IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 10, OCTOBER 2014 2102505

Densification of Nd-Fe-B Powders by Hydrostatic ExtrusionWaldemar Kaszuwara1, Mariusz Kulczyk2, Marcin Kazimierz Leonowicz1,

Tomasz Gizynski1, and Bartosz Michalski1

1Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw 02-507, Poland2Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw 01-142, Poland

Hydrostatic extrusion is a modern method of densifying materials. This method is rapid and permits extruding materials of variousproperties. In the present experiments, Nd-Fe-B powder, provided by Magnequench, was subjected to densification by hot pressingand subsequent hydrostatic extrusion. The powder was initially pressed mechanically and subsequently placed in a copper capsule.The densification was conducted at room temperature and at temperatures above and below the melting point of the Nd-rich phase(TNd ), respectively. When densified at a temperature below TNd , the sample was strongly porous and the powder particles werenot well bonded, whereas at the temperature above TNd the interparticle bonds were good. In scanning electron microscopy imageswe can see solid regions which are fragments of the starting powder particles and, between them, porous regions which also containsmall fragments of the powder particles. It seems that, during the deformation, the surface layer of the polycrystalline powderparticles is segmented into smaller particles. After the extrusion new regions appear, containing the Nd-rich phase which has beenforced out from the intergranular spaces just as it is the case of die-upset forging. The extrusion, even at room temperature, affectsthe magnetic properties of the material, whereas when conducted at higher temperatures, it resulted in a slight decrease of thecoercivity. This can be due to the grains growth when the powder is heated prior to extrusion. No anisotropy of the magneticproperties was observed in the extruded materials.

Index Terms— Hydrostatic extrusion, Nd-Fe-B permanent magnets, powders densification.

I. INTRODUCTION

CURRENTLY a dominating processing method ofNd-Fe-B magnets is (developed in 1984, by Sagawa et al.

[1]) the powder metallurgy route. An interesting alternativefor powder metallurgy are methods which explore plasticdeformation at elevated temperature. Most of the research wasdevoted in this field to die upset forging; although it has asimilar powder metallurgy limitation [2], it does not providepossibility of fabrication of small diameter (below 2 mm) andelongated magnets. Among this group of processing routes hotextrusion seems especially interesting, as it enables processingof magnets having high length-to-diameter ratio and radialanisotropy.

The method of direct extrusion of powders was establishedin the 1980s [3], [4]. Kim et al. [4] extruded powders preparedby liquid atomization, producing material having density of7.5 g/cm3 and radial anisotropy (crystallographic direction[100] was parallel to extrusion direction). Magnetic propertiesincreased with growing deformation ratio.

Later, one could notice high interest in backward extrusionof Nd-Fe-B powders [5], [6], which ended up helping todevelop a highly effective method of fabrication of ring shapedradially anisotropic magnets. For this method, nanocrys-talline melt-spun or microcrystalline powders processed byHydrogenation, Disproportional, Desorption, Recombination(HDDR) method can be applied.

Parameters, which play a decisive role in formation of crys-tallographic anisotropy, in the course of extrusion, compriseRE metal content, temperature, deformation ratio, and extru-

Manuscript received September 9, 2013; revised November 21, 2013;accepted April 6, 2014. Date of publication April 14, 2014; date of currentversion October 8, 2014. Corresponding author: W. Kaszuwara (e-mail:wkaszu@inmat.pw.edu.pl).

Digital Object Identifier 10.1109/TMAG.2014.2317152

Fig. 1. Scheme of HE process.

sion rate. In majority of publications one can find informationthat texture formation is possible for RE content exceeding thestoichiometry of the Nd2Fe14B phase, that is, 26.7 wt% [7].However, one can also find information that the anisotropywas achieved for lower RE concentrations [8], [9].

Hot working, including extrusion, of Nd-Fe-B powders isusually carried out at a higher temperature than the meltingpoint of the Nd-rich phase, that is, 655 °C [10].

