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Avijit Mondal1*, Anish Upadhyaya1and Dinesh Agrawal2

1Department of Materials & Metallurgical EngineeringIndianInstituteofTechnology,Kanpur208016,India

2ThePennsylvaniaStateUniversity,UniversityPark,PA16802,USA*avijitm@iitk.ac.in

Microwave processing is emerging as an innovative and highly effective material processing method offering many advantages over conventional methods, especially for sintering applications. It is recognized for its various advantages, such as: time and energy saving, rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved mechanical properties which lead to better product performance. Major constraints in conventional sintering of refractory material such as tungsten and its alloys are high sintering temperatures and long soaking times which cause abnormal grain growth and lead to poor mechanical properties. They get further aggravated at smaller (submicron and nano) tungsten powder sizes. This study describes recent research findings; W-18Cu and W-7Ni-3Cu alloys have been successfully con-solidated using microwave heating which resulted in an overall reduction of sintering time of up to 80%. The microwave sintered samples exhibited finer microstructure and superior mechanical properties when compared with the conventional samples. Submission Date: 6August2008

Acceptance Date: 7January2009Publication Date:16January2009

Microwave Sintering of w-18cu and w-7ni-3cu alloyS

Keywords: Tungsten heavy alloys, microwave sintering, microstrutures

INTRODUCTION

Tungsten is a refractory metal with a melting point of 3420°C. Because of its high melting point, tungsten and its alloys are mostly consoli-dated through powder metallurgy (P/M) tech-niques, though for some specific applications mechanical alloying and infiltration techniques are also employed. Most commonly, liquid phase sintering (LPS) is used for consolidating these alloys. The technique offers the advantage of rel-atively lower sintering temperatures, enhanced densification, microstructural homogenization and near theoretical density. These alloys are

processed via sintering at temperatures ranging from 1200°C to 1500°C when at least one of the constituents melts and facilitates the sintering process. Depending upon the composition, these alloys also offer a favorable combination of high thermal conductivity, low thermal expan-sion coefficient, high density, high strength and ductility, good corrosion resistance and ease of machinability. These alloys, because of their wide range of properties, are strategically very important and have a wide range of applications, such as electrical contact, heat spreaders, elec-tronic packaging, radiation-shield, counter-bal-anced weights, and kinetic energy penetrator in defense industry [Upadhyaya, 2001]. Because of technological demands and strategic concerns, there is considerable interest worldwide in con-solidating the tungsten based alloys for thermal Guest Editor: Dr. Satoshi Horikoshi, Tokyo University of

Science, Chiba, Japan

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management and defense industry applications. Each application has its own specific require-ments as far as the properties are concerned. In general for most of the applications, near theoretical density, dimensional stability, high hardness, toughness and very high ductility are important [German, 1996]. In order to avoid thermal shock, processing of tungsten heavy alloys in a conventional fur-nace involves heating at a slower rate (<10°C/min) and with an isothermal hold at intermittent temperatures. This not only increases the process time, but also results in significant microstruc-tural coarsening (W grain growth) during sinter-ing, leading to the degradation of mechanical properties. This problem is further aggravated when the initial powder size is extremely fine. Hence, it is envisaged that a fast heating rate would mitigate this problem. One of the tech-niques to achieve fast and relatively uniform sintering is through microwaves. Microwaves interact with individual particulates within the pressed compacts directly, and thereby provide rapid volumetric and uniform heating [Clark and Sutton, 1996]. This technique offers many advantages over conventional methods for con-solidation. The main advantages include rapid sintering, fine microstructure, energy savings and improvements in the mechanical properties of the sintered products. Until recently, micro-wave processing was mostly restricted to ceram-ics, cemented carbides and ferrites [Clark and Sutton, 1996; Gerdes et al., 1996; Agrawal et al., 2001; Rodiger et al., 1998; Porada and Borchert, 1996-97; Tsay et al., 2001]. Applicability of microwave sintering to metals was ignored due to the fact that they reflect microwaves. Roy et al. [1999] reported that particulate metals can be heated rapidly in microwaves. This has led to the use of microwaves to consolidate a range of particulate metals and alloys. Researchers have reported microstructural refinement due to rapid heating, significant reduction in pro-cess time, elimination of brittle intermetallic formation and superior mechanical properties

in microwaves [Saitou, 2006; Jain et al., 2006; Upadhyaya et al., 2008; Mondal et al., 2008]. The present study describes the consolidation of tungsten based alloys using microwave energy. Conventional sintering has also been conducted to compare the effectiveness of the microwave process.

