Melting and Heat Treating Metals Using Microwave Heating
Melting and casting metals using microwave energy has been done at Y-12 National Security Complex (NSC) for over a decade. The technology has emerged from a lab-scale curiosity to a production scale process, and recently was licensed to several companies for commercial use. Metals that have been melted and cast include steels, titanium, zirconium, uranium, copper, brass, bronze, aluminum, and many other metals and alloy systems. Melt sizes range from a few pounds to more than 750 lb (few kg to 350+ kg). The potential of this technology is huge with possible scale-up.
Bulk metals do not readily couple directly with microwave energy at room temperature because they are electrically conductive, and, therefore, readily reflect the incident energy. However, the electromagnetic field generated by the microwave field does allow for the loosely bound electrons to move and concentrate at surfaces, edges and points. This results in discharge of the energy in the form of arcing . However, in the case of powder metals, microwaves couple very efficiently at room temperatures , and sintering and melting can be achieved very rapidly.
Equipment and set up
The microwave unit used for this work is a relatively standard 2.45-GHz multimode cavity equipped with vacuum capability, as well as complete control for argon, air, nitrogen and other atmospheres. The chamber is equipped with a mode stirrer to break up any standing waves and create a multimode, 2.45-GHz field within the cavity. A pair of 6-kW COBER S6F industrial microwave generators is used to provide the microwaves to the cavity. The waveguides are equipped with dual couplers and a pair of Agilent power meters, which supply a signal to an Agilent E44198B EPM power meter. A set of quarter-wave tuning stubs is placed in each waveguide to help tune the cavity and reduce the reflected power. In addition, wave-matching features are included at the windows where the waveguide enters the chamber to prevent heating of the windows. One waveguide is directed into the cavity in transverse magnetic (TM) mode and the second is directed into the cavity in transverse electric (TE) mode.
Melting and casting
Microwave metal melting is counter-intuitive given the common misconceptions about the behavior of metals in microwave fields. Following are a few fundamental principals that make metal melting in the microwave possible.
Three basic elements required to heat and melt bulk metals using microwaves are: a multimode microwave cavity, a microwave-absorbing ceramic crucible and a thermally insulating casket that is microwave-transparent. The metal charge is placed in an open (no lid) ceramic crucible, and the insulating casket is positioned to completely cover the open crucible. The casket and crucible assembly are placed into a high-power multimode microwave cavity (Fig. 1) capable of uniformly heating the crucible to the desired temperature. Microwave energy applied to the cavity is strongly absorbed by the crucible. The metal charge in the crucible is quickly heated by means of radiation, conduction and convection with the heated crucible walls. The thermally insulating casket increases the energy efficiency of the microwave system by trapping the heat generated in the crucible. This method allows metal objects that cannot be directly heated by microwave energy to be melted easily and efficiently.
Heat treating metals
Three distinct methods used to heat treat metals using a microwave furnace are: molten salt-bath processing, granular suscepting media and fluidized bed processing.
Molten salt-bath processing traditionally has been used for such processes as hardening, annealing, carburizing and nitrocarburizing, wherein the workpiece to be heat treated is immersed into the molten salt bath. Traditional eutectic salts used for heat treating include nitrate salts, carbonate salts, cyanides, chloride salts and caustic salts. The type of heat treatment is determined by careful control of the time and temperature, and quenching of the workpiece sometimes follows the heat treatment. An advantage of using traditional salt baths is that they quickly and uniformly heat the workpiece by conduction. Various surface properties can be imparted to the workpiece by controlling the composition of the salt .
The methods traditionally used to heat the salt to the required processing temperature are submerged electrode, internally heated, externally heated, electrical resistance and gas fired, each of which has certain limitations.
The use of microwaves as a heat source for molten salt-bath processing has several advantages including:
- The salt bath can be operated at a wide range of processing temperatures with a single vessel and power supply
- The salt bath can be allowed to cool and solidify when not needed, and easily restarted when needed
- A variety of salt compositions can be used
- The process can be operated as a batch or continuous
- The same microwave power supplies can be used for other heating processes when the salt bath is not in use
This method has been used to melt and remelt traditional eutectic carbonate salt on a laboratory scale. Scale-up to a full size production is being investigated.
