Process improvement design and experimental study on sintered NdFeB mixed powder

Rare earth permanent magnet material is one of the most important basic materials to support modern industry. At present, in the manufacturing process of NdFeB Material by powder metallurgy process, the mixed powder process still has some deficiencies, the powder mixed directly with additives by manual operation is easy to appear partial agglomeration. After sintering, oil spots will be produced on the surface of the products, thus affecting the surface bonding performance of the sintered body after electroplating. In order to solve the above shortcomings, this paper designs a new type of layered spray additive addition method, and verifies the effectiveness of the device for reducing oil stains on the surface of products by comparing the test results. Through the test results, it can be shown that the process of using new atomization to add additives can effectively avoid the problem of local oxidation during one-time addition, and at the same time reduce the phenomenon of powder agglomeration in the process of mixing powder, the proportion of oil spot area on the surface of finished products after electroplating is obviously reduced, and the bonding properties of coating and base are improved.

Development of rare earth permanent magnet materials

Since rare earth permanent magnet materials entered people’s field of vision in 1966, through continuous research and development, rare earth permanent magnet materials have been developed from the first generation SmCo5, the second generation Sm2Co17, to the third generation Nd2Fe14B material [1-2]. Among them, neodymium iron boron permanent magnet materials are characterized by excellent magnetic induction and high coercive force. Market demand and industrial scale are developing rapidly. Application fields include modern industrial motors, Microelectronics motors, new energy motors, modern medical equipment, maglev trains and other technical fields such as light and electricity [3]. In the past 10 years, NdFeB materials in the world are developing rapidly with an average annual growth rate of about 20%. Sintered NdFeB materials have become one of the most important basic materials to support modern industry [4]. At present, mainstream neodymium iron boron materials are produced by powder metallurgy. In the process of powder metallurgy, mixed powder is the key process link that affects the properties of NdFeB materials. The powder granularity and components of various raw materials are distributed evenly through the powder mixing process, and during this process, the powder surface is treated with anti-oxidation treatment, prevent the oxidation of the powder in the back-oriented molding process from affecting the magnetic properties. At present, the addition of additives in the mixing process.

The method is mainly directly added by manual method, which mainly has the following disadvantages. First, in order to prevent powder oxidation, the opening time of the tank mouth is shortened as far as possible, resulting in high operation difficulty; Second, manually added powder is prone to local agglomeration, resulting in uneven distribution of additives, the surface binding properties of sintered body after electroplating are further affected.
Based on the reasons above, this paper will carry out research on the improvement of the technology of ndfeb powder mixing process and design a new type of layered spray additive addition method, and discuss the influence of the new additive adding process on the performance of sintered ndfeb magnet samples by testing and comparing the performance differences of sintered ndfeb magnet samples prepared by direct addition method and stratified spray addition.

Improved design of mixing powder process

At present, the production process of sintered NdFeB materials is shown in the figure, including melting, hydrogen crushing, milling, mixing, press forming, sintering and aging, machining, surface treatment and other steps, among them, the mixing process has an important influence on the uniformity of the components, and then on the performance of sintered NdFeB magnets. After tracking and analyzing the product quality, it is found that there are often a small amount of oil spots on the surface of sintered NdFeB obtained by traditional process production. However, the existence of oil spots affects the stability of product quality and the binding force of surface coating and substrate after electroplating. It brings great hidden dangers to the quality and reliability of products. Through the composition analysis of the oil spot, it is found that the formation of the oil spot is mainly due to the uneven distribution of the admixture, which forms a partial agglomeration and then changes the composition of the micro region. Therefore, in order to reduce the possibility of producing oil spots on the surface of products, the mixed powder process in the existing production process must be improved. Since additives need to be added in the powder mixing process, the current adding methods all adopt manual one-time adding process as shown in Figure 2), while manual adding has obvious deficiencies. First of all, due to the characteristics of neodymium iron boron preparation process, neodymium iron boron alloy needs to go through the hydrogen breaking process, that is, to use the hydrogen absorption characteristics of the rare earth alloy to put the neodymium iron boron alloy in a hydrogen environment, hydrogen enters the alloy along the neodymium-rich thin layer, which makes the alloy expand and burst to break. After hydrogen broken NdFeB alloy is prone to oxidation, so in the addition of additives in the process, the tank opening time is very short, easy to cause difficult operation. Even if the operation is very fast, it will cause partial oxidation of the powder near the tank mouth, which is difficult to avoid; Secondly, manually adding additives will easily cause partial agglomeration of the powder, the requirements for storage time are more strict and not easy to store. Even through subsequent mixing powder processing, Some aggregates still exist, resulting in uneven component of local raw materials. Oil spots are easily produced after sintering, and at the same time affect the binding force of the surface coating and the matrix after electroplating.

