Nd-Fe-B Magnets, Properties and Applications
Michael Weickhmann, Vacuumschmelze GmbH & Co. KG, Hanau, Germany
Abstract
Permanent magnets based on Rare Earth components have become more and more
important in today’s technology. This publication gives an overview of new Nd-Fe-B magnet
grades with improved corrosion resistance and special coating possibilities. In addition, a
detailed analysis is given of the magnetic orientation of axial field die pressed magnets and
their optimised use in a rotor system of embedded magnets.
1.
Magnetic properties of Nd-Fe-B permanent magnets
1.1
Overview of some soft and hard magnetic material
Magnetic alloys are typically described with their characteristically properties such as
Remanence Br for permanent magnets and Saturation Polarisation Js for soft magnetic
materials, each given in [T] or in [Vs/m2], and Coercivity Hc given in [A/cm] or [kOe]. Where
as in soft magnetic materials the Coercivity is kept as low as possible in the range of some
1x10-3 A/cm to reduce the hysteresis losses, the Coercivity of permanent magnets may be as
high as 2650 kA/m to assure stability against opposing fields occurring in e.g. electrical
machines. Fig. 1 gives an overview of characteristic magnetic materials.
Rolling Process
Rapid Solidification
2.5
FeCo
SMC
Pure Fe,
3%FeSi
2.0
C-Steels
Fe
Nanocrystalline
1.5
CoFeNi
FeCoCr
FeCoV
FeMnNiTi
FeNi (medium)
2.0
FeCoVCr
Fe Cr Co
1.5
NdFeB
FeNiAlTi
RE-Co
FeSiAl
1.0
Powder cores
1.0
Ni-Fe
Cr-, Co- Steels
FeNi 80%
AlNiCo
MnAlC
Pt Co
(Permalloys)
0.5
Amorphous
Co-based
Soft ferrites
NdFeB
bonded
Cu Ni Fe
0.5
Hard ferrites
0
0.001
0.01
0.1
1
10
103
100
104
Coercivity Hc /(A/cm)
soft magnetic
semi hard
magnetic
hard magnetic
Fig. 1 Overview of magnetic materials and alloys
0
Remanece Br / T
2.5
Saturation magnetization Js / T
Powder Route
1.2
Maximum Energy density (BH)max
The maximum energy density or energy product is an important property to characterise a
permanent magnet. It depends on the Remanence Br, [Vs/m2], the Coercivity HcB [kA/m]
which best describes the stability of demagnetisation, the permeability µp as well as the
rectangular shape of the hysteresis curve. Research and development is being focussed to
increase the Coercivity by adjusting the alloy composition (not neglecting Js) and by
producing a fine microstructure (since HcJ is reciprocal proportional to the grain size). A high
residual induction may be achieved by producing small crystallites and aligning them
perfectly parallel to gain the best anisotropy. The development of the energy density (BH)max
[kJ/m3] over the time is depicted in Fig. 2.
Fig. 2 Development of permanent magnets and energy density
1.3
Magnetic properties and production process of Nd-Fe-B magnets
By observing various permanent magnetic materials in detail, it may be shown that Rare
Earth based magnets possess the highest energy density available today, with a Remanence
of up to 1.45 T and a maximum Coercivity of 2650 kA/m, while ferrite, for example,
possesses a Remanence of only 0.45 T and a Coercivity of about 300 kA/m; bearing in mind,
that both intrinsic properties are interacting, so that the highest Remanence can not be
achieved at the highest Coercivity.
Nd-Fe-B sintered
VACODYM®
1.40
remanence B r (T)
1.20
1.00
Sm-Co
bonded
0.80
Sm-Co sintered
VACOMAX®
compression
Nd-Fe-B anisotropic
bonded
injection
compression
0.60
Nd-Fe-B isotropic bonded
injection
0.40
sintered
Ferrit
bonded
0.20
0.00
0
400
800
1200
1600
2000
2400
2800
3200
coercivity HcJ (kA/m)
Fig. 3 Overview of permanent magnetic materials as a function of Remanence vs. Coercivity
The following Fig. 4 displays the schematic manufacturing and sintering process of Rare
Earth based permanent magnets. In mass production, typically there are two ways of die
pressing which are common in industry: Axial field die-Pressing called AP and Transverse
field die-Pressing called TP. The TP method involves more manufacturing steps by first
pressing blocks and then slicing them to the desired shape, which is more expensive than AP
pressing, where the die press speed is much higher, thus reducing the costs significantly.
