Churning toward stronger, more uniform aluminum casts

  • 20-Dec-2016 12:03 EST
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Solidified cross-sections from aluminum ingots were made using a conventional method (top) and a new jet metal process developed by MIT researchers. “The introduction of the jet induced a completely different recirculation of the grains, and therefore you get different microstructure all along the section,” said MIT Assistant Professor Antoine Allanore. (All images MIT)

Macrosegregation is the uneven distribution of alloying elements within a solidified aluminum part, creating, for example, copper-poor regions. This is most likely to occur in the center of a casting, where it remains hidden until the casting is reprocessed for another use such as rolling a thick slab into a flat sheet. These unbalanced structures can form on a scale from several fractions of an inch to several yards and they can lead to cracking, shearing, or other mechanical failure of the material.

This issue is thought to be particularly significant as industry moves toward faster production schedules and larger sheet metal runs—for example, parts for pickup trucks and airplane wings. Greater emphasis on aluminum recycling also poses issues where the composition of secondary elements may be unpredictable.

“Analyzing the structure, and in particular the presence of solid grains, formed as the aluminum alloy turns from liquid to solid is difficult because you cannot see through aluminum, the material is rapidly cooled from 700ºC, and differently sized grains are moving as the aluminum solidifies at the rate of about 2 to 3 in/min,” said Antoine Allanore, Assistant Professor of Metallurgy at MIT.

The problem is typically a lack of the alloying element near the center of the solidifying slab or ingot.

“It’s a very perverse situation in the sense that from the outside the solid slab could look very nice, ready to go to the next treatment, and it’s only later on that you discover that there was this defect in a section, or in an area, which basically means a huge loss of productivity for the entire supply chain,” said Allanore.

Over the past three years, Allanore and his student, Samuel R. Wagstaff, were able to pinpoint a single number—the “macrosegregation index”—that quantifies the difference between the ideal chemical makeup and the actual chemical makeup at specific points in the solidification process.

“In our experiments, we did some specific tests at full scale to quench, so to basically sample the molten metal as it’s cast, and we’ve seen grains anywhere between 10 microns up to 50 microns, and those grains are, according to our development, the ones responsible for macrosegregation,” said Allanore.

Their solution is inserting a jet stream to recirculate the hot liquid so those grains get redistributed uniformly as opposed to accumulating in one region of the ingot.

“It’s like a hose of water in a swimming pool,” he said. “From a purely fluid mechanics perspective, the mixture is homogeneous. It’s just a full, complete mixture of the alloying elements and aluminum.”

“The introduction of the jet induced a completely different recirculation of the grains and therefore you get different microstructure, all along the section. It’s not just on the edges or not just in the center, it’s really across the entire section,” Allanore said. The researchers were able to calculate the optimal jet power needed for the most common aluminum alloys, and then tested their predictions.

Centering on metal work

Wagstaff finished his doctorate at MIT in September after three years and now works for Novelis in Sierre, Switzerland. A multi-generational, and successful, interest in metals led him to first working for Novelis at age 14. After he earned his bachelor’s degree in mechanical and aerospace engineering at Cornell University, Novelis offered Wagstaff the opportunity to pursue a PhD to help the company solve the problem of macrosegregation by developing a method to stir aluminum.

“The problem with aircraft-grade or aerospace-grade plate is you have very significant macrosegregation regions in the center of that plate, so you have drastic drops in mechanical properties in the very center,” said Wagstaff. “Our research started with the idea we want to be able to stop macrosegregation.”

“We knew we could figure out how to mix things up and we could stir things around, but being able to compare A to B to C would have been really difficult, and so that’s where the macrosegregation index came from. That’s just a numerical scheme that we invented to compare type A mixing to type B mixing to type C mixing, so then we can somehow relate all of the different mixing parameters together to say this kind of mixing is better,” he said.

The solution was to design a jet that would work with existing direct-chill casting machines.

“All we did was change the jet power as a function of diameter using a magnetic pump to control speed, power, and velocity of that jet throughout the casting,” said Wagstaff. “The great thing about jets is they are pretty well defined, we understand how they expand, how their forces are distributed as a function of time, as a function of space, so they are a relatively easy phenomenon to study. We ended up coupling magnets with the jet and built a noncontact magnetic pump to generate our jet.”

The team developed formulas to calculate how fast and how strong the jet power has to be to prevent clustering of defects in the center for a given set of alloying elements and mold dimensions. While officially the researchers report improvement of 20%, Wagstaff says with optimization of the jet pump, improvement up to 60% is possible.

Grain formation

According to Carolyn Joseph, a grad student in Allanore's group, small variations in individual grains, or microsegregation, can sometimes be healed by reheating the aluminum casting, but when large-scale uneven distribution occurs with a weak centerline, it is impractical because it would take far too long for the copper or other alloying element to migrate through the material.

Using the new jet stirring technique, she takes samples during casting near the two-phase region (slurry), in which grains of solid metal circulate in the liquid aluminum. She does this by rapidly cooling the metal at various locations along the ingot as it is being formed, and she studies the samples under a microscope for differences in grain size, shape, composition, and distribution.

“The size of your solid structure, how fine or coarse it is, depends on the rate at which you’re cooling it,” said Joseph. Microscopic images she made of samples showing large grain structures are evidence those grains were solid in the slurry before it was rapidly cooled.

“In the liquid, they are mixed, the cooper and aluminum form a solution, but when you go from liquid to solid, there is segregation of the alloying elements,” said Joseph. Grains that form early are depleted in copper and tend to cluster in the center of a slab.

“The advantage of this is that it’s an intermediate type of snapshot. Instead of looking at the final cross-section and studying its grain size and composition, we can see it at an intermediate stage, while it’s a semi-solid mixture, what’s going on,” said Joseph. “On the macroscale, you want an even distribution of copper, and that’s what Sam’s mixing has been able to achieve.”

Role in recycling

Allanore believes the jet-stirred aluminum process can also play a role in recycling.

“Not all recycled products of aluminum are the same, because some of them come from a former plane and some of them come from a former beverage can, and these are two different alloys,” he said.

“So when it comes to society being able to recycle and make new high-quality aluminum products, we can clearly see that there is an issue of how are we going to deal with those alloying elements.”

The work that Allanore has done is seen as one example of how researchers can modify existing technologies so that they become more ready to have more recycled material without compromising at the end with the quality of the product that you are making.

“By doing the proper amount of theoretical work and experimental work and working in collaboration, hand-in-hand with industry, we can find these type of solutions that allow higher productivity and more recycled materials, which means less energy and less environmental impact, something very exciting,” said Allanore.

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