The Effect of Surfactant Admixture during Milling on Pressing

ISSN 20751133, Inorganic Materials: Applied Research, 2014, Vol. 5, No. 5, pp. 530–535. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.I. Alymov, A.B. Ankudinov, V.A. Zelenskii, I.M. Milyaev, V.S. Yusupov, T.A. Vompe, 2014, published in Perspektivnye Materialy, 2014, No. 4, pp. 51–57.

NEW MATERIALS PROCESSING TECHNOLOGIES

The Effect of Surfactant Admixture during Milling on Pressing, Sintering, and Magnetic Properties of FeCrCoMoW Alloy M. I. Alymov, A. B. Ankudinov, V. A. Zelenskii, I. M. Milyaev, V. S. Yusupov, and T. A. Vompe Baikov Institute for Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russia email: [email protected], a[email protected], [email protected], [email protected], [email protected], [email protected] Received December 2, 2013

Abstract—The effect of surfactant admixture (stearic and oleic acids) with a variety of milling (dry milling, milling in water and ethanol) on pressing and sintering and magnetic properties of the Fe–30Cr–20Co– 2Mo–2W (wt %) hard magnetic powder alloy was studied. The addition of stearic and oleic acids in the amount of up to 2 wt % during dry or wet milling increases the density of pressed samples. The use of surfac tants during milling in water and ethanol increases the density of sintered samples. Xray diffraction shows that only accumulation of defects takes place in the crystal structure of the particles, and no additional phases form during dry or wet milling of powder mixtures containing a surfactant. The best magnetic hysteresis prop erties were obtained with samples milled in ethanol with 1% stearic acid. Keywords: powder alloys, permanent magnets, surfactants, milling, pressing, sintering, magnetic properties, coercive force, residual induction, maximum energy product DOI: 10.1134/S2075113314050037

INTRODUCTION Wrought hard magnetic materials (HMMs) of the Fe–Cr–Co system are used mainly in production of small permanent magnets (PMs). Forming of magnets from theses alloys usually involves classical pressure metal treatment (rolling, press forming, blanking, etc.) of section iron delivered by metallurgical plants or precise casting using investment patterns. In recent years, the production of PMs from FeCrCo HMM by powder metallurgy techniques has not received enough attention [1]. This seems to be due to the fact that the powder metallurgy technology in production of precipitation hardening HMMs is generally employed to enhance their mechanical properties (strength and plasticity), while this is not a problem with the FeCrCo HMM. The authors of [2–4] dem onstrated that the PMs obtained by the powder metal lurgy technology show almost the same hysteresis parameters as those of the magnets produced from section iron. A lively interest around in the utilization of powder metallurgy methods in production of PMs from HMMs in the Fe–Cr–Co system has been gen erated recently owing to the possibility of using the majority of processing equipment at plants which pro duce PMs of hardmagnetic ferrites and rareearth alloys based on SmCo5, Sm2Co17, and Nd2Fe14B, decreasing the amount of mechanical treatment of PMs, and lowering overall energy consumption in small batch production of PMs [5–8].

Preparation of powders, in particular, mixing and milling prior to pressing, is of great importance in pro duction of powder articles. It is well known [9] that the use of surfactants is one of the primary ways to inten sify a milling process. Fine grinding of materials is the most energyconsuming process. Surfactants sorb at new surfaces generated during grinding of solids and decrease the surface free energy. As they do so, the work produced in forming an additional amount of surface decreases. Surfactants adsorbing at the surface of ground materials penetrate into microcracks, form tight interlayers which decrease the attractive force between solid particles, interfere with closing of microcracks, and produce wedging action, which reduces the energy spent to crush a solid. Films cre ated by surfactants at the particle surface diminish the area and the number of contacts between particles and prevent their sticking and aggregation. Therefore, with allowance for high energy consumption during the milling process, the use of surfactants is often an effec tive route to decrease the final production cost. The aim of the present work is the experimental study of the effect of surfactant admixture at the mill ing stage of powder preparation on consolidation of powders during pressing and sintering, as well as on the magnetic hysteresis properties of the Fe–30Cr– 20Co–2Mo–2W (wt %) hardmagnetic powder alloy.

