Effect of crushing methods on morphology and magnetic properties of anisotropic NdFeB powders

JOURNAL OF RARE EARTHS, Vol. 35, No. 8, Aug. 2017, P. 800

Effect of crushing methods on morphology and magnetic properties of anisotropic NdFeB powders WANG Renquan (王仁全), LIU Ying (刘 颖)*, LI Jun (李 军), CHU Linhua (储林华), YANG Xiaojiao (杨晓娇), QIU Yuchong (邱彧冲) (College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China) Received 31 October 2016; revised 4 May 2017

Abstract: Crushing is a critical factor for the anisotropic NdFeB powders with high magnetic properties. However, the traditional crushing methods of disk-milling (DM) or impact crushing (IC), are difficult to obtain high-performance powders. Herein, a new method of preparing anisotropic powders with high performance was reported, which was through crushing the hot-deformed magnets perpendicular to c-axis direction (shear crushing, SC). The microstructure and alignment of the magnets were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results indicated that the cracks mainly extended along the Nd-rich grain boundary phase and/or the binding interface of ribbons. It not only could protect the magnetic phase from being damaged, but also could keep the grains integrated. Therefore, shear crushing was an effective method for the preparation of highperformance anisotropic powders and then to fabricate the bonded magnets with Br=9.0 kGs, Hci=10.48 kOe and (BH)max=17.7 MGOe. Keywords: hot-deformation; mechanical anisotropy; crushing; microstructure; rare earths

Permanent magnets play a crucial role in modern society as a component in a wide range of devices utilized by many industries and consumers[1–6]. The NdFeB-type magnet can roughly be classified into two categories, the metallic and bonded magnets. The metallic magnet can either be produced by the conventional alloy casting[7] and powder metallurgy method[8,9] or by the hot deformation routes[10–12]. Unlike metallic magnets, bonded magnets possess many advantages, such as flexibility, machinability and weight reduction. To date, a majority of NdFeB bonded magnets also adopts the rapid solidification (or melt spinning) to produce raw powders and, therefore, the magnets are magnetically isotropic[13]. Magnets with better properties can be produced from anisotropic powder which can be obtained by hydrogen decrepitation process[3,14,15] and mechanical crushing[16]. The hydrogen decrepitation process is an effective method to prepare the anisotropic powder by pulverizing the hot-deformed magnets into powder. However, hydrogen drastically reduces the intrinsic magnetic anisotropy of Nd2Fe14B, which in turn substantially lowers the coercivity. In order to recover the magnetic properties, hydrogen must be desorbed from the decrepitated powder. The researches indicated that the desorption of hydrogen only by heating the powder in vacuum to above 600 ºC[14,17], which greatly limited the range of applica-

tion. On the other hand, mechanical crushing by traditional methods, such as a disk mill or jaw and roll crushers, is difficult to obtain the high-performance magnetic powders. The "non-directional" mechanical crushing method is likely to induce transgranular fracture and damage magnetic matrix phase Nd2Fe14B, resulting in the oxidation of powders[16] and decomposition of the soft magnetic α-Fe phase[18]. In order to keep the original texture of hot-deformed magnets, a new way was used to prepare the anisotropic powder. According to the characteristics of mechanical and structural anisotropy of hot-deformed magnets, they were comminuted into powders by crushing perpendicular to c-axis direction (Shear crushing). The mechanism of different crushing methods were discussed according to the morphologies and microstructure of the powders.

1 Experimental MQU-F (Nd13.6Fe73.6Co6.6Ga0.6B5.6, Magnequench Inc.) powders were used as the starting materials. The powders were firstly compacted into fully-dense isotropic magnet (Φ50 mm×H 33 mm) at 700–730 ºC under the pressure of 300 MPa in vacuum. The specimens were then hot deformed (HD) at 830 ºC under the pressure of 135–150 MPa in pure argon with the height reduction of

Foundation item: Project supported by the Foundation of Sichuan Province Development and Reform Commission (2016sfgw001), Sichuan Province Science and Technology Support Program (2014GZ0090, 2016GZ0262), and Sichuan Province Achievement Transformation Fund (2014GPTZ0003) * Corresponding author: LIU Ying (E-mail: [email protected]; Tel.: +86-28-85405332) DOI: 10.1016/S1002-0721(17)60979-7

WANG Renquan et al., Effect of crushing methods on morphology and magnetic properties of anisotropic NdFeB powders 801

68% (named as M1) and 73% (named as M2). Anisotropic powder was prepared by mechanical crushing with different methods, disk milling (DM), impact crushing (IC) (Fig. 1(a)) and shear crushing (DC) (Fig. 1(b)). The powder size was analyzed by sieves with different meshes (325, 200, 140 and 100). The compressive  strength of the hot-deformed magnet was tested by an electronic universal mechanical testing machine (WDW-50E) along the parallel (//P) and perpendicular (⊥P) to the pressing direction under a loading speed of 0.05 mm/min. The magnetic properties of bonded magnets were measured using an AMT-4 type multi-functional magnetization characteristics measuring instrument. The microstructure of the hot-deformed magnet and anisotropic powder were observed by a field emission scanning electron microscope (FESEM, S-4800 and JEOL 7200). The degree of alignment of the bonded magnets was examined by X-ray diffraction (DX-2000) with Cu Kα radiation.

