Application of Rare Earth Permanent Magnet on Magnetic Abrasive Machining

1 Department of Mechanical Engineering, Anshan University of Science and Technology, China
2 Department of Mechanical Engineering, Saitama Institute of Technology, Japan
Keywords: Magnetic abrasive machining, Permanent magnet, Magnetic flux, Coercive force

Abstract. Because of the raw material elements and its purity and so on, the Nd-Fe-B permanent magnet,
the strongest magnetic material, which needs artificial synthesis, can hardly be used directly. The
performance of the permanent magnet has not yet been greatly developed owing to the limitation of the
artificial synthesizing technology, of the powder sintering technology and that of the application. In this
paper, the magnetic abrasive machining method as a new application is put forward, and from this
viewpoint, are discussed the performance and the processing technology of the permanent magnet and the
magnetic abrasive machining method. A sintering route combining the direction heat treatment technique
to increase the magnetic energy is suggested.
Introduction
In the permanent magnet materials domain, the development of the Nd-Fe-B permanent magnet has
now undergone nearly 20 years. Through the raw material depuration, powder smelting, heat
treatment, surface treatment and so on, each kind of technological progress in the Nd-Fe-B permanent
magnet leads to the continuous enhancement of its permanent magnetic performance, which also
needs to develop the new magnetic materials, and that a new application is to be sought for has
become the common request. The magnetic abrasive machining is regarded as one kind of new
permanent magnet application.
The magnetic abrasive machining is a method in which the magnetic field (lines of magnetic force)
is used to precisely machine the surface of the workpiece [1]. When the magnetic abrasive particles
are filled between N-S magnetic poles, owing to the application of magnetism, the magnetic abrasive
particles become a hard magnetic brush along the lines of magnetic force (The magnetization is
generated). When the workpiece is inserted into this magnetic field, and when the relative movement
between the workpiece and magnetic poles is given, the magnetic abrasive particles will press and
polish the surface of the workpiece (Figure 1).
Performance demand of permanent magnet
There are a lot of factors that influence the processing efficiency and the quality of the magnetic abrasive
machining. In the magnetic abrasive
machining, the magnetic abrasive
particles present a free state; rely on the
function of the magnetic field to enclose
on the surface of workpiece. When the
rotation speed of workpiece is too fast,
because of centrifugal function, the
magnetic abrasive particles breaks away
from the restraint of the magnetic force
and flys to the outside. So, the rotation
should be controlled within some certain
speed. Therefore, the magnetic force of
Fig. 1 Principle of magnetic abrasive machining
Key Engineering Materials Vols. 336-338 (2007) pp. 712-714
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permanent magnet is one of the main influencing
factors of polishing efficiency.
Now the experiment shows [2]:
F = K D3 H ( ∂H / ∂x ) (1)
where F is Magnetic force, K is Susceptibility, D
is Iron particles diameter, H and (∂H / ∂x) are
magnetic density and change rate, respectively.
The polishing effect considered, the diameter
of the magnetic abrasive particles and susceptibility
of the material are almost of fixed value, so,
the intensity of magnetic induction of permanent
magnet can only be increased for improving the
polishing efficiency.
The intensity of magnetic induction of permanent magnet is determined by the product (also called
the biggest energy product, which expresses the permanent magnet energy)of the intensity of
magnetic induction and the demagnetization of permanent magnet in the air gap, besides, the intensity
is related to the volume of the magnet and air gap.
Permanent magnet is various in types.The material ingredients of the rare earth and technology of
the artificial synthesis is different. But , its performance index is not different. In order to comply with
the market demand and the enhancement of processing efficiency, the permanent magnet used in
magnetic abrasive machining requires, under the volume small premise, a higher energy product and
higher magnetic-flux density. Seen from Fig. 2, the Nd-Fe-B (neodymium-iron-boron) magnet, a
typical example of the rare earth magnet, and the strongest man-made permanent magnet so far,
demonstrates a higher coercive force and the residual magnetic-flux density, and the Nd-Fe-B magnet’s
magnetic energy product is also higher, and along with the HDDR (hydrogenation disproportionation,
desorption and recombination) technology and the progress in nanometer synthesis magnet