EXPERIMENTAL PROCEDURE

For the present study the W-18Cu powders were supplied by Osram Sylvania, Towanda, USA. The elemental W (d50: 4.2µm) , Ni (d50: 11.0 µm) and Cu (d50: 47.0 µm) powders were supplied by Kennametal-Widia India Ltd., Bangalore; International Specialty Products (INCO), USA; and ACu Powder International, LLC, USA re-spectively. The detailed powder characteristics have been reported elsewhere [Mondal et al., 2008]. The elemental W, Ni and Cu powders were mixed in a required proportion in a turbula mixer for about 60 min to prepare 90W-7Ni-3Cu (wt.%) alloy composition. Powders were pressed in a die of 16 mm inner diameter to make the green compacts of approximately 6 to 8 mm in height. The pressure was applied uniaxially in a 50T hydraulic press with floating die, using zinc stearate as a die wall lubricant. All the samples were compressed at pressures of 200 MPa. To study the densification behavior, the green compacts were sintered using conventional and microwave furnaces. The sintering temperatures selected for the liquid phase sintering of W-18Cu and 90W-7Ni-3Cu were 1300°C and 1450°C, respectively. Sintering was carried out with a constant heating rate 5°C/min in a flowing H2 atmosphere. Microwave sintering of the green compacts was carried out using a multi-mode cavity 2.45 GHz, 6 kW commercial microwave furnace. The temperature of the sample was monitored using an optical pyrometer with the circular crosswire focused on the sample cross-section. The sintered density was obtained by both di-mensional measurements as well as Archimedes’

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density measurement technique. Metallographic techniques were employed on the sintered samples. The scanning electron micrographs of as-polished samples were obtained by a scanning electron microscope (model: FEI quanta, Neth-erlands) in the secondary electron (SE) and back scattered (BSE) mode. Phase determination and phase evolution, if any were studied for all the samples with the help of X-Ray Diffractometer (Model: RICH. SEIFERT & Co., GmbH & Co. KG, Germany). The experimental variables of the XRD are as follows: scan rate-3°/min, tar-get-Cu, and power-30kVx20 mA. Bulk hardness measurements were performed on polished sur-faces of sintered cylindrical compacts at a load of 5 kg using Vickers hardness tester (model: V100-C1, supplier: Leco, Japan). The load was applied for 30 s. Micro-hardness tester (model: Leitz 8299, Germany) was used to perform the micro-hardness tests on the tungsten grains and matrix phases. The loads applied for micro-hard-ness test was 50 g for 15 s. Ten measurements were taken at random locations for each macro and micro-hardness test to determine statistically accurate results.

RESULTS AND DISCUSSION Figs. 1(a) and 1(b) compare the thermal profiles for both the compositions heated in both the conventional and microwave furnace. It is in-teresting to note that for both, the compositions couple with microwaves and heat up rapidly. The overall heating rates in the microwave fur-nace were 15° and 22°C/min for W-18Cu and 90W-7Ni-3Cu, respectively. Taking into con-sideration the slow heating rate (5°C/min) and isothermal holds at intermittent temperatures in conventional furnace, there is about a 70 to 80% reduction in the process time during sintering of all the compositions in the microwave furnace. Despite such a fast heating rate, no micro- or macro-cracking was observed in all microwave-sintered samples. This underscores the efficacy of volumetric heating associated with micro-waves. Figs. 2(a) and 2(b) show the effect of the heating mode upon the sintered density and densification parameter for both the composi-tions. It is noteworthy to mention that in both, a heating mode more than 98% of its theoretical density has been achieved. Microstructural ob-servation was made to confirm the effect of the heating mode on grain coarsening phenomenon and microstructural homogeneity.

Figure 1. Thermal profiles of (a) W-18Cu and (b) W-7Ni-3Cu alloys sintered using conventional and microwave furnaces.

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Fig. 1: Thermal profiles of (a) W-18Cu and (b) W-7Ni-3Cu alloys sintered using conventional and microwave furnaces.

(a) (b)

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Figs. 3 and 4 show representative micro-structures of conventionally and microwave sintered compacts of both the compositions. Absence of porosity in the microstructures re-confirms near-full densification achieved during microwave sintering. For both the composi-tions, microwave sintering results showed sig-nificantly lower grain coarsening. In sintering, grain coarsening mechanisms and their kinetics are time-dependent. Hence, sintering time com-pression in microwave furnace restricts W grain growth. Furthermore, the SEM micrographs of W-18Cu and 90W-7Ni-3Cu compacts show a homogeneous distribution of both the phases irrespective of heating mode. Figs. 5(a) and 5(b) compare the XRD pat-terns of both the compositions. In the XRD pattern of both conventionally and microwave sintered samples, there were no differences as far as phase evolution is concerned. But it is interesting to note that although the sample size and other parameters were constant during the experiment, relative intensity was still relatively lower in microwave sintered samples than in conventional ones. Further investigations are un-derway to understand this behavior during phase evolution [Mondal and Upadhyaya, 2008]. Table 1 summarizes the effect of sintering

mode on the bulk hardness in both the alloys. Vickers hardness measurement of all the samples showed that, irrespective of the composition, microwave sintered samples have higher bulk hardness than their respective conventionally sintered counterpart. This can be attributed to the relatively higher sintered density noted in microwave sintering. Finer grain size is also another factor that contributes to increasing the bulk hardness in microwave sintered compacts. Lower micro-hardness in the matrix phase can be linked to reduced W solubility in the Ni-Cu matrix, which has recently been confirmed through EPMA analysis [Mondal and Upad-hyaya, 2008].