Fig. 1 Schematic diagram of the crushing (a) Impact crushing; (b) Shear crushing

2 Results and discussion 2.1

Preparation of high-performance anisotropic NdFeB Magnets

The bulk anisotropic magnets were prepared by hotdeformation with different height reduction and the appearance of the crack-free HD magnet is shown in Fig. 2(a). The peak magnetic properties with Br=14.28 kGs, Hci=8.89 kOe, and (BH)max=49.2 MGOe were obtained when the deformation reached to 73% (M2) due to its excellent c-axis crystallographic alignment (Fig.2(b)). In comparison to the sample M2, the sample M1 has a relatively lower Br=13.6 kGs and (BH)max=43.8 MGOe, but a higher Hci=12.37 kOe. To some extent, a large plastic deformation is beneficial to obtaining a good texture, but the coercivity decreases with increasing the deformation ratio[19]. The microstructure of the sample M2 in Fig. 2(c) shows that the platelet shaped grains are stacked perpendicularly to the pressing direction. On the other hand, the XRD pattern in Fig. 2(d) presents that the orientation peaks of (004), (105) and (006) become the main peaks, which indicate that a well texture is obtained in sample M2 and in accordance with the microstructure observed above. Similarly, the magnet M1 has a good texture with finer grains and then a higher coercivity with respect to that of magnet M2. In order to resist demagnetization at high temperature, the magnets must have a high coercivity. Thus, we selected the M1 magnets as the precursor to

Fig. 2 Characterization of the hot-deformed magnet (a) Appearance; (b) Magnetic properties; (c) Microstructure; (d) XRD patterns

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prepare the anisotropic powders due to its superior combination of high Br (13.6 kGs) and high Hci (12.37 kOe). 2.2 Morphology of anisotropic powder crushed by different methods 2.2.1 Impact milling The morphology of anisotropic powders crushed by different methods is presented in Fig. 3. From Fig. 3(a), it can be seen that the powders crushed by impacting are mainly composed of many fine particles. The size distribution was further analysed by sieve with different meshes and the results showed that the powder size less than 45 μm reached to 36.23%, as shown in Table 1. These fine powders easily incline to be oxidized due to their large specific surface areas, which result in complete or partial magnetic softening of the individual powders and then deteriorate the magnetic properties. On the other hand, the shape of the powders is very irregular, including many sharp arrises and angles. Those defects would hinder the powder rotation in the following mechanical moulding process under a magnetic field.

2.2.2 Disk milling Pulverizing HD magnets by disk milling has been widely used to produce the anisotropic powder[16]. Fig. 3(c) shows the morphology of the powders and it can be noted that many sharp arriss and angles are also observed on the pulverized particles. However, the proportion of fine powders (<45 μm) drastically decrease comparing with the powders metioned above, which is only about 15%. In addition, from the SEM images, it can be observed that lots of ultra-fine powders adhered to the surface of the large powders, as shown in Fig. 3(d). The size distribution ranges a large scale and, in contrary to the IC powders, the ratio of large size (>104 μm) occupies the most proportion for about 41.3%, as shown in Table 1. 2.2.3 Shear crushing The morphology of anisotropic powders crushed by shear crushing is shown in Fig. 3(e). Compared with the above IC and DM powders, the particles have a relatively regular morphology and exhibit flake-like. Meanwhile, the edges and angles of the powders possess less sharp arrises in appearance than IC and DM powders. Furthermore, the powder surface presented in Fig. 3(f) is relatively "clean". The size distribution mainly focuses on the range from 45 to 150 μm and the proportion of fine powder (<45 μm) is only about 8%, which is far lower than that of the above two powders, as shown in Table 1. 2.3

Fig. 3 Morphology of anisotropic powders crushed by different methods (a, b) IC; (c, d) DM; (e, f) SC Table 1 Size distribution (%) of anisotropic powder prepared by different crushing methods Size range/μm