Granular suscepting media. A part can be quickly and efficiently heated by placing it into a granular material that directly absorbs microwave energy. When metal items are placed into a microwave field, the energy tends to concentrate at edges and points, locations where stresses could initiate a crack and provide a failure mechanism. The use of a granular suscepting media helps to normalize and diminish any nonuniformity in the microwave field, resulting in an evenly heated part. A variety of processing atmospheres can also be used. This processing method provides good thermal conduction and results in an easily controlled process. Distinct characteristics of the processing method include:
- It appears to be completely scaleable; that is, a small, simple unit based on a "home" microwave has been demonstrated, and the data presented below is from a 12-kW production scale microwave system
- A variety of suscepting powders can be used depending compatibility with the metal being processed
- A wide range of temperatures can be achieved and maintained
- It can be operated as a batch or continuous process
- The process can be performed in a standard industrial microwave chamber; it does not require a specially designed unique system
Fluidized-bed processing is a heat treating process in which particles are suspended in a gaseous stream, and the suspended particles behave like a liquid. A variety of processing gases, such as argon, LPG, natural gas, ammonia and nitrogen, can be used to provide the fluidization and to produce the desired surface properties on the treated metal parts. The atmosphere within the furnace can also be varied easily and quickly, according to the heat treatment required. Heat treated parts can be quenched to achieve required properties .
This processing method has advantages in energy use, material characteristics and processing flexibility. Advantages include:
- Possible scaleability; that is, a small, simple unit based on a "home" microwave has been demonstrated
- The thermal conductivity of this method is comparable to those of molten salt methods
- The process does not cause hydrogen embrittlement
- The bed can be heated quickly, used for small production runs and shut off when not in use
- A variety of suscepting powders can be used, depending on compatibility with the metal being processed
- A wide range of processing gases can be used depending on product requirements
- A wide range of temperatures can be achieved and maintained
- It can be operated as a batch or continuous process
The use of microwave energy to heat treat different metals is illustrated in the following case study using the granular suscepting method previously described. The goal was to use microwave energy to anneal cartridge brass (70Cu-30Zn). In the process, it is important to achieve uniform heating throughout the workpiece without having point, edge or skin effects, which cause nonuniform heating of the material, possibly leading to unwanted changes in material properties. Experiments were performed on cartridge brass to compare the results between conventional annealing and microwave annealing. Cartridge brass was selected based on material properties and available data for comparison.
Work was performed in three phases. In Phases I and II, cartridge brass coupons were heated to 425°C (800°F) and to 540°C (1000°F), respectively, and in Phase III, a cartridge brass gear was heated to 650°C (1200°F). In each of these phases, the microstructure and hardness of the microwave heat-treated samples were compared with conventionally heat-treated samples (Table 1). In all three phases, the microstructure of the microwave-processed samples duplicated the microstructure of the conventionally processed samples. Hardness values of the microwave-processed samples were 5 to 15% higher than those of conventionally processed samples in all tests. It is believed that this is a result of slightly longer times at temperature for the conventionally processed samples than for microwave-processed samples.
Homogeneity of the workpiece was achieved and no negative effects from the use of microwaves (no edge effects, surface effects or arcing) were observed. The results of heat treating using microwave processing duplicated those obtained using conventional heat treating methods.
Heat treating at the higher temperatures resulted in a significant change in microstructure from the as-received samples. Figure 2 shows that significant grain growth occurred at 650°C with similar results for both microwave-processed and conventionally processed parts.
To determine the effects of microwave heat treatment on a representative industrial part, tests were conducted on a cartridge-brass gear having rounded, sharpened and flat teeth, as well as typical teeth. In addition, holes of different shapes were cut out of the gear body to give the part greater complexity. Figure 3 shows sample locations where metallographic and hardness tests were performed. This design was selected because it represents a broad range of angles and curvatures a component could have that need to be heat treated. Such a design would clearly show any negative effects caused by the use of microwaves as a heat source.
The gears were heated to a temperature of 650°C and held for 1 hour in both the conventional and microwave furnaces. After heat treatment, the gears were evaluated using the same procedures used on the Phase I and II coupons (Fig. 4).
No negative surface, edge, or point effects were observed by using the microwaves as a heat source for annealing. This shape incorporated several different challenging geometries, which did not inhibit the ability of the microwave to successfully heat treat any of the teeth or the base of the gear. The surface finish of the microwave-annealed gear was comparable to that of the conventionally annealed gear. The microstructure of the gear heated in the microwave furnace showed homogeneity throughout the entire structure. Arcing is most likely to occur at sharp points, but no arcing was observed during the heat treatment in the microwave furnace.
The results of these experiments show that microwaves can be used to successfully heat treat metal components without any negative effects on the metal. Hardness values and microstructures were similar to those obtained using conventional heat treatment in all phases of the project.