June 22, 1999 University Park, Pa. -- Anyone who uses a microwave knows that metals, such as aluminum foil, should not be placed in these ovens, but a team of Penn State material scientists is microwaving a wide range of powder metals and producing machine components with improved properties.
"Solid metals cause problems in microwaves because they reflect, rather than absorb, the microwave radiation," says Dr. Dinesh K. Agrawal, professor of materials, senior scientist and director of Penn State's Microwave Processing and Engineering Center. "Powder metals do absorb microwave radiation and can be heated and sintered, using microwaves."
Sintering is used to manufacture many parts made from ceramics, metals and combinations of materials. Green products composed of fine particles and a small amount of binder compressed and dried, are heated to a point where the binder disappears and, over time and higher temperatures, the material is fully sintered.
A solid, dense material forms after the microwave treatment. Microwave sintering allows complex shapes to be manufactured with savings in time and energy, but conventional sintering can take long periods of time and large amounts of energy.
"Our findings indicate that virtually any powder-metal green body can be sintered in 10 to 30 minutes in an appropriate microwave sintering apparatus," the researchers said in today's (June 17) issue of the journal Nature.
The Penn State research team, including Agrawal; Rustum Roy, the Evan Pugh Professor Emeritus of Solid State; Jiping Cheng, post-doctoral research associate; and Shalva Gedevanishvili, research associate, materials science, used commercial powder metal components of various compositions. Metals included iron, steel, copper, aluminum, nickel, molybdenum, cobalt, tungsten, tungsten carbide, and tin. The components included small gears, rings and tubes. They compared their microwaved products with products produced by conventional thermal sintering.
"We obtained essentially fully dense bodies with substantially improved mechanical properties compared to identical bodies sintered in the conventional manner," said Agrawal. The researchers found a homogeneous microstructure with very little porosity in the microwave sintered products.
The key to microwave sintering of powder metals is the specialized insulated sintering chambers. In conventional thermal sintering, the sintering oven is heated and this heat is transferred to the greenware, but microwaving does not heat the chamber, just the greenware. Without insulation, the heat generated in the greenware would be lost to the inside of the microwave cavity and it would take an enormous span of time to reach the required temperatures. The insulated chambers trap the heat and allow temperatures to rise rapidly.
The researchers can also alter the atmosphere of the chamber to include inert noble gases like argon or neon, hydrogen, nitrogen or forming gas -- 5 percent hydrogen and 95 percent nitrogen.
"Because microwave sintering takes less time and lower energy levels, it is cost effective," says Agrawal. "Commercialization of continuous processing equipment for microwave sintering is currently underway." Microwaving of powder metals, as opposed to solid metals, is possible because of the difference in surface area between fine powder particles and solid substances. While solid metals reflect microwaves, a surface effect on the particles allows them to absorb the microwave energy. The insulation material used in the microwave sintering process neither reflects nor absorbs microwaves, but is transparent to them at low temperatures.
Microwave sintering produces a finer grain size than conventional sintering and the shape of any porosities that do exist differs from the conventional product. The microwave produced porosities led to higher ductility and toughness.
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