Vacuum-Melting
Casting
Production-Process for
Rare-Earth Permanent
Magnets
Crushing
Milling
Aligning
P
P
P
P
isostatic
-
Pressing
H ↑ = AP
H
H
→
die-
Sintering /Annealing
Machining/
Surface-Treatment
H
Magnetizing
H
Fig. 4 Manufacturing process of Rare Earth based magnets
= TP
1.4
Magnetic properties depending on the pressing process AP or TP
The outer axial or transverse field H influences the degree of magnetic orientation within the
pressed magnet (green body), thus the crystallite grain aligns itself into the direction of the
outer applied field. The degree of orientation is significantly influenced by the according
pressing technique AP or TP, resulting in a difference in Remanence of roughly 4% between
AP and TP.
Fig. 5a and 5b displays the Remanence and Coercivity of various alloys and compares the
AP or TP processing techniques respectively.
1,45
1,4
VD 6XX AP
VD 7XX AP
VD 8XX AP
722
745
remanence at 20°C (T)
1,35
764
837
1,3
1,25
776
854
633
863
1,2
655
1,15
872
669
max. continuous temperature @ B/µ0H = -1
Max. kontinuierliche Temperatur @ B/µ0H = -1
1,1
1,05
60°C
1
10
12
80°C
14
16
110°C
18
22
890
688
180°C 200°C 210°C 230°C
160°C
20
881
677
24
26
28
30
32
34
36
38
coercivity at 20°C (kOe)
Fig. 5a Remanence and Coercivity of AP die-pressed alloys
1,5
1,45
722
remanence at 20°C (T)
1,4
VD 6XX TP
VD 7XX TP
VD 8XX TP
745
1,35
837
764
1,3
633
854
776
863
655
1,25
872
669
1,2
1,15
881
max. continuous temperature @ B/µ0H = -1
max. kontinuierliche Temperatur @ B/µ0H = -1
890
677
688
1,1
50°C
70°C 100°C
150°C
170°C 190°C 200°C 220°C
1,05
10
12
14
16
18
20
22
24
26
coercivity at 20°C (kOe)
Fig. 5b Remanence and Coercivity of TP die-pressed alloys
28
30
32
34
36
38
2.0
Corrosion resistance of Nd-Fe-B permanent magnets
Apart from the magnetic properties, the corrosion resistance is an important focus for
applications such as electrical machines in Hybrid vehicles. Special coatings such as
VACCOAT® 20011 have been developed, which are electrically isolating, i.e. non conductive.
To test the corrosion resistance of the basic alloy, uncoated magnets are exposed in a
pressure cooker test at 130°C, 2.6 bar (2.6 x 105 Pa) pressure and 95% relative humidity. By
determining the mass prior to exposure, it is possible to set up a weight loss versus time
diagram by removing the oxides on the surface of the magnets at different time intervals and
determining its bulk mass. This test is commonly known as a HAST Test (Highly Accelerated
Stress Test). The weight loss per surface area in [g/cm2] of magnet (since corrosion always
takes place at the surface of the magnet) over the exposure time t in [days] may be plotted
according Fig. 6. While (old) traditional magnets show a specific weight loss of more than
1000 mg/cm2 after 14 days of exposure, new alloys such VACODYM®© 6xx and 8xx show a
specific weight loss of up to 1 mg/cm2 after 10 days of exposure. This behaviour is
comparable to SiFe laminated electric steel sheet, although its morphology is different.
M a ssen verlust / w e ig ht loss (m g/cm2 )
0,1
VACODYM 6XX, 8XX
1
VACODYM 7XX
10
VACODYM 642
100
herkömmliches
traditional Nd-Fe-B
1000
0
2
4
6
8
10
12
14
16
18
Auslagerungsdauer (Tage) / exposure time (days)
20
Fig. 6 HAST Test analogous to IEC 68-2-66 (130°C; 95% moisture; 2.6 bar pressure)
2.1
Corrosion resistance and salt-spray resistance of VACCOAT® 10047 und 20011
coating.