The effect of deformation rate is not completely clear.Most of the authors claim that this rate should be as low aspossible, because the process of texture formation is diffusionbased [11]. In most cases, the optimal deformation rate isregarded to be 1–5 × 10−2 mm/s [12], [13]. However, someresearchers reported also texture-related enhanced remanencefor 10−1 s−1 [13]. Similar rates are applied for backwardextrusion 10−2–10−4 s−1 [14]. There are also reports onappropriate crystallographic and magnetic anisotropy achievedfor substantially higher rates. In [15], the remanence morethan 1 T was obtained for die-upset forging with the rate of16 mm/s.

In this paper, application of hydrostatic extrusion (HE) fordensification of Nd-Fe-B powders was studied.

The hydrostatic extrusion was performed at the Instituteof High Pressure Physics, Polish Academy of Science (themethod was described in detail in [16]). During the HE(Fig. 1), the billet (3) is surrounded by fluid (2) compressed

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2102505 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 10, OCTOBER 2014

in container (4) by a plunger (1). The fluid pressure forces thebillet through the die (5) to create the extruded product (6).Because of the high hydrostatic pressure, nucleation, growth,and coalescence of material imperfections is suppressed; thus,brittle solids and powder materials can be processed withthe achievable extrusion ratio, substantially higher than in theconventional direct extrusion process.

We are not aware of papers which report on the applicationof this method for Nd-Fe-B powders. HE is characterizedby high deformation rate and high deformation ratio. Thus,the current results present the influence of these parameterson the properties of Nd-Fe-B material, hot deformed in theparameters range, which has not been so far reported.

II. EXPERIMENT

For this research, Nd-Fe-B melt-spun isotropic powderproduced by Magnequench was used. Its composition (wt%)was: 31.6% Nd, 60.17% Fe, and 5.95% Co. Additions of 1.1%Eu, 0.51% Ga, and 0.18% Al were also detected. The chemicalcomposition was studied using a method of X-ray FluorescentSpectroscopy (XRF Bruker S4 Explorer). The polycrystallinepowder particles had the shape of about 25 μm thick flakes.They were composed of grains around 40 nm in size. Thepowder was initially cold pressed to green density 65% oftheoretical value. The compacts were subsequently placed incopper capsules. The diameter of the capsule and compactedpowder core were 50 and 22 mm, respectively. The capsuleswere sealed under vacuum, and the blocking copper plugwas soldered with electron beam. The Nd-Fe-B powder didnot have direct contact either with air or with pressurizedliquid within the entire technological process. During theHE processes carried out at elevated temperature, the billetis heated up in an external oven and transferred to the die (thedevice works in vertical mode). After closing, the chamberis filled with oil at room temperature. Within the time oftransferring from the oven to the beginning of extrusion thebillet cools down, depending on its size, by 100 °C–150 °C.The extrusion lasts about 15 s. In the case of double-stageprocess, the initially extruded rod was repeatedly located ina copper capsule. Detailed control of the billet temperatureduring the process is impossible. The values mentioned inthis paper are temperatures, to which the billet was heatedup before the extrusion—they are not real temperatures of thematerial during extrusion.

Three single-stage processes were performed: one at roomtemperature and two after heating the billet to 700 °C and800 °C, respectively (specimens denoted in Table I as 20,700, and 800, respectively). Additionally two double-stageprocesses were done: first extrusion at room temperature and800 °C, followed by a second run at room temperature in bothcases (specimens denoted in Table I as 800–20 and 20–20).The first stage extrusion carried out at room temperature wasperformed at 82 mm/s (deformation rate 1.27 s−1). For theother processes, the extrusion rate was 52 mm/s (deformationrate 1.02 s−1).

The magnetic properties of the samples were examinedusing a Lake Shore vibrating sample magnetometer (VSM).

