Materials Force Equipment Development

Manufacturing Engineering, Oct 2007 by Morey, Bruce

Layered composites in some MMCs present even more of a challenge, because the individual layers of composite, titanium, and or aluminum have different properties, and thus are machined in a different way. Approaching layered composites with a single tool is challenging, which in turn causes a bit of difficulty in creating a "single tool" solution to produce holes as a standard off-the-shelf product, says Sandvik's Standridge. "I think the whole tooling community is working on solutions to drill composites effectively," says Standridge. "This is true for metal-matrix as well as the traditional carbon fiber-epoxy composites used in aerospace."

Increasing demand is spurring new ways to create titanium alloys. Demanding applications are increasingly satisfied with metal-matrix composites. One company, Dynamet Technology Inc. (Burlington, MA), has advanced powder metallurgy to both meet demand for titanium alloys and titanium-based metal-matrix composites. They now offer powder titanium-alloy parts that are as fully dense and strong as parts made from traditional wrought material. In their CHIP process, Ti powder is blended, cold isostatically pressed, sintered in a vacuum furnace, and then finally hot isostatically pressed (HIP), according to Sue Abkowitz, vice president technology and operations at Dynamet Technology.

Most exciting about this process is its ability to produce metal-matrix composites just as easily as titanium alloys. Using the same CHIP process, elemental titanium powder added with alloy powders and ceramic particles creates an MMC they call CermeTi, for CeramicMetal-Titanium.

Published data for a version using titanium-carbide reinforcement shows a 13% increase in yield strength and a 15% increase in elastic modulus over Ti6A4V alone. "More importantly for many applications, increased wear resistance is the key property provided by the carbide particles in the matrix," explains Abkowitz. The improved wear resistance and higher modulus makes the properties more like steel, while retaining the much lighter weight and corrosion resistance of titanium. Abkowitz says applications for CermeTi include aerospace parts, sporting goods, medical implants, and industrial tooling, including shot sleeve liners for aluminum die casting, where the high-temperature erosion resistance is especially useful.

CHIP is a near-net shape process-finish machining is required to complete the part, whether composite or pure alloy. As with composites in general, machinability is a concern. "There are some issues machining this material, but the good news is that for anyone already familiac; with machining titanium, the leap to CermeTi MMC is not great," explains Abkowitz. "It acts more like a metal than a composite material." This is because the reinforcement volume is not high and is a particulate rather than fiber.

Bruce Morey

Contributing Editor