Typically, in a conventional (electrically heated) furnace compacts are radiatively heated during sintering. Consequently, to prevent a thermal gradient within the compact, a slower heating rate coupled with an isothermal hold at intermittent temperatures is provided that increases the process time, thereby contributing to coarsening. To eliminate this problem, a faster heating rate during sintering is envisaged. Fast heating rates in a conventional furnace, however, result in a thermal gradient within the compacts and lead to compact distortion and inhomogeneous microstructure.
One of the techniques to achieve relatively homogeneous as well as fast sintering in compacts is through microwave heating. Microwaves directly interact with the particulates within the pressed compacts and thereby provide rapid volumetric heating. This reduces processing time and results in energy savings. In addition, the uniform heating minimizes problems such as localized microstructural coarsening, thereby yielding better properties. Microwave sintering is being widely applied for materials synthesis. Until very recently, however, use of microwaves was restricted only to ceramics. Roy et al. have shown that metal powders can also couple with microwaves and can therefore be heated efficiently and rapidly to higher temperatures. This has direct application using microwaves for consolidating particulate ferrous and nonferrous alloys. This paper describes the microwave sintering of stainless steel and its composites, bronze and tungsten heavy alloys. The properties of the microwave-sintered compacts have been compared to those prepared by conventional sintering.
Microwave Heating of StainlessAs-received gas-atomized 316L [Fe-16.5Cr-12.97Ni-2.48Mo-0.93Si-0.21Mn-0.025C-0.008S (in wt.%)] and 434L [Fe-17Cr-1Mo-0.2Mn-0.02C-0.02S-0.02P] stainless steel powders (supplier: Ametek Specialty Metal Products, U.S.) were uniaxially compacted at 600 MPa. The as-pressed stainless steel compacts were sintered in both the solid state (1200°C) and supersolidus (1400°C) conditions. Sintered stainless steel is used to manufacture automotive exhaust flanges.
Figure 1 compares the thermal profiles of the 316L and 434L compacts sintered in a conventional and microwave furnace. It is interesting to note that both 316L and 434L compacts couple with microwaves and are heated rapidly. The overall heating rate in a microwave furnace was around 45°C/minute, which results in about 90% reduction in the process time during sintering of stainless compacts.
In another set of experiments, the effect of heating mode was evaluated on austenitic stainless steel with varying yttria alumina garnet (YAG) addition (5 and 10 wt.%). Figures 3a and 3b show the effect of YAG addition on the hardness of austenitic stainless steels sintered in conventional and microwave furnaces. Regardless of the heating mode and sintering temperature, YAG addition leads to an increase in the hardness. For compacts consolidated at 1200°C (2192°F), the hardness of microwave-sintered compacts is higher. This is due to the higher densification of microwave-sintered 316L and 316-YAG compacts during microwave heating.
Microwave Sintering of BronzeIn Cu-Sn based bronzes (for automotive bearing applications), sintering usually results in compact swelling. It is a well-known fact that the size of copper (Cu) and tin (Sn) powders and heating rate influence the size and size distribution of porosity in the microstructure. Furthermore, a high heating rate is expected to restrict Sn diffusion into Cu, thereby minimizing compact swelling. This can make such alloys amenable for structural applications. In order to validate the above hypotheses, a microwave furnace offers a viable technique to heat the compacts more rapidly and uniformly.
Microwave Sintering of W-Ni-Fe AlloyTungsten heavy alloys (WHA) typically contain 80-98 weight% tungsten (W) interspersed in a matrix consisting of relatively lower-melting transition elements such as Ni, Fe, Cu and Co. These alloys offer a unique combination of properties, such as high density (16-18 g/cm3), higher mechanical properties, good corrosion resistance and easy machinability. This makes them suitable for ordnance applications and as counterbalance weights. Owing to the refractoriness of tungsten (m.p. 3420°C), these alloys are usually consolidated through liquid-phase sintering. To avoid thermal shock, processing of WHAs in a conventional furnace involves heating at a slower rate (<10°C/minute). This not only increases process time, but also results in significant microstructural coarsening (W grain growth) during sintering and leads to degradation of mechanical properties. Hence, it is envisaged that a fast heating rate would mitigate this problem.
