Application and Prospect of additive manufacturing technology in permanent magnet material preparation
The application of additive manufacturing (3D printing) technology in the research of permanent magnetic functional materials is a hot topic. At present, the main research directions of 3D printing permanent magnet materials are preparation methods of permanent magnet precursor powder, room temperature adhesive printing technology, thermal adhesive printing technology, sintering printing technology and so on. This paper systematically summarizes the research status of permanent magnet materials prepared by different 3D printing technologies, and analyzes the existing problems and prospects. At the same time, the preliminary results of alnico permanent magnet material prepared by laser melting sintering technology are introduced. The experiment shows that alnico permanent magnet with the same performance as conventional casting process can also be prepared by 3D printing technology.
Permanent magnet materials are widely used in aviation, aerospace, navigation, weapons and other national defense and military industries, as well as military and civilian applications such as instrumentation, energy transportation, medical equipment, and electronic communications. There are mainly three common molding methods for permanent magnet materials: sintering molding, hot pressing molding, and bonding molding [1,2,3,4,5,6,7,8,9,10].
Additive manufacturing (AM), also known as 3D printing or rapid prototyping (RP), is a bottom-up material accumulation manufacturing technology. The advantage of additive manufacturing technology in material preparation is reflected in the ability to achieve near-net molding of complex shapes and manufacturing difficult-to-process products, which is conducive to saving resources, reducing energy consumption, and improving production efficiency[11,12,13,14,15,16 ,17,18,19,20,21,22,23]. At present, the commonly used 3D printing technology is characterized by rapid printing of single or small batches, which has played a significant role in product innovation. The use of 3D printing permanent magnet materials can realize the near net shape of complex-shaped magnets. It can not only reduce the waste of raw materials and energy consumption in the process of preparation and use, but also does not require machining, which reduces labor costs [24,25]. The 3D printing technology related to the preparation of permanent magnet materials can be divided into 6 types according to the molding method, and can be divided into three types according to the process conditions: The first type is room temperature bonded magnets, which include three-dimensional printing (3DP) and direct inkjet printing. (Direct-write 3DP) two molding methods; the second type is thermally bonded magnets, which include large area additive manufacturing (BAAM) and fused deposition three-dimensional printing (FDM) two molding methods[17,26] ; The three types are fusion sintered magnets, which include selective laser melting (SLM) molding and electron beam melting molding (EBM) two molding methods, as shown in Table 1. While comparing and analyzing the application status of these three types of technologies, this article also introduces the preliminary results of using SLM technology to prepare Al-Ni-Co magnets.
Preparation of 13D printing permanent magnet precursor powder
Table of Contents
- Preparation of 13D printing permanent magnet precursor powder
- 3D printed bonded magnet
- Melting and sintering 3D printing permanent magnets
The permanent magnetic powder commonly used in the 3D printing process is mainly Nd-Fe-B magnetic powder, manufactured by Magnequench. The scanning electron microscope (SEM) photos of the two typical magnetic powders are shown in Figure 1. Figure 1(a) is the SEM photo of MQP-15-9HD neodymium iron boron powder, the powder shape is flake. Figure 1(b) is the SEM photo of MQP-S-11-9 neodymium iron boron powder, its appearance is approximately spherical. Table 2 lists the performance parameters of the two magnetic powders. It can be seen that the comprehensive magnetic properties of the flake NdFeB powder are better than the spherical NdFeB powder.
According to different 3D printing technologies and molding methods, flake and spherical powders are suitable for different printing methods. In order to obtain higher performance printing magnets, Direct-write 3DP, BAAM, FDM and other bonded 3D printing technologies are used flake magnetic powder, and technologies like 3DP, SLM, EBM and other technologies need to evenly spread the magnetic powder on the substrate, so the magnetic powder needs to be approximately spherical, that is, similar to MQP-S-11-9 neodymium iron boron powder, as shown in Figure 1. (b) Shown.
