EPMA-Proceedings-2009-Sintering.pdf

5/23/2018 EPMA-Proceedings-2009-Sintering.pdf

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Euro PM2009Field Assisted Sintering

Manuscript refereed by Professor Bernd Kieback, Fraunhofer IFAM/TU Dresden,Germany

Sintering of WC-12Co Powders with Different Particle Sizes byPulsed Electric Current Sintering and Effect of Powder

Characteristics on Microstructure and Mechanical Properties

M. E. Curaa, J. Lagerbomb, J. Lottaa, X.W. Liua, J. Syrena, T. Ritvonenb, U. Kanervaa, Y. Gea,

R. Ritasaloa, O. Sderberga, E. Turunenb, S-P. Hannulaa

aHelsinki University of Technology, Department of Materials Science and Engineering,POB6200, FI-02015, Espoo, Finland; bVTT, Technical Research Center of Finland,POB1300, FI-33101, Tampere Finland

Abstract. WC-12Co powders were prepared from three different WC carbide and metallic Co powdergrades (nano, fine and coarse) by vibrator milling for 5, 15, or 30 min in argon. Powders werecharacterised by laser diffractometer, SEM/EDS, and XRD. Nano-Co was cubic in the milled mixtures,

while the other Co grades transformed to hexagonal. All the powders were pulsed electric currentsintered by using the processing parameters 1453 K, 3 min, 50 MPa, and 100 K/min. Densificationwas studied by Archimedes method. Microstructure of the compacted samples was analysed bySEM/EDS, and XRD. The WC grain size and porosity were measured from SEM pictures by applyingthe image analysis. The mechanical properties of the compacts were studied with HV1 microhardnesstest and by nanoindentation for one sample. Nearly full density was obtained in compacts with fine-WC/nano-Co-powder, while the highest hardness 23 GPa was obtained with powders containingnano-WC.

1. IntroductionHard metals and their composites are commonly used in harsh environments such as drilling,

metal cutting, wire drawing, mining, metal extrusion due to their high hardness and wear resistance,good chemical stability and high melting point. In cemented carbide hard metals carbide particles areembedded in a continuous metal binder skeleton [1-3]. These materials are produced by powder

metallurgy methods including liquid phase sintering, hot pressing and hot isostatic pressing. Relativelynew sintering methods such as pulsed electric current sintering, high-frequency induction heatingsintering, electric discharge sintering and microwave sintering are also used for consolidation [3-8].The mechanical properties of WC-Co cermets can be controlled by the characteristics of startingpowders and cobalt content [9]. Size of WC particles has a great influence on hardness, strength andwear-resistance of the final material [2,4,6]. Lately considerable efforts in research have beendemonstrated on producing, processing and consolidating of ultrafine and nanocrystalline WC-Copowders [3,4,6,10]. During sintering grain growth occurs and it is usually not possible to observenanocrystalline structure afterwards. This is more severe in conventional techniques, and more rapidfor finer powders [11]. Coarsening of grains is believed to be result of grain boundary migration andOstwald ripening process [11,12]. Grain growth can be controlled by addition of different carbidephases (VC, Cr3C2, NbC, Mo2C, etc.) as grain growth inhibitors and by controlling sintering parameters[13-15]. In the recent years sintering techniques exhibiting rapid heating and shorter sintering periodshave been of interest i.e. pulsed electric current sintering (PECS) also known as spark plasmasintering (SPS) or field assisted sintering technique (FAST). It is a relatively new method for sinteringof wide variety of materials and has attracted a lot of attention in the last decade. It uses Jouleheating, provided by electric current flowing through two electrodes and powders in graphite mouldsbetween them. It is fast and requires lower sintering temperatures and durations than conventionalmethods. Uniaxial pressure is applied for compressing the powder and to provide a continuous contactbetween the two electrodes [2-10]. In the present study, WC 12 % wt Co powders with differentparticle sizes were consolidated by PECS. Effect of powder characteristics on densification andhardness has been studied. Microstructure examination and grain size analysis were conducted forthe sintered pieces.

2. Experimental

The nano WC powder was milled from the fine powder having nanofractioned structure with a

particle size of 200 nm (Zhuzhou Cemented Carbide Works Import & Export Company). The otherstart powders used were the fine size WC DS80 powder (0.8-0.9 m, H.C. Starck), coarse technicalgrade (2-5 m, H.C. Starck), nano cobalt with average particle size of ~28 nm (variation 0-60 nm;specific surface area 40-60 m2/g, NaBond Technologies Co., Ltd), fine Co was S-80 (4 m) and


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coarse Co S-320 (7.9 m, OMG Kokkola Chemicals Ltd) were used.The WC-12Co powder mixtureswere prepared by using Spex 8000 high energy vibratory ball mill. Three different mixing times, 5, 15and 30 min were applied, and mixing was carried out in argon. The powder size distribution of thepowders was studied by laser diffractometer (Lecotrac LT100). Powder morphology was examinedwith the scanning electron microscope. Consolidation of the powders was carried out with pulsedelectric current sintering equipment (FCT HP D 25-2) sintering was carried out at 1453 K for 3 min,

under the vacuum of 7 Pa. Powders were pressed with a pressure of 50 MPa, heating rate was 100K/min and pulse/pause ratio was 10:5. The samples were compacted in 20 mm diameter graphitemoulds and 18 g of powder was used in every experiment. After sintering all the samples were groundand polished with the diamond discs (120-1200 grits) and diamond paste (6, 3 and 1 m) respectively,and a final polishing was made by colloidal silica (0.16 m, pH 9.8). The densities of the sinteredcompacts were measured by applying the Archimedes method (Sartorius CPA224S, 0.1 mg). Whileevaluating the theoretical density, the measured density was compared to the value of 14.46 g/cm 3[5].Grain size measurements were made by image analysis from SEM images of sintered compacts [16-18].The Vickers microhardness measurements with 1 kg load were measured with the Zwick & CoZ323 apparatus. Nanoindentation was also performed to the compact sintered from 30 min millednano WC and nano Co grade powder. Nine indents were made with Berkovich tip and the applied loadwas 2000 N. The XRDs of the materials were measured with Philips PW 3830 powder diffractometer(CuK radiation).

3. Results and Discussion

3.1 Powder characteristicsThe powder grades for WC and cobalt used in the experiments were nano, fine and coarse. The

particle size distributions after grinding are given in Table 1. The lowest size measured was 0.183 0.894 2.765 for nano grade and highest was 0.443 3.847 15.91 for the coarse grade as in d10d50 d90. Three different milling times (5 min, 15 min and 30 min.) apparently did not have anysignificant effect on particle size, the values being close to each other. However, longer milling timehelped breaking the agglomerates which improved the densification. Especially in nano gradepowders agglomeration was in large scale before milling.

Table 1. Powder grades, their particle size distributions and properties of sintered compacts.

Sample

Powder Compact

Raw materialpowders (type)

Millingtime

(min)

Particle size distributiond10-d50-d90(1. peak)

(m)

DensityHardness

(HV1)Measured

(g/cm3) T.D.1nn

200 nm WC 28 nm Co

5 0.773 2.268 14.61 (0.315) 13.9215 0.0429 96.3 0.3 2304.0 57.0

2nn 15 0.183 0.894 2.765 (0.223) 14.0719 0.0307 97.3 0.2 2225.1 54.0

3nn 30 0.256 1.040 1.619 (0.289) 14.1400 0.0395 97.8 0.3 2294.8 79.3

1nf200 nm WC

4 m Co

5 0.303 1.562 12.73 (0.243) 14.0777 0.0827 97.4 0.6 2105.8 65.9

2nf 15 0.298 1.215 7.003 (0.289) 14.0477 0.0232 97.1 0.2 2123.1 45.9

3nf 30 0.180 0.747 3.829 (0.223) 14.0445 0.0408 97.1 0.3 2196.4 48.2

1nc200 nm WC

7.9 m Co

5 0.193 0.961 12.99 (0.223) 14.0133 0.0553 96.9 0.4 2295.3 88.4

2nc 15 0.185 0.857 8.489 (0.223) 14.0402 0.0380 97.1 0.3 2274.3 66.6

3nc 30 0.159 0.696 6.335 (0.204) 14.0303 0.0349 97.0 0.2 2285.0 83.8

1fn 0.8-0.9 mWC

28 nm Co

5 0.228 1.195 12.73 (0.315) 14.3777 0.0418 99.4 0.3 1996.6 89.0

2fn 15 0.256 1.040 1.619 (0.289) 14.3902 0.0684 99.5 0.5 1951.6 129.6

3fn 30 0.216 0.999 2.017 (0.265) 14.3955 0.0267 99.6 0.2 2045.9 77.9

1ff 0.8-0.9 mWC

4 m Co

5 0.215 0.935 2.630 (0.289) 14.2091 0.0366 98.3 0.3 1877.0 38.3

2ff 15 0.260 1.027 2.548 (0.315) 14.2122 0.0360 98.3 0.2 1832.8 30.0

3ff 30 0.250 1.001 2.328 (0.315) 14.2336 0.0235 98.4 0.2 1862.5 49.6

1fc 0.8-0.9 mWC

7.9 m Co

5 0.243 1.037 4.952 (0.289) 14.1662 0.0353 98.0 0.2 1862.5 49.6

2fc 15 0.119 0.409 1.838 (0.172) 14.1433 0.0492 97.8 0.2 1878.5 74.1

3fc 30 0.210 0.973 5.279 (0.243) 14.0759 0.0604 97.3 0.4 1847.6 44.0

1cn2-5 m WC

28 nm Co

5 0.756 1.690 10.42 (0.289) 13.9157 0.0629 96.2 0.4 1384.1 139.8

2cn 15 0.498 1.553 11.13 (0.289) 13.8975 0.0488 96.1 0.3 1320.5 108.4

3cn 30 0.606 1.763 11.63 (0.289) 14.0665 0.0344 97.3 0.2 1397.4 31.8

1cf 2-5 m WC

4 m Co

5 0.374

2.623

11.48 (0.243) 13.8649 0.0342 95.9 0.2 1395.8 87.72cf 15 0.306 2.382 - 10.64 (0.243) 13.9379 0.0731 96.4 0.5 1366.1 65.9

3cf 30 0.384 2.113 - 9.071 (0.315) 13.9447 0.0477 96.4 0.3 1432.3 52.1

1cc2-5 m WC

7.9 m Co

5 0.443 3.847 15.91 (0.315) 13.7535 0.0450 95.1 0.3 1369.9 51.9

2cc 15 0.322 3.102 14.73 (0.243) 13.8941 0.0553 96.1 0.4 1417.7 54.6

3cc 30 0.278 2.321 11.77 (0.243) 13.8328 0.0325 95.7 0.2 1442.6 59.5

According to Figure 1, the powders deagglomerate as a function of milling time. Thus, 30 min millingtime was accepted as optimum duration in this study and microstructural evaluation was carried outwith compacts of 30 min milled powders. The morphologies of selected powders are given in Figure 2.

