This study investigates the effect of heating mode on the sintering of tungsten-copper alloys containing up to 30？wt.%？Cu. The sinterability of the W-Cu system consolidated in a 2.45？GHz multimode microwave furnace has been critically compared with that processed in a radiatively heated (conventional) furnace. The as-pressed W-Cu alloys can be readily sintered in microwave furnace with substantial (sixfold) reduction in the processing time. As compared to conventional sintering, microwave processing results in greater densification, more homogenous distribution of the binder phase, and smaller tungsten grain size. The densification in compacts increases with increasing Cu content. For all compositions, the electrical conductivity and hardness of microwave sintered W-Cu alloys are higher than those of their conventionally sintered counterparts. This study investigates the effect of heating mode on the sintering of tungsten-copper alloys containing up to 30？wt.%？Cu. The W-Cu alloys were sintered in a 2.45？GHz microwave furnace with substantial (sixfold) reduction in the processing time. As compared to conventional sintering, microwave processing results in greater densification, more homogenous distribution of the binder phase, and smaller tungsten grain size. This results in higher electrical conductivity and hardness of the microwave sintered W-Cu alloys. 1. Introduction Tungsten-copper alloys are widely used for a range of applications, such as electrical contact materials, thermal-management devices, and in ordnance applications [1–3]. On account of the refractoriness of the major phase ( : 3420°C), these alloys are usually processed through liquid phase sintering [4]. Due to lack of solubility between W and Cu, it is rather difficult to attain full densification in this system. For ensuring complete densification through liquid phase sintering, sufficient amount of melt and its homogeneous distribution are required [5]. Unfortunately, due to the density difference between the constituents, it is rather difficult to ensure Cu homogeneity. Thus, the W and Cu powders are often comilled, which, in turn, results in contamination from the milling media and results in subsequent degradation of property, such as conductivity [6]. Alternatively, Cu coating over the tungsten has shown to enhance densification during sintering [7]. However, such a technique is not economically viable for upscaling. Another approach for ensuring densification in the W-Cu system is through the addition of transition metal additives that activate the sintering kinetics [8–10].
References
[1]
A. Upadhyaya, “Processing strategy for consolidating tungsten heavy alloys for ordnance applications,” Materials Chemistry and Physics, vol. 67, no. 1–3, pp. 101–110, 2001.
[2]
A. Upadhyaya, “Processing strategy for consolidating tungsten heavy alloys for ordnance and thermal-management applications,” Transactions of the Indian Institute of Metals, vol. 55, no. 1-2, pp. 51–69, 2002.
[3]
R. M. German, K. F. Hens, and J. L. Johnson, “Powder metallurgy processing of thermal management materials for microelectronic applications,” International Journal of Powder Metallurgy, vol. 30, no. 2, pp. 205–215, 1994.
[4]
A. Belhadjhamida and R. M. German, “Tungsten and tungsten alloys by powder metallurgy: a status review,” in Tungsten and Tungsten Alloys Recent Advances, A. Crowson and E. S. Chen, Eds., pp. 3–20, TMS, Warrendale, Pa, USA, 1991.
[5]
J. L. Johnson, J. J. Brezovsky, and R. M. German, “Effect of liquid content on distortion and rearrangement densification of liquid-phase-sintered W-Cu,” Metallurgical and Materials Transactions A, vol. 36, no. 6, pp. 1557–1565, 2005.
[6]
J.-C. Kim, S.-S. Ryu, Y. D. Kim, and I.-H. Moon, “Densification behavior of mechanically alloyed W-Cu composite powders by the double rearrangement process,” Scripta Materialia, vol. 39, no. 6, pp. 669–676, 1998.
[7]
A. Upadhyaya and C. Ghosh, “Effect of coating and activators on sintering of W-Cu alloys,” Powder Metallurgy Progress, vol. 2, no. 2, pp. 98–110, 2002.
[8]
J. L. Johnson and R. M. German, “A theory of activated liquid phase sintering and its application to the W-Cu system,” in Proceedings of the International Conference & Exhibition on Powder Metallurgy & Particulate Materials, vol. 3 of Advances in Powder Metallurgy and Particulate Materials, pp. 35–46, Metal Powder Industries Federation, June 1992.
