International Journal of Refractory Metals and Hard Materials
Microwave sintering of tungsten
Introduction
Tungsten, has the highest melting point (Tm = 3683 K) among all the metals and is generally processed by conventional powder metallurgy route, its melting and casting being extremely difficult. Conventional sintering requires high sintering temperatures (T > 2773 K) and relatively long soaking times to achieve densities above 90% of theoretical.
Owing to its good high temperature properties, tungsten becomes the obvious choice in wide range of high temperature applications. In order to meet the property requirements of these applications apt processing method is to be sought since the processing route plays a vital role in determining the final properties of a sintered material. Conventional methods, are either time-consuming or very expensive, and often produce coarse microstructure. Hence, new sintering techniques providing rapid densification of sintered parts with a fine grain size are sought. One such recent technological innovation is the use of microwave energy for sintering metal powders.
Microwave sintering is fundamentally different from conventional sintering [1]. In conventional sintering heat transfer takes place by radiation/conduction, the heat energy being transferred from the sample surface to the core by conduction. On contrary, during the microwave sintering procedure, the whole of the sample absorbs microwaves and gets heated up by volumetric heating. The electromagnetic energy of the microwaves is converted to thermal energy [1]. Other distinct advantages include faster heating rates leading to finer microstructures and hence better properties, reduced processing time and cost [2].
Though most of the work in microwave technology is focused on ceramics, recently it has been proven that microwave can also be used for processing most of the metal powders. For example, it is reported that copper steel (MPIF FC-0208 composition) and nickel steel powder (FN-0208) sintered by microwave technique have exhibited higher sintered density, better hardness and flexural strength, thus yielding superior mechanical properties than the conventionally sintered samples [1]. Similarly, tungsten heavy alloy (92.5W–6.4Ni–1.1Fe) has also been sintered in microwave and is found to have better hardness, tensile strength and elongation when compared to conventionally sintered heavy alloy [3].
The main objective of the present study is to understand the effects of high-energy milling on the sinterability of tungsten in microwave and also to make a comparative assessment between as-received (coarser) and activated (finer) tungsten powder in terms of their response to microwave sintering, sintered density and microstructure.
Section snippets
Experimental
In the present study, microwave sintering of as-received tungsten and activated tungsten powder is carried out in a 3 kW, 2.45 GHz microwave furnace. As-received powder of 99.95% purity and average particle size of 5–7 μm (by Fisher Sub Sieve Sizer) is isostatically compacted at 250 MPa into compacts of 40 mm diameter and 40 mm height. Cumulative particle size of 5–50 μm determined by laser diffraction sizing (LDS) and the derivative curve are shown in Fig. 1b, Fig. 1c, respectively.
To study the
Results and discussion
Particles of as-received powder have a cuboidal morphology and cumulative particle size of 5–50 μm LDS (Fig. 1a, Fig. 1b). Properties of the tungsten powder used in the present study are tabulated in Table 1. High-energy milling is performed on the as-received powder in order to activate it by reducing its particle size.
The particle morphology of activated powder (as shown in Fig. 2) is found to be platelet-like. The crystallite size measured using XRD is as low as 17 nm indicating significant
Conclusions
In the present study, microwave sintering of both as-received tungsten and activated tungsten powder is successfully accomplished, as both the powders responded to microwave sintering at 2073 K. While the as-received powder got densified to 85% of theoretical density (TD), the density of the compact made from activated powder is calculated to be 93% of TD. The activated powder shows more densification because of the reduced particle size and higher specific surface energy (because of the
Acknowledgements
Authors would like to thank Defence Research and Development Organization for sponsoring the activity through a research project. Authors are thankful to the Director, DMRL for his encouragement and valuable guidance. Authors are also thankful to Dr. T.P. Bagchi and Shri U. Ravi Kiran, Scientists of DMRL, for providing valuable guidance and information pertaining to some of the experiments. Thanks are also due to Dr. Shivanand Borkar of M/s Pradeep Metals, Mumbai, India for extending the
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