Experimental and theoretical investigation on microwave melting of metals
Introduction
Melting metals in conventional furnaces such as electric arc furnace, cupola furnace, blast furnace, induction furnace, crucible furnace etc. consumes significant amount of energy. Additionally there are possibilities of material and energy losses and some safety risks (Moore et al., 2003). In order to overcome the inherent disadvantages of conventional melting, one or more of the advanced melting technologies such as electron beam melting, infra red melting, plasma melting, microwave melting, solar melting etc. are preferred according to the specific requirements and applications. Microwave heating receives considerable attention due to its major advantages such as high heating rates, reduced processing time, low power consumption and less environmental hazards (Jones et al., 2002). During microwave heating, large amount of heat may be generated for a lossy material throughout the volume, whereas for conventional heating, the material is heated via an external heat source and subsequent radiative transfer. Microwave finds an important application in various non-conventional heating methods. In food processing, microwave finds application in drying of foodstuffs (Khraisheh et al., 1997), preparation of activated carbons having the properties of high surface area and pore volume (Ji et al., 2007), processing of high melting temperature glasses at high heating rates (Almeida et al., 2007), curing of polymers in order to improve their strength (Yarlagadda and Hsu, 2004) and treatment of wastes which were more energy efficient than conventional methods (Appleton et al., 2005). One of the largest application areas of microwaves is in ceramics processing, which includes sintering and quality improvement of certain ceramic materials (Menezes and Kiminami, 2008) and also joining of ceramics where materials processed with microwave exhibited better joint and flexural strength (Yarlagadda and Soon, 1998).
Although microwave can heat many materials, certain difficulties are encountered in heating metals and alloys. Microwaves cause sparking of metallic materials and most metals are known to reflect microwaves, as their skin depth is of the order of few microns (Gupta and Wong, 2007). Skin depth is a measure of depth of microwave penetration in which the field is attenuated by 1/e of its value at the surface (Metaxas and Meredith, 1983). It was reported that microwave sintering of metal powders can be achieved, and that microwave sintering yielded fast heating rates, uniform microstructure and shape retention (Leonelli et al., 2008), increased sintered density, higher flexural strength and uniform distribution of pores (Anklekar et al., 2001). Saitou (2006) demonstrated that the rate of microwave sintering was higher than that of electric furnace sintering for iron, nickel, copper and stainless steel powders and reported that microwave radiation did not change the sintering mechanism. Microwave heating of metal particles was modelled using finite difference time domain (FDTD) technique (Mishra et al., 2006). Microwave sintering of metal–matrix composites such as copper–graphite composites was performed and the resulting finer microstructure enhanced the performance of the composites (Rajkumar and Aravindan, 2009). Sintering of other metal–matrix composites such as Al/SiC, Ti/C were achieved at high temperatures (>933 K) with low input power (<1 kW) using microwaves (Leparoux et al., 2003). Successful microwave brazing of nickel–titanium alloy has also been reported (Eijk et al., 2008). Microwave heating of metals was extended to melting process by researchers of Oak Ridge Y-12 National Security Complex (TN, USA) for melting and casting of steels, titanium, zirconium, uranium, copper, brass, bronze, aluminium of varying masses, from few kg to 350 kg (Ripley and Oberhaus, 2005). Agrawal (2006) reported that bulk metals such as aluminium, copper and stainless steel can be melted. While the published reports demonstrate that it is possible to melt metals, the detailed heating characteristics have not been reported so far.
Hybrid microwave melting of lead, tin, aluminium and copper was experimentally investigated by varying microwave power level and the load and the results are reported here. The experimental results were successfully modelled by lumped parameter model. The results showed that the heat absorption was a strong function of temperature.
Section snippets
Materials and methods
A 1.3 kW water cooled Thermwave microwave oven (Research Microwave Systems, New York, USA) consisting of the following components was used.
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Microwave power unit (furnace).
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Microwave power controller.
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Thermocouple assemble (Type S; temperature range 1800 K) with a gas inlet. The gas inlet was needed so that the metals that would normally oxidize on heating can be heated in presence of an inert gas such as nitrogen or argon. The thermocouple was shielded with platinum foil to prevent arcing and
Microwave melting of tin (melting point 505 K)
Fig. 2 shows the temperature profile vs. time for tin samples at different loads and power levels. Tin granules of average diameter 3 mm were used for melting. It was known that some of the submicron and nanosized metal powders undergo volumetric heating when subjected to microwaves. For relatively coarse powders (>100 μm), the heating may be conductive from the outside (skin) to the interior of the powder (Mishra et al., 2006). Hence, for metal particles of size in the range of 3 mm, melting was
Conclusion
Microwave melting of tin, lead, aluminium and copper was accomplished at high heating rates, with clean and controlled process conditions. The melting time for lead and tin were not affected by the increase in load up to 150 g irrespective of the power level used. On comparing with a conventional furnace, microwave melting was found to be twice faster, consume less energy and safer to handle. The experimental results were modelled by lumped parameter model using a heat source which varies cubic
Acknowledgement
The authors would like to thank Council of Scientific and Industrial Research (CSIR), India for financing this project (22(0433)/07/EMR-II).
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