This paper describes the sintering behavior of a range of ferrous and nonferrous alloys in a microwave furnace. As compared to conventional (radiatively heated) furnace heating, particulate metals are rapidly heated to high temperatures in microwaves. This results in up to a 90% reduction in the total sintering time. This paper also compares the effect of heating mode on the densification, microstructure and mechanical properties of the compacts.

Fig. 1. Effect of heating mode on the thermal profiles of 316L and 434L stainless steel compacts

In recent years, there has been an increasing trend to consolidate ferrous and nonferrous alloys through powder metallurgical (P/M) processing. As compared to conventional casting techniques, P/M offers advantages such as relatively lower processing temperature, near-net shaping, greater material utilization (>95%) and a more homogenous microstructure. The properties of a P/M-processed component depend on proper optimization of the sintering cycle.

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.[1] 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.[2] Roy et al.[3] 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.[4] 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.

Fig. 2. Optical micrographs of (a) 316L and (b) 434L compacts sintered at 1400°C for 1h in conventional (left) and microwave (right) furnace

Microwave Heating of Stainless

As-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.[5] 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.

Fig. 3. Effect of YAG addition on the bulk hardness of austenitic stainless steel sintered in conventional and microwave furnace at (a) 1200°C and (b) 1400°C

For microwave sintering, a slight difference in the heating rates for the 316L and 434L compacts is observed, which is attributed to the dependence of microwave-metal coupling on the composition. Figures 2a and 2b compare the optical micrographs of conventional and microwave-sintered 316L and 434L compacts. It is quite evident from Figure 2 that microwave sintering restricts microstructural coarsening for both grades of stainless steel.[6]

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.[7] 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.

Fig. 4. Effect of YAG addition and sintering mode on the (a) ultimate tensile strength and (b) elongation of austenitic stainless steel sintered in conventional and microwave furnace at 1400°C

Figures 4a and 4b compare the strength and ductility of 316L and 316L-YAG compacts sintered at 1400°C (2552°F) using both conventional and microwave furnaces. It is interesting to note that in microwave-sintered compacts, the ductility increases with increasing YAG content. In conventionally sintered samples (Figure 4b), the ductility progressively deteriorates with YAG addition. Besides interacting with stainless steel, microwaves also couple strongly with the ceramics. Hence, ceramic additives, such as YAG, act as sort of a hot spot in the compact, which leads to better interfacial bonding and results in corresponding increases in the ductility.

Fig. 5. Thermal profiles of elemental copper and tin powder compact heated in microwave furnace

Microwave Sintering of Bronze

In 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.

Fig. 6. Heating profiles of Cu-10Sn bronze in microwave and conventional furnace

Figure 5 shows the thermal profiles of elemental copper and tin powders heated in a microwave furnace. It is worth nothing that both powders strongly couple with the microwave, and within five minutes the compact temperature reaches near melting point.[8] Figure 6 compares the microwave and conventional heating profiles of premixed Cu-10Sn compact. For microwave heating of bronze, the sintering time reduces by 70%, whereas for tungsten heavy alloy it reduces by about 45% as compared to the conventional sintering. The sintered density, densification parameter and mechanical properties of microwave-sintered bronze are summarized in Table 1. Swelling is observed for conventionally sintered samples, whereas no swelling occurs in microwave-heated samples. The latter can be attributed to a faster heating rate, which minimizes the diffusion of tin into copper. As compared to conventional sintering, microwave-sintered bronze has similar ductility and significantly higher hardness and tensile strength.

Microwave Sintering of W-Ni-Fe Alloy

Tungsten 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.

Fig. 7. Comparison of heating profile and power consumption of microwave and conventionally sintered 92.5W-6.4Ni-1.1Fe alloy

In a series of experiments, the sintering response of a 92.5W-7.5(Ni-Fe) alloy with a non-optimal matrix composition consolidated through microwaves and its densification, microstructure and mechanical properties are compared to conventionally sintered compacts. Figure 7 compares the heating profiles and power consumption of 92.5W-6.4Ni-1.1Fe compacts during microwave and conventional sintering. These compacts couple with microwaves and rapidly heat up.

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.

Fig. 8. SEM micrographs (a) microwave (b) conventionally sintered 92.5W-6.4Ni-1.1Fe alloy

Figure 8 compares the microstructures of conventional and microwave-sintered alloys. Clearly, microwave sintering results in significantly lower tungsten grain coarsening. Table 2 compares the mechanical properties of microwave- and conventionally sintered alloys.[9] The higher bulk hardness and tensile strength in the microwave-sintered compact can be attributed to a more refined grain size. Based on these results, it is evident that microwave sintering offers a potentially viable means to consolidate WHA for structural and high-performance applications.

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.

Fig. 9. XRD analysis of tungsten heavy alloy heated in conventional and microwave furnaces.

Summary

Both 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

Acknowledgements
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: anishu@iitk.ac.in

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: sintering, microwave sintering, grain coarsening, intermetallics

SIDEBAR: Experimental Setup

Unlike 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 FibrefraxTM boards 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.