The military truck industry relies upon iron and steel components that are hardy enough to withstand blasts on the battlefield. But the strength of such parts comes with a tradeoff; the dense materials weigh down vehicles making them less maneuverable than desired.
Manufacturers for years have been seeking lighter materials that offer the same protective qualities of steel and iron. Many of them are turning to aluminum and magnesium alloys that are reinforced with small particles for strength. But these composites require expensive and complicated tooling processes to cast or machine the material into parts.
A team of researchers at the University of Wisconsin-Madison has come up with an experimental casting technique that doubles the strength of alloys by incorporating tiny ceramic particles into the molten metals, or melts. The new manufacturing process could open the doors for commercial-scale production of metal matrix nanocomposite components in the coming decade.
Nanoparticles are widely used in polymers and plastics, but are rarely mixed in with metals from a casting standpoint. When added to molten metals, nanoparticles tend to clump together. Dispersing them evenly throughout the mixture strengthens the cast metal. But it is difficult to separate the particles, which are smaller than blood cells.
“One of the biggest problems in the nanotechnology field is dispersion of the nanoparticles,” says Xiaochun Li, a materials engineering professor at the university.
He and his team discovered that employing high-intensity ultrasonic waves — a process called cavitation — inside the molten metal solves that problem. The sound waves generate bubbles that expand and contract rapidly at 20,000 cycles per second. Eventually they burst, and create a huge shock wave, says Li, the lead investigator. The energy generated by that “micro nuclear bomb” is enough to disperse nanoparticle clusters evenly within the melt, he adds. So far, the team has been able to produce two-pound ingots.
The National Institute of Standards and Technology in January awarded $10.1 million to the team to continue the research. Wisconsin-based companies including aluminum casting specialist Eck Industries Inc., truck manufacturer Oshkosh Corp. and Houston, Texas-based Nanostructured & Amorphous Materials Inc. are contributing resources to the five-year project. The goal is to achieve mass production of lightweight, strong-performance aluminum and magnesium nanocomposites.
Oshkosh Corp. is committing $50,000 worth of engineers’ time. One of its contributions is a computer program that simulates casting. Based upon data inputs, the software will animate the pouring of the molten metal into a sand mold and will predict any defects in the resulting casting.
Adding particles to castings can make a metal less fluid and more difficult to pour into a mold, says Bob Hathaway, vice president of the materials engineering group. The computer program will model that effect to predict how the various melts fall into the mold.
“We’re going to be doing a lot of modeling and simulation upfront to determine what’s going to work,” he says.
At Oshkosh’s failure analysis lab, engineers will use a scanning electron microscope to analyze the samples.
The intent is to determine which alloys in the aluminum and magnesium families are better suited for nanoparticles. “If we’re successful right out of the chute, then we’d spend a lot of time looking at those alloy systems,” says Hathaway.
Foundries currently depend upon a method called stir casting to make metal matrix composites. In this process, a metal melts in a furnace. Salt grain-sized particles are stirred in and the molten mixture is poured into a mold. The particles disperse homogeneously. The problem with this process arrives after casting. If the component needs holes for bolts, for example, an expensive drilling tool is required because conventional machines cannot cut into them.
The cavitation process would allow production foundries to use their current equipment, researchers say. Ultrasonic processing equipment can be readily incorporated into the melting equipment, says David Weiss, vice president of sales and engineering at Eck Industries.
“We believe that this ultrasonic cavitation technique will be applicable to a broader class of casting than pre-form technologies,” he says. Castings can be selectively reinforced by placing small particles into the mold before molten metal is added, he says. When these “pre-forms” are used, the molten metal must be forced into the mold in a pressurized “squeeze-casting” process. Non-reinforced areas can be machined normally, but the price per part becomes prohibitively expensive.
Nanocomposites will have a huge impact on the military vehicle industry, officials say. Oshkosh manufactures the mine-resistant ambush-protected all terrain vehicle for the Defense Department. The truck, which is being driven by troops in Afghanistan, has an independent suspension with upper control arms made out of cast iron.
Hathaway points out that if the cast iron could be swapped out with an aluminum nanocomposite down the road, the part’s weight would be cut by half. “That increases payload capacity and also keeps the vehicle within its weight target,” he says.