Table 1 3D printing permanent magnet material technology classification
|3D printing permanent magnet technology||English name||Type||Printing temperature||Material Science||Characteristic|
|Three dimensional printing bonding molding ||Three-dimensional printing (3DP)||Bonded permanent magnet||Room temperature||Binder, polyurethane resin, all kinds of NdFeB powder||It can be cured at 100-150 ℃, with low solid content (45%) and no damage to the structure of magnetic powder|
|Direct ink jet printing molding ||Direct-write three-dimensional printing (direct-write 3DP)||Bonded permanent magnet||Room temperature||Epoxy resin, polyether amine, acetone, Nd-Fe-B powder||100 drying treatment; low solid content (40%) |
|Large area additive manufacturing ||Big area additive manufacturing (BAAM)||Bonded permanent magnet||～200 ℃||Nylon 12, Nd-Fe-B powder||The comprehensive magnetic properties and mechanical properties are better than those of hot extruded Nd-Fe-B products|
|Fused deposition 3D printing ||Fused deposition||Bonded permanent magnet||About 100 ℃||A mixture of silane coupling agent γ – APTS, NdFeB powder and polybasic acid salt||Polymer heating and spraying, fuse manufacturing, material extrusion|
|Laser melting sintering of permanent magnet ||Selective laser||Sintered permanent magnet||600～1200 ℃||Al-Ni-Co, Nd-Fe-B permanent magnetic metal powder||The powder is irradiated by laser and melted by high energy of laser|
|Electron beam melting molding ||Electron beam||Sintered permanent magnet||600 ～ 1200 ℃ after preheating||Al-Ni-Co, Nd-Fe-B permanent magnetic metal powder||The powder is melted by bombarding it with electron beam|
Figure.1 SEM image of NdFeB magnet powder
Table.2 Performance parameters of magnetic powder
For flake magnetic powder, the precursor needs to be prepared before bonding 3D printing. In BAAM printing technology, the precursor is composed of 65% NdFeB and Sm-Fe-N mixed magnetic powder and 35% nylon 12. In the FDM method, the preparation process of the precursor is to first treat the NdFeB powder with a silane coupling agent γ-APTS to strengthen the interface bonding and prevent oxidation. The above-mentioned magnetic powder and polycaprolactone (PCL) are mixed for 10 minutes. Pelletizing is carried out in a screw extruder, and then the masterbatch is mixed with PCL and extruded in a single screw extruder to obtain filaments with a diameter of 1.75mm [19,20]. In Direct-write 3DP, the precursor is composed of epoxy resin (used as a binder), polyetheramine (curing agent), acetone, and mixed magnetic powder. The mixed magnetic powder used for printing is made of melted and quenched MQP-15 -9HDNdFeB powder and strontium ferrite powder are mixed .
At this stage, the main preparation processes of spherical powder are plasma rotating electrode method (PREP), plasma atomization method (PA), crucible vacuum induction melting gas atomization (VIGA), crucible electrodeless induction melting gas atomization (EIGA), plasma ball Chemical method (PS), their respective characteristics are shown in Table 3 .
Among them, the gas atomization method is more suitable for preparing spherical powders of magnetic materials. For example, the literature  reported that Magnequench used a gas atomization method to prepare Nd7.5Pr0.7Zr2.6Ti2.5Co2.5Fe75B8.8 spherical powder, which is currently the only commercial powder suitable for selective laser melting preparation method.