The overall heating rate in the microwave furnace was 20°C/minute. Consequently, there is about a 75% reduction in the process time of W-Ni-Fe compacts in a microwave furnace. It is interesting to note that heating of the same compact in microwaves is achieved at much lower power consumption. This can be attributed to the fact that in microwaves the sample heats up and acts as the source of heat. Consequently, the effective thermal-mass reduction lowers the required power input. Both conventional and microwave-sintered samples attained >98.5% density.
Figure 9 compares the XRD patterns of the conventional and microwave-sintered WHA. As compared to the microwave-sintered alloy, the XRD pattern of a conventionally sintered compact exhibits intermetallics (FeNi, NiW, Fe7W6). These intermetallics impart brittleness to the alloy and are attributed to slower thermal cycle. The present composition had a Ni:Fe ratio of 6:1. Therefore, the intermetallic formation was expected during conventional sintering. An implication of the result is that microwave sintering can be utilized effectively even for non-stoichiometric binder composition.
SummaryBoth ferrous and nonferrous alloys can be successfully processed through microwave sintering. This material is often utilized for automotive components such as gears, connecting rods, synchronizer hubs, valve seats and more. As compared to conventional heating, microwave sintering results in a significant reduction in the processing time (ranging from 70-90%). This restricts microstructural coarsening and results in significant improvement in the mechanical properties. It is therefore envisaged that microwave sintering offers a potentially viable means to consolidate particulate metals, alloys and composites for high-performance applications.IH
The author gratefully acknowledges the financial support from INDO-US Science and Technology Forum (IUSSTF), New Delhi, India. The author thanks Prof. Dinesh Agarwal (Penn State University, U.S.), Mr. G. Swaminathan (BHEL, India), and Prof. Y. Bienvenu (Ecole des Mines, France) for their technical feedback and assistance.
For more information:Contact Anish Upadhyaya, associate professor, Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India; tel: 91-512-2597672; fax: 2597505; e-Mail: firstname.lastname@example.org
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SIDEBAR: Experimental SetupUnlike ceramics, sintering of metal-powder compacts necessitates a controlled-atmosphere cavity. For conducting such an experiment, a 1.1-kW microwave furnace with a 2.45-GHz multi-mode cavity was indigenously designed in collaboration with Bharat Heavy Electrical Ltd. (BHEL), Hyderabad, India. The detailed description of the furnace (SinterwaveTM) construction is given elsewhere. Figures A and B show the schematic setup and photograph of the microwave furnace used for the present investigation. The size of the metallic cavity was 40×62 cm2. A high-density (99.9%), doubly recrystallized alumina tube of 7.1 cm diameter and 67 cm in length was positioned at the center of the furnace by drilling holes on the side faces, with ends projecting outward on both sides (Figure B). The alumina tube was cooled from the end by flanges with flowing water through them. Through leakproof fittings, provision was made for gas inlet and outlet. All the sintering experiments were conducted in hydrogen.
The full-density alumina tube prevents any gas leakage. Furthermore, pore-free alumina is transparent to microwaves and does not heat up. Thus, the temperature rise during sintering is solely due to microwave/metal interaction. An insulation package made of FibrefraxTMboards was used to surround the tube at the center of the cavity to minimize heat loss. The design was such that it could be used both with and without susceptor or secondary couplers. The present sintering studies were conducted using the former configuration with SiC plates. Unlike conventional sintering, thermocouples cannot be used to measure temperatures in microwave furnaces. The temperature measurement was done using the M680 infrared pyrometer from Mikron Inc. The infrared pyrometer output was coupled with PC-based data-acquisition and display software.