Solidifying New Alliances

What's old is new again. Metal casting, an integral step in every type of metal processing and manufacturing operation, dates to the Bronze Age when early man began pouring copper and tin allows. While the art behind metal casting is age-old, the science is still being investigated, and the applications are ever changing. These range from precision shape casting of single-crystal turbine blades to advanced remelting and casting of ingots for production of all wrought-rolled, extruded, and forged-alloys.

Advances in metal casting are leading to new levels of electrical, magnetic, chemical, and structural performance. They may also be more environment-ally friendly. With lighter metals, for example, lighter cars can be manufactured. The weight savings, in turn, will lead to reduced energy usage and a smaller environmental footprint. Researchers at Purdue's new Center for Metal Casting Research (PCMC) are drawing on the long-standing traditions of their discipline to help shape the future.

The center was formed in response to what Keith Bowman, head of the School of Materials Engineering, sees as an emerging area of need. "We recognize that there is an opportunity to apply many advances in materials science to the challenges of metal casting, including new computational tools and advanced experimental equipment," he says.

Introducing the Center for Metal Casting Research

David Johnson plays with fire on a daily basis. Going back to the basics in his metals research, he stands in front of a zone-melting furnace solidifying ingots of metal in search of stronger, but less brittle, high-temperature materials. This focus on metallurgy is where the discipline of materials engineering got its start in the early 1900s, and it remains an evolving area with a rich future.

Johnson, an associate professor of materials engineering, is a key player in the newly established PCMC, which will conduct interdisciplinary, industry- driven research and education programs to advance metal casting practices and develop new commercial solidification processes. Researchers will initially focus on four areas: lightweight shape castings; solidification processing of wrought alloys; crystal growth and investment casting of superalloys and intermetallics; and specialty alloys and processes. Johnson is involved in the crystal growth area, undertaking zone melting with equipment that can form complex titanium aluminide intermetallics for aerospace applications, as well as nickel and cobalt-based superalloys.

Lighter materials, Greater energy savings

Qingyou Han, an associate professor of mechanical engineering technology with a courtesy appointment in materials engineering, will lend his expertise in developing lighter metal parts for the automotive industry to the new center by developing die casting processes that can make car parts from heat-treatable aluminum alloys. He is collaborating with materials engineering researchers Kevin Trumble, the new center's director, and Matthew Krane on the development of ultrasound assisted die-casting technologies to make lightweight metal castings with high internal integrity and that are pressure-tight, leak-proof and heat-treatable.

Die casting involves injecting molten metal into a steel die. It is more cost-effective and less energy intensive than sand casting. One of the goals of the research is to achieve a 30 percent increase in metal yield per mold by replacing sand castings with high-quality die castings. The benefits would trickle down from the metal casting industry to the automotive industry to the consumer to the environment. Johnson and Krane are collaborating on a project associated with the investment casting of TiAl alloys to produce aligned lamellar, two-phase microstructures that exhibit very low creep-strain rates at high temperatures, thereby extending the operating envelope of the engine or the lifetime of the parts. In particular, they are investigating crystal growth of TiAl alloys through ceramic performs with the goal of understanding the initial transients in the dendritic growth patterns and solid-liquid composition fields so that larger processing windows for potential single-crystal growth can be developed. The project fits well with Johnson's primary interest—the development of high-temperature structural materials, especially those based on intermetallic alloys. Among its

applications is the aerospace industry and its need for jet engine materials that can withstand heat and oxidation and perform with dimensional stability.

One center, multiple approaches

All of the Purdue Engineering researchers working in metallurgy are driven by the end goal of producing better and stronger materials and optimizing processes; each will bring different talents to the new center.

Krane, an associate professor in materials engineering with a courtesy appointment in mechanical engineering, studies materials processing and solidification at the micro- and macroscale and

focuses on the prediction of macroscopic transport phenomena as well as microstructure development during the processing of materials. The morphology, the crystal structure, and the freezing/melting behavior of the dendritic structures and the mass diffusion fields around them have significant influence on the properties of the final cast part. The goal is to understand how to produce microstructures that improve metal quality and to pass this learning on to industry.

One of Krane's projects, for example, looks at how to prevent defects in ingots. This research examines vacuum arc remelting, a semi-continuous process widely used to improve the cleanliness

and refine the structure of ingots of specialty steels, superalloys, and titanium-based alloys. The process is performed in vacuum via melting the consumable electrode and collecting the solidifying metal in a water-cooled copper crucible. The heat required for remelting is released by an electric arc between the consumable electrode and liquid metal. The final internal structure of the ingot is greatly influenced by fluid motion in the liquid metal, which is driven by electromagnetic and buoyancy forces. The interaction of the flow and the solidifying metal results in ingot scale macrosegregation, which is a serious defect that compromises the quality of the produced ingots.

To study these types of problems, Krane employs both numerical simulation and bench-top experiments in his lab, as well as in-plant tests on location with industry partners. His work, and that of his colleagues, is designed to "diagnose problems in processing and suggest solutions."

Allying with industry

In the last century, the discipline of what is now commonly known as materials engineering has grown from its natal roots in metallurgy to embrace ceramics and polymers and emerging materials. Those who choose to focus on metals point to the myriad applications of their chosen material—from the automotive industry to biomedical processing and its manufacture of artificial hip joints to the demanding needs of the international steel industry.

"Some people think metallurgy is old hat, that nothing interesting is going on. There is a lot of very interesting science going on in metallurgy and its practical applications," says Krane, who on a recent Friday morning could be found on the phone talking new science with engineers at an Indiana steel company. The new PCMC is ideally positioned to partner with metal processing industries. With global economies contracting, it is ever more important for companies to improve competitiveness through increased yield and quality and decreased costs. The center can provide companies a partner for either pre-competitive or proprietary research in the science and engineering of metal casting processes, which is usually much cheaper than doing research in-house. The combination of experimental and numerical capabilities and expertise enables the center to tackle a wide range of such problems. When the current economic difficulties recede, the center will be well positioned as a source of new employees for industry and a source of expertise to improve processes and products. Target industries include everything from jet engine manufacturers and their part and materials suppliers, producers of metal products for the energy production sector, transportation industries, and the makers of biomedical implants.  "We're trying to cast a broad net with the center," Krane says. "We want to provide a place where we can look at the science that connects the processes and helps people with applications of the process."

– Linda Thomas Terhune