TABLE I

PROCESSES OF HYDROSTATIC EXTRUSION AND THEIR SELECTED

PARAMETERS. (do AND d f STAND FOR INITIAL AND

FINAL BILLET DIAMETERS, RESPECTIVELY)

TABLE II

COERCIVITY HC , REMANENCE BR , CRYSTALLITE SIZE D, LATTICE

DEFORMATION ε AND DENSITY ρ FOR SPECIMENS

OBTAINED BY HYDROSTATIC EXTRUSION

Samples were characterized by X-ray diffraction (XRD) withCuKα radiation. The size of the Nd2Fe14B crystallites wasdetermined using the Williamson-Hall method, and on thebasis of TEM images (number in parenthesis in Table II).The thermal analysis was performed in a differential scanningcalorimeter (DSC) at a heating rate of 40 K/min. The mag-netic properties of the sinters were examined using a VSMmagnetometer, and their structure was analyzed by scanningelectron microscopy (SEM) (Zeiss). Density of the materialwas measured using helium pycnometer.

III. RESULTS AND DISCUSSION

After extrusion, the materials had a form of copper rod withan Nd-Fe-B core. To extract the magnetic core, the copper shellwas mechanically removed. The powder, which was subjectedto single-stage extrusion with a density of 5.48 g/cm3 only,was reached at room temperature.

The material was not compact enough to prepare from itmetallographic specimen. The materials extruded at 700 °Cand 800 °C were substantially better densified and maintainedtheir consistency after removal of the copper shell. Theirmicrostructures are shown in Figs. 2 and 3. One can notice thatthe specimen 700 in fact was processed at a lower temperaturethan the melting point of the, existing in the alloy, Nd-richphase. At low magnification one can see flakes of the startingpowder. Some of the flakes were fractured to several pieces.The specimen showed large, dense areas, separated from each

KASZUWARA et al.: DENSIFICATION OF Nd-Fe-B POWDERS BY HYDROSTATIC EXTRUSION 2102505

Fig. 2. SEM, BSE contrast microstructure of the specimen extruded in asingle stage at 700 °C.

other by cracks and pores. After extrusion at 800 °C the flakesexhibited visible refinement. One can see their deformed,bulk parts, with very fine particles between them. At highmagnification one can notice small, bright areas, constitutingprecipitates of the Nd-rich phase (Fig. 3). In the course ofextrusion, this phase is liquid and is partially extracted outfrom the grain boundaries to feel small pores.

The rods fabricated in two-stage processes were welldensified (Fig. 4). They possessed very few large pores(10–100 μm), small pores (below 1 μm) and, not observedbefore, spherical pores having diameter 1–2 μm. Large andspherical pores form bands perpendicular to the extrusiondirection. In both specimens, one can see almost bulk areas,being initial flakes and areas, which are filled with smallparticles, which are fragments of initial flakes. The areasof Nd-rich phase, which had been removed from the grainboundaries, were visible only in the specimens extruded inone stage heated to 800 °C.

Phase structure of the materials subjected to extrusion wasinvestigated by XRD. Examples of the diffraction patterns areshown in Fig. 5. On the patterns of all the samples studied onecan notice reflections representing only the Nd2Fe14B phase.Relations of the intensity of particular reflections were alsothe same for all the specimens and consistent with that forthe initial powder. There is no evidence for existing texturein the extruded alloys. To further prove it, a cube sample

Fig. 3. SEM, BSE contrast microstructure of the specimen extruded in asingle stage at 800 °C.

2 × 2 × 2 mm, was cut off from the material extruded at800 °C. The remanence on the cube was measured paralleland perpendicular to the extrusion direction. Thanks to therectangular shape of the specimen there was no necessity ofconsidering the shape coefficient. The remanence values, mea-sured in both directions varied less than 1%. This definitelyconfirms lack of texture in the extruded magnetic material.

The only visible differences on the diffraction patterns resultfrom different width of the peaks. This fact gives evidenceof various crystallite sizes and deformation of the crystallinelattice. Hydrostatic extrusion enables us to produce very highdeformations, which in some cases can even lead to partialamorphization of the material. Thus, the material, which wasdouble extruded at room temperature and which experiencedthe greatest deformation, was subjected to microcalorimetricinvestigation. The DSC traces did not show any effects ofphase transformation in a range of crystallization temperatureof Nd2Fe14B phase. One can conclude from this fact thathydrostatic extrusion, in the applied conditions, does not leadto amorphization of the material.