Figure 1.Particle size distributions after 5, 15 and 30 minutes of milling of nano WC and Co powder.


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Figure 2. SEM images of powders with different WC and Co grades, milled for 30 min:a) 3nn b) 3nf, c) 3nc, d) 3fn, e) 3ff, f) 3fc, g) 3cc .

The XRD study did not show significant change of the structure after different milling times, andtherefore, only the patterns for the fine-WC with the different cobalt grades and for the fine-Co mixedwith different WC grades are shown in Figure 3. Cubic cobalt was observed only in the nano-sized

cobalt containing powders after all milling times, while in all the other powders the cobalt structure washexagonal independently of the milling time. This is in accordance with the fact that the smaller grainsize favours the cubic structure in cobalt [1]. However, the original cobalt powder was in the cubicform, and it transformed to the hexagonal form due to the mechanically induced transformation;according to [19] even nano-cobalt would transform to hexagonal after 10 hours milling. Whencomparing the different WC-grades in mixture with fine-Co (Figure 3b), no remarkable difference in theWC peaks of XRD could be observed. In coarse WC grade powders metallic tungsten and W 2Cobserved in the XRD, are probably due to poor carburisation during production process.

Figure 3. XRD diffractograms for (a) for the fine-WC mixed with different cobalt grades and(b) for the fine-Co mixed with different WC grades. All the powders were milled for 30 min.

3.2 Densification and MicrostructureSintering temperature, time and pressure were kept constant (1453 K, 3 min, 50 MPa,

respectively) during all compactions in order to observe the effect of initial powder characteristics. Theheating rate was 100 K/min in order to keep thermal cycle relatively short. In the conventional sinteringdensification starts already at heating stage and this contributes the grain growth. As faster heatingrates are applicable in pulsed electric current sintering, therefore both the fast heating and the shortprocessing time enables the retaining of the original grain size.

The density comparison of sintered compacts from different grade powders is given in Figure 4.Fine WC containing powders were observed to result in the highest density. In this grade the densityvalues were increased for the smaller particle size of cobalt. Compacts made from powders milled for30 min showed either clearly better or nearly the same densities as those milled for 15 min. Breakingof agglomerates can be the reason for this. The microstructures of the bulk materials for the powdersshown in Figure 2 are given in Figure 5. Despite of the high theoretical density values of the sinteredmaterials, discontinuity of WC structure is observed in SEM images as the WC particles on the verysurface may be pulled out during sample preparation (due to the long polishing stage). Those samples


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having these Co lakes in the microstructure showed higher standard deviation during hardnessmeasurements (Table 1, Figure 4). Also, grain growth is observed in the sintered compacts.Coarsening of grains was more severe in those samples compacted from nano-powders. The averagegrain sizes of compacts sintered from powders 3nn, 3nf, 3nc, 3fn, 3ff, 3fc, 3cc were 0.4, 0.37, 0.41,0.50, 0.53, 0.52, and 0.56 m, respectively.

Nano WC Fine WC Coarse WC

92,0

93,0

94,0

95,0

96,0

97,0

98,0

99,0

100,0

Density,

T.D.

%

5 min

15 min

30 min

Figure 4.Densities of sintered compacts.

Figure 5. SEM images of bulk materials compacted of the powders shown in Figure 2:a) 3nn b) 3nf, c) 3nc, d) 3fn, e) 3ff, f) 3fc, g) 3cc.

In Figure 6, the XRD patterns of the powders are compared to those of the respective bulkmaterials. The hexagonal Co of the powders is transformed to cubic above 683 K which is clearlyexceeded in the sintering process [1-19], and since no reverse transformation exists during cooling,the observed cobalt in the bulks is cubic. In the sintered compacts of fine WC and nano Co, -carbidephase of Co3W3C is observed (Fig. 6b). During high temperature processing WC particles can partiallydissolve in the binder phase and form W 2C and carbon rich -phases (CoxWyC) [8]. Formation of firstis normally due to low C content. The amount is related with particles size of the components andheating rate.

Figure 6. XRD diffractograms for the powders and bulks of (a) the nano-WC grade combined to different Cogrades, and (b) the fine-WC mixed with different Co grades. All the powders were milled for 30 min, and the bulks

were pulsed electric current sintered with 1453 K, 3 min, 50 MPa, and 100 K/min.


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3.3 Mechanical Properties

Measured Vickers microhardness values are presented in Figure 7. They are in goodaccordance with earlier reported values [5-7]. Microhardness values close to 2300 HV1 (e.g. 23 GPa)were measured for nano-grade materials which is higher than published results for WC 12%wt Comaterial made with pulsed electric current sintering [5,6,13,14]. On the other hand, rather lowhardness values in the range of 1300-1400 HV1 (~14 GPa) were measured for those samples

containing coarse WC grade. Even though the fine grade materials had the highest densification, theirhardness values were lower than nano-grade ones. Bulk materials prepared from 30 min milledpowders had slightly higher hardness values probably due to deagglomeration during milling and morehomogenous distribution of carbide and metal powders. Nanoindentation of the 3nn compact wasmostly made on WC particles resulting in values between 16.6-34.3 GPa range (Figure 8). Thesevalues were comparable with values from literature [20].

0

500

1000

1500

2000

2500

3000

Hardness,HV1

5 min

15 min

30 min

Figure 7.Vickers microhardness (HV1) of the sintered compacts.

SPS_WC-12Co: Berkovich tip / Pmax = 2000N / Load control

1.6 m

1

2

43

5

6 7

8

9Point H(GPa) Er(GPa) E(GPa)

1 21.0 376.6 528.7

2 16.6 286.6 360.2

3 19.1 292.0 369.3

4 30.0 439.9 673.0

5 29.1 414.5 612.1

6 21.4 365.8 506.4

7 22.0 359.9 494.5

8 34.3 400.3 579.9

9 30.3 381.0 538.0

Literature values of *

WC: 12-24 / 700 GPaCo: 2 / 200 GPa

Figure 8.Nanoindentation results of the 3nn bulk material. (a) SPM images of the measuring spot before

indentations; (b) the same after the test.

4. ConclusionsWC-12Co powders prepared from the nano-Co exhibited the cubic cobalt still after the longest

milling time of 30 min, while in the other powders cobalt was transformed to hexagonal phase even

with the shortest milling time (5 min). Pulsed electric current sintering produced fully dense compactsin three minutes at 1453 K from the mixed fine grade WC and the nano grade Co powders. In theother Co grade mixtures, hexagonal cobalt was transformed into cubic cobalt during sintering process.The highest microhardness (23 GPa) value was achieved for compacts sintered from the nano-WCand the nano-Co powder mixtures. Hardness value measured by nanoindentation of the WC particlesin the same material was as high as 34.4 GPa. The grain sizes of final compacts were between 0.37and 0.56 m which indicates little grain growth during sintering.

Acknowledgements

The authors thank acknowledge also the Finnish Funding Agency for Technology and Innovation(Tekes), and the consortium of Finnish Companies, for funding the study. Also, the Center of NewMaterials at TKK is acknowledged for the partial funding of the SPS equipment.


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References

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2. Zhao J.F., Holland T., Unuvar C., Munir Z.A., Sparking plasma sintering of nanometric tungstencarbide. Int. J. Refract. Metal. Hard Mater. 27, 130-139.

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properties of nanocrystalline cemented tungsten carbide

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synthesis and consolidation of materials: A review of the spark plasma sintering method,J.Material Science, 2006, 41, 763-777

5. Sivaprahasam D, Chandrasekar SB, Sundaresan R. Microstructure and mechanical propertiesof nanocrystalline WC12Co consolidated by spark plasma sintering. Int. J. Refract. Metal. HardMater. 2007, 25, 144152.

6. Kim H-C., Shon I-J., Yoon J-K., Doh J-M., Consolidation of ultra fine WC and WC-Co hardmaterials by pulsed current activated sintering and its mechanical properties. Int. J. Refract.Metal. Hard Mater. 2007, 25, 46-52.

7. Kim H.C., Jeong I.K., Shon I.J., Ko I.Y., Doh J.M., Fabrication of WC-8 wt.%Co hard materialsby two rapid sintering processes. Int. J. Refract. Metal. Hard Mater. 2007, 25, 336-340.

8. Picas J.A., Xiong Y., Punset M., Ajdelsztajn L., Forn A., Schoenung J.M., Microstructure andwear resistance of WC-Co by three consolidation processing techniques. Int. J. Refract. Metal.Hard Mater. 2009, 27, 344-349.

9. Huang S.G., Vanmeensel K., Li L., Van der Biest O., Vleugels J., Influence of starting powderon the microstructure of WCCo hardmetals obtained by spark plasma sintering. Mat. Sci. Eng.A, 2008, 475, 87-91.

10. Zhao S.X., Song X.Y., Zhang J.X., Liu X.M., Effects of scale combination and contact conditionof raw powders on SPS sintered near-nanocrystalline WC-Co alloy, Mat. Sci. Eng. A 2008, 473,323-329.

11. Wang X., Fang Z.Z., Sohn H.Y., Grain growth during the early stage of sintering of nanosizedWC-Co powder. Int. J. Refract. Metal. Hard Mater. 2008, 26, 232-241.

12. Weidow J., Norgren S., Andrn H-O., Effect of V, Cr and Mn additions on the microstructure ofWC-Co. Int. J. Refract. Metal. Hard Mater (2009) doi:10.1016/j.ijrmhm.2009.02.002.

