The microwave oven has been a convenience for consumers for some time, but the technology is now making its way into manufacturing processes.
A team of Penn State material scientists are 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 Dinesh Agrawal, professor of materials, senior scientist and director of Penn States 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 at which the binder disappears and, over time and higher temperatures, the material is fully sintered.
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. The research shows that virtually any powder-metal green body can be sintered in 10 to 30 minutes using the microwave technology.
The research team tested commercial powder metals, including iron, steel, copper, aluminum, nickel, molybdenum, cobalt, tungsten, tungsten carbide and tin. It manufactured small gears, rings and tubes, then compared them to products made by conventional thermal sintering.
The key to microwave sintering of powder metals is the specialized insulated sintering chambers. Without insulation, the heat generated in the part would be lost to the inside of the microwave cavity and it would take an enormous amount 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 such as argon, 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. The microwave produced products with higher ductility and toughness compared to those made using conventional methods.
Todd Shryock (email@example.com) is SBNs special reports editor.