Layer by Layer
The parts in jet engines have to withstand staggering forces and temperatures, and they have to be as light as possible to save on fuel. That means it’s complex and costly to make them: technicians at General Electric weld together as many as 20 separate pieces of metal to achieve a shape that efficiently mixes fuel and air in a fuel injector. But for a new engine coming out next year, GE thinks it has a better way to make fuel injectors: by printing them.
To do it, a laser traces out the shape of the injector’s cross-section on a bed of cobalt-chrome powder, fusing the powder into solid form to build up the injector one ultrathin layer at a time. This promises to be less expensive than traditional manufacturing methods, and it should lead to a lighter part—which is to say a better one. The first parts will go into jet engines, says Prabhjot Singh, who runs a lab at GE that focuses on improving and applying this and similar 3-D printing processes. But, he adds, “there’s not a day we don’t hear from one of the other divisions at GE interested in using this technology.”
These innovations are at the forefront of a radical change in manufacturing technology that is especially appealing in advanced applications like aerospace and cars. The 3-D printing techniques won’t just make it more efficient to produce existing parts. They will also make it possible to produce things that weren’t even conceivable before—like parts with complex, scooped-out shapes that minimize weight without sacrificing strength. Unlike machining processes, which can leave up to 90 percent of the material on the floor, 3-D printing leaves virtually no waste—a huge consideration with expensive metals such as titanium. The technology could also reduce the need to store parts in inventory, because it’s just as easy to print another part—or an improved version of it—10 years after the first one was made. An automobile manufacturer receiving reports of a failure in a seat belt mechanism could have a reconfigured version on its way to dealers within days.
Additive manufacturing, as 3-D printing is also known, emerged in the mid-1980s after Charles Hull invented what he called stereolithography, in which the top layer of a pool of resin is hardened by an ultraviolet laser. Various methods of 3-D printing have become popular with engineers who want to create prototypes of new designs or make a few highly customized parts: they can make a 3-D blueprint of a part in a computer-assisted design program and then get a printer to spit it out hours later. This process avoids the up-front costs, long lead times, and design constraints of conventional high-volume manufacturing techniques like injection molding, casting, and stamping. But the technology has been adapted to only a limited set of materials, and there have been questions about quality control. Building parts this way has also been slow—it can take a day or more to do what traditional manufacturing can accomplish in minutes or hours. For these reasons, 3-D printing hasn’t been used for very large runs of production parts.
But now the technology is advancing far enough for production runs in niche markets such as medical devices. And it’s poised to break into several larger applications over the next several years. “We’ve come to the point when enough critical advances are happening to make the technology truly useful in manufacturing end-use parts,” says Tim Gornet, who runs the Rapid Prototyping Center at the University of Louisville.
Several techniques can be used to “print” a solid object layer by layer. In sintering, a thin layer of powdered metal or thermoplastic is exposed to a laser or electron beam that fuses the material into a solid in designated areas; then a new coating of powder is laid on top and the process repeated. Parts can also be built up with heated plastic or metal extruded or squirted through a nozzle that moves to create the shape of one layer, after which another layer is deposited directly on top, and so forth. In another 3-D printing method, glue is used to bind powders.
Aerospace companies are at the forefront of adopting the technology, because airplanes often need parts with complex geometries to meet tricky airflow and cooling requirements in jammed compartments. About 20,000 parts made by laser sintering are already flying in military and commercial aircraft made by Boeing, including 32 different components for its 787 Dreamliner planes, according to Terry Wohlers, a manufacturing consultant who specializes in additive processes. These aren’t items that have to be mass-produced; Boeing might make a few hundred of them all year. They’re also not critical to flight; among them are elaborately shaped air ducts needed for cooling, which previously had to be manufactured in multiple pieces. “Now we can optimize the design of these parts for weight, and we save material and labor,” says Mike Vander Wel, director of Boeing’s manufacturing technology strategy group. “In theory, this is the ultimate manufacturing method for us.” Though the speed limitations of 3-D printing might keep it from ever producing the majority of Boeing’s parts, Vander Wel says, the approach is likely to be used in a growing proportion of them.