13. Huang S.G., Liu R.L., Li L., Van der Biest O., Vleugels J., NbC as grain growth inhibitor andcarbide in WC-Co hardmetals. Int. J. Refract. Metal. Hard Mater. 2008, 26, 389-395.

14. Huang S.G., Li L., Vanmeensel K., Van der Biest O., Vleugels J., VC, Cr3C2and NbC dopedWC-Co cemented carbides prepared by pulsed electric current sintering. Int. J. Refract. Metal.Hard Mater. 2007, 25, 417-422.

15. Huang S.G., Vanmeensel K., Li L., Van der Biest O., Vleugels J., Tailored sintering of VC-dopedWC-Co cemented carbides by pulsed electric current sintering. Int. J. Refract. Metal. HardMater. 2008, 26, 256262.

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Manuscript refereed by Dr Jrgen Schmidt, Fraunhofer Inst i tute, Germany

Electro Discharge Sintering as a Process for Rapid Compaction inPM-Technology

P. Schtte1, J. Garcia1,2, W. Theisen1

1Chair of Materials Technology, Ruhr University Bochum, D- 44780 Bochum, Germany2Helmholtz-Zentrum Berlin fr Materialien und Energie GmbH, D-14109 Berlin, Germany

AbstractElectro Discharge Sintering (EDS) is used as a rapid compaction process for metal powders.The application of a constant pressure and a current pulse, in the amount of 3 x 103 ampere,yields to a dense microstructure. The process takes place within milliseconds. The newsintering technique discloses the potential to avoid diffusion-controlled changes in themicrostructure, which take place in conventional sintering techniques. In this work firstinvestigations of the physical densification on metal powders, produced with the EDStechnique, are presented. Correlations between sintering parameters (energy, pressure),powders characteristics (grain size, alloying elements) and physical properties (density) areshown.

IntroductionThe technique of EDS has been introduced in 1976 by Clyens, Al-Hassani and Johnson [1].In the beginning of the development stage porous bodies were consolidated. This wasrelated to low energy sintering machines using capacitors that provided 80 F [2]. Recentpapers describe the current assisted consolidation of metal powders to highly dense parts.Densification of aluminium [3], WC-Co [4], AISI M2 high speed steel [5] as well as titanium,tin and zinc [6] are presented. First investigations regarding process theory and technology

are carried out by [7, 8]. Although the Field Activated Sintering Technology (FAST)processes have been extensively investigated, a fully understanding of the processphenomena is still lacking.

MaterialsIn this paper atomized steel powders are investigated with a main focus on the cold worksteel 1.2380. This steel is widely used for wear resistant applications and usuallyconsolidated by hot isostatic pressing [9]. After heat treatment 1.2380 presents smallcarbides in a martensitic matrix. To investigate the influence of the powder size on the EDSconsolidation behaviour, different grain size fractions were sintered. Furthermore the steelpowders 1.4404 and 1.8905, with grain size as-atomized, were sintered. They where chosento investigate the consolidation behaviour as layer assembly or intermixture of dissimilar

materials. The investigation of the movement of different alloying elements (C, Cr, V) allowconclusions of the diffusion process during sintering. The chemical compositions of thematerials investigated are shown in Table 1.

Table 1: Materials and their composition in wt %

C Cr V Mo Ni Mn

1.2380 2,30 13,00 4,00 1,00 0 0,30

1.4404 0,02 18,00 0 2,30 12,00 2,00

1.8905 0,13 0,20 0 0 1,00 0,95 Experimental Procedure

The setup for EDS experiments is given in a schematic drawing in Fig. 1 a). A nonconductive forming die made from silicon nitride is filled with powder. At top and bottomhighly conductive copper pressure dies allow to apply a compressive force (via hydraulic)


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and an electrical current of some hundred thousand amperes. The current is discharged bycapacitors and converted by transformers. In Fig. 1 b) shows the pressure and currentregime versus time. At first the powder is compressed with constant pressure that is limitedby the pressure strength of the copper dies. Then the DC current is discharged. Thetemperature can be observed on a macroscopic and microscopic scale. The samples for theresults presented in this paper were produced using an industrial Capacitor DischargeWelding unit provided by Manfred Schlemmer GmbH. 7500 F capacitors provide 40000 Wsat 24 V output voltage. Energy and pressure can be varied in 1 % and 1 kN steps. Theforming die used for the experiments has a 16 mm inner diameter. For all experiments 10 gof powder were used for the consolidation experiments resulting in a sample height of about5 mm. The whole process takes place at atmospheric condition without any shielding gas orvacuum atmosphere.

Figure 1:a) Schematic assembly of the EDS machine, b) Characteristics of EDS process parameters

Experimental MethodsCompacted samples were cut, embedded in resin and polished using diamond andaluminium oxide suspension up to 0.25 m grain size. The polished samples were etchedwith V2A pickle to develop the microstructure. The light optical photographs were takenusing an Olympus BX-60M microscope. For higher magnifications a Scanning ElectronMicroscope LEO 1530 VP is used. X-Ray diffraction measurements are carried out using aSiemens D-500. Vickers Microhardness is measured with a Paar Physica MHT-10 microhardness tester under a load of 490 mN.

Results and DiscussionProcess Parameters and Consolidation Behaviour

The initial experiments were designed to determine the optimal densification parameters. Fora better detection of microstructural changes the powder grain size was chosen to be lagerthan 250 m. In Fig. 2 it is shown that the density of the microstructure correlates with theincrease of the discharge energy reaching full dense microstructure at 70 % of dischargeenergy. Additionally the discharge current and the displacement of the pressure die into theforming die disclose a nearly linear performance with increasing discharge energy. Furtherincrease of the discharge energy or multiple discharge pulses did not change the results atthat stage of experiments. As seen in Fig. 2 the interparticle pores between the powdergrains are subsequently reduced with increased discharge energy. As a main densificationprocess neck formation is observed. With increasing discharge energy the necks becomelarger until all the pores are filled up. Although a constant pressure is applied the spherical

shapes of the powder grains remain unchanged, which indicates that no plastic deformationoccurs to the powder grains by the applied pressure under the conditions investigated. Fig. 3shows the microstructures near the surface of the specimen using different grain size


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fractions at 50 and 70 % densification energy. The low discharge energy leads to anenhanced densification especially near the sample surface. This tendency is observed for allpowder grain sizes. For that reason a discharge energy of 70 % was chosen for the followingexperiments. The higher porosity near the surface indicates that densification proceeds fromthe core to the case of the sample. The same behaviour is described in former literature [10]as a result of the electromagnetic pinch effect. It is specified that the pinch effect aids to fillthe adjacent interparticle pores which has the highest influence in the middle of the sampleand decreases towards the surface. This was observed on materials consolidated by using aEDS technique where the current is directly discharged from the capacitors, which leads to ahigh discharge voltage. In contrast to this, the setup used for these investigations generateslow discharge voltages but significantly higher current, so that joule heating is the initialactivator for densification. Once the discharge energy is sufficiently high the surface nearmicrostructure will be densified by filling up the interparticle pores with molten material whichhas been produced at the surface of the powder grains.

Figure 2:Correlation of discharged energy, electrical current, indentation of the die and the sampledensity (1.2380)

Figure 3: Coherence ofdischarged energy, grain sizeand micro structure at thesurface (1.2380)

Microstructure formation and evolutionThe dense consolidated microstructure of 1.2380 cold work steel is shown in a smallmagnification in Fig. 4a). Fig. 4b) shows the region between two consolidated grains (former


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interparticle pore). On the left and right side the microstructure of the cold work steel inatomized condition is present. It consists of a eutectic carbide network surrounding metalcells. Depending on the high solidification rate of a powder grain during atomization thenetwork is fairly small. In the middle of Fig. 4b) the arrow points to an area where a liquidphase has been present during EDS showing a much finer carbide network. This is due to asmaller volume of liquid metal in EDS than in atomization and a higher cooling rate broughtabout by a self-quenching effect. X-Ray diffraction measurements were carried out on1.2380 cold work steel in order to clarify if a microstructural change other than a finer shapeof the carbide network occurs due to consolidation by EDS. For reasons of comparisonpowders in the state of as-atomized and heat-treated (HT=hardening and tempering) areinvestigated after consolidation with EDS. The results are shown in Fig. 5 where as-atomizedpowder (fast solidification from liquid phase) (1) presents MC and M7C3carbides, numerousaustenite and very few martensite. After heat-treatment (2) all retained austenite hastransformed to martensite and MC plus M7C3carbides. During the EDS process the materialthat filled up the interparticle pores experienced fast melting and solidification. Therefore itwas assumed that the phases should be equal to the state as-atomized. This is generally thecase as proven by a consolidated sample (3) consisting of as-atomized powder. By

comparing the heat-treated powder before (2) and after EDS consolidation (4) a clearabnormality is shown. Again austenite peaks are present, which proves that changes in themicrostructure occur during EDS. These results correspond with the change ofmicrostructure shown in Fig. 4.

Figure 4: a)Powdergrains with filled upinterparticle pores b)Change of microstructuredue to liquid phases(1.2380)

Figure 5: X-Raydiffraction of powder as-atomized (1) powderheat-treated (2) EDSconsolidated powder as-atomized (3) and EDSconsolidated powderheat-treated (4); all1.2380


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Microhardness MeasurementsThe atomized powder grains and the consolidated microstructures were characterised bymicrohardness measurements (Table 2). The microhardness of 1.2380 powder is the lowest(517 HV 0.05) principally due to the high amount of retained austenite that is present afteratomisation. The filled-up interparticle pores show a slight increase of the microhardness,which might result from the finer carbide network. After hardening and tempering themicrohardness increases up to 840 HV 0.05 because of the martensitic metal matrix withoutretained austenite. If this heat treated powder is consolidated with comparable EDSparameters, a density of only 95 % is reached. The hardness measured at the filled-upinterparticle pores of the consolidated heat-treated powder increases to 978 HV 0.05. Thereason for this has not been fully investigated up to now. One possible explanation might bethe difference in thermal conductivity. The powder in the hardened and tempered state isfully martensitic and due to that it shows a higher thermal conductivity than the powder in theas atomized state which is predominantly austenitic. This leads to a higher cooling rate of theliquid interparticle material due to a stronger self-quenching effect.

