Microwave Sintering, Brazing and Melting of Metallic Materials

Microwave energy has been in use for variety of applications for over 50 years. These applications include communication, food processing, wood drying, rubber vulcanization, medical therapy, polymers, etc. In the last two decades microwave heating has been also applied very effectively and efficiently to heat and sinter ceramic materials. Microwave heating is recognized for its various advantages, such as: time and energy saving, very rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved mechanical properties, better product performance, etc. The most recent application of microwaves has been in the field of metallic materials for sintering, brazing/joining and melting. Several common steel compositions, pure metals and refractory metals have been sintered in microwaves to nearly full density with improved mechanical properties. Many commercial powder-metal components of various alloy compositions including iron and steel, copper, aluminum, nickel, Mo, Co, Ti, W, Sn, etc., and their alloys have also been sintered in microwaves producing better properties than their conventional counterparts by using a 2.45 GHz multimode microwave system. This work has been further extended to join and braze bulk metal pieces, especially super alloy based turbine blades. Further, in a specially designed microwave cavity, even the bulk metals can be made to couple with the microwave field and melted. The implications of these findings are obvious in the field of powder metal technology. Introduction Microwave energy has been in use for over 50 years in a variety of applications such as communications, food processing, rubber vulcanization, textile and wood products, and drying of ceramic powders. Widespread use of microwave home ovens has in fact revolutionized homecooking. The use of the electromagnetic spectrum in the “microwave” region for many energyintensive technologies has been led by the consumer acceptance of microwave home ovens. Use of microwave technology in material science and processing is not rather new. The areas where it has been applied include: process control, drying of ceramic sanitary wares, calcination, and decomposition of gaseous species by microwave plasma, powder synthesis, and sintering of oxide ceramics and some non-oxide systems [1-4]. Microwave technology is attractive because it has many obvious advantages when compared with conventional methods, such as: very short cycle time resulting in energy savings as high as 90% over conventional methods, rapid heating rates, finer microstructures, and hence, improved mechanical properties and environmental friendliness [4]. Researchers in academia and industry have been working in the area of microwave processing of a variety of materials for many years. Most of this work is confined mainly in the area of ceramics. However, now it has been shown that the microwave energy can, in fact, be used to sinter virtually 183 Sohn International Symposium ADVANCED PROCESSING OF METALS AND MATERIALS VOLUME 4 NEW, IMPROVED AND EXISTING TECHNOLOGIES: NON-FERROUS MATERIALS EXTRACTION AND PROCESSING Edited by F. Kongoli and R.G. Reddy TMS (The Minerals, Metals & Materials Society), 2006 all powdered metals as efficiently and effectively as in the ceramic systems. This has opened up an entirely new research area to investigate the advantages of microwaves for metallic materials to meet the challenging and growing needs in many metallurgical applications. This paper describes the latest developments in the area of microwave processing of metallic material, especially at the Penn State University in the last few years. In many conventional methods involving resistant/radiation/convection heating, the thermal energy is absorbed on the surface of the work-piece and then it is transferred towards the inside via thermal conductivity; so there is an energy transfer through the thermal conductivity mechanism in these methods, and therefore the process is slow. Such methods are not very energy efficient. On the other hand in case of microwave (and RF-induction) heating, the electromagnetic energy is absorbed by the material as a whole (also known as volumetric heating) due to microwave-matter coupling and deep penetration, and then is converted in to heat through dielectric (in case of ceramics), magnetic permittivity/eddy currents (metals) loss mechanisms. Since there is an energy conversion and no thermal conductivity mechanism involved, the heating is very rapid, uniform and highly energy efficient. These two processes are fundamentally different in their heating mechanisms, and hence often result in a vastly different product. Due to the internal heating in the microwave processing, it is possible to sinter many materials at a much lower temperature and shorter time than required in conventional methods. The use of microwave processing reduces typical sintering times by a factor of 10 or more in many cases, thereby minimizing grain growth. Thus, it