Sumanth Shankar: Innovative Solutions in Aluminum Casting (October 2005)

Innovative Solutions in Aluminum Casting
A Focus for New Orlick Research Chair

    The bare walls of his office and the still-empty bookshelves bear testimony to Dr. Sumanth Shankar’s recent arrival at McMaster. He hasn’t yet had time to unpack his books, but already the newly appointed Braley-Orlick Chair in Advanced Manufacturing has some big plans for his work at the MMRI. An expert in the solidification processing, casting and heat treating of aluminum and other light metals, his interests neatly mesh with the research agendas of several other recent recruits to the Faculty of Engineering, which is acquiring a growing reputation for its expertise in metallurgy, casting, heat treating and metal forming.
    Dr. Shankar brings to McMaster his experience as a research scientist at Worcester Polytechnic Institute (WPI) in Massachusetts, where he was involved in several projects working with aluminum alloys and was the co-inventor of a technique that enables the casting of aluminum-based wrought alloys. Dr. Shankar’s research interest solidification processing focuses on the process by which metals and metallic alloys transition from liquid to a solid state. Understanding how this happens has practical importance in manufacturing because the desired physical properties of the final cast part depend not only on the chemical composition of the alloy, but also the process by which the metal solidifies.
    This process is particularly interesting to the automotive and aerospace industries, which would like to be able to cast aluminum alloys with higher strength to weight ratio and superior performance properties. Currently, there are a number of aluminum-based wrought alloys that have the superior physical properties manufacturers are looking for, but they cannot be successfully cast because of defects that develop during the solidification process.

Casting Wrought Alloys

The defects that arise in the casting of these alloys result from the way in which aluminum solidifies when it is poured into a mold. In the mold, the metal cools from the outside-in, as heat is extracted from the mold walls. As the metal cools (more quickly at its outer surface), a tree-like “dendritic” structure of solid-phase material forms in the microstructure of the metal, trapping liquid-phase material in closed spaces. It is those closed spaces that become the site of “holes” (defects) as the solidification process continues

Optical Micrographs of cast aluminum wrought alloy samples. Image at left is from a conventional cast sample showing dendrites of primary aluminum and image at right is from a sample cast using CDS method showing globular/non-dendritic primary aluminum phase.

While at WPI, Dr. Shankar and his colleagues developed a novel approach to casting that disrupts the formation of this dendritic structure, replacing it with a more uniform globular microstructure where liquid phase metal is not trapped and defects do not form. This process, called Controlled Diffusion Solidification (CDS), involves mixing two streams of liquid metal of different compositions and temperatures. The violent mixing process and the modified temperature gradient within themold results in a non-dendritic microstructure that is free of the defects traditionally associated with the casting of wrought alloys.
The “Eureka” moment for Dr. Shankar and his colleagues was when they realized it might be possible to modify the casting process so that the metal cools from the inside-out, rather than the outside-in. By refusing to accept the constraints of traditional casting techniques they were able to accomplish what had previously been impossible. As a result, aluminum and other light-metal alloys may now find many new applications in important areas of manufacturing.

Industry Support

Dr. Shankar’s pursuit of this promising line of research has the support of one of Canada’s leading auto parts manufacturers, Orlick Industries. Orlick is providing financial support and has donated a die casting machine and two industrial furnaces to Dr. Shankar’s lab, which will be used to continue his investigations into CDS and other related projects. Looking toward the future, Dr. Shankar would like to see a research centre established at McMaster that addresses solidification and thermal processing issues. Such a centre would have a cross-disciplinary character, drawing on the expertise of researchers in several engineering and physical sciences departments. Critical to this vision would be support from diverse industry sources, including primary metal producers, casters, heat treaters and manufacturers, all of whom share a community of interest in the outcome of the research.

For more information, please contact: Dr. Sumanth Shankar, Department of Mechanical Engineering, shankar@mcmaster.ca