technology, the magnetism performance also obtains the rapid enhancement, and it is possible for the
permanent magnet to obtain bigger magnetic field strength in the very small space [3].
Moreover, when the magnetic abrasive machining processing is used, the magnetic abrasive
machining working temperatures generally reach 200 centigrade because of the cooling function of
the polishing liquid. However, Curie temperature of the Nd-Fe-B permanent magnet may reach 312
centigrade, which conforms to the magnetic abrasive machining requirement in the real working
condition.
Nd-Fe-B permanent magnet
Nd-Fe-B material is a type of intermetallic compound, which has a composition of two rare earth
atoms: 14 iron atoms and one boron atom. Besides the main phase
Nd2Fe14B in Nd-Fe-B permanent
magnet material, a small number of Nd-rich phase (Nd2FeB3), boron-rich phase (Nd2Fe7B8) as well as
other phases exist. The Nd-rich phase provides the pinning of the domain walls so that Nd-Fe-B
magnet has high coercive force. The main phase Nd2Fe14B is a square crystal structure, which is
extremely important for obtaining the high magnetic properties (see Fig 3). Among them, the main
phase and the rich Nd phase are two most important phases of determining Nd-Fe-B permanent
magnet magnetism performance [4]. The main phase Nd2Fe14B only has the single axle aeolotropism
hard magnetism phase in the Nd-Fe-B permanent magnet and the main phase has higher saturation
magnetization intensity, magnetism crystal, and the dissimilitude field of each phase.
The Nd-Fe-B permanent magnet manufacture technology is divided into two kinds: sintered and
bonded. Generally speaking, Nd-Fe-B sintered magnet is the compact aeolotropic magnet, which is
made with the powder metallurgy method, but the Nd-Fe-B bonded magnet is obtained by means of
instant cooling of the microcrystal powder, and caking the powder into magnet lumps again with the
Fig. 2 Deperm curve of main permanent magnet
713 High-Performance Ceramics IV
polymer or other adhesive. Thus, Nd-Fe-B bonded magnet
is the non-compact isotropism magnet. Consequently,
the permanent magnetism performance of Nd-Fe-B
sintered magnet is higher than that of the Nd-Fe-B
bonded magnet.
In the production of Nd-Fe-B permanent magnet, the
rich Nd phase is separated out and agglutinated, and the
main phase surface magnetic domain occurrence spot is
eliminated. The rich Nd of the Nd-Fe-B permanent is
non-ferromagnetism magnet, therefore, there is a
problem in reducing the magnetization, and the rich Nd
can easily cause the Nd-Fe-B permanent oxidized, so it
is necessary to reduce the rich Nd as much as possible.
Here two methods are recommended.
First, in the alloy that is nealy composed of the
Nd2Fe14B composition, the primary crystal iron is
separated out by means of the Strip Casting control.
Simultaneously, the Nd2Fe14B will be heated up to 1027K or above in H2 so that the base body will be
decomposed into very small NdH2, α-Fe and Fe2B organizations, then dehydrogenation processing
will be completed within the same temperature range. During the dehydrogenation, Nd also produces
very small Nd2Fe14B chemical compounds that combine with each other, thus the higher coercive
force
of permanent magnet powder will be obtained. But, in the hydrogenation process, the part of
hydrogen atom will enter into the magnet’s crystal lattice. In such a case, the dehydrogenation
processing will be very difficult.
Second, the aeolotropism powders can be obtained by increasing Ga and Co elements in the
Nd-Fe-B alloy. By compressing these powders, the raw material powder uniformity of the granularity
can be enhanced; the sintered body organization may become very tiny, and makes the coercive force
bigger so much so that the bigger magnetic energy product will be obtained.
Conclusions
The magnetic abrasive machining requires that the permanent magnet performance should be the high
magnetic energy product. Therefore, the sintered Nd-Fe-B permanent magnet is widely chosen, and
this method has already received good processing effect in the actual application. If the raw material is
chosen, the rich Nd2Fe14B which enhances the magnetic energy product should be chosen. The
magnetic abrasive machining is one kind of new application domain for permanent magnet, but the
magnetic abrasive machining technology, compared with the traditional technology, still has
shortcomings, for example, the processing efficiency is not higher. One of the reasons is that the
magnetic energy product and the surplus magnetic-flux density of permanent magnet are not ideal.
The new manufacturing technology will continue to be found in the near future, so that the higher
magnetism performance of the permanent magnet can be developed.
References

Key Engineering Materials Vols. 336-338 (2007) pp. 712-714
online at http://www.scientific.net
© (2007) Trans Tech Publications, Switzerland