[9]
D.-G. Kim, G.-S. Kim, M.-J. Suk, S.-T. Oh, and Y. D. Kim, “Effect of heating rate on microstructural homogeneity of sintered W-15？wt%？Cu nanocomposite fabricated from W-CuO powder mixture,” Scripta Materialia, vol. 51, no. 7, pp. 677–681, 2004.
[10]
V. Gauthier, F. Robaut, A. Upadhyaya, and C. H. Allibert, “Effect of Fe on the constitution of Cu-W alloys at 1200°C,” Journal of Alloys and Compounds, vol. 361, no. 1-2, pp. 222–226, 2003.
[11]
J. L. Johnson and R. M. German, “Factors affecting the thermal conductivity of W-Cu composites,” in Proceedings of the International Conference & Exhibition on Powder Metallurgy & Particulate Materials, vol. 4 of Advances in Powder Metallurgy and Particulate Materials, pp. 201–213, Metal Powder Industries Federation, May 1993.
[12]
J. L. Johnson, J. J. Brezovsky, and R. M. German, “Effects of tungsten particle size and copper content on densification of liquid-phase-sintered W-Cu,” Metallurgical and Materials Transactions A, vol. 36, no. 10, pp. 2807–2814, 2005.
[13]
R. M. German and S. Farooq, “An update on the theory of liquid phase sintering,” in Sintering'87, S. Sōmiya, M. Shimada, M. Yoshimura, and R. Watanabe, Eds., vol. 1, pp. 459–464, Elsevier Science, New York, NY, USA, 1988.
[14]
M. Gupta and W. L. E. Wong, Microwaves and Metals, John Wiley & Sons, Singapore, 2007.
[15]
K. R？diger, K. Dreyer, T. Gerdes, and M. Willert-Porada, “Microwave sintering of hardmetals,” International Journal of Refractory Metals and Hard Materials, vol. 16, no. 4–6, pp. 409–416, 1998.
[16]
R. Roy, D. Agrawal, J. Cheng, and S. Gedevanishvili, “Full sintering of powdered-metal bodies in a microwave field,” Nature, vol. 399, pp. 668–670, 1999.
[17]
K. Saitou, “Microwave sintering of iron, cobalt, nickel, copper and stainless steel powders,” Scripta Materialia, vol. 54, no. 5, pp. 875–879, 2006.
[18]
R. M. Anklekar, D. K. Agrawal, and R. Roy, “Microwave sintering and mechanical properties of PM copper steel,” Powder Metallurgy, vol. 44, no. 4, pp. 355–362, 2001.
[19]
M. Gupta and W. L. E. Wong, “Enhancing overall mechanical performance of metallic materials using two-directional microwave assisted rapid sintering,” Scripta Materialia, vol. 52, no. 6, pp. 479–483, 2005.
[20]
S. S. Panda, V. Singh, A. Upadhyaya, and D. Agrawal, “Sintering response of austenitic (316L) and ferritic (434L) stainless steel consolidated in conventional and microwave furnaces,” Scripta Materialia, vol. 54, no. 12, pp. 2179–2183, 2006.
[21]
S. S. Panda, V. Singh, A. Upadhyaya, and D. Agrawal, “Effect of conventional and microwave sintering on the properties of yttria alumina garnet-dispersed austenitic stainless steel,” Metallurgical and Materials Transactions A, vol. 37, no. 7, pp. 2253–2264, 2006.
[22]
C. Padmavathi, A. Upadhyaya, and D. Agrawal, “Corrosion behavior of microwave-sintered austenitic stainless steel composites,” Scripta Materialia, vol. 57, no. 7, pp. 651–654, 2007.
[23]
A. Upadhyaya and G. Sethi, “Effect of heating mode on the densification and microstructural homogenization response of premixed bronze,” Scripta Materialia, vol. 56, no. 6, pp. 469–472, 2007.
[24]
A. Upadhyaya, S. K. Tiwari, and P. Mishra, “Microwave sintering of W-Ni-Fe alloy,” Scripta Materialia, vol. 56, no. 1, pp. 5–8, 2007.