0–45

45–75

75–104

104–150

IC

36.23

28.64

12.85

22.27

DM

14.97

27.91

15.84

41.28

SC

7.96

28.01

29.89

34.14

Magnetic properties of bonded magnets prepared with different crushing powders

Bonded magnets (Φ10 mm×H 8 mm) were prepared with epoxy binder using hot-compression molding technique. For epoxy, 2.5 wt.% epoxy was mixed with the powder, which was compression molded at a pressure of 400 MPa in a magnetic field of 2 T (transverse to the compression axis) at the temperature of 120 ºC in argon atmosphere. The magnetic properties of the bonded magnets made by different kinds of powders are shown in Table 2. The Hci and Br of bonded magnets are 6.60, 8.79 and 10.48 kOe, 7.9, 8.8 and 9.0 kGs respectively, corresponding to the magnets made by IC, DM and SC powders. The Hci of bonded magnets significantly reduced comparing with the HD magnet (12.37 kOe), especially in the bonded magnets prepared by DM and IC powders and it decreases by 28.9% and 46.6%, respectively. However, it only reduces by 15.3% in the SC bonded magnet. In addition, the results in Table 2 show that the highest value of Br is obtained in the bonded magnets prepared by SC powders. As aforementioned, the SC powders have more regular shape and less sharp edges and angles, which can reduce the obstacle or friction between powders and are beneficial to the process of magnetic and mechanical alignment. The XRD patterns shown in Fig. 4 are also consistent with the result. The intensity ratio of (006) and (105), [I(006)/I(105)], represents

WANG Renquan et al., Effect of crushing methods on morphology and magnetic properties of anisotropic NdFeB powders 803 Table 2 Magnetic properties of the bonded magnets prepared by different crushing methods Methods

Density/ 3

Br/

Hci/

(BH)max/ MGOe

(g/cm )

kGs

kOe

IC

5.78

7.9

6.60

12.1

DM

5.79

8.8

8.79

14.2

SC

5.81

9.0

10.48

17.7

Fig. 4 XRD patterns of the bonded magnets made by different anisotropic powders (1) IC; (2) DM; (3) SC

the degree of orientation to some extent[20]. As shown in Fig. 4, the value of I(006)/I(105) in bonded magnet made by SC powder is 1.01 with respect to that of 0.92 and 0.80 in DM and IC magnets, respectively. Meanwhile, some rare earth oxides, NdxOy, were found in the IC and DM magnets, as presented in Fig. 4(1) and (2). Since the fine powders possess a large specific surface area and its high surface activity, the element Nd in Nd-rich phase and the matrix phase Nd2Fe14B is probably prone to oxidation in the crushing process, which would result in the coercivity decreasing rapidly.

Fig. 5 Compressive strength of hot-deformed magnets in different directions (a) Perpendicular to c-axis; (b) Parallel to c-axis

3 Discussion The magnetic properties of bonded magnets sensitively affected by the crushing method, which has a close relationship with the anisotropic microstructure of hot deformed magnets. Fig. 5 shows the compressive strength of hot deformed magnets in different directions. The compressive strength of magnet reaches to 810 MPa at the direction of paralleling to the c-axis while it is only 710 MPa in the perpendicular direction. The reports[21,22] indicated that the compressive strength of HD magnet even reaches up to 1980 MPa in the direction of paralleling to the c-axis versus 985 MPa in the perpendicular direction. The mechanical anisotropy of HD magnets mainly originates from the structural anisotropy, the Nd-rich phase and the binding interface between the starting ribbons. The Nd-rich phase, about 2–5 nm thickness[23], distributes along the grain boundaries, as shown in Fig. 6(c). Its Vickers hardness is only about 262, which is far below that of the matrix phase (HV950)[24].

Fig. 6 Microstructure of the hot deformed magnets (a) Parallel to pressure; (b) Perpendicular to pressure; (c) Schematic diagram of HD magnets

Once the cracks generated in the Nd-rich phase and/or the weak binding interface, they will expand rapidly along the grain boundaries and/or interface due to the anisotropic grain growth in HD magnets[25]. According to the SEM micrographs shown in Fig. 6(a) and (b), the morphology of fractured surfaces shows a typical intergranular fracture behavior. As a result, it could not only obtain the powder with regular shape and integrity grains, but also make the crushing process easier. In contrast, the

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HD magnets crushed through impacting are very difficult and the grains are mainly broken by transgranular fracture. Consequently, many fine and irregular shape powders are obtained in this method. The effect of DM falls in between the above two methods and the powders possess better properties than that of IC powders.

4 Conclusions The characteristic of mechanical and structural anisotropy in HD magnets was exploited to prepare anisotropic powders. The results indicated that the cracks expanded preferentially along the Nd-rich grain boundary and the binding interface when crushing perpendicular to c-axis of the HD magnets. As a result, the powders could keep more relative integrity grains and have a more regular shape than that of the ones obtained by IC and DM methods. Finally, the bonded magnets fabricated by the anisotropic powders crushed by shearing process had better magnetic properties with Br=9.0 kGs, (BH)max= 17.7 MGOe and Hci=10.48 kOe.

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