The high-coeacivity magnets show a growing demand in recent years due to the rapid development of the hybrid electric vehicle (HEV) and the electric vehicle (EV). In order to ensure the high-temperature stability, the manufacturer will use Dysprosium element to substitute part of Neodymium element. As a kind of heavy rare earth element, the introduction of Dysprosium will greatly improve the raw material cost of the magnet, and also reduce its remanence and maximum energy product.
In this circumstance, the founder of NdFeB magnet, Dr. Masato Sagawa, has been launched the grain boundary diffusion (GBD) technology and the pressless process (PLP) technology to optimize the content of Dysprosium element in magnet.
What is pressless process (PLP) magnet?
The pressless process (PLP) is a kind of near net-shape manufacturing technique. The initial production of the pressless process (PLP) magnet is very close to final shape, therefore reducing machining and grinding cost dramatically. The shape and dimensions of the final magnet, and shrinkage degree of the NdFeB powder after sintered are taken into consideration while mold designing.
The intrinsic coercivity and maximum energy product of Dy-free pressless process (PLP) magnet already reached 1512kA/m and 398kJ/m3 in Sagawa’s previous work.
The technical process of pressless process (PLP) magnet
The highlights of pressless process (PLP) magnet
1.The grain size much lower than traditional technology.
For ferromagnetic material, the coercivity increased with decreasing grain size under the certain range of dimension, then declined sharply when grain size less than the critical size of single-domain. Reducing the grain size is also an extremely effective way to enhance the coercivity of NdFeB magnet.
The typical grain size of simple NdFeB is around 5μm, and far away from its critical size of single-domain. The smaller grain size means the larger specific surface area, hence more easily to cause surface oxidation. Sagawa obtained ultrafine NdFeB grain by innovative HD (hydrogen-decrepitation) + HDDR (hydrogenation-disproportionation-desorption-recombination) + helium jet milling process. It should be noted that the helium gas is more suitable than nitrogen gas to fabricate ultrafine powder due to its lighter mass fraction, and will also improve the particle size distribution of jet-milled powder.
2.No longer need pressing equipment.
The most obvious feature of pressless process (PLP) magnet is “pressless”. The “pressless” means the manufacture process of pressless process (PLP) magnet is no longer needed the pressing equipment. Most of pressing equipment can’t achieve oxygen-free condition completely.
The shape of pressless process (PLP) magnet includes block and breadloaf. How to improve the shape freedom is the main problem faced by pressless process (PLP) magnet in its future development.
3.The whole process under the argon atmosphere.
The working atmosphere of the pressless process (PLP) system is argon gas. The argon atmosphere will ensure the oxygen content in the low level and eliminate the generation of rare earth nitride, then avoid deteriorating of coercivity.
