Sintered Neodymium Magnets: Properties, Grades, and Manufacturing

Raw material including PrNd alloy, iron, copper, DyFe, etc. are mixed and melted together according to a certain proportion. These are heated in a vacuum furnace and a stream of molten metal is forced under pressure onto a cooled drum where it is rapidly cooled at approximately 100,000 degrees-per-second. As a result, strip cast NdFeB ingot is produced for further processing

After melting and casting, the NdFeB alloy ingot is subject to hydrogen atmosphere. Hydrogen slowly absorbs into the solid alloy and makes it brittle, which upon milling becomes fine powder. The process is therefore called hydrogen decrepitation.

The NdFeB powder produced from hydrogen decrepitation is protected in an inert gas atmosphere and handled in glove boxes before going to the automated press. The powder are then fed into a grinding chamber, where the high-velocity gas streams are introduced to cause particles clash violently against each other. As a result, extremely fine powder having a size ranging from 3 to 5 μm is produced.

The powder from jet milling is pressed. During the pressing, a strong magnetic field is applied in order to rotate the powder particles so that their main crystal axes are aligned. That’s required to achieve anisotropy, which means that every piece of powder in the finished magnet will contribute to the magnetic field. Without the alignment, the c-axis (crystal axis) would be more or less randomly distributed, and only those particles which are roughly in the direction of the magnetization field would be magnetized, resulting in a significantly weaker magnet.

The pressed NdFeB block is bagged and immersed into a cold isostatic press (CIP) under huge pressure. This process intends to remove the remaining air gaps within the block. NdFeB blocks coming out of CIP become a bit smaller than it was before it goes in.

The pressed blocks are placed in a sintering furnace at an extremely high temperature just below the melting point of the NdFeB alloy. At this temperature of over 1000 degree, the individual atoms have a lot of motion, which enables the NdFeb alloy to develop their full magnetic and mechanical properties. The magnetic domains maintain the same orientation they had before sintering. At this temperature, shrinkage of NdFeB blocks takes place and full density is achieved. 

After sintering, there are pent-up stresses in the metal from all the movement during sintering, so the blocks are heat-treated again in a step fashion at lower temperatures to reduce the stresses. The blocks are ramped up to a high holding temperature for a set time and then they are ramped down to a lower holding temperature. Once the holding time is achieved, the now stress-free blocks are slowly cooled to room temperature.