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EXPERIMENTAL During preparation of powder mixtures for the experimental investigations, the following powders were used: VS carbonyl iron, PKhS1 chromium, PK1 cobalt, and tungsten, molybdenum, and silicon powders of direct reduction with a particle size smaller than 40 μm. Homogeneous powder mixtures for the milling experiments with surfactants were obtained in a C2.0 turbulent mixer. The powders were mixed in a 200 mL glass vessel. Balls 3 mm in diameter made of ShKh15 steel in the amount of 200 g were put in the vessels together with the powders. The mixing was carried out for 80 min, and no intervening pauses were made. The milling experiments with the prepared powder mixtures were carried out in a Pulverizette7 mill. The milling was performed at a rate of 600 rpm for 15 min. The powders were treated with the addition of surfac tants, namely, stearic and oleic acids. In addition to dry milling, wet drying in distilled water and ethanol was performed. The hardened steel milling containers 80 mL in volume were filled with 25 g of the treated powder mix ture. Stearic or oleic acid was next added in the amount of 0.3, 1, and 2% of the mass of the treated powder, and 150 g of balls 3 mm in diameter made of ShKh15 steel were poured into it. In the case of wet treatment, 20 mL of the requisite liquid was poured into the drums. To minimize powder oxidation, the milling containers were evacuated in a box, which was then filled with highpurity argon. This process was repeated two times. After it, the milling containers were covered, and the mixture was treated in a plane tarytype mill. From all treated powders, experimental specimens were pressed and sintered. Several refer ence specimens were made from powders milled with no surfactant addition. The powders were pressed on a KNUTH130042 handpower press in a composite die 13.6 mm in diameter under a pressure of 600 MPa. The pressing was uniaxial and unilateral. Before pressing, the die was treated with zinc stearate to decrease lateral fric tion. It was taken 10 g powder of mixture for filling. The diameter of all the resulting pressed samples was about 13.6 mm, and the their height was 10.6 to 13.0 mm (this parameter depends on the properties of the pressed powders, in particular, the milling medium and the type of surfactant, and determines the density of the pressed sample). The absolute density was calcu lated by dividing the pressed sample mass by its vol ume, specified by the geometry characteristics, which were measured with a micrometer with a scale division of 0.01 mm. The precision of density measurement was ~0.3%. The relative density was calculated by divid ing the absolute density value by the theoretical one. The samples were sintered in a vacuum shaft fur nace SShV1.25/25 I1 under a vacuum no worse than 10–2 Pa. The following sintering mode was used: INORGANIC MATERIALS: APPLIED RESEARCH

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—Heating to T = 450°C for 30 to 40 min; —Heating from 450°C to the sintering tempera ture for 1.5 h; —Holding at 1300°C for 2.5 h; —Cooling to T = 450°C for 1.5 h; —Slow cooling of the samples in a furnace to room temperature Tr. The density of the sintered samples was calculated by dividing their mass by the volume determined hydrostatically. The random error of the absolute and relative densities obtained by such a technique was ~0.2%. The magnetic hysteresis properties were measured by Permagraph L. The sintered powder samples were treated according to the following mode: quenching from 1250°C into water, heating to 720 to 730°C, holding for 3 to 5 min, cooling to 620°C in a magnetic field at a rate of 180 deg/h, and cooling from 620 to 500°C under no magnetic field at a rate of 8 deg/h. All thermal treatments, including the thermomagnetic one, were performed on a laboratory unit with an elec tromagnet generating a magnetic field with a strength of H = 320 kA/m. The Xray diffraction (XRD) analysis was carried out on an ARLX’TRA diffractometer equipped with a Peltier detector in CuKα radiation in an angle range of 35–125 deg. The XRD patterns were interpreted by their comparison with the PDF2 database cards of the International Center for Diffraction Data (ICDD). EXPERIMENTAL RESULTS AND DISCUSSION Figure 1 shows the dependence of the density of pressed and sintered samples on the content of stearic acid introduced into the powder mixture prepared for pressing during milling. We see that, in all cases, the addition of the surfactant increases the density of pressed samples. However, there is a fundamental dif ference between the density change behavior for sin tered samples subjected to dry and wet milling. The density of sintered samples increases if, upon milling in a wet medium, a greater amount of stearic acid is added. Another picture is observed with dry milling: as the stearic acid content is raised, the density of sin tered samples decreases, although remaining rather high on the whole. In the case of wet milling, a direct relation between the densities of sintered samples and initial pressed powder compacts is observed. Pressed samples with a higher density transform into samples with a lower porosity after sintering. In the case of dry milling, we have the opposite: pressed samples with a lower den sity transform into denser sintered samples upon sin tering. All sintered samples, except for those prepared from powders subjected to milling in water, have a No. 5

