Strong metal parts made by microwaving.

A metal plate or piece of aluminum foil placed in a microwave oven gives off a frightening shower of sparks. Surprisingly, though, researchers at Pennsylvania State University in State College have found that microwaving can be a fast, inexpensive way to make metal parts such as gears.

By pressing metal powder into a mold and heating the object in an insulated microwave chamber, the researchers can fuse, or sinter, the particles into complex shapes. "People had always thought that metals wouldn't work in microwaves," says Dinesh K. Agrawal, but powdered metals absorb rather than reflect the energy, obviating the sparking problem.

Compared with sintering done in a furnace, microwaving can save both time and money. "In a conventional furnace, you heat the entire chamber before you heat the product, so you waste a lot of energy," says Agrawal. "In microwave sintering, we are heating the part only."

An object that would take 5 to 10 hours to make in a furnace requires only 90 minutes in the microwave oven. The microwave energy absorbed by the powder instantly converts to thermal energy, thus heating the product, says Agrawal. "The whole process is very rapid and very efficient," he concludes. He and his colleagues describe the technique in the June 17 NATURE.

Moreover, the parts have much better mechanical properties than conventionally sintered ones. The powder particles fuse into a dense solid with a fine microstructure. The quick process doesn't allow enough time for individual metal grains to grow, and "the finer the grains in the product, the stronger it will be," Agrawal explains. The researchers are now developing the technique for use on a large scale.