Effect of Process and Trace Elements on the Properties of NdFeB

- Nov 01, 2017 -

Permanent magnetic materials have become important material bases for modern science and technology, such as computer technology, information technology, aerospace technology, communication technology, transportation technology, office automation technology, home appliance technology and human health and health care. Commonly used permanent magnet materials are mainly ferrite, alnico and rare earth permanent magnet. Rare earth permanent magnet materials include samarium cobalt and NdFeB, etc., of which NdFeB has a record high remanence, high coercivity and high magnetic energy product, known as a new generation of rare earth permanent magnet or third generation rare earth permanent magnet.


NdFeB magnets according to its preparation process can be divided into sintered NdFeB magnets, bonded NdFeB magnets and hot deformation NdFeB magnets. The magnetic energy product of the sintered NdFeB magnets is high, and the current level of international laboratory research has reached 444kJ / m (55.8MGOe), and the industrial production has reached 414 kJ / m <3> (52MGOe), but the majority of our country NdFeB production enterprises, due to obsolete production equipment, backward technology, resulting in low product performance (magnetic energy products generally in 278-320kJ / m <3> (35-40MGOe) around), and the performance of the stability and consistency is poor, Has been unable to enter the mainstream applications of NdFeB magnets.

Sintered NdFeB permanent magnet material with its high magnetic energy product, low cost and good processing performance and obtain a rapid promotion and application. With the expansion of its application field, its comprehensive performance requirements are also increasing, especially NdFeB permanent magnet motor application environment requirements more and more demanding, high heat resistance sintered NdFeB magnets demand More and more intense. High coercivity sintered magnets, if combined with a low temperature coefficient, can make NdFeB magnets at higher temperatures, especially in the use of more than 200 ℃ temperature sintered NdFeB magnets, the scope of its application will be more The more broad. This is because although the Sm-Co magnet can work at temperatures above 300 ° C, it is expensive because of the need to add a large number of strategic elements Co. Therefore, in the temperature range of 200 ~ 300 ℃, sintered NdFeB magnets have a great advantage.


First, its high magnetic properties, generally its maximum magnetic energy products are greater than 30MG0e, and Sm-Co magnet magnetization energy is much lower. Second, the sintered NdFeB magnets do not contain or contain only a small amount of Co, Ni and other strategic elements, the price is cheaper than the Sm-Co, together cost is much higher. Moreover, the sintered NdFeB magnets have good mechanical and mechanical properties, which are better machinability than Sm-Co magnets, thereby reducing the reject rate and improving the production efficiency. Therefore, the development of high coercivity sintered NdFeB magnets is an important direction for future competition. The hard magnetic properties of NdFeB magnets mainly depend on Nd <2> Fe <14> B (2: 14: 1) main phase. Adjusting the main component in the magnet can improve the intrinsic magnetic parameters and optimize the microstructure of the magnet grains can improve the macroscopic hard magnetic properties. We use advanced production equipment, take a new magnet preparation process, by adjusting the process parameters, optimize the composition formula, add trace elements and other means, so that the hard magnetic properties of the magnet can be greatly improved. Our main research work is as follows:

(1) The alloy magnets with Nd <33.5> Dy <0.99> Fe <, bal> .Al <0.52> Cu <0.1> B <1.15> (wt%) were prepared by conventional powder metallurgy method. The effects of the addition of trace elements Dy, Cu and Al on the microstructure and magnetic properties of the magnet have developed a high coercivity magnet. The results show that the addition of Dy, Cu and Al by conventional process is particularly effective for improving the coercivity of sintered magnets. The addition of Dy not only enhances the anisotropy field of the magnet, but also finishes the grain and suppresses the precipitation of soft magnetic a-Fe in the alloy, which effectively improves the microstructure of the magnet and enhances the coercivity of the magnet. There are two reasons for the addition of Cu to increase the coercivity of the magnet: one is Cu into the main phase, occupies the J <2> crystal position, reduces the plane anisotropy, helps to improve the uniaxial anisotropy field The coercivity of the magnet. On the other hand, Cu has a small grain size, and the specific surface area of the grains increases. For the high-performance magnets with low neodymium content, the neodymium-containing phase distribution is more uniform. The special wettability of Al plays a good wetting effect on the grain boundary and forms a new Al-rich neodymium phase at the grain boundary, making the grain boundaries clear and more effectively reducing the exchange between grains The demagnetization effect of the coupling increases the coercivity. Using magnetic microscope to analyze the structure of magnetic domain, we can find that the width of magnetic domain is much smaller than that of average grain size, which shows that almost all of the sintered NdFeB grains are multi-domain structures under thermal demagnetization.