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98

72 97

78.6%

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72 80 68

99.4%

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77.9% 78 77

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76 98.0

75 74 0

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97 0.5 1.0 1.5 Oleic acid content, %

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97.8%

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(c) Pressed sample density, %

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99.7% 99 98

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97 76 96 72


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0.5 1.0 1.5 Oleic acid content, %

2.0

Fig. 1. Dependence of the density ρ of pressed and sintered samples on the stearic acid content: (a) dry milling, (b) milling in water, and (c) milling in ethanol.

Fig. 2. Dependence of the density ρ of pressed and sintered samples on the oleic acid content: (a) dry milling, (b) mill ing in water, and (c) milling in ethanol.

density higher than 97.4%. The sintered samples obtained from powders subjected to milling in water with ≥1% stearic acid retain a high density of more than 98%. However, when the stearic acid is added in lower amounts or it is not added at all, the density of sintered samples decreases to ~75%. The reason for such a significant increase in porosity is not clear and calls for additional research.

Figure 2 demonstrates the dependence of the den sity of pressed and sintered samples on the content of oleic acid introduced into the powder mixture pre pared for pressing during milling. We see that, as in the case of the stearic acid, the addition of the oleic acid increases the density of pressed samples. However, the density change curves for sintered samples subjected to dry and wet milling differ. The density of sintered

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THE EFFECT OF SURFACTANT ADMIXTURE DURING MILLING ON PRESSING I, cps

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(a)

1200

800 400 0 40

60

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2θ, deg

40

60

80 (c)

100

2θ, deg

40

60

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100

2θ, deg

I, cps 600

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0 I, cps

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400 200 0 Fig. 3. XRD patterns of (a) initial powder mixture, (b) powder mixture after dry milling, and (c) powder mixture after milling in ethanol.

samples increases if, upon milling in a wet medium, a greater amount of oleic acid is added. For milling in a dry medium, as the stearic acid content is raised, the sintered sample density changes only a little, although being higher than that for sintered samples milled without the oleic acid. In the case of wet milling, pressed samples with a higher density transform into samples with a lower porosity upon sintering. In the case of dry milling with the addition of 0.3 to 2% oleic acid, the sintered sam ple density depends on the pressed sample density INORGANIC MATERIALS: APPLIED RESEARCH

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weakly. All sintered samples, except for those prepared from powders subjected to milling in water, have a density higher than 96.0%. For the samples obtained from powders subjected to milling in water, the sin tered sample density strongly depends on the oleic acid content added during milling. For example, its value is ~98% when the oleic content is ~2%. When no oleic acid is added during milling, the sintered sample density decreases to ~77%. The reason for such a sig nificant increase in porosity is not clear and calls for additional research, just as in the case of stearic acid. No. 5

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Magnetic hysteresis parameters of the samples Surfactant addition during milling, %

B r, T

(BH)max, kJ/m3

HcB, kA/m

Dry milling with stearic acid 2 0.65 62.42 1 0.40 50.60 0.3 0.87 66.72 No surfactant 0.96 67.72 Dry milling with oleic acid 2 0.61 60.37 1 0.38 49.63 0.3 0.82 65.02 No surfactant 0.93 65.03 Milling in water with stearic acid 2 0.57 64.79 1 0.65 65.75 0.3 0.62 62.12 No surfactant 0.60 60.24 Milling in water with oleic acid 2 0.53 61.49 1 0.61 63.25 0.3 0.60 61.01 No surfactant 0.61 60.12 Milling in ethanol with stearic acid 2 0.83 69.39 1 1.0 60.14 0.3 0.90 68.96 No surfactant 0.91 65.24 Milling in ethanol with oleic acid 2 0.73 66.33 1 0.91 58.34 0.3 0.90 58.93 No surfactant 0.81 60.25

13.8 6.0 24.4 27.7 13.1 5.6 23.4 26.5 13.0 15.7 13.2 12.2 12.2 14.7 12.1 12.1 24.4 32.8 28.0 26.5 23.4 29.8 28.9 23.5

Br is the residual induction, HcB is the coercive force, and (BH)max is the maximum energy product.

