It's amazing what you can do with a kitchen appliance these days. Not only can a high-power microwave oven be used to cook food, it can be harnessed for joining, carburizing, sintering, brazing, nitriding, and annealing metal parts. Potentially, it also can be used to add hard-material coatings, or to decrystalize metal surfaces as a rust preventative. "Our work on microwave heat-treating began with a kitchen microwave," says Dominique Tasch, v.p. and general manager, Microwave Technologies, Dana Corp. (Toledo, OH; www.dana.com), "but metals and microwaves don't mix." It took the addition of a quartz vessel and a plasma to make the whole thing work.
"The plasma can be created from adding most any gas," says Tasch, who explains its job is to absorb the microwaves and prevent their reflection back to the magnetron. (It's the reflection of microwaves off aluminum foil that causes damage to the magnetron in kitchen microwaves.) Parts are placed in a quartz glass container that is used to contain the plasma at atmospheric pressure once the container has been purged of air through the introduction of argon. This is why the process is called "AtmoPlas," for "atmospheric plasma." According to Tasch: "Others have created a plasma microwave process, but they've only been able to do it by drawing down the pressure in the containment vessel to maintain the plasma. Our technology doesn't need this added complexity and expense."
Temperatures in excess of 1,300?C are possible with this process, and coatings that dissociate in plasma can be added to coat parts. The compact containment area—not much larger than the part undergoing the process—means the procedure is very energy efficient, does not give off excess heat, and does not require the microwave reactor be kept running as is necessary with a heat-treating oven. "The unit has 65% to 80% wall plug efficiency and +95% plasma heat efficiency," says Dr. Devendra Kumar, director, R&D, Emerging Microwave Technologies, Dana Corp. "So you heat the part, not the volume, just as the plate gets hot while the microwave stays cool when you cook your dinner."
Multiple magnetrons are used to generate the microwave energy, and it's not unusual to find 2.45-GHz magnetrons—like those used in home microwaves—in Dana's test units. Which means it won't be necessary to reinvent existing microwave technology in order to bring Dana's units to production. "We've already investigated ganging a number of reactors together for high-volume manufacturing," says Mike Dougherty, Sr., manager, Applications Engineering, Microwave Processing Technology, Dana Corp., "where the parts would be bar coded so each one could run a different process, if necessary." And Dana has signed an agreement with ALD Vacuum Technologies (Hanau, Germany) to develop the AtmoPlas technology for heat-treating and carburizing. However, Dana is investigating new uses that include the creation of carbon nanotubes—a byproduct created when acetylene is used to create the plasma—and investigating whether this device can be utilized to improve in-cylinder combustion and exhaust aftertreatment for both gasoline and diesel engines. "We've only just scratched the surface," says Tasch.—CAS