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High Density Infrared Processing:
Opportunities in Aluminum
12-27-02
When you hear the term "infrared", you might think of the warmth from the sun or stealthy images seen through night vision goggles. But rapid heating using infrared energy promises to play an increasingly important role in the aluminum industry if R&D work currently underway continues to show potential. Specifically, the developments in the area of high density infrared processing look promising for a range of applications.
Infrared is energy similar to visible light, but with a longer wavelength. Infrared energy is invisible to the human eye. Like the sun, infrared rays heat the earth, people and objects directly. The energy emitted is safely absorbed by surfaces that warm up. Radiant heating does not require any medium for transmission, since it is a form of electromagnetic radiation. As a result, any type of processing atmosphere, including vacuum, may be used.
Infrared technology is currently used in a wide range of industrial applications, including drying solder resists in the electronics industry, browning and sterilization in food processing, curing of coatings, and plastic thermoforming. Infrared heating has many advantages over other heating methods, including:
- Non-contact heating
- Controllable temperature gradients, allowing the potential for zone heating
- Low operating costs
- Inherently clean process
The primary innovation that makes this of interest to metals industries is the advent of high density infrared technology, producing a high heating rate capability on the order of 50-400oC/sec. For example, facilities in the Infrared Processing Center at Oak Ridge National Laboratory include both tungsten-halogen and plasma lamp systems. The tungsten-halogen lamps can produce a theoretical power density of 500 W/cm2. While design considerations limit the actual power density to roughly 40 W/cm2, this is still twice that of conventional heat sources. The plasma lamp system produces flux densities up to 3500 W/cm2, which greatly enhances to potential heating rate, and therefore the range of processes and product sizes. The heating systems can be devised in a variety of furnace shapes: simple unidirectional or bi-directional panels, box or tube furnaces, spot and line heaters, or systems for inner diameter heating. The plasma-based systems can be used in a scanning mode with robotics at widths as large as 38 cm.
Applications of high density infrared technology have been investigated in a number of materials and market areas. Success has been achieved with coating and surface modification, die heating, joining, and heat treatment. Joining applications include advanced materials joining, automotive parts, and titanium to steel for armor applications. High density infrared heating has also been used for powder metallurgical products, with the rapid heating allowing the fusing of powders into a solid mass for direct production of refractory metal sheet, for example, without significant oxidation being encountered.
Unidirectional heating is used for rapid preheating of dies used for isothermal forging of medical implants, saving time and energy over conventional electric or gas heating processes. These heaters cut the time to 10 minutes from 4 hours, enhance die life, and are roughly twice as energy efficient as conventional electric heaters.
Applications of specific relevance to the aluminum industry include the coating of H-13 steel pins used in die casting with a chromium carbide coating to reduce the propensity for soldering that can occur between the steel pin and the molten aluminum. The rapid heating of the infrared system is used to fuse the coating without changing the underlying steel base metal properties. Another application of interest is the use of the localized heating to facilitate thermal forming of tubes without the need of dies or tooling.
Recently, the Department of Energy's Industrial Materials for the Future Program
awarded a grant to ORNL and the University of Tennessee to study surface treatments
on refractories used in aluminum processing. High density infrared processing
will be used to apply corrosion resistant, high emissivity coatings on refractories
that will reduce porosity in the refractories. Further information on the program
is available at the Advanced Industrial Materials Program web site at www.eere.energy.gov/industry/imf.
The rapid heating rate capability of high density infrared processing also offers a set of interesting opportunities in the area of advanced heat treatment. By rapidly heating wrought products, the potential to obtain a finer recrystallized grain size exists. This type of processing could be implemented for flat rolled products by incorporating a high density infrared unit just prior to the conventional heat treat furnaces, using the former for rapid heat up and the latter for the soak portion of the cycle. Even without the potential microstructural benefits, the rapid heating would offer the potential for reduced heat treat cycle times and higher product throughput.
A CD-ROM describing the facilities and developmental efforts at the ORNL Infrared Processing Center can be obtained from Dr. Craig Blue, ORNL Infrared Processing Center, at blueca@ornl.gov.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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