Large Block, High Density, Low Weight Loss Rate and Excellent Plating Process for High Grade NdFeB Magnet

Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

Large Block, High Density, Low Weight Loss Rate and Excellent Plating Process for High Grade NdFeB Magnet LIN Xi-feng, DING Kai-hong, CHEN Gang, CUI Sheng-Ii, PENG Zhong-jie, ZHAO Da-jun, LV Si-jing, CHEN De-jin, LI Zhong-hua, WANG Yong-jie, WANG Shuang-zhu (Yantai Shougang Magnetic Materials Inc, 888 Yongda Street, Fushan District, Yantai City, Shandong 265500, China)

Abstract: In order to produce N44SH and N40UH high-grade magnet, both strip casting and hydrogen decrepitation processes were adopted, and the subsequent milling, sintering and heat-treatment processes were improved. The density of N40UH magnet is risen up to 7.65 g/cnr', Weight loss rate after the Highly Accelerated Stress Test (HAST) for 48 hours is less than 0.2 mg/cnr', The microstructure of magnet shows that the grain size is between 5 to 8 urn. The rich-Nd phase is fme and uniform, the pinhole is very few, such magnet can be used for wind power generator after phosphoruster treatment directly, and it can also be used in hydrogen atmosphere of hybrid car after surface treatment.

Key words: high grade Nd-Fe-B magnet; low weight loss rate; high density

1 Introduction

high grade magnets as Eqn.(l).

NdFeB sintered magnet came out in 1983. Because

(BH)maiMGOe)+Hej(kOe)

~

64

(1)

the magnetic performance of this magnet is higher than

where (BH)max is expressed in MGOe at room tem-

other magnets, and its content of Co is lower than

perature, Hej is expressed in kOe at room temperature.

Sm2C017, so its cost is lower. NdFeB magnet is widely used in MRI, VCM, power generator, CDIDVD pick up,

Such high-grade rare earth magnet is used for wind generator, servomotor, elevator motor and etc.

communication, EV, hybrid car and etc [II.

Strip casting and hydrogen decrepitation have

With global economy development the energy con-

been adopted for high-grade magnet process; sub-

sumed becomes larger and larger. The coal and oil as

sequent milling and compressing processes to prepare

unreproduceable resources become less and less. How

the big block magnet have been improved as well. The

to save energy and nature resources becomes an urgent

size of produced magnet is 130x70x38 mnr'. The

problem, which stands in front of us. Rare earth magnet

weight of each block is 2.5 kg. The magnetic perfor-

motor due to its higher efficiency is considered as ener-

mances of both N44SH and N40UH are within the

gy saving tool; consequently, the market demand for

range given by discriminant (A) of high-grade magnet.

rare earth magnet used for motor increased very sharply.

Their density reached 7.65 g/cnr', weight loss rate for

There are some general requests for rare earth magnet

HAST test for 48 hours is less than 0.2 mg/crrr'. The

motor, they are as follows: 1) the size and the power of

endurance of NiCuNi plating layer against Salt Spray

rare earth motor become larger; 2) the efficiency be-

Test (SST) is 120 hours, and its endurance against

comes higher; 3) the total working time becomes longer.

Pressure Cooking Test (PCT) is 240 hours.

Consequently, some new magnets with big size, high magnetic performance, high density,low weight loss rate,

2 Procedure

and high reliable of plating layer must be developed.

2.1 Procedure for high-density rare earth magnet

As it is well known that there is a discriminant for -296-

For Nd-Fe-B sintered magnets, its residual mag-

Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

netization (M r ) is expressed by Eqn.(2) [21• d--

Mr=A(l-fJ)-Cos8Ms

do

ticles. In result of AGG magnetic performance becomes (2)

where M s -

saturation magnetization; lati . re anve density;

d do -

A-

The powder size distribution during jet milling process should be checked and adjusted in advance. The magnet powder obtained after improvement is in the range of Xl 0;::2 urn and X9~9 urn, Fig.3 and Fig.4

volume fraction of correct magnetic domain;

CosO -

poor", Fig.2 is SEM image of AGG

are powder size distribution curve before and after im-

alignment degree;

provement.

