Aluminum recycling has become a successful business since its inception a century ago. Nearly a third of the aluminum produced in the United States is made from aluminum scraps that have been recycled in a process—usually remelting—that uses only 5 to 10 percent of the energy it takes to extract aluminum from mined bauxite ore.

But there are limitations to the remelting of aluminum, says Wojciech Misiolek, the director of Lehigh's Institute for Metal Forming and the Loewy Chair in Materials Forming and Processing.

Significant metal losses can occur when aluminum is remelted. Machine chips and other fine pieces of scrap are difficult to remelt. Contamination and impurities can cause the mechanical properties of recycled aluminum, especially its strength and ductility, to be inferior to those of pure aluminum. And remelting itself is energy-intensive.

In an effort to make aluminum production a more sustainable endeavor, materials scientists are investigating solid state recycling of aluminum chips by extrusion, which promises higher metal yield and lower heat use—and therefore energy consumption—than conventional recycling based on remelting.

The role of the extrusion die

A group that includes researchers from Lehigh and the Technical University (TU) of Dortmund in Germany has found that using special extrusion dies improves the mechanical properties of recycled aluminum by allowing more strain and shearing during metal flow.

Using transmission electron microscopy (TEM), the group is comparing the evolution of the microstructure, including the size and distribution of critical grain boundaries, of aluminum that is extruded through three different types of dies.

In the past year, the group has reported its results twice in Materials Science and Engineering A and once in the CIRP Annals of Manufacturing Technology. Both journals are published by Elsevier.

In its experiments, the group processed machined aluminum chips into a cylindrically shaped billet and extruded the billet, pushing it through a die.

As aluminum flows through a die, it deforms, changing in shape and size. Shearing occurs as the rate of flow increases and decreases, causing the fracture of an oxide layer, which covers each chip and which can act as a barrier that prevents bonding.