During the past several years, a variety of vehicle components have been made lighter-weight thanks to weight-saving materials and designs. However, the automotive industry has yet to find a viable, cost-competitive alternative for heavy cast iron brake rotors. Automotive researchers know that much lighter-weight metal matrix composites (MMCs), such as those made of aluminum, could replace heavier brake assembly components in certain vehicles. In fact, a 50 percent reduction in mass of these brake components could save consumers 0.25 to 0.35 miles per gallon in a mid-sized car. MMCs also can favorably impact the life cycle of brake systems - for example, a 50 percent increase in wear life can double the time between rotor change-outs and reduce vehicle downtime as well as particle emissions to the environment.
Now researchers from Pacific Northwest National Laboratory (PNNL) are collaborating with industry partner Arconic to provide an alternative reinforcement to SiC. Titanium diboride - or TiB2 - has shown finer particle size, better homogeneity, and possible lower costs when paired with aluminum. Their goal is to reduce the weight of brake rotors by greater than 50 percent, while improving performance, wear life, and life cycle. The team is taking a multi-step approach in this two-year project, beginning with production of a 50-volume-percent reinforced master alloy - liquid aluminum infiltration of a TiB2 preform - at the Arconic Technology Center. The team will then use PNNL stir-casting technology and equipment to cast four MMC billets from the Al-TiB2 master alloy and an A-356 aluminum block, followed by machining the billets into subscale rotors. The team will also perform material characterization and wear testing, pairing the rotors with several different pad compositions to determine effect and optimal friction. Lastly, the resulting rotors and pads will be weighed and measured to establish mass gain or loss - culminating in determination of overall feasibility for using TiB2 as a reinforcement for Al MMC brake rotors.
During the project’s first year, the team fabricated the master alloy - constituting approximately 50 percent aluminum and 50 percent TiB2 based on microstructure and density measurements. The master alloy and A356 block were sectioned and combined in a crucible at ratios to produce 5 vol% reinforced melts. A series of stir casting trials helped to establish the necessary mixing times, melt temperatures, mixing paddle geometry, and mixing paddle speeds. The melts were poured into open steel book molds, producing rectangular castings approximately 10-inch by 7 inch by 1 inch. While these castings revealed minor (less than 5 percent) porosity, the team determined that casting of industry components would be via squeeze casting or high-pressure die casting, where porosity would be minimal. This year, the team will continue casting trials to produce the MMCs at a higher particle loading of 10 – 15 vol%. They will then perform machining of the subscale rotors and the wear testing. PNNL uses a rapid mixing and stir-casting technology, developed under a previous DOE-sponsored project, to efficiently combine the ceramic TiB2 particles and molten aluminum together into a composite metal. The process takes only a fraction of the time to manufacture a product compared to other commercial processes - resulting in reduced labor and product costs.
By combining two novel technologies - lower cost TiB2 reinforcement particles and rapid stir-casting techniques - PNNL and its material supply partner are hoping to make available a high-performance, stronger and stiff, yet lightweight, material option for future chassis component designs.