/3 - volume fraction of non-magnetic phase. From Eqn.(2) above, it can be found that if to keep

The powder size distribution has been improved,

others factors unchanged except density, then M; would

therefore no AGG grain can be found in produced

increase with density correspondingly. The micro-

sintered magnet. The density of N44SH magnet is 7.60

structure of the conventional magnet showed that its

g/cm'', and density of N40UH is 7.65 g/cnr'. The uni-

grain size is around 10-20 urn. In order to increase density of magnet, usually the sintering temperature should be higher. But high temperature is always harmful for coercive force of magnet. The density of conventional magnet is usually less than 7.5 g/cm'', As for the magnet made from strip casting alloy, the grain size of final magnet is fine, normally the grain size can be controlled between 5 to 8 urn due to the powder size is smaller and more uniform than what made by conventional process. Thus, high-density magnet can be made without high temperature sintering. By this

Fig. 2 SEM image for AGG in sintered magnet

mean the coercive force of magnet is higher even Density distribution

though its density reaches 7.65 g/cnr'. Fig.l is SEM

2.5 . . - - - - - - - - - - - - - - .

image of strip casting alloys. It reveals the fine and

2 >-

~ 1.5

uniform microstructure of strip casting alloys.

~

g

During producing process of high grade Nd-Fe-B

Il:

magnet, the abnormal grain growth (AGG) would be

1

0.5

o L........a.JULIU

appeared, if powder size distribution is an undesirable

",'" ""-:. ,'Y cf'i

"".

one, i.e, there are some smaller size particles (less than

,.

","

",.

'I'" 'I"" ,10'~ rC"~o, ".",,,, ....._,,'Y

.,.

0,'

(11m)

1 urn) and some large size particles (bigger than 15 urn) in the powder, and the sintering temperature closed to the critical limit, then big particle would expand rather

Fig. 3 Powder distribution after improvement

quickly during sintering at a sacrifice of the small parDensitydistribution 2.5 . . . . - - - - - - - - - - - - - - .

2

se 1.5 ::::l

~ 1 u, 0.5

o L.......I&U ",IJ< <::J":-

<::J'

,'Y .....~ ",."," .,."", 0,',,'>,10'~rC"'Co, ~.",,,, 'C" ....'Y (lJm)

Fig. 4 Powder distribution before improvement

Fig. 1 SEM image for strip casting a1ioy

-297-

Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

formity of grain size is excellent, the max grain size is 8

time hrs

urn, and the average grain size is 5 urn. Fig.5 is SEM

48

image of sintered magnet.

96

144

192

240

~ ~ 0.5

Due to high density of magnet its contained hole is

~

much less, the magnetic performance and weight loss rate is much improved as well.

~'"

1. 5

.9

2

~

2.2 Low weight loss rate magnet Fig.6 and Fig.7 is the weight loss test result for

~ 2.5

N40UH and N44H magnet (test condition; HAST for

~

-.av~t- 1_ I • max

tI

-

-

• min

3"-------------'

240 hours, 120 'C,100% humidity,2 atm).

N44SH

From Fig.6 and Fig.7, it is clear that for N40UH and N44SH magnet weight loss rate for 240 hours is only 1.16-1.67 mg/cm2 and 1.97-2.65 mg/cm'', respec-

Fig. 7 HAST test result for N44SH magnet

tively.

2.3 High grade sintered NedFeB magnet

Table 1 Magnetic performances of high grade

Processing routes for high-grade magnet consists

sintered Nd·Fe·B rare earth magnet

of strip casting, hydrogen decrepitation, jet milling,

No

Grade

Temp

B,

Hej

BHm

Density

'C

kGs

kOe

MGOe

gleml

N44SH

20

13.24

21.58

42.63

7.60

transfer die pressing, vacuum sintering and heat treatment. The magnetic performances of obtained magnets are listed in Table 1. Sum of (BH)max+Hej for N44SH

2

N44SH

120

11.63

12.01

31.26

7.60

and N40UH is 64.21 and 64.24, respectively. They

3

N40UH

20

12.56

25.66

38.58

7.65

both are in the range of discriminant (A).

