MANUFACTURING
MAKING LIGHT WORKBlasting metal powder with lasers to make precision parts Fabricating precisely shaped metal components has always been a challenge for industry. Making tools and dies is especially difficult because they are built of very hard alloys, and machining is liable to introduce microscopic cracks and weak points. Now researchers at Sandia National Laboratories have developed a novel technique for fabricating highly accurate, complex parts directly out of powdered metal. The approach might not be limited to metals: the Sandia team believes its method could in time yield parts that seamlessly blend metals and ceramics in variable proportions. The technique is known as laser-engineered net shaping, or LENS. A continuous thin stream of finely powdered metal in argon gas is directed onto a working surface, where it is illuminated by an industrial-strength laser. The laser melts a few milligrams of the powder, so the molten material fuses onto the surface. The surface can be swiftly moved around in a horizontal plane by computer-driven actuators. Parts are built up by driving the working surface so as to overlay consecutive layers of metal in the desired pattern. In a few hours, for example, LENS can build a hollow bar of tool steel 20 centimeters long with a complex cross-section, a task that would be a much larger project using conventional approaches. Superalloys and even high-melting-point materials such as tungsten can be shaped with the technique. Some samples the Sandia group built in two hours would be "exceedingly difficult" to make with any other method, the workers maintain. Sandia is investigating LENS because of its potential to make components for weapons systems, but the approach has started to attract the interest of civilian industrial giants. Kodak has used LENS to fabricate dies at possibly lower cost than standard procedures, and 3M and other companies are also investigating its potential for making tools and dies. A key advantage, according to Sandia team leader Clinton L. Atwood, is that metal parts fabricated with LENS are "fully dense"--they contain essentially no detectable pores or cracks. Moreover, because only a tiny amount of material is molten at any instant, the melt cools in a fraction of a second. Eric M. Schlienger of Sandia notes that as a result, less soluble components of a molten alloy do not have time to separate out, which can occur in casting. The net effect is that LENS-made parts are stronger and harder than would be expected from standard material samples, and they do not shrink or warp, because they have little internal stress. The surfaces of LENS-made parts are about as smooth as those of cast parts, but that should be improvable, Atwood states. One experimental rig that employs a 300-watt laser lays down about a third of a cubic inch of material an hour. But the process is "very scalable," according to Atwood--larger systems can accurately deposit metal at a higher rate. The key to success, the team says, is ensuring very smooth delivery of the powder to the working area. Although most components of a LENS system are available commercially, the Sandia group had to build its own high-performance powder systems. Ceramics can also be built up in a layering process similar to that used in LENS, notes Sandia's Duane B. Dimos. Delivered in a supersonic jet, the particles melt on impact with the working surface without the need for a laser. Sandia researchers plan to combine ceramic deposition and LENS within the next three years. Parts made of variable metal-ceramic blends might then be possible. Various research groups and companies around the world are using powders in different ways to make complex parts. Some, for example, press polymer-coated metal powders into shape, then later heat the part to fuse the particles together. But Atwood's team may be unique in using pure metals to make finished parts directly. "In five to 10 years this will be very common in manufacturing," Atwood declares. --Tim Beardsley in Albuquerque, N.M. |