3D printed bonded magnet
Room temperature bonding 3D printing permanent magnet
Room temperature bonding 3D printing permanent magnet materials are divided into 3DP molding and Direct-write 3DP molding, and the molding temperature is at room temperature. The technical process of the 3DP molding method is (shown in Figure 2(a))  : First, the print nozzle passes through a print bed with a certain particle size of spherical permanent magnetic material powder and deposits a layer of binder on it to make the permanent magnet The material powders are bonded together, and then a layer of permanent magnetic material powder is laid on this layer by the powder spreader, and a permanent magnetic material product with a complex shape is obtained through the bonding between each layer of powder. Put the body and surrounding powder into an oven at 100～150℃ and bake for 4～6h to dry the binder, and then use a brush to remove excess unbonded magnetic powder. In order to improve the strength of the printed body, the printed body is immersed in polyurethane resin. The immersion time is about 5 minutes. The time should not be too long to prevent damage to the structure of the product. Repeated immersion for about 40 minutes until the body Until the immersion liquid is no longer absorbed, the body is completely immersed. The relative density can reach 50%. Afterwards, post-heat treatment is performed as needed or the material itself.
As a kind of additive manufacturing technology, Direct-write 3DP has been widely used in electronic circuits and flexible devices. At the beginning of printing, use the Slic3r tool path to design the model, and slice the model into the printing code of the first-generation software. Secondly, the printing paste is prepared. First, polyetheramine and acetone are added to the epoxy resin to obtain a pre-mixed solution. Then the magnetic powder was added to the above-mentioned pre-mixed solution, stirred at atmospheric pressure in a planetary centrifugal mixer for 60 seconds, and then the printing slurry was put into a plastic syringe, and the pressure, inter-stage movement and pressure modulation were all controlled by a computer system . Then, the nozzle continuously prints the bonded magnet at a certain speed (30mm·s-1). Finally, after printing on the substrate, the printed parts are cured for 24 hours at room temperature to obtain the finished product, as shown in Figure 2(b). The solid content in 3DP molding is relatively low, about 45% (volume fraction). How to increase the solid content in the future has become an important development direction for the development of the preparation technology of permanent magnet materials using 3DP molding. In Direct-write 3DP, printing speed, layer height, and nozzle diameter are key factors that affect printing quality. Because the epoxy resin plays the role of physical cross-linking between layers, Direct-write 3DP has higher bending strength than products prepared by 3DP molding methods, but printing filaments may also fall to the cured layer. The above results in the phenomenon that the bonding between layers is not tight, resulting in low bonding strength and poor printing results. The maximum solid content of the magnetic material formed by this method is 40% (volume fraction), which still has a gap compared with traditional injection molding. Therefore, the direction of future technology exploration is mainly to improve the solid content and stability of the slurry.
Table.3 Comparison table of preparation process of spherical magnetic powder
|Milling method||Energy consumption of milling||Fine powder yield||Base metal and cost||Advantage||Disadvantage|
|Plasma rotating electrode method (PREP)||Moderate-high||Low||Specific process bar, higher cost||Clean surface, high sphericity, few associated particles, no hollow/satellite powder, good fluidity, high purity, low oxygen content, narrow particle size distribution||The powder size is relatively coarse, the yield of fine-grained powder is low, and the cost of fine powder is high|
|Plasma atomization method (PA)||Moderate-high||Moderate||Thin line, higher cost||The yield of powders below 45 μm is extremely high, and there is almost no hollow sphere gas entrainment, which is better than the gas atomization method. The TC4 alloy used in Arcam electron beam forming is prepared by this method||Slightly poor sphericity, satellite powder, higher wire cost|
|Crucible vacuum induction melting gas atomization (VIGA)||Low-moderate||Higher||Ingredients or master alloys, lower cost||High fine powder yield, less than 45 μm can be used for laser selective melting, low cost||Slightly poor sphericity, more satellite powder, high hollow powder ratio of 45～406 μm powder, air entrainment, not suitable for electron beam selective melting molding, direct hot isostatic pressing and other powder metallurgy fields|
|Crucible-free electrode induction melting gas atomization (EIGA)||Low-moderate||Moderate||Conventional bars, lower cost|