4

N40UH

150

10.88

8.46

27.42

7.65

Fig.8 to Fig.ll are the B-H curves for N44SH and N40UH magnet at room temperature and high temperature.

2.4 Preparation of big block magnet With development of rare earth magnet motors, the size of magnet becomes larger and larger. We have to solve this problem by the following means: Pressing machine used is different from the conventional one. The major difference is the strength Fig. 5 SEM image for high density magnet

of magnet field, which reaches up to 2 Tesla, and the max press force is up to 60 tons. It is able to make big

48

~0.5 ~

13 1

~

~1.5

..s

i

2

time hrs 96 144

192

240

1-------- - 1-- I-

-I

•ave

emax • min

block magnet with high density and high grade by this new pressing machine. The weight for each block is around 2-3 kg. The magnetic uniformity for one big block is listed in Table 2, the position of samples is shown in Fig.l2. From the data of Table 2 it is clear that the difference of Br is 0.08 kOs, the difference of R ej is

~2.5

3L.---

----J

N40UH

0.44 kOe, and the magnetic uniformity for each block is excellent. Table 3 is the magnetic uniformity test result for a

Fig. 6 HAST test resuit for N40UH magnet

mass product batch. From Table 3 it is clear that the -298-

Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

uniformity of Br-Mr = 0.17 kGs; the uniformity of Hcj-tlilcj = 1.3 kOe for one mass product batch.

Fig.13 is magnetic performance distribution for one batch; Position of each sample is shown in Fig.14.

~---------:::::::~••

. .. .. I

I

....

-........... -........ ....,- ......

I

- - '....... ..•....

.... ... ....

l.1a 4.1,...

lid

..- ,.1.. ..

8fC

taM t< Ita

110I)

110I)

I I 111

-

4UI

IIIIIOlIt

t....

oM 1M • .4J

,,

I I I I I

• m

ItNIIIJ ItNIIIJ

• ....... 0rMIl

m .......

Fig. 8 N44SH B-B curves at 20 "C

...

""

.. .. I

I

I

I ..,

-- - .. - - -..... ................... ......

...• """ .......

--

...

lid

.. ..- -........... 2ft

tUl

lUI

lUI

•

f.D I.tI

.....

11IOII

Fig. 9 N40UH B·B curves at 20 "C

-299-

• I t.I I iIDt' I I 1771

.,'" .,

I. I

m .....

IIrMIt

ItolWII lIIMIt

m

IWIII

Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

Fig. 10 N44SHB·H curves at 120 "C

Fig.11 N40UHB·H curves at 150 "C

2.5 High reliability of surface coating technologies For Nd-Fe-B rare earth magnet surface treatment technologies are very important. An Automatic plating line controlled by computer was built in our Plant. The

process route includes degreasing, ache wishing, activation, abstergent, plating dark nickel, plating copper, plating bright nickel. Based on the fact that magnet has higher density and hole in it is much less, then the

-300-

Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

Table 2 MapeUt performanceuniformityfor

Table3 Magnetic Uniformityfor a mass product batch

one big block

No. c3

Br/kGs

Hc/kOe

BHn/MGOe

13.06

21.50

41.61

Br/kGs

Hc/kOe

(BH)...../MGOe

12.53

25.06

38.9

7.64

2

13.00

21.91

41.63

2

12.54

25.47

38.84

7.65

3

13.05

21.84

41.84

3

12.56

25.34

38.67

7.64

4

12.94

22.11

41.47

4

12.55

25.27

38.83

7.65

5

13.01

21.97

41.43

5

41.83

No

p/(g·cm

)

