FeCrCo sintered magnet and a-Fe/NdFeB compound magnet

Microstructure and magnetic characteristics of Sintered FeCrCo.magnet produced in the way of SPS

In general, FeCrCo magnet is rolled into sheet metal then punched into different shapes, the application range is constrained if adopting this way, thus a new way named powder metallurgy is developed for making bigger piece of FeCrCo magnets Songyuanyuzhi and other Japanese carry out the processing for FeCrCo magnet by adopting MK-3.2-Ⅱ pulse electrifying and pressing sintering machine. The powders of raw material are made by spraying, which is with 25% Cr, 13%Co-Fe, in terms of mass, and having an average granule diameter of 80μm. To pressurize 39MPa, the temperature is 923K-1323k within a temperature rising time of 9min, temperature keeping time of 7min. Then it comes to homogenization handling, with heating temperature of 1423K, temperature maintaining time of 1 h, after it achieves the anisotropic magnetic property it comes to the regulation of magnetic field, with the exterior field of 0.4T, heating temperature of 928K, and temperature keeping time of 2h, the aging is the last processing step, with sample sized of D20mm*H40mm. To make comparison, powders of same particles size will be used, and samples will be made in exterior heating furnace using traditional sintering technique. Scanning electric lens will be used to observe the organization and structure for the samples, all FeCrCo powders formed by particles close to sphere shape will be electrifying pressurized and sintered using technique of SPS and spraying powder crushing under 1073k,l to make it above 1173K, then as being sintered, the gap will be considerably reduced to have a intensity up to 98%, which is bigger than the 96.2% obtained under traditional sintering technique. For the latter type, the  sintering temperature should be up to ~1700K, which means with SPS way the temperature is 500K low. For the powder of smaller particle diameter such as 20μm, the bigger the density of sintered magnet compared to the particle diameter (80μm),the bigger the density for the sintering density, the latter one is less than 90%. The density of FeCrCo sintered magnet produced with SPS under 1223K is up to 98%, Br=1.25T, (BH)max=29.1kJ/m3, HC = 40.0kA/m, a far better magnetic property.

Following is the related sintering temperature and property:

SPS Temp.

Br

Hc

(BH)max

Relative density

/k

/T

/kA·m-1

/kJ·m-3

/%

1173

1.03

40.0

26.1

97.1

1223

1.25

40.0

29.1

98.0

1273

1.20

40.0

28.8

98.0

1323

1.05

36.0

18.4

97.6

1723

1.13

40.0

24.0

96.6

Both the magnet produced by using SPS technique with transmission electric lens (under sintering temperature of 1223K)and the one made by traditional technique share one common feature that they are FeCrCo sintered magnet formed by highly uniform homogeneousa-Fe crystals.

Magnetic property for α-Fe/ Nd2Fe14B compound magnet produced with hot-forging technique.

Y. Kawashita with other researchers in Nissan Motor Co., Ltd have developed the anisotropic compound magnet by using hot-forging technique. Experimentation for the production as follows:
Firstly using single roller melt-spun technique(with roller speed Vs=24/m/s to make melt-spun strips, the original alloy isNd10Fe80Co5B5, then powders needed for hot-forging processing will be made within Ar atmosphere, which has a particle size less than 350μm, then put 10g such powders into the cylindrical vessel, which is made of soft steel and has inner diameter of 14mm, height of 30mm. The vessel will be vacuumized and heated with high frequency induction at a speed of 50℃/s.The hot-forging machine is called crank mechanism, with a ho-forging strain speed of 150/s, compression rate of 90-95%,and hot-forging temperature range within 600-1100℃. To make comparison, some melt-spun strips would be vacuum annealed firstly. The hot-forged samples are of slab shape, should be solidified with epoxy resin and measured with VSM for its magnetic property, while DSC(Differential Scanning Calorimeter)will be used to measure its crystallization process, XRD(X-Ray diffraction)together with scanning electric lens and transmission electric lens for measuring its microstructure. The measuring and analysis shows that: Br and Hcj for the quenched strips is quite low, as the annealing temperature increases, the Br will reach 0.81T, Hcj up to 448kA/m, until temperature is up to 700-800℃.However, during 600-750℃, for the hot-forged samples, its Br=0.81, Hcj=460kA/m. As proved by the analysis that the anisotropic index J’S= J’S // /J’S⊥,the hot-forging temperature is 600~950℃, J’S ≈1.05, if temperature up to 1000℃ or more, then the J’S will achieve 1.25~1.35, which means that at this time the hot-forged samples are anisotropic magnets. The Br and Hcj of hot-forged sample will decrease as the hot-forged temperature increases, the ultimate approx. values are: Br=0.3T, Hcj=30kA /m, which means the crystal granule has grown up remarkably, while the magnetic property would decrease. The anisotropic hot-forged sample has a phasic crystal granule size of nearly 1μm for hard magnetic type, while the soft magnetic phasic crystal will extend along the normal line to the hot-forging direction. According to the inspection result with X-ray, it shows that the preferred magnetizing axis for the hard magnetic phase of the slab shaped hot-forged sample is parallel to the hot-forged exterior forcing direction. The reason thatα-Fe/Nd2Fe14B compound magnet induces anisotropy to create mechanism is due to the rotation on crystal brims of under single axis stress at 1000℃ or higher temperature. According to above description, we can see that magnetic property for hot-forged sample at 1000℃ or higher temperature will be improved.

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