Sintering High Tungsten Content W-Ni-Fe Heavy Alloys by Microwave Radiation
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MICROWAVE (MW) sintering has attracted attention in powder metallurgy (PM) in the few past decades.[1–5] Since the first report by Roy et al.[6] in 1999, MW sintering of powdered metals has been widely studied,[7–11] showing notable advantages over conventional (CV) sintering such as enhanced densification, refined microstructure, and improved mechanical property. These advantages are mainly derived from its unique heating mechanism. Different from heat transmission to materials from external heating elements via conduction, convection, or radiation in CV sintering, the heat in MW sintering is generated by direct conversion of the electromagnetic energy within the work piece.[12–14]
CHENGSHANG ZHOU, formerly with State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, P.R. China, is now Graduate Student with the Department of Metallurgical Engineering, University of Utah, 135 S 1460 E Rm 412, Salt Lake City, UT 84112. JIANHONG YI, formerly with State Key Laboratory for Powder Metallurgy, Central South University, is now Professor with the School of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, P.R. China. Contact e-mail: [email protected] SHUDONG LUO, formerly with the State Key Laboratory for Powder Metallurgy, Central South University, is now Post-Doctoral Research Fellow with the School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, The University of Queensland, Brisbane, QLD 4072, Australia. Manuscript submitted January 6, 2013. Article published online September 6, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
Tungsten heavy alloy (WHA) is a group of two-phase composites which consists of high-strength tungsten solid grains (88 to 98 pct W, in mass, unless otherwise stated) and ductile matrix containing W, Ni, Fe, Cu, or Co. This alloy exhibits a unique combination of density, strength, ductility, machinability, and corrosion resistance. The fabrication of WHAs involves isothermal sintering at temperatures above the eutectic temperature [1738 K (1465 C) for W-Ni-Fe[15]], where the formation and spreading of wetting liquid largely contributes to the densification.[16] In this regard, it is challenging to sinter WHAs containing high tungsten content. Furthermore, tungsten grain undesirably coarsens via grain coalescence during sintering of these alloys, affecting their mechanical properties.[17,18] Either increasing the sintering temperature or prolonging the isothermal holding time is a practical solution to insure full densification, but at the cost of microstructural coarsening.[17,18] This makes MW sintering attractive for the fabrication of high tungsten content WHAs in virtue of the advantages observed with MW sintering over CV sintering. To date, a few attempts have been reported on MW sintering of WHAs. Upadhyaya et al.[19] first sintered 92.5W-6.4Ni-1.1Fe at 1773 K (1500 C) for 20 minutes at an average heating rate of 20 K/min (20 C/min) using a 2.45 GHz multi-mode MW fur
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