Aluminum matrix composites seem like they should be more popular. Made from reinforcing particles or fibers encapsulated in a matrix of aluminum, these materials tend to weigh less and have better strength, stiffness, and dimensional stability than traditional aluminum alloys and, sometimes, even steel. They should be a natural fit for engine components and other fast-moving parts that see high stresses or have precision clearances. "Things with inertial issues can always benefit from being lighter and smaller," points out Bill Satzer, technical manager for 3M's metal matrix composites lab in St. Paul, MN. Combining mass reduction with increased stiffness, meanwhile, can also help improve damping characteristics and fight NVH. For all their potential, though, these materials have traditionally cost too much for all but a few niche applications in auto racing and aerospace, "The big bugaboo has been cost," admits Dr. William Harrigan, president of MMC Engineering Inc, a developer of metal matrix composite technology in North Ridge, CA. "The costs have traditionally been at least double what automotive OEMs want." This substantial cost barrier still lingers, but it now promises to come down a bit thanks to recent technology improvements.
A rounder alumina
MMC Engineering, for one, recently developed new aluminum composites that highlight some of the progress in discontinuously reinforced materials—those with particle or whisker reinforcements. The company's latest materials feature aluminum oxide reinforcements in 7000 Series aluminums. MMC fabricates these materials using powder metal methods, which can turn out near net shape parts and reduce secondary machining operations. And Harrigan adds that powder metal parts usually have lower residual stresses than parts produced through casting or other liquid-state processes. "Powder metal is more expensive than casting," he says. "It also produces parts with better mechanical properties." But it's the reinforcement itself that really sets the MMC Engineering's material apart.
Aluminum oxide has long been seen as a low-cost alternative to the pricey boron carbide and silicon carbide reinforcements that have made their way into jet, Formula One, NASCAR, and motorcycle racing engines over the years. "But efforts to develop a high-strength composite with alumina as a reinforcement have been unsuccessful until now," Harrigan says, explaining that previous alumina reinforcements in powder metal tend to produce an undesirable microstructure that ultimately limits strength.
MMC Engineering's latest composites, however, use a unique form of alumina from Alex Ventures (Glenwood Springs, CO). Called Alumina EX, this reinforcement has a spherical shape rather than the sharp-cornered, irregularly-flat shape of traditional alumina, boron carbide, and silicon carbide reinforcements. Without getting too bogged down in the metallurgical details, Harrigan contends that the spherical shape prevents the microstructure problems found with traditional alumina. Tensile strength improvements result. In tests conducted by Harrigan, for example, composites made from Alumina EX and 7000 Series aluminum generated yield strengths in excess of 550 MPa—strength levels comparable to materials reinforced with silicon carbide and boron carbide. The only property trade-off associated with Alumina EX is a slightly lower elastic modulus than composites made with silicon and boron carbide. "But that can be easily designed around," Harrigan says.
With mechanical properties in the same ballpark as previous composites, the prime advantage of materials reinforced with spherical alumina may be the contribution to cost savings. Harrigan estimates that the price of silicon carbide and boron carbide aluminum composites represents "way less than half" of the cost of using these materials. Machining costs make up most of the remaining cost, which should come as no surprise given that high-strength aluminum composites normally need to be machined with diamond tools. Composites using the spherical alumina, by contrast, can be machined with conventional carbide tools, and "have a machineability similar to 4000 Series aluminum," Harrigan reports.
As for the ability to drop aluminum matrix composites into assemblies with other materials, Harrigan notes that composites technology enables predictable reductions in the thermal expansion through adjustments in the amount of reinforcements. For example, he says, a typical aluminum CTE of 23 ppm/° C can be reduced to as little as 12 ppm/° C for composites with a high volume of reinforcements. "Realistically, we're usually talking 16-17 ppm/ deg C.," he says. "We can match the CTE of steel in most cases."
Getting some fiber
Aluminum composites reinforced with continuous fibers haven't really taken off either because of their own high prices and unfamiliar design characteristics. But the materials scientists at 3M have plans to change that situation with a growing line-up of composites based on Nextel ceramic oxide fibers within various aluminum alloys. These materials typically have about three times the stiffness of conventional 7000 Series aluminum and weigh at least 40% less than steel, according to Satzer. In a properly designed part, they can beat steel's strength and stiffness too. These composites have been around for a few years and still cost several dollars per pound more than the ferrous materials they usually compete against. Yet their price-performance picture continues to change for the better.
One reason is that MMC parts now lend themselves to selective reinforcement with the help of finite element analysis. "Finite element analysis lets us put the fibers in high-stress areas where they can do the most good," Satzer says. Closely matching the amount and location of costly reinforcements to the actual application requirements can potentially knock down the material's direct cost, but reduced machining costs promise the greater savings: For those MMC parts that can't be cast net shape—and many can—selective reinforcement can keep the fibers away from areas that need machining. Another of 3M's cost-reducing strategies has been making MMC parts easier to manufacture. "Our general approach is not to invent new casting processes for our materials but to make our materials adapt to existing processes," says John Skildum, 3M's business development manager for metal matrix composites. To make MMC technology easier to drop into established casting operations, 3M plans to offer pre-cut, reinforcement preforms in standard shapes and sizes.
These cost-limiting factors have already started to help aluminum composites shine in those applications where mechanical performance trumps material price. In racing engines, for instance, the aluminum composites have seen use in valve train components—including push rods for Buell racing motorcycles—where low mass and stiffness translate to faster, more efficient valve operation, especially as the RPM climbs. "That ultimately means more power," Skildum says. A more mundane, but potentially much larger, application for aluminum composites may turn up elsewhere on the car. 3M and an unnamed brake supplier have been pursuing selectively reinforced aluminum composites as a replacement for cast iron in brake calipers. "The technical argument is very compelling," says Satzer. Aluminum composite calipers weigh 50% less than cast iron. At the same time, the composite has enough strength and stiffness to replace cast iron without an increase in part size. "It fits in the same package size as the cast iron calipers," Satzer says.
One remaining barrier that continuously reinforced aluminum composites will have to overcome has less to do with money than understanding. Satzer often has to remind engineers that these materials are anisotropic—with strength and stiffness properties that differ in the longitudinal and transverse directions. One of 3M's materials, for example, has a tensile strength of 240 ksi and a transverse strength of just 20 ksi. "Teaching people to design with anisotropic material has been one of the biggest issues we've encountered so far," he says.