is possible to produce fine microstructure in microwave sintered metal parts. Microwaves are electromagnetic radiation with wavelengths ranging from about 1 mm to 1 m in free space and frequencies between 300 GHz to 300 MHz, respectively. However, only very few frequency bands in this range are allowed for research and industrial applications to avoid interference with communication. The most common microwave frequency used for research is 2.45 GHz ( ~ 12.25 cm), the same as for the domestic microwave ovens; the other allowable frequencies are 915 MHz ( ~ 32.8 cm), 30 GHz ( ~1 cm) and 83 GHz for some specific applications. The use of microwaves in the sintering of ceramic materials is relatively new. The first reported use of microwave in ceramics goes as far back as in 1968 [5], however, the real activity in the field picked up momentum only in the late 1970s and continued with great enthusiasm and excitement in the 1980s. Several earlier excellent reviews by Clark and Sutton [1], Schiffman [2], Katz [3], Sutton [4, 6], have summarized the status of microwave processing research till 1996. During this period, besides the full commercialization of microwave drying and food processing, the microwave heating and sintering of traditional and special/advanced ceramics, composites, and glass ceramics: alumina, uranium oxides, silica, zeolites, barium titanates, ferrites, glass-ceramics, hydroxyapatite, etc., were investigated. Two recent reviews [7, 8] have summarized the latest developments in the field of microwave sintering and synthesis of inorganic solids. Some of these latest developments include microwave sintering of cemented tungsten carbide [9-13], development of transparent ceramics [13, 14], and the sintering of metallic materials [15, 16]. This paper briefly summarizes the work on metallic materials performed at the Penn State University. Microwave Processing of Metallic Materials Microwave processing of materials was mostly confined until 2000 to ceramics, semiconductors, inorganic and polymeric materials. There have been very few detailed reports on microwave processing of metals. The main reason for this lack of work in microwave heating/sintering of metals was due to the misconception that all metals reflect microwave and/or cause plasma formation, and hence cannot be heated, except exhibiting surface heating due to limited penetration of the microwave radiation. This observation is evident from the conventional view shown in a plot (Figure 1) between microwave absorption in solid materials and electrical conductivity [17]. It is evident 184 from this plot that only semiconductors are good microwave absorbers, ceramics/insulators are transparent in microwave, and the metals should reflect microwaves. However, the researchers did not notice that this relation is valid only for sintered or bulk materials at room temperature, and not for powdered materials and/or at higher temperatures. Now it has been proved that all metallic materials in powder form do absorb microwaves. The cause for this phenomenon is not yet very well explained. Figure 1. Microwave Energy absorption is a function of electrical conductivity At 2.45 GHz it is observed that the skin depth in the bulk metals is very low (of the order of a few microns), and hence very little penetration of microwaves takes place. However, in the case of fine metal powders rapid heating can occur. A theoretical model developed predicted that if the metal powder particle size is less than 100 m, it will absorb microwaves at 2.45 GHz. It was further observed that the degree of microwave absorption depends upon the electrical conductivity, temperature and the frequency. In magnetic materials other manifestations of the microwave coupling include hysteresis losses, dimensional resonances, and magnetic resonances from precession of magnetic moments of unpaired electrons [18]. The earliest work of microwave interaction with metallic powders is reported by Nishitani [19], who reported that by adding a few percent of electrically conducting powders such as aluminum, the heating rates of the refractory ceramics is considerably enhanced. Walkiewicz et al. [20] likewise simply exposed a range of materials, including six metals to a 2.4 GHz field, and reported modest heating (but not sintering) in the range from 120 C (Mg) to 768 C (Fe). Whittaker and Mingos [21] used the high exothermic reaction rates of metal powders with sulfur for the microwave-induced synthesis of metal sulphides. Sheinberg et al. [22] heated Cu powders coated with CuO to 650°C but did not report any sintering of them. Narsimhan et al. [23] succeeded in heating Fe alloys in a microwave oven only up to 370°C in 30 minutes. But in all these studies no sintering of pure metal or alloy powders was reported. It was only in 1998 in this laboratory that the first attempt at microwave sintering of powder metals took place [15], and since then many other researchers have reported successful sintering of many metallic materials [24-26].