Table 2: Microhardness of embedded powder (as-atomized and heat-treated), filled-up interparticlepores (after EDS using as-atomized and heat-treated powder)

Hardness HV 0.05

1.2380 Powder 517

1.2380 Filled-Up Interparticle Pores 568

1.2380 Powder HT 840

1.2380 HT Filled-Up Interparticle Pores 978

Bonding and Diffusion BehaviourFig. 6 shows both the consolidated microstructures and the fracture surfaces of a1.2380/1.8905 intermixture and a pure 1.2380 sample. Because of the high density achieved(> 99 %) a homogeneous fracture surface is expected, but can not be found at theintermixture of 1.2380/1.8905. Due to the enormous differences of the material strength, thefracture surface shows a ductile fracture behaviour of 1.8905 and no failure of 1.2380

powder grains. It can also be observed that no interconnection of the different powdermaterials has developed during EDS consolidation. The fracture surface of pure 1.2380shows a transcrystalline fracture as it is demanded from satisfactory consolidated powders.In order to investigate the diffusion and bonding behaviour, especially of the powderintermixtures, EDX mappings are carried out. One example of consolidated intermixture from1.8905/1.4404 is shown in Fig. 7. The arrow in Fig. 7 points at a filled-up interparticle pore.As shown in the element allocations, a mixture of elements from the powder components isonly present in areas that were liquid during consolidation. Diffusion of elements along grainboundaries seems not to be noticeable with EDX measurement. Future measurements withhigh resolution microscopy will be carried out in order to investigate the diffusion behaviourand element distribution.

Figure 6: Comparison of themicrostructure and the fracturesurface of 1.2380/1.8905

intermixture and a sample ofpure 1.2380


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Figure 7: EDX- Mapping of1.4404 (top) and 1.8905(bottom) intermixture

ConclusionsIt has been shown that Electro-Discharge-Sintering is a suitable technique to consolidatecold work steel powder within a very short time (510 ms). Optimal parameters were foundto sinter dense samples of the steel powders used. Independent from powder grain size,densification increases with increasing discharge energy from the core towards the surfaceof the samples. It can be assumed that the interparticle pores are filled up with moltenmaterial from the powder grain boundaries during the consolidation process. The materialsolidifies rapidly due to self-quenching and shows a finer carbide network. Using X-Ray

diffraction it was observed that changes in the microstructure occur after consolidation.Samples showed retained austenite after consolidation, whereas none was present in theinitial heat-treated powders. Although the microstructure of the powder intermixtures wasdense, fracture surfaces showed weak adhesion in between the powder grains for thedissimilar powders as achieved.

AcknowledgementsThe authors thank for the financial support of the German Federal Ministry for Education andResearch (BMBF), sponsoring the consortium project 03X3515.

References

1. Alp, T., Al-Hassani, S.T.S., Johnson, W., J. Mat. Techn. 107 (1985) 186-194.2. Clyens, S., Al-Hassani, S.T.S., Johnson, W., Int. J. Mech. Sci.18 1 (1976) 37-44.3. Qiu, J., Shibata, T., Rock, C., Okazaki, K., Mat. Trans., JIM 38 3 (1997) 226 - 2314. Grigoriev, E.G., Rosliakov, A.V., J. Mat. Proc. Techn. 191 (2007) 182-1845. Fais, A., Maizza, G., J. Mat. Proc. Techn.202 (2008) 70-756. Rajagopalan, P.K., Desai, S.V., Kalghatgi, R.S., Krishnan, T.S., Bose, D.K., Mat. Sci.Eng.,A280 (2000) 289-2937. Yavuz, N., Etemoglu, A.B., Gullu, E., Can, M., Metall,58 (2004) 295-3008. Ryabinina, O.N., Raichenko, A.I., Burenkov, G.L., Translated from PoroshkovayaMetallurgiya, 11 167 (1976) 16-219. Berns, H., Theisen, W., Ferrous Materials Steel and cast iron, SpringerVerlag Berlin(2008) 242-243

10. Alp, T., Can, M., Al-Hassani, S.T.S., Powder Met.30 1 (1987) 29-36


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Manuscript refereed by Professor B ernd K ieback, Fraunhofer IFAM/TU Dresden,

Germany

Induction Sintering of Fe-2Cu PM Compacts

Uur AVDAR*,Enver ATK** Celal Bayar University, Department of Mechanical Engineering, Muradiye CampusManisa/TURKEY

Abstract:

Fe powders mixed with 2% Cu by weight were compacted with 600 MPa to form 3-pointbending samples having dimensions of 10x10x55 mm. Compacted powders were sintered byusing medium-low frequency (30 kHz, 12kW) induction energy in conveyor system. Sinteringprocess was completed in several sintering durations from 400 to 1300 seconds at 1120oCunder atmospheric environment (open air). Micro structural and mechanical properties ofsintered samples were investigated. Maximum stress for 3-point bending was achieved at

1000 seconds of sintering duration at 1120 C. The mechanical properties of inductionsintered samples were compared with conventionally sintered samples at 1120 C for 30minutes of sintering duration under argon atmosphere. The maximum stress values forinduction sintered sample were nearly similar to conventionally sintered samples.

Keywords:Induction, Sintering, Powder Metal.

1. INTRODUCTIONThe sintering process was generally done in the sintering furnaces. The sintering furnacecontrols heat and time during the sintering loop. Additionally, it keeps the atmosphere andprovides the possibility of heat treatment after the sintering. The sintering process was donein a batch furnace or continuous furnace [1,2].

The sintering process of an iron-based powder metal sample in a classical furnace wascarried out at 1120C in 30 minutes. The reason for the induction sintering was to decreasesintering time.

As an alternative to classical sintering methods, novel rapid sintering processes have startedbeing used. These are; induction heating, microwave heating, plasma heating and laserheating [3].

Based on the literature induction sintering was studied in 1980s by W. Hermel et al. aboutinduction sintering fundamentals, applications [23, 24] and A. Salak et al. about inductionsintered iron based samples [25]. Sintering by induction was patented in 1988 in the USAunder the name of Induction sintering process and apparatus [4]. The most significantstudies done on sintering by induction after that date have been done under the name of highfrequency induction heat sintering which are called HFIHS. Parallel to our study, M.Nakamura et al. [5] took out the patent of rapid sintering of iron and steel by induction heatunder hydrogen and nitrogen gases in 2003.

HFIHS (High Frequency Induction Heat Sintering) [7 - 18]:HFIHS method is a method of rapid sintering like SPS (Spark Plasma Sintering) method. Ascompared to SPS, HFIHS is a new method of sintering [6].

This method is generally applied to ceramics, composites and bio materials. For example;

are used in materials such as WC-Co [7], uranium oxide (UO) [8], composite like WC-Ni,

Fe, Co [9, 10], WC-Mo2C [11], WC-TiC [12], WC-8%Co [13], 8YSZ-Fe2O3[14], TiB2-WB2[15],NbSi2-Si3N4 [16], bio ceramics like Hydroxyapatite (Hap) [17] and Al2O3-(ZrO2+3%Y2O3)


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[18]. When the powders which are generally placed between Al2O3 double axis blocks in theplate are pressed as double axis, they are sintered by high frequency induction coil.

Cavdar et al. studied iron and iron based powder metals for sintering samples with low-medium frequency induction sintering [19, 20, 21].

2. MATERIALS AND METHODThe samples used in tests were produced as Fe + 2% Cu (weight) of ratio %2 copper (0.74gr.) and iron powder (36.26 gr.). Zinc stearate of the ratio 0.8% (0.296 gr.) by weight wasused as the lubricant.

The powder metal used in the samples are ASC10029 (The characteristics are shown atTable 1) produced by Hgens. The powder metal was thoroughly mixed for 20 minutes in25 cycle / min to have a homogeneous mixture. The powder metal samples having the sizeof 10x10x55 mm and weight of 37 g, were produced by pressing at a pressure of 600 MPausing one axis press.

Table 1.The Characteristics of Iron Powder [26].

SPECIFICATION RESULT %

MN MAX

GRANULOMETRY

B.S.S. MESH MICRONS

85 180 0.0 0.0

100 150 0.5 0.3

150 106 12.5

200 74 19.1

300 53 19.8

350 45 11.6

-350 45 30.0 40.0 36.7

PHYSICAL PROPERTIES

Apparent Density g/cc 2.55 2.75 2.69

Flow Secs 30 26

CHEMICAL ANALYSIS

Copper % 99.00 99.74

Oxygen % 0.15 0.08

All sintering temperatures were applied at 1120C. A group of pressed metal powdersamples were sintered in classical sintering furnace in argon gas environment andatmospheric environment (open air) in 30 minutes. The other group was sintered by 12 kW,

30 kHz (medium-low frequency) induction generator and atmospheric environment (openair). Sintering process was completed in several sintering durations from 400 to 1300seconds. The samples temperature was measured by infrared thermometer during the


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induction sintering process. While samples temperature was measured by thermocouple,temperature was affected by magnetic flux. Therefore temperature was measured by infraredpyrometer.

Figure 1.Induction Machine and Sintering Process (Celal Bayar University, research

laboratory, Muradiye Campus-Manisa-Turkey)

3. RESULTS AND DISCUSSION5 different samples were used for all induction sintering experiments, average of theexperiments results were shown in the figures. According to the classic sintering experimentsresults were used in average of 3 samples results in the figures.

3.1. 3 point bending results for 2% Cu based iron powder samples.3 point bending tests was performed in Autograph Shimadzu AG-IS 100 kN universal testmachine to compare the sintered samples by the classical sintering process and sinteredsamples by induction sintering. Bending strength values of powder metal samples are shownFigure 2. The percentage of the break strain values of Powder metal samples is shown in

Figure 3.

Figure 2.Bending strength of %2 Cu based iron samples.


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Figure 3.The percentage of the break strain values of %2 Cu based iron samples.

3.2. Vickers hardness resultsMicro hardness measurement was performed with Future Tech FM-700 for using 10 sec., 50

grf .The results are given in Table 2.