Boeing’s main rival, the European Aeronautic Defense and Space Company (EADS), is using the technology to make titanium parts in satellites and hopes to use it for parts it makes in higher volume for Airbus planes. “We don’t yet know what the extent of our use of additive-layer manufacturing there will be yet, but we don’t see any show stoppers,” says Jon Meyer, who heads research on 3-D printing at EADS’s Innovation Works division in England.
GE’s jet engine division may be closer than anyone else to bringing 3-D-printed parts into large-scale commercial production. In addition to the fuel injector, GE is also laser-sintering titanium into complex shapes for four-foot-long strips bonded onto the leading edge of fan blades. These strips deflect debris and create more efficient airflow. Until now, each one has required tens of hours of forging and machining, during which 50 percent of the titanium was lost. By switching to 3-D printing, the company will save about $25,000 in labor and material in each engine, estimates Todd Rockstroh, the GE consulting engineer who heads the effort. The blade edge and the fuel injector will start appearing in engines as early as 2013, and they will be integrated into full-scale production runs in the thousands by about 2016.
Meanwhile, says Rockstroh, the company hopes to gain design flexibility by using 3-D printing for more parts. When it recently discovered that a stem in the fuel injector was subjected to excessive levels of heat stress, a redesigned version came out of the printer within a week. “Before, we would have had to redesign 20 different parts, with all the associated tooling,” says Rockstroh. “It might not have even been possible.” And using 3-D printing to corrugate the insides of some parts can reduce their weight by up to 70 percent, which can save an airline millions of gallons of fuel every year. That prospect has GE looking for ways to print everything from gearbox housings to control mechanisms. “We’re going on a major weight-reduction scavenger hunt next year,” Rockstroh says.
Automobiles could similarly benefit from lighter parts, and the University of Louisville’s Gornet notes that printing processes could cut the weight of valves, pistons, and fuel injectors by at least half. Some manufacturers of ultraluxury and high-performance cars, including Bentley and BMW, are already using 3-D printing for parts with production runs in the hundreds.
CHALLENGES TO OVERCOME
If it weren’t for the limitations of the technology, 3-D printing would already be much more broadly used. “Speeds are atrociously slow right now,” says GE’s Singh. Todd Grimm, who heads an additive-manufacturing consultancy in Edgewood, Kentucky, estimates that the time it takes to produce a part will have to improve as much as a hundredfold if 3-D printing is to compete directly with conventional manufacturing techniques in most applications. That won’t happen in the next few years.
Another problem: for now, only a handful of plastic and metal compounds can be used in 3-D printing. In laser sintering, for example, the material must be able to form a powder that will melt neatly when it is hit with a laser, and then solidify quickly. The compounds that meet the necessary criteria can cost 50 to 100 times as much by weight as the raw materials used in conventional manufacturing processes, partly because they’re in such low demand that they’re available only from small specialty suppliers.
As demand increases with new applications, however, supplier competition should pull prices down dramatically. And the list of available materials is slowly expanding. GE is trying to use ceramics, which would open up new possibilities in engines and medical devices, among other areas.
Simple experience, too, will do much to improve the technology. So far, manufacturers don’t have enough data to predict exactly how a part will turn out and how it will hold up, or how production variables—including temperature, choice of material, part shape, and cooling time—affect the results. That can be frustrating, says Singh: “3-D printing often ends up being a black art. A part is made out of thousands of layers, and each layer is a potential failure mode. We still don’t understand why a part comes out slightly differently on one machine than it does on another, or even on the same machine on a different day.” For example, the layering process tends to build up interlayer stresses in unpredictable ways, so that some parts end up distorted. Porosity can vary within parts as well, leading to concerns about fatigue or brittleness. That could be a big problem in aircraft engines or wing struts. “We know how to make the metals strong enough,” says Boeing’s Vander Wel. “But we worry about the unpredictability. Can we repeat a result to get 100 parts that are exactly the same? We’re not sure yet.”
Even with these challenges, time is on the side of 3-D printing, says Vander Wel, and not just because the processes are improving. Engineers are understandably reluctant to embrace a new technology for critical parts when their deadlines and reputations, not to mention the lives of people in airplanes, are at stake. “But younger designers are adapting more quickly,” he says. “They’re not so quick to say, ‘It can’t be built this way.’”
David H. Freedman, a science journalist based in Boston, wrote about optogenetics in the November/December 2010 issue of TR. His latest book is Wrong: Why Experts Keep Failing Us.
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