(2) Nd <31.0> Dy <1.08> Fe <, bal> .Nb <.0.5> Al <(d) was prepared by Hydrogen Decaffitation (HD) process and Jet Milling (JM) 0.34 & gt; B & lt; 1.1 & gt; (wt%) magnets. The effects of hydrogen treatment on the microstructure of the magnet were studied. The results show:

a: The composition of the alloy can be made close to the composition of the main phase Nd <2> Fe <14> B, and the main phase grain is small, the neodymium-rich phase is distributed uniformly to avoid the soft magnetic a- Fe phase. The microstructure of the alloy prepared by this process satisfies the requirements of high-performance NdFeB magnets for the alloy, and opens up a new way to prepare high-performance NdFeB magnets. Adding some alloying elements can improve the magnetic properties of sintered NdFeB.

B: Nb was added into the alloy to form two kinds of compounds of NbFe and Nb <2> Fe <3>, which prevented the precipitation of a-Fe to some extent and refined the grain of sintered body. And the addition of Nb to make the sintered magnets grain shape more regular, the size tends to be consistent, intergranular Nd-rich distribution is more uniform. Nb mainly in the neodymium phase, sometimes associated with Dy, and Dy mainly into the main phase. In the milling process by adding Dy <2> O <3>, can improve the coercivity of sintered body, hinder the growth of particles. By observing the magnetic domain, the reason of the magnetic domain and the shape of the magnetic domain are analyzed. The crystal morphology is connected with the corresponding magnetic domain structure. It is found that the composite structure of Dy, Nb and Al makes the magnetic domain structure of the magnet clearer. The magnetic texture has been formed. It also explains the relationship between magnetic domain structure and magnetic properties. We also studied the relationship between the pulsed magnetic field and the degree of orientation. The results show that the repeated pulsed magnetic field orientation process is very beneficial to improve the orientation of the billet.

 

(3) 35UH high coercivity sintered NdFeB material was prepared by replacing the rare earth with heavy rare earth, reducing the oxygen content and low temperature long time fine grain liquid phase sintering technology. The effect of adding trace elements on the coercivity of magnets was studied. By adding Tb to prepare high temperature and high coercivity magnets, the problem of high temperature stability of NdFeB magnets was solved, and the high coercivity magnets The It is found that the addition of Tb improves the magnetic properties of the sintered NdIFe-B magnets because the addition of Tb improves the microstructure of the magnet by systematically studying the effect of Tb on the sintered NdFeB magnets. In particular, the addition of Tb makes the size of the grains in the sintered NdFeB magnets coincide, so that the grain shape is obviously regular. Add Tb can also refine the grain, improve the Hci and demagnetization curve of the square. For the sintered NdFeB magnets, the grain size is relatively large (μm), the non-magnetic phase separation between the grains, the interaction between the crystal grains is relatively weak, the main effect of coercivity is long-range static magnetism effect. The grain size is reduced, the grain refinement and uniformity, the long-range magnetostatic interaction between the grains gradually weakens, the coercivity of the magnet increases greatly, and the remanence is weakly strengthened; the remanence of the oriented magnet is obviously larger than that of the unoriented magnet, This field theory gives a better explanation. In addition, the demagnetization curve of the sample also shows that the effect of the applied orientation field on the magnet performance is similar to that of the interphase exchange coupling interaction.


Article from NdFeB Industry Network

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