Table 2. Average hardness values of the Powder metal samples.

Samples Names Ratio hardness values (HV)

Classical Sintering (without argon gases) 136

Classical Sintering (with argon gases) 142

400 Sec. Induction sintering (without argon gases) 152




3.3. Micro structure photographs of the samples.Induction sintering samples micro structure photos are shown in figure 4.

A B


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C D

Figure 4. A) 400 second induction sintering sample (sintering in open air), B) 700 second inductionsintering sample (sintering in open air), C) 1000 second induction sintering sample (sintering in open

air), D) 1300 second induction sintering sample (sintering in open air) photo

4. CONCLUSIONIron based samples included %2 Cu were sintered with induction sintering method.During the active induction heating system, the more sample strength is increased the more sinteringtime is. The maximum stress value is observed in 1000 seconds induction sintering process. Whensintering time passes 1000 second, maximum stress value is decreased.

A maximum stress value of classic sintering in atmospheric environment (open air) is reached in 700second induction sintering process.

A maximum stress value of classic sintering in argon environment is reached in 1000 second inductionsintering process.

The percentage of the highest break strain value obtained induction sintering process in atmosphericenvironment (open air) is reached in consequence of 1300 second induction sintering. With this value,it can be achieved only to the percentage of the break strain value obtained by classical sintering inatmospheric environment (open air).

According to the Average Vickers hardness values of induction sintering are harder than classicsintering in both argon and atmosphere environments hardness. The more induction sintering time isincreased, the hardness value is increased.

According to the mechanical properties expected from the sample induction sintering parameters mustbe able to be optimized.Induction sintering parameters could be optimized according to mechanical properties of samples.

SPECIAL THANKSTo Prof. Dr. Cevdet Meri ([email protected]) and Prof. Dr. Halun KARACA([email protected] ) for their academic helps,

To Tozmetal Inc. (http://www.tozmetal.com/english.htm ), Ayta Ata ([email protected]) for theirhelp as the provider of the metal powders,

4. REFERENCES1. German, R.M., Powder Metallurgy Science, MPIF, USA, 1984.2. Randall M. German; translator editors: S. Sarta, M. Trker, N. Durlu, Toz Metalrjisi ve

Paralkl Malzemeler lemleri TTMD, Ankara, Trkiye, 2007.3. Randal M. German, Sintering theory and practice The Pennsylvania State University Park,

Pennsylvania, A willeyinterscience publication, Jon Wiley & Sons, INC. 1996 USA, pp. 313-362, 373-400, 403-420.

4. United States Patent, Patent Number: 4,720,615. Inventor: Jerry R. Dunn, Boaz, Ala;Assignee: Tocco, Inc., Boaz, Ala; Appl. No: 770,768; Date of patent: Jan.19.1988, USA.

5. M. Nakamura et al., I. Japan Soc., Iron and steel rapid sintering of steels by induction

heating in hydrogen-nitrogen Patents, Metal-powder.net, p. 33, March 2003, MPR.6. H.C. Kim, D.K. Kim, K.D.Woo, I.Y.Ko, I.J. Shon; Consolidation o f binderless WC-TiC by high

frequency induction heating sintering, Elsevier, International Journal of Refractory Maters &Hard Materials 26 (2008) 48-54, Republic of Korea, accepted 18 January 2007.


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7. Hwan-Cheol Kim, Dong-Young Oh, In-Jin Shon, Sintering of nanophase WC - 15 vol. %Cohard metals by rapid sintering process, Elsevier, Refractory Metals & Hard Materials 22(2004)197-203, South Korea, accepted 22 june 2004.

8. J.H. Yang, Y.W. Kim, J.H. Kim, D.J. Kim, K.W.Kang, Y.W. Rhee, K.S. Kim, K.W. Song; Pressure less rapid sintering of UO2assisted by high frequency induction heating process,The American Ceramic Society, 91 [10] 3202-3206, 2008.

9. In-Jin Shon, In-Kyoon Jeong, In-Yong Ko, Jung-Mann Doh, Kee-Do Woo; Sintering behaviorand mechanical properties of WC-10Co, WC-10Ni and WC10Fe hard materials produced byhigh-frequency induction heated sintering Elsevier, Ceramic International xxx(2008) xxx-xxx,Republic of Korea, accepted 3 November 2007.

10. H.C. Kim, I. J. Shon, Z. A. Munir, Rapid sintering of ultra - fine WC - 10 wt % Co by high -frequency induction heating, Elsevier, International Journal of Refractory metals & HardMaterials 24 (2006) 427-431, accepted 5 July 2005.

11. Hwan-Cheol Kim, Hyun-Kuk Park, In-Kyonn Jeong, In-Yong Ko, In-Jin Shon; Sintering ofbinderless WC-Mo2C hard materials by rapid sintering process, Elsevier, CeramicInternational xxx (2007) xxx-xxx, Republic of Korea, accepted 27 march 2007.

12. H.C. Kim, D.K. Kim, K.D.Woo, I.Y.Ko, I.J. Shon; Consolidation of binderless WC -TiC by highfrequency induction heating sintering, Elsevier, International Journal of Refractory Maters &Hard Materials 26 (2008) 48-54, Republic of Korea, accepted 18 January 2007.

13. H.C.Kim, I.K.Jeong, I.J.Shon, I.Y. Ko, J.M.Doh; Fabrication of WC-8wt.%Co hard materialsby two rapid sintering processes Elsevier, International Journal of Refractory Metals & HardMaterials 25 (2007) 336-340, Republic of Korea, accepted 4 September 2006.

14. I.J. Shon, I. K. Jeong, J.H. Park, B.R. Kim, K.T. Lee; Effect of Fe2O3addition onconsolidation and properties of 8 mol% yttria- stabilized zirconia by high-frequency inductionheated sintering (HFIHS), Elsevier, Ceramics International xxx(2008)xxx-xxx, accepted 6November 2007.

15. M.Shibuya, M.Ohanagi Effect of nickel boride add itive on simultaneous densification andphase decomposition of TiB2-WB2solid solutions by pressure less sintering using inductionheating, Elsevier, Journal of the European Ceramic Society 27 (2007) 301-306, Japan,accepted 6 May 2006.

16. H.K. Park, I.J. Shon, J.K. Yoon, J.M. Doh, I.Y. Ko, Z.A. Munir; Simultaneous synthesis andconsolidation of nanostructured NbSi2-Si3N4composite from mechanically activated powders

by high frequency induction-heated combustion Elsevier, Journal of Alloys and Compounds461 (2008) 560-564, Available online 25 June 2007.

17. K.A. Khalil, S.W. Kim, N.Darmaraj, K.W.Kim, H.Y. Kim; Novel mechanism to improvetoughness of the hyroxyapatite bio ceramics using high-frequency induction heat sintering,Elsevier, Journal of Materials Processing Technology 187-188 (2007) 417-420.

18. K.A. Khalil, S.W.Kim; Mechanical wet-milling and subsequent consolidation of ultra-fine Al2O3-(ZrO2+Y2O3) bio ceramics by using high-frequency induction heat sintering, Science Press,Trans. Nonferrous Met. Soc. China 17(2007) 21-26, accepted 13 September 2006.

19. U. Cavdar, E. Atik; Sintering with induction, International Powder metal Congress &Exhibition, Rostengarten congress center, Euro PM2008, Mannheim/Germany, September29th - 1st October, Vol.3, p.p: 33-38.

20. U. Cavdar, E. Atik; The effects of powders size in Induction sintering, in Turkish, 12.International Materials Symposium, October 15-17 2008, Pamukkale University Congress &

Cultural Center, Denizli, Turkey, vol.2 p.p: 1286-1290.21. U. Cavdar, E. Atik; Induction sintering of %3 cu contented iron based powder metal parts5th

International Powder Metallurgy Conference Turkey, TOBB University, Ankara/Turkey, 8-12October 2008.

22. U. Cavdar, E. Atik; Iron based powder bushings sintering via induction generator in Turkish,5th International Powder Metallurgy Conference Turkey, TOBB University, Ankara/Turkey, 8-12 October 2008.

23. W. Hermel, W. Foerster, G. Leitner, K. Voigt, Short time induction sintering of TiC- and WC-based hard metals, Metal Powder Report, Shrewsbury, England, p 10, v 1. 110. 13.

24. W. Hermel, G. Leitner, and R. Krumphold, "Review of Induction Sintering: Fundamentals andApplications," Powder Metal., 23 [3] 1305 (1980).

25.A. Salak, G. Leitner, W. Hermel. Properties of Induction-sintered Fe-Mn-C and Fe-Mn-Cu-Csteel in the sintered and forged states. Powder Metallurgy International. 1981. Vol 13. N 1.Pp:21-4-45.

26.http://www.hoganas.com/
http://www.hoganas.com/http://www.hoganas.com/http://www.hoganas.com/http://www.hoganas.com/


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Euro PM 2009Field Assisted Sintering

Manuscript refereed by Dr Jrgen Schm idt, Fraunho fer IFAM, Germany

Spark Plasma Sintering Behavior of Ceramic Biocomposites Basedon Hydroxyapatite Nanopowders

Gingu O1

., Lupu N2

., Tanasescu S3

., Rotaru P1

., Harabor A1

., Mangra M.1

, Ciupitu I.1

,Sima G.1, Olei A.1, Bucse I.1

1University of Craiova, 13 A.I.Cuza, 200585, Craiova, Romania2Institute of Technical Physics, Blv. Mangeron, Iasi, Romania3Institute of Chemical Physics I.G. Murgulescu, Splaiul Independentei, Bucharest, Romania

Abstract: The advanced materials used for biomedical applications are characterized by thenanostructure feature. The implant biocomposites used for bone tissue reconstruction arerecommended to be nanostructured improving the functionalisation of the interface betweenthe implant and the bone tissue. The paper presents the experimental results concerning the

elaboration of biocomposite materials processed by Spark Plasma Sintering (SPS).Micrometric Ti powders and nanometric hydroxyapatite powders have been mixed and coldcompacted at 120 MPa. Nanostructured sintered biocomposites are processed by SPS invacuum. The elaborated biocomposites are analyzed from the point of view of structural(SEM) and mechanical (microhardness) properties. The experimental data are compared tothe similar ones obtained for the same materials processed by other technology routes.