[25]
P. Mishra, G. Sethi, and A. Upadhyaya, “Modeling of microwave heating of particulate metals,” Metallurgical and Materials Transactions B, vol. 37, no. 5, pp. 839–845, 2006.
[26]
A. Mondal, D. Agrawal, and A. Upadhyaya, “Microwave heating of pure copper powder with varying particle size and porosity,” The Journal of Microwave Power and Electromagnetic Energy, vol. 43, no. 1, pp. 5–10, 2009.
[27]
G. Prabhu, A. Chakraborty, and B. Sarma, “Microwave sintering of tungsten,” International Journal of Refractory Metals and Hard Materials, vol. 27, no. 3, pp. 545–548, 2009.
[28]
S. D. Luo, J. H. Yi, Y. L. Guo, Y. D. Peng, L. Y. Li, and J. M. Ran, “Microwave sintering W-Cu composites: analyses of densification and microstructural homogenization,” Journal of Alloys and Compounds, vol. 473, no. 1-2, pp. L5–L9, 2009.
[29]
A. Mondal, A. Upadhyaya, and D. Agrawal, “Microwave sintering of refractory metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe alloys,” The Journal of Microwave Power and Electromagnetic Energy, vol. 44, no. 1, pp. 28–44, 2010.
[30]
A. Upadhyaya and R. M. German, “Densification and dilation of sintered W-CU alloys,” International Journal of Powder Metallurgy, vol. 34, no. 2, pp. 43–55, 1998.
[31]
J. L. Johnson and R. M. German, “Solid-state contributions to densification during liquid-phase sintering,” Metallurgical and Materials Transactions B, vol. 27, no. 6, pp. 901–909, 1996.
[32]
V. V. Panichkina and N. I. Filipov, “Structure formation processes in the sintering of disperse copper-tungsten pseudoalloys,” Science of Sintering, vol. 26, pp. 269–275, 1994.
[33]
G. S. Upadhyaya, Sintered Metallic and Ceramic Materials, John Wiley & Sons, New York, NY, USA, 2000.
[34]
A. P. Savitskii, Liquid Phase Sintering of the Systems with Interacting Components, Russian Academy of Sciences, Tomsk, Russia, 1993.
[35]
H. Okamoto, P. R. Subramaniam, and L. Kacprzak, Binary Phase Diagrams, vol. 2, American Society for Microbiology, Materials Park, Ohio, USA, 2nd edition, 1990.
[36]
A. Upadhyaya and R. M. German, “Shape distortion in liquid-phase-sintered tungsten heavy alloys,” Metallurgical and Materials Transactions A, vol. 29, no. 10, pp. 2631–2638, 1998.
[37]
R. M. German, P. Suri, and S. J. Park, “Review: liquid phase sintering,” Journal of Materials Science, vol. 44, no. 1, pp. 1–39, 2009.
[38]
B. D. Storozh and P. S. Kislyi, “Sintering of tungsten-alumina cermets in the presence of a liquid phase,” Soviet Powder Metallurgy and Metal Ceramics, vol. 13, no. 9, pp. 712–716, 1974.
[39]
R. Bollina and R. M. German, “Heating rate effects on microstructural properties of liquid phase sintered tungsten heavy alloys,” International Journal of Refractory Metals and Hard Materials, vol. 22, no. 2-3, pp. 117–127, 2004.
[40]
S.-M. Lee and S.-J. L. Kang, “Theoretical analysis of liquid-phase sintering: pore filling theory,” Acta Materialia, vol. 46, no. 9, pp. 3191–3202, 1998.
[41]
A. Mondal, A. Upadhyaya, and D. Agrawal, “Microwave sintering of W-18Cu and W-7Ni-3Cu alloys,” The Journal of Microwave Power and Electromagnetic Energy, vol. 43, no. 1, pp. 11–16, 2009.
[42]
W. J. Huppmann, “The elementary mechanisms of liquid phase sintering: II. solution-reprecipitation,” Zeitschrift fuer Metallkunde, vol. 70, pp. 792–797, 1979.
[43]
S.-C. Yang, S. S. Mani, and R. M. German, “The effect of contiguity on growth kinetics in liquid-phase sintering,” Journal of Metals, vol. 42, no. 4, pp. 16–19, 1990.
