer wherein the magnetic domains of said magnet are predominantly radially aligned with respect to the magnet curvature, the method comprising:

• forming at least one curved cavity substantially in the shape of the magnet between a tubular ferromagnetic sheath member having a working surface shaped to define the outer curved surface of the magnet and a composite forming mandrel having a working surface shaped to define the inner curved surface of the magnet, the mandrel having a ferromagnetic core slidably retained in a non-magnetic sleeve;
• introducing rare earth-transition metal powder into the cavity in which the powder particles have single magnetic domains;
• pressing the powder in the direction of the axis of the magnet to a density of at least about 50% of the rare earth-transition metal density whereat the powder particles become mechanically interlocked with each other;
• subjecting the powder in the cavity to a magnetic field before said particles become mechanically interlocked such that the magnetic domains of the powder particles are radially magnetically aligned with respect to the magnet curvature and such that the powder particles have residual magnetism after the field is removed;
• removing the magnetic field;
• withdrawing the magnetic core from the mandrel, the mandrel sleeve preventing disturbance of the magnetically aligned powder and
• thereafter withdrawing the mandrel sleeve such that the powder particles remain magnetically attached to the tubular sheath in a cohesive mass in the magnet shape, and
• heating the powder mass to promote densification to at least about 90% of rare earth-transition metal density and to bond the magnet to the sheath.

A method of forming rare earth-transition metal powder into a thin, curved permanent pole magnet for an electrical motor, wherein said magnet is metalurgically bonded to a ferromagnetic sheath member that serves as a backing for said magnet and as a flux path for the motor, the method comprising:

• inserting into a tubular steel sheath member a composite mandrel having a cylindrical ferromagnetic core slidably retained within a thin, non-magnetic sleeve, said mandrel having at least one axial slot in its peripheral surface which in combination with the inner wall of the sheath forms a thin curved cavity in the shape of the magnet;
• filling the cavity with rare earth-transition metal powder in which the powder particles have single magnetic domains;
• applying a magnetic field to the powder to align the easy axes of the particles radially with respect to the magnet, the mandrel core providing a low reluctance path for the field flux and a high flux density in the cavity,
• pressing the aligned powder in the direction of the magnet axis to compact the powder mass to a density of at least about 50% of the density of the rare earth-transition metal and to fix the original powder particles in their magnetically aligned positions, the particles having sufficient residual magnetism to adhere to said casing upon removal of said field,
• removing the applied magnetic field,
• withdrawing the ferromagnetic core portion from the mandrel, and thereafter withdrawing the non-magnetic sleeve portion such that said the rare earth-transition metal powder particles are magnetically maintained in a cohesive mass against the sheath in the configuration of the magnet;
• and
• heating the powder mass at a pressure greater than atmospheric pressure to promote the particles to coalesce to a density of at least about 90% of their theoretical density and to metallurgically bond the magnet to the sheath.