1. IntroductionThe hydroxyapatite, with chemical formula:Ca5(PO4)3(OH) (acronym HA), is one of

the well-known biomaterial resembling to human hard tissue [1], being used for bonerepairing by implants and grafts. But the excellent HAs biocompatibility is counterbalanced

by its poor mechanical properties: elastic modulus (40-117) GPa, compressive strength294MPa, bending strength 147 MPa, Vickers hardness 3,43 GPa. These properties aredifferent from the hard tissues ones in comparison with the main mechanical properties of thecortical bone (the denser one among the bones) which are: elastic modulus (4-27) GPa,compressive strength (100-160) MPa, tensile strength (45-175) MPa, shear modulus (2-9)GPa, shear strength (50-70) MPa [2].

Because of this behavior, HA is usually used as a matrix for biocomposite materials.The reinforcing component could be:

polymeric, i.e. collagen type I (a natural polymer found in bones) providing the followingadvantages: an accelerated osteogenesis process, high stability, slow 3D swelling andhigh mechanical wet properties [3], [4], [5];

ceramic, i.e. zirconia, providing high fatigue resistance at high pressure [6] as well as high

bending strength, microhardness and Young modulus [7];metallic alloys based on Ti, Ni, Co-Cr-Mo, stainless steel, amalgam [8] also for increasingthe mechanical properties [9];

non-metallic, as carbon nanotubes (SWCNT or MWCNT) up to 7% vol. with advantageouseffects regarding the improvement of the fracture toughness and flexural strength by 50%and 28% respectively because of MWCNT capacity to absorb some fracture energy andprevent propagation of cracks [10].

In the same time, due to recent advances in nanostructured materials synthesis andcharacterization techniques, metallic and ceramic nanostructured biomaterials are the key tothe future of the biomedical industry [11]. The nanostructure characteristic of thesebiocomposites is necessary to be fulfilled because of the superior properties provided incomparison with the micro-structured similar materials [12]. Thus, HA nanocrystalline pasteis used as autogeneous bone graft with a good success rate in maxillofacial and stomatologysurgery and orthopaedics, providing an accelerated fracture healing and bone density


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increase at a high degree of tolerance [13, 14]. Despite of this, HA nanocrystalline isrecommended always to be combined with some form of stable osteosynthesis, preferablywith an angularly stable plate, due to its lack of dimensional stability [15].

In this paper the research focuses the PM processing and characterisation of thebiocomposite material based on HA nanopowders reinforced by micrometric Ti powderparticles. The Spark Plasma Sintering (SPS) process is applied to process the ceramicmatrix of HA nanopowders that will provide the main properties of the composite from thepoint of view of the biocompatibility with the human hard tissues as well as of thenanostructured feature. The experimental results are compared with similar materialsprocessed by other technological routes.

2. Materials and experimental procedureThe raw materials used for the research are: HA nanopowders having less than 200

nm particle size (BET), type Aldrich, and micrometric Ti powder particles (~ 100 micronsparticle size).

The PM route to process biocomposites type HA/Ti includes the following steps:2.1 the calcination treatment of HA powder particles (in air, at 9000C for 1 hour) in order to

release the H2O content. Beyond this main effect, an undesirable one is theagglomeration of the nanopowders. Fig. 1 presents the particle size distribution of HAnanopowders before (a) and after (b) the calcination treatment. The analysis has beendeveloped on BROOKHAVEN 90 PLUS BI-MAS equipment, which measures by laserscattering the powders particle size between 2 nm2 m.

Fig.1a Particle size distribution of HA nanopowders before calcination (initial)


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Fig.1b (Continuation) Particle size distribution of HA nanopowders

2.2 the wet mixing of HA nanopowders and Ti micrometric ones as 3:1 ratio in a planetaryball mill type Fritsch / Pulverisette 6. The mixing parameters are: balls/powder mixtureratio = 2:1; rotation = 200 rpm, mixing atmosphere = air; mixing medium = wet (ethanol).

The mixing stage is a critical one in order to provide the dispersion of calcinated HAnanopowders that show the conglomeration behavior during the calcination treatment,Fig.1.b. The effect of the mixing process is visible from the optic and electronicmicroscopy images (see below).

2.3 the mixture drying in air at 2000C, overnight, for alcohol evaporation;2.4 the dry mixing of HA/Ti mixture for 1 minute in Fritsch / Pulverisette 6, balls/powder

mixture ratio = 2:1; rotation = 200 rpm, mixing atmosphere = air;2.5 the vacuum SPS process for biocomposites sintering. The sintering parameters are

presented in Tab. 1 and one of the thermal cycles is presented in Fig.2.

Tab.1 SPS parameters for HA/Ti processing

Nr.crt. Sample type Sintering conditions

atmosphere temperature [ C] time [min.]

1

HA/Ti (3/1) vacuum

1.000 10

2 20

3 1.100 10

4 20


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Fig.2 Thermal cycle for HA/Ti biocomposite SPS-ed at 10000C/10 minutes

After biocomposite processing, optical and electronic microscopy analysis underlinethe morphological aspects of the sintered samples. Also, microhardness tests provide

information regarding this mechanical characteristic of HA/Ti processed by SPS.

3. Experimental results and discussions

The aim of calcination process is to decrease as much as possible the water contentfrom the HA nanopowders in order to improve its behavior during sintering process that canbe negatively influenced by H2O content. Because of the hydroxyapatites hygroscopicity, itshows some agglomerates even in initial stage (< 200 nm, as elaborated powder), the meandiameter of HA nanopowders being 401,8 nm (Fig. 1a). Once the calcination treatmentdevelops, the agglomerates grow up to clusters of 992,7 nm (Fig. 1b).

Thus, the mixing process plays an important role in dispersing of HA nanopowders

clusters among the micrometric Ti powder particles. The homogeneous structure of HA/Tipowder mixture can be observed in Fig.2 which presents the biocomposite processed bySPS at 10000C/10 min.

The samples porosity is quite low and Ti reinforcing component (grey spots) ishomogeneously dispersed in HA matrix (white spots), Fig. 2.

From the picture above it can be drawn that SPS process is able to keep the initial HAnanopowders in the submicronic range. In comparison, Fig. 4 presents the microstructure ofthe same biocomposite processed by classic sintering at 12000C/2 hours.


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Fig.2 SEM (BSE) for HA/Ti processed by SPS at 10000C/10 min, providing a homogeneousstructure regarding the reinforcing component (Ti) in the ceramic matrix (HA)

Fig.3 SEM (BSE) of HA/Ti elaborated at 10000C/10 min showing the presence of submicronicHA powder particles after SPS processing

HA matrix

Ti reinforcement

Submicronic HA

powder particles

pore


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Fig. 4 SEM (BSE) of HA/Ti processed by classic sintering at 12000

C/2h

The next research will provide more information regarding the effect of SPS on wearbehavior as well as the biocompatibility tests results, in comparison with similar materialsprocessed by other PM technologies.

4. ConclusionsPM biocomposite materials HA/Ti are able to be processed by SPS technique,

especially in the case of HA nanostructured matrix reinforced by metallic (Ti) micrometricpowders. The main advantages of spark plasma sintering consist in low porosity andnanostructured feature of the biocomposite structure that could have a great impact on itsmechanical and biocompatible properties.

AcknowledgementsThe authors would like to thank to Prof. Mario Rosso and his collaborators from

University Politecnico of Turin (www.polito.it), DISMIC (Department of Materials Science andChemical Engineering), Italy, supporting the SEM analysis.

References1. L.L. Hench, Bioceramics Journal of the American Ceramic Society 81 (1998) 1705-17282. www.eng.tau.ac.il/~gefen/BB_lec1.pdf3. J. Xie, M.J. Baumann, L.R. McCabe, Osteoblasts respond to hydroxyapatite surfaces with

intermediate changes in gene expression, J. Biomed.Mater.Res. A 71 (2004) 108-1174. A.C. Lawson, Y.T. Czernuszka, Collagen-calcium phosphate composites, Proc. Int. Mech. Eng.

Part H, J. Eng. Med. 212 (1998) 413-4255. A. Scabbia, L. Trombelli, A comparative study on the use of HA/collagen/chrondroitin sulphate

biomaterial (Biostite&reg.) in the treatment of deep intraosseous defects, J. Clin. Periodontal 31(2004) 348-355

6. E. Chang, W.J. Chang, B.C. Wang, C.Y. Yang, Journal of Materials Science Medicine 8 (1997)193-200

7. X. Miao, Y. Chen, H. Guo, K.A. Khor, Ceramics International 30 (2004) 1793-17968. M. Niinomi, Science and technology for Advanced Materials 4 (2003) 445-4549. R. McPherson et al., J.Mater.Sci.Med. 1995, 327-33410. Y.H. Meng et al., J.Mater.Sci.Mater.Med., 2008, 19, 75-8111. K. Jurczyk et al., European J.Med.Res., 11, Suppl.II, 2006, 133], [B.C. Ward, T.J. Webster,

Mat.Sc.Eng., C27, 2007, 57512. K. Niespodziana et al., Rev. Adv. Mater. Sci. 18 (2008), 236-240], [Y.H. Meng et al.,

J.Mater.Sci.Med. (2008), 19:75-8113. F.D. Burstein, Cleft Palate Craniofac.J., 37, 2000, 1],14. R. Schnettler et al., Eur.J.Trauma, 30, (4), 2004, 219

15. F.-X. Huber et al., J.Mater.Sci.Mater.Med., 2008, 19, 33-38
http://www.eng.tau.ac.il/~gefen/BB_lec1.pdfhttp://www.eng.tau.ac.il/~gefen/BB_lec1.pdfhttp://www.eng.tau.ac.il/~gefen/BB_lec1.pdf


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Manuscript refereed by Professor Bernd Kieback, Fraunhofer IFAM/TU Dresden,Germany

Consolidation Process of Tungsten Carbide - Cobalt Powder byElectro-Discharge Compaction

E.G. GrigoryevMoscow Engineering Physics Institute115409, Kashirskoe sh., 31, Moscow, Russia

ABSTRACT:

The consolidation of high strength structure of WC-Co composite material is investigated andoptimal operating parameters are defined. Tungsten carbide cobalt composites wereproduced by the method of high voltage electrical discharge together with application ofmechanical pressure to powder compact. Pulse current parameters were measured byRogovsky coil. The temperature evolution during electro-discharge compaction process wasmeasured by means of thermocouple method. It was studied densification process of powder

material during electro-discharge compaction by means of high-velocity filming. Themicrostructure of WC Co cemented carbides was investigated by optical microscopy.Densification to near theoretical density in a relatively short time can be accomplished withinsignificant change in grain size. We have found that the powder densification process haswave nature in electro-discharge compaction. I defined the wave front velocity ofdensification process and the pressure amplitude in wave front to be subject to parameters ofelectro-discharge compaction.