Measurements of the magnetic properties revealed that theextrusion results in a decrease of the coercivity and increaseof the remanence (Table II). The remanence of the extruded,at room temperature, material grows with increasing den-sity. For the processes carried out at elevated temperaturesthe remanence decreases. The higher the temperature the

2102505 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 10, OCTOBER 2014

Fig. 4. SEM, BSE contrast microstructure of the specimen extruded in adouble stage. (a) Specimen 20–20. (b) Specimen 800–20.

Fig. 5. (a) XRD diffraction patterns of initial powder. (b) and (c) Specimenextruded in double stage process at room temperature and at 800 °C.

greater the remanence drop (despite the increase in den-sity). An important factor, leading to drop of the coercivity,is the grain growth occurring during heating of the bil-let before extrusion. The crystallite size of the Nd2Fe14Bphase and the values of lattice deformation are collected inTable II. In the case of the materials double-stage extruded

(800–20), the sample diameter was too small to obtain areliable XRD pattern; thus the crystallite size was assessedon a basis of TEM image (values in parenthesis).

The crystallite size in initial powder was calculated tobe 39 nm. The same result was achieved for the specimensingle-stage extruded at room temperature. After double-stageextrusion, at the same conditions, the crystallite size was44 nm. This difference is too small to justify the judgmentthat the crystallite size really changed. For the specimens,which before extrusion were heated to temperatures 700 °Cand 800 °C, a clear crystallite growth is evident. This growthis proportional to the temperature value. This is the factor thatdeteriorates the coercivity after extrusion at high temperatures.

The effect of hydrostatic extrusion on lattice deformationis also clear. A single-stage extrusion, at room temperature,results in three times higher deformation than the value for theinitial powder. After the second extrusion, at the same condi-tions, this value about 10 times exceeds the one for the startingmaterial. Hydrostatic extrusion at high temperature also resultsin increased lattice deformation. Exceptionally high deforma-tion value was obtained after extrusion at 800 °C; however, inthis case some contribution can come from thermal stress. Onecannot, however, definitely state that the lattice deformationscan affect the magnetic properties of the material.

On the basis of the obtained results we can state that, in thecase of extrusion at high temperatures coercivity drop resultsfrom excessive grain growth, whereas when the process iscarried out above the melting point of the Nd-rich phase acontribution from extracting of this phase to pores and surfaceis also important. This, however, does not explain the reasonfor coercivity decrease after the processes carried out at roomtemperature. SEM studies did not show large precipitates ofthe Nd-rich phase, which are present in the material extrudedat 800 °C. One can suspect that that extrusion results inchanges in the area of grain boundaries of the Nd2Fe14B phase,the quality of which plays a crucial role in controlling thecoercivity of magnets. The Nd-rich phase may lose its integrityand cause increased proportion of magnetically nonisolatedgrains between the grains of the Nd2Fe14B. High strain actingin the materials extruded at low temperature, however, does notallow for preparing thin foils for TEM studies, which wouldverify this thesis.

IV. CONCLUSION

Hydrostatic extrusion enables densification of Nd-Fe-Bpowder to the density close to the theoretical value, evenat room temperature. Because of the high process rate itis impossible to completely reduce the porosity, even ata temperature as high as 800 °C. Therefore, the diffusionprocesses are substantially suppressed and do not occur in asufficient degree, which would provide strong crystallographicand magnetic anisotropy.

Some features of morphological texture, which manifestthemselves in a distribution of porosity perpendicular to theextrusion direction, were observed. On extrusion at 800 °C, theareas of the Nd-rich phase, having 1 μm size, squeezed out ofthe grain boundaries, are observed. The drop of coercivity,in the specimens extruded at room temperature, show that

KASZUWARA et al.: DENSIFICATION OF Nd-Fe-B POWDERS BY HYDROSTATIC EXTRUSION 2102505

at these experimental conditions, the isolating, paramagneticlayer between the Nd2Fe14B grains, is damaged.

Hydrostatic extrusion enables simple fabrication of densemagnets; however, it does not provide formation of crystallo-graphic and anisotropy and enhanced remanence.

ACKNOWLEDGMENT

This work was supported by the European Union inOperational Programme Innovative Economy POIG underGrant 01.01.102.00/10. The powders were provided byMagnequench.

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