|Plasma spheroidization (PS)||Moderate-high||Moderate||Non-spherical powder||The powder has regular shape, high spheroidization rate, smooth surface and good fluidity. Can prepare high melting temperature refractory metals, such as tantalum, tungsten, niobium and molybdenum||Long heating cycle, easy to cause volatile elements to be emitted, irregular powder, large surface area, high oxygen content|
Figure.2 Process flow chart of 3DP(a) and Direct-write 3DP(b)
Thermally bonded 3D printed permanent magnets
Thermal bonding 3D printing technology refers to bonded permanent magnet materials formed at 80-200°C, including two molding methods: BAAM and FDM. BAAM technology is currently a promising molding method for exploration, and it can be used to manufacture anisotropic, near-net-shaped bonded NdFeB. The basic process of the BAAM method is shown in Figure 3(a) . First, the composite magnetic particles composed of the mixed magnetic powder of NdFeB and SmFeN and nylon 12 are loaded into the printing cylinder that can be heated, and the cylinder is heated to 250°C. In the state, the composite magnetic particles can become a fluid state. A certain magnetic field is applied to the printing port, and then pressure is applied to squeeze out the melted composite fluid in the barrel to achieve three-dimensional printing. The advantage of this method is that there is no need for printing filaments, the nozzles are directly used to heat the magnetic particles to extrude them, and an anisotropic NdFeB magnetic product is printed under the action of a magnetic field. The raw material has good fluidity, and the magnetic properties, mechanical properties, and microstructure of the product are obviously better than those of NdFeB magnetic parts by extrusion molding, which is suitable for industrial production of large-size anisotropic NdFeB magnetic products.
Figure.3 Thermal bonding 3D printing process
As shown in Figure 3(b), the principle of FDM is  : The prepared precursor (filament) is melted into a liquid through the extrusion head of the heater, and the molten thermoplastic material is extruded through the nozzle. , The extrusion head moves accurately along the contour of each section of the part, and the semi-flowing thermoplastic material is deposited and solidified to cover the built part, and it solidifies rapidly within 0.1s. Each time a layer is formed, the workbench Then the height of one layer is lowered, and the nozzle scans and spins the cross section of the next layer, and repeats the deposition layer by layer until the last layer, so that layer by layer from bottom to top, it is stacked into a solid model or part. In FDM molding, each layer is stacked on the upper layer, and the upper layer plays the role of positioning and supporting the current layer, and it is extruded layer by layer in a prescribed manner to solidify and deposit on the platform. on.
The density of NdFeB products manufactured by BAAM under laboratory conditions is about 5gcm-3, which is equivalent to the density of products produced by injection molding, Hcj=11.0kOe, Br=7.2kG, saturation magnetization is about 7.9kG, and injection molding In comparison, BAAM has higher coercivity and remanence, the product (BH)max = 11.0MGOe . After testing, the average Young’s modulus of the product is 4.29GPa, the ultimate tensile strength is 6.60MPa, and the ultimate strain is 4.18%, showing good magnetic and mechanical properties.
BAAM technology can print anisotropic magnets. The future direction of BAAM technology exploration is mainly to study the type of binder, solid content, magnetic field distribution during printing, and the selection of molding temperature to further improve the remanence and coercivity of BAAM magnets. Strength, magnetic energy product and mechanical properties, and reduce defects between layers to improve mechanical properties. The FDM technology process is clean, simple, easy to make materials and does not produce garbage; high dimensional accuracy, good surface quality, easy to assemble, and can quickly build bottle-shaped or hollow parts; raw materials are provided in the form of reel wire, which is easy to handle and quickly replace. At present, good performance indicators have been achieved in the exploration process of the preparation of small magnetic materials. However, due to the obvious stripes on the surface of the molded part and the low strength in the direction perpendicular to the cross-section, it is necessary to design and fabricate a support structure. The molding speed is relatively slow, which is not suitable for building large parts.
Melting and sintering 3D printing permanent magnets
SLM fusion sintered permanent magnet
SLM technology is a technology in which metal powder is completely melted and solidified by cooling under the heat of a laser beam. Under the action of high laser energy density, the metal powder is completely melted and can be welded with solid metal metallurgy after cooling. SLM technology is precisely through this process, a rapid prototyping technology that accumulates three-dimensional solids and does not require adhesives.