Special electric insulating coatings, such as VACCOAT® 10047 and 20011 were optimised
during the last years, so that an excellent salt-spray-resistance according ASTM B 117 or
DIN 50021 and temperature resistance of up to 200°C are achieved. Fig. 7a and Fig. 7b
show different magnets after exposure of up to 500 hours under Salt-Spray or 12 days after
HAST.
©
licensed by Hitachi Metals
VACODYM® 677 Magnets with Al-Spray-Coating VACCOAT® 10047
After 500 h salt spray test
After 12 d pressure cooker test
Segmented magnets have been glued
(130°C / 2,7 bar / 100% !! humidity)
and coated with VACCOAT ® 10047
Fig. 7a Salt-spray test DIN 50021 and HAST (130°C; 2,7 bar 100% moisture)
Edge Protection VACCOAT® 20011 – barrel coating process
Fig. 7b Micrograph of VACCOAT® 20011 coated magnet with homogeneous coating even
around sharp edges
3.0
Hot / Cold Side effect and field profile on AP die pressed magnets
A detailed analysis of Axial-field die-Pressed magnets reveals a different field distribution on
the upper and lower side caused by the position of the pressing mould within the die, thus a
divergence of the orientation field towards the upper punch. Fig. 8 shows a schematic setup
of the pressing tool with the field distribution within the mould.
Dimple or
groove to
mark the cold
Side
1
1.
2.
3.
4.
5.
4
5
1
2
coil
coil
punches
die
support
filling plate
Magnet
Field distribution in a
Axial Field Pressing tool
1
1
3
2
2
5
1
Fig. 8 Schematic setup of a pressing tool and magnetic orientation-coil
The divergence of field orientation within the magnet may adequately be simulated using a
FEA (Finite Element Analysis) model, by subdividing the entire magnet in six sub-magnets,
where the left and right sub-magnets positioned near to the upper punch have an inclination
of magnetic orientation of 20° respectively.
Fig. 9 FEA model simulating the field-profile of the upper and lower side of a magnet.
Evaluating the field profile at a distance of the width of a magnet above the upper side (Coldside upper punch) of the magnet and the lower side (Hot-side lower punch) of the magnet
respectively, we obtain a difference of induction of up to 14% as shown in Fig. 9. The
following Fig. 10 renders a field profile of AP die pressed magnets versus a strict diametrical
parallel oriented magnet, cut out of block material at the same Remanence level.
Fig. 10 Typical field distribution of upper and lower side of AP die pressed magnet
versus a strict diametrical parallel oriented magnet cut out of a block.
3.1
Hot- / Cold Side effect on AP die pressed magnets while magnetising
When magnetising embedded magnets in a rotor using a magnetisation head, the field
distribution of the head which co-aligns with the impregnated orientation of an AP die pressed
magnet can be observed. The orientation of the left or right subsection of the magnet obeys
the same direction respectively. To saturate the magnet fully, the magnitude of the outer flux
need to be at least 2500 kA/m at every position within the magnet, which is shown in Fig. 11
by arrows.
Fig. 11 Field profile using a magnetisation head for embedded magnets.
3.2
Hot / Cold Side effect on AP die pressed magnets in a motor application
Using a FEA model of a rotor with embedded magnets and a slot-less stator, one is able to
analyse the air gap induction and compare an AP magnet with a strictly diametrical parallel
oriented magnet cut out of a block material at the same Remanence level. The difference of
air gap induction may be up to 2%, which is almost the same achieved with TP transverse
die-pressed magnets. Fig. 12 a,b,c show this analysis in detail.
Fig. 12 a Magnet strictly parallel oriented
Fig.12 b AP orientation with Hot / Cold Side effect
Fig. 12 c Comparison of air gap induction
4.0
Summary
New Nd-Fe-B alloys and appropriate coatings may help to improve the performance of e.g.
permanent electric synchronous machines in respect of all environmental conditions i.e.
moisture or even oily conditions, as well as mounting magnets in such a way, that its intrinsic
field distribution due to the axial field die-pressing technique is being exploited to increase
the air gap flux density significantly at reasonable costs.