Published online 17 June 1999 | Nature | doi:10.1038/news990617-2

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Machining metals by microwave

If you thought that microwave ovens were useful for nothing more than baking potatoes, think again. Reseachers at Pennsylvania State University report in the 17 June issue of Nature that they can bake up metal engine parts, too.

Dinesh Agrawal and colleagues show that a blast of microwaves will cook a powdered metal into a solid metal object. By filling moulds with the powder and treating it with a dose of microwaves, they were able to produce cast metal parts without any need for conventional melting and pouring. Those traditional images of a metalworking shop - a hellish forge of Vulcan with molten metal running in glowing rivers - might be a thing of the past.

The use of microwaves to process materials is nothing new, but it has not previously been possible to use it on metals. Ceramic powders are commonly formed into solid bodies using microwave energy to heat the grains, causing them to become welded together, a process called 'sintering'. Despite the intuitive association of ceramics with brittle, fragile artefacts, they include some of the hardest materials known. These high-performance ceramics, such as silicon carbide, are used for making components of jet engines, turbines, automobile turbochargers and cutting tools.

Microwave sintering is really little different from heating up a pie. The powder is placed in a microwave field, and the grains heat up as they absorb the radiation. This can raise the temperature to over a thousand degrees - hot enough that the atoms in the material become mobile, so that adjacent grains can 'flow' together. One advantage over conventional methods of heating is that the microwaves penetrate the entire sample and are absorbed throughout, rather than the heat being transferred gradually from outside inwards by conduction. This means that the energy is absorbed more efficiently, and the processing time can be short. Just a minute or so can be sufficient to sinter a ceramic powder into a solid mass of fused grains.

Microwave processing is now used for a huge variety of materials: it can be used to treat wood and textiles, to process food, to 'cure' plastics and even to instigate chemical reactions. But for metalworking, there's a problem: metals are reflective. Microwaves tend to bounce off them like light from a mirror. This has discouraged researchers from trying to use microwaves for forming metal objects (and explains why you should never put metal objects in your domestic microwave.)

But Agrawal and colleagues show that the pessimism was misplaced. Yes, a sheet of metal will reflect microwaves, but in powdered form it seems that metals are no longer so reflective. The researchers were able to make dense sintered bodies from powders of just about every metal they tried, including iron, steel, cooper, aluminium and nickel. Just a ten- to thirty-minute blast of microwaves was enough, and in several cases the microwave-sintered metals were denser and harder than those sintered by conventional heating.