In the direct metal SLM 3D printing process of NdFeB, the study found that the magnetic properties of the material are greatly reduced due to the formation of impurity phases during the printing process. According to the binary phase diagram shown in Figure 4(a), by appropriately adjusting the crystallization conditions, NdFeB magnets can be directly 3D printed and exhibit good magnetic properties, which are even higher than bonded magnets within a certain temperature range. . In the selective laser melting process, the recrystallized alloy is partially melted. It is a challenge to find the appropriate printing parameters. Based on the complex peritectic crystallization phase diagram of NdFeB, the high cooling rate of the microscopic molten pool produces a peritectic reaction to form Nd2Fe14B (F). The key to intermetallic phase and small grain size contributes to the improvement of coercivity. Studies have shown that heat treatment after printing does not improve the performance of NdFeB magnets. Compared with bonded magnets or raw magnetic powders, the temperature sensitivity of (BH)max of SLM printed magnets is higher, which may be related to the smaller lattice distortion during rapid cooling . Some experimental results show that the local high tempering gradient will affect the anisotropy of the magnet, which may be one of the possible ways to generate anisotropy in 3D printed magnets, which is beneficial to improve the magnetic properties.
Figure.4 (a) Pseudo-binary phase diagram of sintered NdFeB, (b) complex structure print
As shown in Figure 4(b), the researcher  printed an arbitrary complex structure and tested the sample, and found that the channel inside the magnet can cool the magnet and keep it close to room temperature during operation. It is difficult to achieve by traditional sintering methods. This cooling method can also be applied in motors, generators, actuators or sensors. Compared with the as-cast and sintered states, the remanence of SLM sintered Al-Ni-Co permanent magnets is as high as 0.90 T, which is close to 1.06 T of the directional solidification, high-texture and anisotropic cast Al-Ni-Co. In addition, the 3D-manufactured Al-Ni-Co permanent magnet has the same high coercivity of 300 kA·m-1, which is consistent with the coercivity of Al-Ni-Co with the same casting process . This shows that 3D printing technology is more suitable for the preparation of Al-Ni-Co magnets. Al-Ni-Co cylinders made of Al-Ni-Co magnets prepared by SLM technology are heat treated under different conditions. Researchers have found that Al-Ni-Co magnets are very sensitive to annealing time and annealing temperature .
It can be seen from exploratory experiments that SLM can not only reduce processing and post-processing costs, but also has the potential to provide the same level of coercivity and remanence as the existing commercially available Al-Ni-Co, and even better magnetic properties [twenty one].
Figure.5 (a) SEM image of Al-Ni-Co powder, (b) printed Al-Ni-Co sample, (c) printed magnet z-axis topography
Recently, I tried to use SLM technology to print Al-Ni-Co magnets. First, Al-Ni-Co powder was prepared, and the morphology is shown in Figure 5(a); the Al-Ni-Co magnetic powder below 80 μm is loaded into the material cylinder, and the thickness of the powder layer is 40 μm, and the height of the cylinder is Under the condition of 40 μm and printing power of 190 W, the process of raising the cylinder, spreading powder and melting is repeated to form a three-dimensional entity layer by layer. Figure 5(b) shows the printed product prepared by the SLM method. Figure 5(c) is the topography of the z-axis plane (along the height direction) after printing. It can be seen from the figure that although the 3D printed Al-Ni-Co has some cracks, overall there are few holes. The density of 3D printed Al-Ni-Co material is equivalent to the traditional casting process, about 7.3g·cm-3. Magnetic testing showed that the 3D printed Al-Ni-Co is close to the cast magnet.