INTRODUCTION

The methods of consolidation of powder materials, based on various techniques oftransmission of electric current pulses through a powder under mechanical pressure, arewidely studied in many research centers. Nowadays these methods are especially important

because it makes possible to obtain the bulk nanostructure materials. These methodsinclude field-assisted sintering technique (FAST), plasma assisted sintering (PAS), sparkplasma sintering (SPS) [1], electric-discharge sintering [2], electro-discharge compaction(EDC) [3], etc. The large number of these methods is related to the wide range of variation inthe electrical parameters of the action on a powder. The efficiency of electric-pulse methodsis determined by the multifactor effect on consolidated materials [4]. To obtain materials withrequired properties, one has to know the macroscopic processes occurring in the bulk of aconsolidated sample. For example, the kinetics of consolidation of powder materials in thesemethods is significantly different, and their duration changes from several tens of minutes forelectric-discharge sintering and spark plasma sintering [1, 2] to several milliseconds forelectro-discharge compaction [3]. In this paper, we report the results of studying themacroscopic phenomena occurring under electro-discharge compaction of conducting WCCo powders. Today WCCo composites are extensively used to enhance the wearresistance of various engineering components, e.g. cutting tools and dies. Cementedcarbides are used throughout industry for high wear, abrasive applications as a result of theirextreme hardness. Apart from the high hardness, WC has other unique properties such ashigh melting point, high wear resistance, good thermal shock resistance, thermal conductivityand good oxidation resistance. Matrices of ductile metals, such as cobalt (Co), greatlyimprove its toughness so that brittle fracture can be effectively avoided during operation.Electro-discharge compaction (EDC) is advanced method which can produce near-net-shapecompacts of high relative density much more rapidly than others conventional processessuch as pressure-less sintering, hot press and HIP. This method has demonstrated apotential to provide distinct technological and economical benefits in the consolidation of

difficult-to-sinter powders, including short processing times, fewer processing steps,elimination of the need for sintering aids, and near net shape capabilities.The principle of EDC is to discharge a high-voltage (up to 30 kV), high-density current (~100kA/cm2) pulse (for less than 300 s) from a capacitor bank through the powders under


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external pressure, resulting in a temperature rise of more than 2500 K, instantaneously toweld grains of powders together. This discharge time is long enough for densification, yet tooshort for extensive grain growth as exhibited in conventional technique. In this way full ornear full densification may be achieved with minimal undesirable microstructural changesdue to short consolidation time. WCCo powders could be consolidated into solid bulks byelectro-discharge compaction (EDC) with densities close to theoretical.

EXPERIMENTAL PROCEDURES

Dense WCCo composites were fabricated by an EDC method. The schematic of theElectro-Discharge Compaction (EDC) system is shown in Fig. 1.

Fig. 1. Schematic of EDC apparatus1charging unit, 2capacitor bank, 3trigatron switch, 4control system, 5electricaldischarge ignition system, 6pulse electrical discharge registration system, 7punch -

electrode, 8die , 9powder.

Electro-discharge compaction (EDC) apparatus for powder consolidation consists basically ofcharging unit (1); a bank of capacitors (2) and trigatron switch (3) to connect a powdercolumn (9) suddenly across the charged capacitor bank. The capacitor bank consists of thirty200 F capacitors that can store up to 6 kV. EDC uses the pulse current generated from thecapacitor bank to quickly heat a powder column subjected to constant pressure during theprocess. Powder column was a circular cross-section rod of diameter ~ 10 mm and lengthfrom 10 to 15 mm. In this process the WCCo powder is poured into an electrically non-

conducting ceramic die (8). The ceramic die is plugged at two ends with molybdenumelectrodes-punches (7) and an external pressure up to 400 MPa is applied to the powder onair-operated press. A high voltage capacitor bank is discharged through the powder. Weused commercial WCCo powders (grain size WC < 5m) as starting material for electro-discharge compaction. Characteristics of the chemical composition of these powders areresulted in Table 1.

Chemical element WC Co free carbon total oxygen

mass % ~ 80 20 0.101 0.13

Table 1. The chemical composition of WC - Co powders

Table 1 gives the general values before the electric discharge compaction.The discharge current is measured by a toroidal Rogowsky coil (6) around the powdercolumn. An oscillograph showing a typical output from the Rogowski coil is shown in Fig. 2.


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EDC applies a high-voltage, high-density current pulse to the powder column under externalpressure for a very short period of time. This method uses the passage of the pulse electriccurrent to provide the resistive heating of the powder by the Joule effect. Joule heatingoccurs at the inter-particle contact to instantaneously weld powder particles, resulting indensification.

Fig. 2. Typical pulse current traces from registration system (Rogowski coil)(Peak currents:150 kA , 280 kA , 3110 kA)

The achieved WC-Co powder compact density as a result of EDC process depends onapplied external air-operated pressure, magnitude and waveform of pulse current thatdepends on RLCparameters of the electrical discharge circuit.

Density measurements after EDC process were performed using the Archimedes principle indistilled water.The temperature evolution during electro discharge compaction process was measured bymeans of thermocouple method. A standard temperature curves at electro dischargecompaction are shown in Figure 3.

Fig. 3. Standard temperature curves of powder column side surface during EDC process(at constant pressure P = 200 MPa)


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Temperature variations 1, 2, 3 correspond of powder column side surface during EDCprocess. Temperature dependence 1 was got at amplitude of discharge pulse current equal

95 kA /cm2, 2 90 kA /cm2, 3 85 kA/cm2. The powder material densification process duringof EDC takes place at approximately constant temperature. It follows from the measuredtemperature curves (Fig. 3).

RESULTS AND DISCUSSION

The most important factor which determines the success of the EDC process is theinstantaneous current density in the powder column [5]. Fig. 4 shows the effect magnitude ofpulse current on the resultant WCCo composite densities after EDC (external pressure200 MPa). This experimental dependence has a maximum on fixed peak current density(Fig.4).

Fig. 4. WCCo compact density dependence from current density

The resultant WCCo composite density increases within the current density region: from 75kA/cm2to 95 kA/cm2. The resultant density reaches the maximum value at ~95 kA/cm2anddrastically decreases beyond 100 kA/cm2.Representative micrographs of WCCo compact samples were observed on the polished

cross-sections using an optical microscopy. There is a difference between microstructures atthe edge and inside the sample depending on the distribution of magnetic pressure inducedby the pulse current. The distribution of magnetic pressure (pinch effect) is defined by thedistribution of a current density in the powder compact. The distribution of magnetic pressurehas a parabolic profile with maximum inside sample (~ 18 MPa) when a current density hasthe homogeneous distribution in the powder column [5]. The magnetic pressure is morehomogeneous in powder compact volume when the skin effect is strong. Fig. 5 shows thetypical WC-Co compact structure with the strong skin effect. In this case, the externalcompressed-air pressure was 200 MPa, and the peak pulse current density was 95 kA/cm 2.


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Fig. 5. WC-Co compact structure after EDC with a strong skin effect

We studied the influence of pulse current density and external pressure on the kinetics ofpowder material densification at electro discharge compaction. Consolidation of powdermaterial takes place at constant pressure P during of the electro discharge compactionprocess. Dependences of WC-Co compact density versus the time during EDC process areresulted in Fig. 6.

Fig. 6. Variations of powder column relative density ()as a function of time in EDC process

at constant external pressure.

On Fig. 6 curves: 1 (75 kA/cm2), 2 (90 kA /cm2) were got at constant pressure P = 200 MPa.The densification time tdof powder material is 6 ms < td< 16 ms for all our experiments. Theresults of experiments show that motion of punches in the process of the electro dischargecompaction takes place with steady speed. The value of speed depends on amplitude ofpulse current and external pressure. The magnitude of external pressure determines initialspecific resistance of powder column and, accordingly, amount of heat, selected in powdermaterial. With the increase of pressure the specific resistance of powder column goes downsharply, that results in the less heating of powder material. The densification of thecompacted material takes place due to an intensive plasticstrain which depends on external

pressure P andyield stress of powder matter (T) (Ttemperature).

Therefore speed of plastic flow of the compacted powder material, and, consequently, speedof change of length of the compacted powder column is determined by the temperature at
http://lingvo.yandex.ru/en?CardId=SeWllbGQgc3RyYWlu;L0H;3;0;1;0;4;5http://lingvo.yandex.ru/en?CardId=SeWllbGQgc3RyZXNz;L0H;3;0;1;0;2;3http://lingvo.yandex.ru/en?CardId=SeWllbGQgc3RyZXNz;L0H;3;0;1;0;2;3http://lingvo.yandex.ru/en?CardId=SeWllbGQgc3RyYWlu;L0H;3;0;1;0;4;5


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EDC process. The speed of densification depends on a dimensionless parameter

= (T)/P.It should be noted that kinetics of process of densification of powder material at EDCsubstantially differs from kinetics at PAS and SPS [1]. It is related to distinction in speed ofinput energy and in the parameters of influence by external pressure on powder material.

CONCLUSIONS

The response of WC-Co powder column (loaded external pressure) to high energy electricaldischarge has been described and understood in terms of peak current density and externalpressure. It was found that the density of WC-Co powder column reached its maximumvalues at applied pressure P = 200 MPa and high voltage electrical discharge parameters(the peak pulse current density = 95 kA/cm2). There is an upper limit of the discharge voltagebeyond which the powder material disintegrates like an exploding wire. Attempts ofcompacting WC-Co powder composites by EDC method would give future fruitful results.

