A new method for mixing aluminum with copper and manganese significantly improves distribution of alloying elements in cast aluminum parts.

The improved process could help reduce defects and manufacturing waste, and enhance productivity. Massachusetts Institute of Technology (MIT) Assistant Professor of Metallurgy Antoine Allanore and PhD student Samuel Wagstaff announced experimental results and their theoretical explanations in a series of journal articles.

Continuous aluminum casting line. Credit: Wikimedia CommonsDirect-chill casting, the standard process for casting aluminum slabs, involves mixing alloying elements, generally copper or manganese, into liquid aluminum. The mixture is rapidly cooled. Quality problems crop up when the additives do not distribute evenly, resulting in weaknesses in the casting. Defective casts that end up on the scrap heap can eat into profits and slow down production.

Allanore and Wagstaff developed a system for classifying the distribution of alloying solutes as well as a novel mixing mechanism. Macrosegregation refers to the tendency of the solutes to cluster in portions of a casting, rather than to distribute evenly through the piece. Most often these clusters form in the center of the casting. Determining where the clusters occur is difficult or impossible; the answer might come only when a defect leads to mechanical failure.

By developing a “macrosegratation index” – a measure of alloying element distribution – the team had a standard yardstick to use to compare the effectiveness of mixing processes. Lower index numbers indicate samples that approximate a theoretical ideal mixture and therefore are higher quality.

Another MIT researcher, Materials Scientist Carolyn Joseph, tested an alloy containing 4.5% copper at different stages of cooling. Studying castings at intermediate stages of cooling allowed her to see changes in grain composition and location that resulted from different mixing processes.

Since alloying elements tend to cluster in a casting’s center, Allanore and Wagstaff concentrated on preventing these specific defects. They opted for a jet that would work with existing direct-chill casting machines. Using a magnetic pump, the team controlled jet power as a function of diameter.

The results of Joseph’s analyses showed the researchers which jet settings reduced central clustering the most effectively, improving alloying element distribution up to 20%. Wagstaff believes that an up to 60% improvement is achievable with pump optimization.

Improving the quality of castings made from “virgin” aluminum has obvious benefits for industry and for customers who expect high quality products. Allanore sees a place for this new stirring method in processing recycled aluminum. A pot of recycled metal can contain everything from a soda can to airplane parts, each made from different alloys. More effective mixing could result in better-quality products cast from recycled aluminum.

Allanore and Wagstaff conducted their research at the aluminum products company Novelis Solatens Technology Center in Spokane, Wash. “By doing the proper amount of theoretical work and experimental work and working in collaboration, hand-in-hand with industry, we can find these types of solutions that allow higher productivity, more recycled materials which means less energy and less environmental impact, something very exciting,” Allanore says.