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19HARD-MAGNETIC MATERIAL FROM A MECHANOACTIVATED SPHERICAL POWDER OF THE ALLOY 25XK15ME
V. A. Zelensky*", V. S. Shustov", A. B. Ankudinov", I. M. Milyaev", and M. I. AlymovA
aBaikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, 119991 Russia
bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: zelensky55@bk.ru
DOI: 10.24411/9999-0014A-2019-10203
The hard-magnetic single-phase alloys of the Fe-Cr-Co system occupy a well-deserved place among hard-magnetic materials due to their high mechanical properties with a sufficiently high level of magnetic hysteresis properties (residual induction Br, coercive force Hcb, maximum energy (BH)max) [1, 2]. The production of magnets is carried out by casting or powder metallurgy. In the case of powder manufacture, in some cases it is more economically advantageous to give up the use of elemental powders, and to use powders of alloys with the required chemical composition. In the present work the alloy powder obtained by spraying the melt of a given composition with a stream of gas is selected as the starting material for the manufacture of samples of a hard-magnetic material. The cooling of the particles after spraying was performed at high speeds, which often leads to high hardness of the powder due to quenching. The use of such powders for the manufacture of details by powder metallurgy methods is associated with a many of difficulties. In particular, the compaction of such powders is complicated due to the spherical shape of the particles and the high strength of the powder due to quenching [3]. In this paper, an effective method for preparing the initial charge is proposed, which allows forming defect-free powder compacts and obtaining high-quality hard-magnetic material.
Spherical powder of 25XK15ME alloy with an average particle size of about 20 |im was used. Its chemical composition is presented in Table 1.
Table 1. Chemical composition (wt %) of 25XK15^E powder._
Fe C Cr Co V Al Si Nb S P O2 N2 base 0.053 24.9 15.1 0.88 0.58 0.68 0.81 0.0074 0.03 0.47 0.23
Pressing a spherical powder in the state of delivery, despite the high pressure (up to 600 MPa) and the use of a split matrix, makes it impossible to obtain samples without defects. Pressings are stratified, and pieces of compacts are disintegrated into powder with a weak touch. To increase the compressibility of the powder, various methods were used: annealing of powder at a temperature above the recrystallization temperature; used plasticizer; the powder was mechanically activated in a planetary mill using balls of different sizes. However, the annealing did not improve the pressing - the same separation of compacts was observed as on the original powder. The introduction of a plasticizer (polyvinyl alcohol was used) made it possible to obtain high-quality compacts and, after sintering, to obtain defect-free samples of a hard magnetic-alloy. However, the magnetic hysteresis characteristics (especially the maximum energy product) at the same time turned out to be unsatisfactory.
Mechanical activation of the charge was performed on a planetary mill Pulverizette-7 at a speed of 500 rpm. 10 g of powder and 70 g of steel balls with a diameter of 3 or 5 mm were taken for filling, the time was varied and was 20, 40, and 80 min. After mechanical activation,
iSHS 2019 Moscow, Russia
the powders were pressed. Analysis of the quality of compacts shows that the use of 5-mm balls for mechanical activation is preferable. Grinding with 3 mm balls for 20 min as well as on the powder without grinding makes it impossible to get defect-free pressing. The results of the experiments are presented in Table 2, where the magnetic hysteresis characteristics of the sintered samples are also presented. As can be seen, their values have acceptable values for technical use and are comparable with the properties of hard-magnetic materials obtained by casting methods and powder metallurgy using elemental powders [4-6].
Table 2. Properties of compacts and magnetic hysteresis characteristics of sintered samples. Grinding time, min 20 40 80
Diameter of balls, mm 3 5 3 5 3 5
Quality of pressing unsatisfactory satisfactory satisfactory good good good
Br, T - 1.185 1.23 1.25 1.21 1.18
Hcb, kA/m - 42.4 43.0 42.7 43.0 43.1
(BH)max, kJ/m3 - 27.6 30.6 31.7 30.7 31.8
Figure 1 shows the hysteresis curve of a hard-magnetic material synthesized from a powder, processed for 80 min with 5-mm balls. The density of obtained material is about 99%. Figure 2 shows the SEM images of spherical particles of the initial powder and the powder after grinding for 40 min with balls with a diameter of 5 mm. It is clearly seen that grinding leads to coarsening of powders due to cold welding and a drastic change in the shape of particles compared with the original spherical powder. The scatter of particles in size becomes significantly smaller in comparison with the original spheres, where the diameter of the main mass of particles lies in the range from 2 to 20 |im. The fact that there are no small particles (less than 10 |im) in the treated powders also indicates that cold welding took place during grinding. during which small spherical particles become part of large ones.
Fig. 1. Hysteresis curve of a sample from mechanically activated powder treated for 80 min using 5-mm balls.
(a) (b)
Fig. 2. Powders: (a) initial spherical; (b) grinding for 40 min with balls 0 5 mm.
Thus, it has been shown that the rational method of preparing a charge from spherical powder, obtained by the method of melt sputtering with a gas jet, is the mechanical activation procedure. Materials synthesized from mechanically activated mixtures have high magnetic properties that ensures their technical application. The best result of magnetic hysteresis characteristics was obtained during mechanical activation for 80 min: Br = 1.18 T; Ясв = 43.1 kA/m; (BH)max = 31.8 kJ/m3.
This work was supported by the Russian Foundation for Basic Research, grant no. 18-03-00666 with the involvement of state assignment number 075-00746-19-00.
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