FIG.1 production process of sintered NdFeB

Figure.2 process of manual one-time addition of admixture
The above shortcomings exist in the manual addition of additives, so the improvement of the current powder mixing process has become a top priority. At the beginning of the design, the new addition method of admixture should mainly correct the shortcomings existing in the manual addition method. First of all, in the process of mixing powder in the powder mixer, oxygen-free method should be adopted when additives are added to avoid oxidation of powder, that is, nitrogen can be used as the bearing gas to be added as additives. Secondly, because the concentration of additives will cause the phenomenon of agglomeration, so the new adding method should avoid the concentration of addition, should take in the mixing process of powder to add. Finally, because the admixture is liquid, atomization of the admixture can also effectively improve the mixing effect of the admixture and the powder. The new type of admixture atomization adding method designed according to the above thinking is shown in figure 3. In order to realize the atomization of additives, as well as the functions of multiple timing and quantitative control of additives, a mixing powder improvement device suitable for the existing neodymium iron boron product production process is specially designed. The structural schematic diagram of the device is shown in figure 4. The device is mainly composed of tank body, power nitrogen, spray gun, air pressure valve, time controller and other components. The pipeline of the device mainly consists of two parts. One is connected with liquid additive and the other is connected with power gas. The additive can be sprayed and added through these two parts. The tank is fixed on the main engine of equipment and connected to the spray gun through air pipe. Liquid level keeps differential pressure with the spray gun so that additives can flow into the spray gun naturally. At the same time, the circuit control is adopted, and the time interval is set according to the demand. Each interval is a certain time, and the continuous spray is a certain time, and the pressure and flow can be adjusted freely. In addition, the total amount of additives can be controlled by the liquid level alarm on the one hand, and on the other hand, each addition can be calculated through the time interval, thus achieving the ideal atomization effect.

Figure.3 process of adding admixture by multiple atomization

Figure.4 Structure of admixture atomization adding device

Experiments

In order to verify the influence of the new sub-spray additive adding device designed in this paper on the performance of ndfeb products, a comparable control test is designed for verification. Three groups of samples with different formulas are prepared by manually directly adding additives and new layered spray adding additives respectively. Study on the influence of different additives on the performance of neodymium iron boron products.

Raw materials

For experimental verification, we take three kinds of NdFeB powder with different brands, which numbers are A, B and C. The main chemical components of NdFeB powder are shown in Table 1.
Table.1 main chemical composition of NdFeB powder

Chemical Constitution	PrNd	Gd	B	Fe	Others
Content/wt%	29.5	3	1	64.5	2

Test methods

Take the three different brand hydrogen broken powder numbered A, B and C 30kg each, weigh the three kinds of powder and divide them into two parts 15kg each, numbers A1, A2, B1, B2, respectively, C 1, C 2. Among them, A1, B1 and C1 powder are mixed by conventional method and then added by manual method, and then mixed with a mixer for one and a half hours, A2, for B2 and C2 powder, respectively use the same equipment of grinding powder with the atomization adding device which has been improved, and separate the additives in the process of making powder, after finishing making powder, mix with the same mixer for one and a half hours. After 6 groups of fully mixed samples are pressed into columns by a press, they are put into the same sintering furnace to burn columns with a diameter of 40mm and a length of 50mm. After sintering, 6 groups of samples were processed into 39mm * 5mm round cake respectively. After electroplating, the proportion of oil stains on the surface was tested. The surface of the sample was observed by scanning electron microscope.

Results and discussion

Analysis of influence of additives added by different methods on oil stains on product surface

6 groups of samples numbered A1, A2, B1, B2, C1 and C2 are mixed, pressed forming, sintering, machine addition and electroplating, through inspection, the oil spots on the surface of the sample are compared

The proportion and the specific results are shown in Table 2. It can be seen from the result of the proportion of oil spot that the oil spot of the sample added with additives by adopting the newly designed multiple atomization adding device is obviously reduced, and no oil spot is found on the surface of B2 sample, the proportion of oil spot on the surface of A2 and C2 samples is less than 0.1%. Through the scanning electron microscope comparison of A1 and A2 samples in figure 5, it can be seen that the oil spot area on the surface of A2 sample obviously decreases, and the non-neodymium iron boron phase in the oil spot area, that is, the white phase in SEM photos is dispersed in the metal equivalent, and no large area abnormal area is found. Comparing the test results and SEM photos, it can be shown that the new atomization adding device can effectively improve the mixing effect of NdFeB powder and admixture, and obviously reduce the possibility of generating oil spots on the surface of the sample, the stability of product quality is improved.

Table.2 The proportion of oil spots on the surface of samples prepared by different methods

Sample Number	A1	A2	B1	B2	C1	C2
Ratio/%	3.5	0.1	2.7	0	2.9	0.08

Fig.5 oil spots on the surface of sintered products with different additives added by different methods

Conclusion

The appearance of the powder was tested after sintering; The comparison of the test results can show that using the new adding method designed in this article to add additives, due to powder oxidation, the bad proportion of “oil spot” caused by agglomeration can largely reduce the proportion of oil spot on the surface of sample from over 2% to under 0.1%, and even eliminate it. Through time-period and layered atomization, the process of adding additives can effectively avoid the problem of local oxidation in one-time addition, and reduce the phenomenon of powder agglomeration in the process of mixing powder, make all kinds of raw materials’ powders mix evenly and have uniform composition, which avoids the appearance of bad “oil spot” on the surface of finished products after electroplating and improves the bonding performance of the coating and the base body.
Author :Deng Zhiwei,Sun Haoran,Chen Jie,Yan Runpu,Yu Xiaojun. 1007-1865(2018)09-0087-02.
Source: China Permanent Magnet Manufacturer – www.rizinia.com
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