EBM electron beam melting molding
The working principle of EBM electron beam melting is similar to SLM except that the heating source is changed from a laser beam to an electron beam. The technical principle is: spread the metal powder in a high vacuum chamber, and use the electron beam emitted by the electron gun to bombard the powder to make the electrons A technology in which kinetic energy is converted into heat energy and the powder is melted and solidified by cooling. Under the action of electron beam, the metal powder is completely melted, and its vacuum degree is required to reach 1.33×10-2～1.33×10-4 Pa. It is generally used to produce active metals and refractory metals such as W, Ti, V, etc., and can also be used Produce high-quality bearing steel and ultra-low carbon stainless steel.
The vacuum environment and high temperature conditions of electron beam melting (EBM), the preheating of the powder layer, are more conducive to the manufacture of rare earth permanent magnet materials that are sensitive to oxidation . However, so far, there has been no report on the use of EBM to manufacture magnetic materials, but it is only used to prepare non-rare earth permanent magnets. For example, Arcam uses EBM to produce non-rare earth MnAl(C) permanent magnets, and has obtained preliminary results , The results show that MnAl(C) magnets can be prepared by EBM method.
Compared with EBM, SLM has higher energy, and the substrate does not need to be preheated. EBM needs to heat the substrate because of the lower printing energy, but because of its higher vacuum, it is more suitable for permanent magnet materials. The typical flow chart of printing, SLM and EBM is shown in Figure 6. In summary, at present, the 3D printing technology used to prepare permanent magnet materials in the laboratory research stage can be divided into three categories: room temperature bonding, thermal bonding 3D printing permanent magnets and fusion sintering 3D printing permanent magnets. The distribution of remanence and intrinsic coercivity of permanent magnetic materials prepared by similar 3D printing technology is shown in Figure 7. Among them, the blue area is the Al-Ni-Co permanent magnet material prepared by the SLM method, which has high remanence, low coercivity, and more concentrated performance distribution; the gray area is the Nd-Fe-B magnet prepared by the SLM method. The distribution difference is large, which may be related to the solidification rate during the printing process; the orange area is the Nd-Fe-B magnet prepared by the Direct-write 3DP method, which has higher coercivity but low remanence; the yellow area is prepared by the BAAM method The Nd-Fe-B magnet of Nd-Fe-B has higher coercivity; the Nd-Fe-B prepared by FDM method in the green area has higher comprehensive performance, the reason is that the FDM method uses higher performance magnetic powder.
Figure.6 3D sintered magnet process (including laser sintering and electron beam sintering)
Figure.7 The distribution of magnetic properties of magnets prepared by three types of printing methods (SLM: Selective Laser Melting, FDM: Fused Deposition Three-dimensional Printing, BAAM: Large Area Additive Manufacturing, Direct 3DP: Direct Inkjet Printing)
- 1. At present, the exploration and preparation of 3D printing permanent magnet materials under laboratory conditions are mainly NdFeB. The results show that the magnetic properties of the 3D printing magnet prepared by FDM technology with flake magnetic powder are relatively high, and the magnetic properties of the magnet prepared by direct write 3DP method are relatively low. Baam technology is suitable for preparing large size anisotropic rare earth permanent magnets, so baam technology can focus on further improving the comprehensive magnetic properties and mechanical properties of magnets.
- 2. For 3D printing bonded magnets, improving the stability and proportion of magnetic powder is the development trend. SLM and EBM belong to the melting sintering method, which can produce high-density printed magnets. The focus of the research is how to control the grain size and enhance the magnetic anisotropy to prepare practical permanent magnet materials with higher performance.
- 3. For SLM technology, 3D printing AlNiCo magnet is studied, which is close to traditional ingot al Ni Co material in density and coercivity, which lays an experimental foundation for further improving the comprehensive magnetic properties of magnet by adjusting printing parameters and controlling solidification characteristics.
Author: Zhu Xiaoyu Liu Tao Wang Lei Wang Shuai Fang Yikun Zhu Minggang (Chinese Journal of rare earth, 2020, 38 (06), 715-723 DOI:10.11785/S1000-4343.20200601)
Source: China Permanent Magnet Manufacturer – www.ymagnet.com
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