REFERENCES

1. Zhang J., Zavaliangos A., Groza J. (2003) P/M Sci. Technol. Briefs 5 (3): 172. Raichenko A.I. Fundamentals of Powder Sintering by Passing an Electric Current (1987)

Metallurgiya, Moscow3. Grigoryev E. G., Rosliakov A.V. (2007) J Mater. Proc. Techn. 191 (1-3): 1824. Baranov Yu.V., Troitskii O.A., Avraamov, Yu.S., Shlyapin A.D. Physical Bases of Electric-Pulse and Electroplastic Treatments and New Materials (2001):Publisher MGIU, Moscow5. Shakery M., Al-Hassani S. T. S., T. J. Davies: (1979) Powder Met. Int. 11: 120


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Manuscript refereed by Professor B ernd K ieback, Fraunhofer IFAM/TU Dresden,

Germany

Microstructure and Mechanical Properties of Fe3Al Bonded WCComposites Obtained by Pulsed Electric Current Sintering

S.G. Huang,O. Van der Biest,J. Vleugels

Department of Metallurgy and Materials Engineering (MTM), Katholieke Universiteit Leuven,Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium

AbstractIn the present study, pulsed electric current sintering (PECS) or spark plasma sintering(SPS), was applied to make 5-20 vol% Fe3Al bonded submicrometer sized WC composites.This technique offers a unique combination of rapid heating/cooling and short processingtime, which makes it an interesting tool to consolidate WC-FeAl composites with limited WCgrain growth. The WC based composites with 5-20 vol% Fe3Al binder were consolidated by

PECS in the solid state at 1200C for 4 min. The microstructures and basic mechanicalproperties were assessed. Microstructural analysis revealed a homogeneous Fe3Al binderdistributions and limited WC grain growth, together with some residual Al 2O3 clusters. Anexcellent Vickers hardness (HV10) of 25.6 GPa and flexural strength of 1 GPa were achievedfor the WC-5 vol% Fe3Al composite. With 20 vol% binder phase, the hardness decreased to19.9 GPa, while the strength slightly increased to 1.2 GPa.

1. IntroductionIntermetallic aluminides are attractive for applications as high-temperature structural materialdue to their good strength at intermediate temperatures and excellent corrosion resistance atelevated temperatures in oxidizing, carburising and sulfidising atmospheres. Iron aluminide

has been investigated for high-temperature structural applications [1] and binder phase forceramic particles such as WC, TiC, TiB2and ZrB2[2,3].So far, ceramic particles reinforced iron aluminides are commonly produced by liquid

phase sintering or melt infiltrating of a porous ceramic skeleton [4]. By liquid phase sintering,iron aluminide bonded TiC and WC composites with up to 60 vol% carbide were producedand the fracture toughness of WC-40% Fe60Al40 composites was in the range of conventionalWC-Co cemented carbides [2]. The wear resistance of the 40 vol% Fe60Al40 bonded WCcomposite was superior [2] and the oxidation resistance is 4 times higher [4] than for WC-40vol% Co cemented carbides. Residual porosity however was unavoidable at higher ceramiccontents, mainly due to the low solubility of TiC and WC in liquid iron aluminide and the lackof substantial dissolution-reprecipitation [4]. Nearly fully dense composites with 70-90 vol%WC could be produced by the pressureless melt infiltration process [5].

In the present study, the possibility of solid state densification of 5 - 20 vol% Fe3Al bondedsubmicrometer WC composites by means of pulsed electric current sintering (PECS), alsoknown as spark plasma sintering (SPS), was assessed. This technique offers a uniquecombination of rapid heating/cooling and shorter processing times, which makes it aninteresting tool to consolidate WC-Fe3Al composites with limited WC grain growth. Theinfluence of the binder content on the microstructure development and mechanical propertieswas investigated.

2. Experimental procedureStarting powder mixtures composed of submicrometer sized WC (grade CRC015, WolframBergbau und Htten GmbH, Austria, FSSS = 0.5 m) with 5, 10 or 20 vol% N2-atomizedFe3Al powder (AGH University of Science and Technology, Poland, -325 mesh) or 5, 10 and

20 vol% Co (Umicore grade Co-HMP, Belgium, FSSS = 0.55 m,) were prepared by wet-mixing in ethanol on a multi-directional mixer (type T2A, Basel, Switzerland) for 24 h usingWC-7.5 wt% Co milling balls (grade MG15, Ceratizit). The suspension was dried in a rotating


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evaporator at 65C. Pulsed electric current sintering (Type HP D25/1, FCT Systeme,Rauenstein, Germany) was performed in a dynamic vacuum of 4 Pa. A pulsed electriccurrent was applied with a pulse/pause duration of 10/5 ms throughout all the experiments.Around 32 g of powder mixture was poured into a cylindrical graphite die with an inner andouter diameter of 30 and 56 mm and sintered for 4 min at 1200C under a pressure of 90MPa, with a constant heating rate of 200C/min and an initial cooling rate of 200C/min.Graphite paper inserts were used to separate the graphite die/punch set-up and the powdermixture. A two-colour pyrometer (400-2300C, Impac, Chesterfield, UK) was focused at thebottom of a central core hole in the upper punch, just 2 mm away from the top surface of thesample. The die/punch/powder assembly is described elsewhere [6].

The bulk density of the sintered samples was measured in ethanol. The microstructure ofpolished surfaces was examined by scanning electron microscopy (SEM, XL30-FEG, FEI,Eindhoven, the Netherlands). The average WC grain size was determined from SEMmicrographs using Image-pro Plus software [7] to measure the linear intercept length of atleast 200 WC grains for each material grade. The reported values are the actually measured

average intercept length. Phase identification was conducted by a - X-ray diffractometer

(XRD, Seifert, Ahrensburg, Germany) using Cu K radiation (40 kV, 40 mA). The Vickers

hardness (HV10) was measured (Model FV-700, Future-Tech Corp., Tokyo, Japan) with anindentation load of 98.1 N. The fracture toughness, KIC, was calculated from the length of theradial cracks originating from the corners of these indentations according to the formulaproposed by Shetty et al. [8]. The reported values are the mean and standard deviation offive indentations. The flexural strength at room temperature was measured in a 3-point

bending test (Instron 4467, PA, USA) on rectangular bard (25.0 3.0 2.0 mm), with a spanlength of 20 mm and a cross-head displacement of 0.1 mm/min. The reported values are themean and standard deviation of 5 measurements. All sample surfaces were ground with adiamond grinding wheel (type D46SW-50-X2, Technodiamant, The Netherlands) on a Junggrinding machine (JF415DS, Gppingen, Germany).

3. Results and discussion

3.1. Phase diagram information of the WC-Fe3Al systemIn order to select a proper processing temperature and estimate the solid solubility of Fe3Al inWC, Thermo-Calc software [9] was used to calculate the phase diagram of the WC-Fe3Alsystem. The solubility of WC in the solid bcc structured Fe 3Al is 3.5 wt% at 1200C andincreases to 8.6 wt% at 1440C at the three phases equilibrium of WC, bcc and liquid. Thesolubility of WC in the Fe3Al binder phase however is substantially lower than in theconventional WC-Co material. At 1200C, solid fcc-Co dissolves nearly 8.5 wt% WC and thesolubility reaches 12 wt% at 1310C in the solid binder. Based on the phase diagramsinformation, a substantial difference in WC grain growth is to be expected in the Fe 3Al andCo bonded WC composites sintered both in the solid and liquid states. In the present study,a solid state densification temperature of 1200C was selected to consolidate the Fe 3Al andCo binder composites.

3.2. WC and Fe3Al starting powdersThe morphology of the WC and Fe3Al starting powders, as well as the WC-10 vol% Fe3Alpowder mixture is presented in Fig. 1. The WC starting powder is in the form of softagglomerates of 200 nm sized WC grains. The N2 gas-atomised Fe3Al starting powderexhibits a round shape and the diameter varies from 10-30 m. Due to the inherent roomtemperature brittleness of Fe3Al powder, the low energy mixing process caused the break upof the Fe3Al agglomerates into sub-micrometer size grains and even dispersion in the WCmatrix, as shown in Fig. 2.c. A small amount of coarse and oxygen-rich grains were alsoobserved in the powder mixtures, as confirmed by EDS point analysis (see Fig. 1.d). Theoxygen content in the Fe3Al powder was reported to be 0.07 wt% [10]. Aluminides easily form

an impervious Al2O3layer that can provide excellent corrosion resistance in a wide range ofcorrosive environments [11,12]. This alumina layer however was experienced as an obstacleduring liquid phase processing of Fe3Al with 70 vol% WC/TiC, resulting in an inhomogeneouslow density microstructure [4]. In this study, the influence of the Al2O3 particles on the


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densification behaviour of the powder mixtures is less profound due to the relatively lowcontent.

(a) (b)

(c) (d)Fig. 1. Morphology of WC (a), Fe3Al (b) and mixed WC-10 vol% Fe3Al (c) starting powders,

together with an EDS point analysis of the labelled grain in (d).

3.3. Microstructure and phase constitution

The bulk density of the sintered composites is summarized in Table 1. According to ASTMB276-91 [13], the apparent porosity of the composites is better than the A02B00C00 level,indicating near full density can be obtained by solid state PECS at 1200C. Plasticdeformation of the Fe3Al binder above 900C could assist densification of the WC-Fe3Alcomposites [14,15]. Fig. 2 shows the low magnification images of the composites with 5 and20 vol% Fe3Al, PECS at 1200C for 4 min. Beside the bright WC-Fe3Al composite matrix, athird phase forming dark contrast clusters can be clearly differentiated. The dark clusters arealigned perpendicularly to the PECS pressing direction. Compositional analysis using energydispersive spectrometry allowed to identify the black phase as pure Al2O3. Besides the Al2O3in the Fe3Al powder, oxygen pick-up during powder processing could be an additional sourcefor Al2O3 particle formation during PECS. The observation of the aligned Al2O3 strings isconsistent with the work of Maziasz et al.[16], illustrating a nearly continuous film of Al2O3

particles at the Fe-Al particle boundaries after extrusion below 1100

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