CONCLUSIONS

W-18Cu and 90W-7Ni-3Cu alloys couple very well with microwaves and are sintered to nearly full density in a multimode microwave furnace. As compared to sintering in a conventional (ra-diatively-heated) furnace, microwave sintering results in a nearly 80% reduction in the sintering time. Despite higher heating rates, no micro- or macro-cracking was observed in microwave-sin-tered samples. This underscores the efficacy of volumetric heating associated with microwaves.

Figure 2. Effect of heating mode (CON: conventional; MW: microwave) on the sintered density (SD) and densification parameter (DP) of (a) W-18Cu and (b) W-7Ni-3Cu alloys.

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Fig. 2: Effect of heating mode (CON: conventional; MW: microwave) on the sintered density (SD) and densification parameter (DP) of (a) W-18Cu and (b) W-7Ni-3Cu alloys.

(a) (b)

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Figure 4. SEM micrographs of (a) conventional and (b) microwave sintered W-7Ni-3Cu alloys.

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Fig. 4: SEM micrographs of (a) conventional and (b) microwave sintered W-7Ni-3Cu alloys.

(a) (b)

Figure 5. XRD patterns of conventionally and microwave sintered (a) W-18Cu and (b) W-7Ni-3Cu alloys.

(a) (b)

Figure 3. SEM micrographs of (a) conventional and (b) microwave sintered W-18Cu alloys.

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Fig. 3: SEM micrographs of (a) conventional and (b) microwave sintered W-18Cu alloys. (a) (b)

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For both the compositions, microwave sintering results in a more refined microstructure, which in turn, leads to enhancement of hardness.

ACKNOWLEDGMENTS

This collaborative research was done as a part of the Center for Development of Metal-Ceramic Composites through Microwave Processing, funded by the Indo-US Science and Technol-ogy Forum (IUSSTF), New Delhi. The authors thank Dr. D. Houcke and Mr. P. Sedor of Osram Sylvania, Towanda, USA for supplying the powders.

REFERENCES

Agrawal, D.K., A.J. Papworth, J. Cheng, H. Jain, and D.B. Williams (2001). “ Microstructural Examina-tion by TEM of WC/Co Composites Prepared by Conventional and Microwave Processes.” Proc. 15th International Plansee seminar, v.2, G. Kneringer, P. Rodhammer and P. wilhartitz (eds.), Plansee AG, Reutte, Austroa, pp.677-684.

Clark, D.E and W. H. Sutton (1996). “Microwave Pro-cessing of Materials.” Ann. Rev. Mater. Sci., 26, pp. 299-331.

Gerdes, T., M. W. Porada, K. Rodiger, and K. Dreyer (1996). “Microwave Sintering of Tungsten Carbide – Cobalt Hardmetals.” Mater. Res. Soc. Symp. Proc., 430, pp.175-180.

German, R.M. (1996). Sintering Theory and Practice. J. Wiley, New York.

Jain, M., G. Skandan, K. Martin, K. Cho, B. Klotz, R. Dowding, D. Kapoor, D. Agarwal, and J. Chang (2006). “Microwave Sintering: A New Approach to Fine-Grain Tungsten-I.” International Journal of Powder Metallurgy, 42(2), pp.45-50.

Mondal, A., D. Agarwal, and A. Upadhyaya (2008). “Microwave Sintering of Tungsten Based Alloys.”

International Conference on Tungsten,Refractory and Hardmaterials VII , Washington DC, USA.

Mondal, A., and A. Upadhyaya (2008). “Phase Evolution of Tungsten Alloys during Microwave Sintering.” unpublished research.

Porada, M.W., and R. Borchert (1996-1997). “ Microwave Sintering of Metal-Ceramic FGM.” Proceedings of the 4th International Symposium on Functionally Graded Materials, AIST Tsukuba Research Center, Tsukuba, Japan, October 21–24, 1996-1997, pp. 349-354.

Rodiger, K., K. Dreyer, T. Gerdes, and M. W. Porada, (1998). “Microwave Sintering of Hardmetals.” In-ternational Journal of Refractory Metals & Hard Materials, 16, pp.409-416.

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Table 1. Effect of heating mode on the hardness of W-18Cu and 90W-7Ni-3Cu compacts.

Heating mode W-18Cu 90W-7Ni-3CuBulk hardness Bulk hardness Micro-hardness

W grains MatrixConventional 325±12 301±21 445±15 333±13Microwave 376±15 328±12 442±9 312±18