Aluminum is ubiquitous in our modern age, but it's surprisingly hard to produce alloys for it without putting up with significant waste from bad mixtures. MIT researchers Antoine Allanore and Samuel R. Wagstaff have been studying how aluminum alloys harden and have come up with a way to use jets to produce more even distributions of copper and manganese in castings.

Direct-chill casting is a semicontinuous way of producing aluminum alloy ingots by quickly cooling the molten metal as it's poured into chilled molds. It's a very effective process, but according to Allanore and Wagstaff it leaves something to be desired in terms of quality control. Though an ingot may look perfectly sound on the outside, it may have patches high or low in copper or manganese ranging in size from inches to feet, resulting in weaker slabs of cast metal. This can mean large amounts of aluminum being relegated to the scrap pile.

The problem is these patches often aren't visible on the surface of an ingot because they tend to form in the center of the casting. This means they remain hidden until the casting is rolled. This slows down production and hinders the making of large slabs for trucks or airplane wings.

A scanning electron microscope image shows large-grain structures contrasting with finer-grain structures in a sample taken during a direct-chill aluminum alloy casting(Credit: MIT)

To produce a more uniform casting, the MIT researchers turned to the macrosegregation index, which is a way of measuring how much an actual mixture differs from an ideal chemical makeup. By manipulating this, the alloy mix can be made 20 percent more even.

"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, and the material is rapidly cooled from 700º C (1,292⁰ F), and differently sized grains are moving as the aluminum solidifies at the rate of about two to three inches per minute," Allanore says. "The problem is typically a lack of the alloying element near the center of the solidifying slab or ingot."

To gain more control, Allancore and Wagstaff took samples of the molten metal at various times as it was cast and studied the formation and migration of grains. It's this process that causes the macrosegregation, so the researchers tried to prevent this using a jet stream to circulate the metal to keep the alloy homogeneous. This also caused the grains to move about and changed the hardened metal's microstructure, so it's more uniform throughout the cross section.

"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," Wagstaff says. "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 non-contact magnetic pump to generate our jet."

As part of this, the team came up with a numeric index that helped them to gauge how far a sample was deviating from an ideal mix. Combined with the jet, they say an improvement of up to 60 percent it possible. The jet stirring can also improve 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," Allanore says. "So when it comes to society being able to recycle and make new aluminum products of high-quality, we can clearly see that there is an issue of how are we going to deal with those alloying elements. The work that we have done, I believe, is one example of how we 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."

The research was published in Metallurgical and Materials Transactions B.

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