Evidently, the nature of these phenomena is the same for both acids. The initial powder mixture and powders treated under various conditions were studied by XRD analy sis. Figure 3 shows the XRD patterns of the initial powder mixture and two powders milled with the addi tion of 1% stearic acid in a dry medium and in ethanol. The interpretation of these XRD patterns demon strates that no phases appear in the structure of milled powders. It is seen that, after milling, the intensity of the XRD peaks decreases and a line broadening effect is observed for the milled powder mixtures compared

to the initial powder mixture. The observed changes are due to reduction in the dimensions of the coherent scattering regions and accumulation of defects in the crystal structure of particles. In the table, the magnetic hysteresis properties (residual induction Br, coercive force HcB, and the maximum energy product (BH)max) for the samples obtained by treatment with surfactants are listed. From the table, it follows that the magnetic properties strongly depend on the milling medium and type and concentration of the surfactant used. In the case of water as a milling medium, the lowest magnetic hys teresis parameters were obtained. The best results were achieved for the milling in ethanol with 1% stearic acid. The magnetic hysteresis properties of this sample are as follows: Br = 1.0 T, HcB = 60.14 kA/m, and (BH)max = 32.8 kJ/m3 at a density of ρ = 98.9%. The milling with 1% oleic acid yields high magnetic hyster esis parameters as well. However, they are worse than those of the sample prepared from the powder with the addition of stearic acid. CONCLUSIONS 1. The Fe–30Cr–20Co–2Mo–2W (wt %) hard magnetic alloy was obtained by the powder metallurgy methods from elemental powders treated together with surfactants (oleic and stearic acids) in various media (dry milling, milling in water and ethanol). The best magnetic hysteresis properties were obtained upon milling in ethanol with 1% stearic acid and were the following: Br = 1.0 T, HcB = 60.14 kA/m, and (BH)max = 32.8 kJ/m3 at a density of ρ = 98.9%. 2. The use of stearic and oleic acids under dry and wet milling in the amount of 2 wt % increases the pressed sample density. The application of surfactants during milling in water and ethanol increases the sin tered sample density. 3. It was established that dry and wet milling of powders with the addition of surfactants results in accumulation of defects in the crystal structure of par ticles, while no additional phases form. ACKNOWLEDGMENTS This study was conducted under Government con tract no. 14.513.11.0028. REFERENCES 1. Kaneko, H., Homma, M., and Nakamura, K., New ductile permanent magnet of Fe–Cr–Co system, Proc. AIP Conf. “Magnetism and Magnetic Materials,” 1971, no. 5, pp. 1088–1092. 2. Green, M.L., Sherwood, R.C., and Wong, C.C., Pow der metallurgy processing of CrCoFe permanent mag net alloys containing 5–25 wt % Co, J. Appl. Phys., 1982, vol. 53, pp. 2398–2400.


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THE EFFECT OF SURFACTANT ADMIXTURE DURING MILLING ON PRESSING 3. Alymov, M.I., Ankudinov, A.B., Zelenskii, V.A., Milyaev, I.M., Yusupov, V.S., and Ustyukhin, A.S., Effect of alloying and sintering regime on magnetic hysteresis properties of Fe–Cr–Co powdered alloy, Fiz. Khim. Obrab. Mater., 2011, no. 3, pp. 34–37. 4. Milyaev, I.M., Alymov, M.I., Yusupov, V.S., Zelenskii, V.A., Ankudinov, A.B., and Milyaev, A.I., The effect of silicon and molybdenum on magnetic hysteresis qualities of powder 22Kh15KA hardmag netic alloy, Poroshk. Metall. Funkts. Pokr., 2011, no. 4, pp. 54–57.


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5. Ryaposov, I.V. and Shatsov, A.A., Peculiarities of alloy ing, structure and properties of powdered hardmag netic alloy with increased exploitation characteristics, Perspekt. Mater., 2009, no. 1, pp. 57–61. 6. Shatsov, A.A., Powder materials of the Fe–Cr–Co sys tem, Metal Sci. Heat Treat., 2004, vol. 46, pp. 152–155. 7. Shatsov, A.A., RF Patent 2334589, 2006. 8. Shatsov, A.A., RF Patent 2038918, 1991. 9. Saryanarayana, C., Mechanical alloying and milling, Progress Mater. Sci., 2001, vol. 46, pp. 1–184.

Translated by Z. Smirnova

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