12.6

25.48

38.85

7.65

6

13.06

21.10

6

12.61

25.36

38.9

7.66

7

13.00

21.22

41.86

7

12.57

25.14

38.77

7.64

8

13.02

21.74

42.04

Max

12.61

25.48

38.90

7.66

9

13.05

21.94

42.24

Min

12.53

25.06

38.67

7.64

10

13.06

21.92

43.02

AVE

12.57

25.30

38.82

7.65

11

13.03

21.78

42.36

R

0.08

0.42

0.23

0.02

12

13.03

22.30

42.80

13

13.04

21.84

42.27

14

13.11

21.49

42.62

15

13.03

21.82

42.43

16

12.98

21.86

42.25

17

13.00

21.85

41.60

18

13.01

21.75

41.81

19

13.04

21.89

42.61

20

13.06

21.00

42.25

21

13.06

21.20

42.14

22

13.01

21.63

41.63

23

13.06

21.09

41.86

24

13.03

21.55

42.20

25

13.06

21.30

41.70

Max

13.11

22.30

43.02

Min

12.94

21.00

41.43

Aver

13.03

21.66

42.06

R

0.17

1.30

1.59

Fig. 12 Positionof samples reliability of coating layer is improved remarkably, and the bonding strength of plating layer is much improved as well. SEM image for plating layer is shown in Fig.15. Table 4 showed reliability of different coating.

Uniformity of magnetic performance test result

3 Result and Discussion (1) Add metal Dy and Tb to replace Nd can improve

13.4

anisotropy field as well as the intrinsic coercive force. But the B, and BHm will decrease correspon- dingly'". (2) Curie temperature, temperature coefficient and weight losses rate can be improved by adding metal Co

13.3

S.... I::l:l

---1

c-----------

1_ ~ Br_

25

-..-!icL

24

13.2

23

13.1

22 21

13

to replace Fe, but the coercive force will be decreased

12.9

correspondingly. Although add metal AI, Cu is able toreduce the influence [5,61.

12.8

,

-----

----

20

----

19 bo

'\

,~

,'"

,b

,"

rvrv

(3) Adding metal Ga is able to create more stable boundary phase or intergranular compounds, thus the Fig. 13 Magnetic performance distribution

weight loss rate can be improved as well [71•

-301-

~ :g

Proceedings of 19th International Workshopon Rare Earth Permanent Magnets & Their Applications

(5) Higher magnetic field and stronger compressing pressure is necessary for making big block magnet with good magnetic performance and better uniformity. Table 4 Reliability of different coating Plating

Thickness

SST

80 'C*900/0

Bonding

humidity

strength

Hour(s)

MPa

240

3000

2(HO

24

600

3Q-40

PCT

type um

Hour(s) Hour(s)

Ni

10-20

120

Zn

10-15

48

Epoxy

10-25

500

Fig. 14 Position of each sample References: [1]

Y. LUO, Proc. of 2004 China Magnet Symposium (Xi'an,

[2]

ZHOU Shou-zeng, DONG Qing-fei. High-Grade Mag-

China, May, 2004).

netics (2nd edition), Beijing, Metallurgy Industry Press, 2004, p.55-56. [3]

ZHOU Shou-zeng, Dong.Qingf. High-Grade Magnetics (2nd edition), Beijing, Metallurgy Industry Press, 2004, 524-526.

[4] Fig.15 SEM image of NiCuNi coating

1657. [5]

(4) Improvement of powder size distribution is

M. Tokunaga, Trans. Mat. Res. Soc. Jpn., vol.14B, Translation in Advanced Materials'93, lIB, p.1001.

able to avoid AGG come out. Thus critical sintering

[6]

temperature can be increased, then density of magnet is higher and magnet performance is improved. The

Lemke, H.Thomas, G, Scripta, Materialta 37, 1997, 1651-

M. Katter, Seminar UK Magnetics Society 6th Oct., Birmingham (1999).

[7]

W. Rodewald, M. Kalter, Evolution of Alternative Proce-

weight loss rate and the reliability of coating would be

ssing Routs for Nd-Fe-B Magnets, Vacuumshmelze

improved for highdensity magnet as well.

GmbH.

-302-