Pressless Process in Route of Obtaining Sintered NdFeB Magnets Highlights / Pressless Process Magnets, PLP magnets
•A review of the pressless process for NdFeB magnets in the world and Russia is given.
•Enhancement of the alignment degree by application of pulsed magnetic field is studied.
•Reduction of the friction forces via addition of internal lubricants is proposed.
•The simulation of the magnetic alignment of Nd–Fe–B uniaxial particles is presented.
•A reason of anisotropic shrinkage of the powder at sintering is suggested.
A short review on the pressless process (PLP) involved in the manufacture of sintered Nd–Fe–B magnet is given. Two approaches to increasing the degree of powder alignment with a high filling density ρf in PLP-containers are proposed. (1) An increase in the pulse duration of applied magnetic field from 3.6 to 6.5 ms enhances the magnetic alignment of magnets prepared from the powder with ρf=2.5 g/cm3 and ρf=3 g/cm3 by 3% and 11%, respectively. (2) Addition of internal lubricants such as zinc stearate or esters reduces friction forces between the powder particles and, when the concentration of lubricants is bellow a critical concentration Ccr, increases Br and (BH)max by 5–7%. Simulation of the magnetic alignment of uniaxial particles demonstrates that a decrease in the coefficient of friction between the powder particles from 0.9 to 0.6 caused by the lubricant addition enhances the alignment degree. Contact dilatometry was used to study the anisotropy of densification of PLP-powders upon sintering. It has been shown that the anisotropy of the powder shrinkage is formed at the first stage of sintering at the temperature about 800 °С and is caused by the capillary action in the Nd-rich liquid.
It is demonstrated how the high degree of powder alignment in PLP magnets can be achieved by loading the powder into a container placed in a magnetic field of moderate strength. The strip-cast alloy with a composition of 30.00 Nd, 1.95 Dy, 66.42 Fe, 0.99 B, 0.54 Co, 0.1 Ga (wt%) was subjected to hydrogen decrepitation and then milled in a vibratory mill in toluene to an average particle size of 2.9 µm determined by the FSSS method. The powder was compacted in the magnetic field of 0.2 – 1.2 T to the filling density 2.6 – 3.2×10³ kg/m³. It is shown that loading the powder into a container placed in a magnetic field enhances the degree of powder alignment in sintered Nd-Fe-B magnets produced from non-pressed powder. At the filling density less than 3.2×10³ kg/m³, the density of magnets is high but insufficient, because of the formation of magnetostatic chains of particles, which impedes the powder compaction. The simulation by the discrete-element method qualitatively proves that the magnetostatic interaction of the chains of particles that are formed in the course of loading in the magnetic field stimulates a decrease in the density of the sintered magnets and its non-uniform distribution over the sample. As a result of the optimization of the parameters of the alignment and compaction of the powder loaded in a magnetic field, PLP magnets with Br ≥1.34 T, Нc ≥950 kA/m, (BH)max ≥340 kJ/m³, and the degree of alignment exceeding 96% were produced.
Nd–Fe–BMagnetic alignmentPressless process (PLP)LubricantDiscrete element method (DEM)Anisotropic shrinkage
What is pressless process (PLP) magnet?
The pressless process (PLP) is a kind of near net-shape manufacturing technique. The initial production of the pressless process (PLP) magnet is very close to final shape, therefore reducing machining and grinding cost dramatically. The shape and dimensions of the final magnet, and shrinkage degree of the NdFeB powder after sintered are taken into consideration while mold designing.
The intrinsic coercivity and maximum energy product of Dy-free pressless process (PLP) magnet already reached 1512kA/m and 398kJ/m3 in Sagawa’s previous work.
The technical process of pressless process (PLP) magnet
The highlights of pressless process (PLP) magnet
Preparation of sintered Nd-Fe-B magnets by pressless process
Pressless process used for the preparation of sintered Nd-Fe-B magnets has been studied. The effect of the average particle size D av, filling density ρf, and powder-texturing conditions for obtaining density ρ and desired magnetic hysteretic properties of sintered magnets has been investigated for both traditional technology (TT) and low-oxygen technology (LOT). The ρf magnitude ensuring the optimum relation between the density ρ of sintered magnets and their degree of texture was shown to be 2.5–3.0 and 2.2 g/cm3 for the TT and LOT, respectively. At lower ρf magnitudes, no required density of sintered magnets is reached, whereas at higher filling densities, a low level of texture and low remanence are realized. Optimum parameters of pulsed field inducing the high degree of texture in powders have been determined. The following properties were achieved for magnets prepared by pressless LOT: B r ≥ 14.2 kG, H c ≥ 8 kOe, and (BH)max ≥ 47.9 MG Oe.
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