MIT researchers use jet technology to make stronger more uniform aluminium alloys

Aluminium is a ubiquitous part of our modern age, but it’s hard to produce alloys for it without putting up with significant waste from bad mixtures. Over the past three years, Massachusetts Institute of Technology (MIT) assistant professor of metallurgy Antoine Allanore and his student Samuel R. Wagstaff have been studying how aluminium alloys harden and have come up with a ways to use jets to produce more even distributions of copper and manganese in castings.

Industrial aluminium slabs are typically produced by blending small amounts of copper or manganese in a reservoir of molten aluminium that is rapidly cooled, a process known as direct-chill casting. It is very effective process, though it may have some patches often aren’t visible on the surface of the ingot, resulting weaker slabs of cast metal. This can mean large amounts of aluminium being relegated to the scrap pile, which in turn slows down production and hinders the making of large slabs for trucks and airplane wings.

The researchers 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.

Allanore explained “We have now tested the technology all along the supply chain, and we did confirm that the 20 percent improvement in macrosegregation index was good enough to allow further increase in productivity,”


He further stated,"Analyzing the structure, and in particular the presence of solid grains, formed as the aluminium alloy turns from liquid to solid is difficult because you cannot see through aluminum, and the material is rapidly cooled from 700º C (1, 292° F), and differently sized grains are moving as the aluminium solidifies at the rate of about two to three inches per minute."

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.

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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 per cent could be achieved. The jet stirring can also improve recycling.

Allanore says. "So when it comes to society being able to recycle and make new aluminium 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."