New technique makes die castings stronger, lighter, and “greener”

Engineers at the Commonwealth Scientific and Industrial Research Organization (CSIRO), the national government body for scientific research in Australia, have developed what they call a “green” die-casting technology. Suitable for aluminum and magnesium alloys, the technology reduces the amount of metal it takes to cast a part and, hence, cuts melting and recycling costs, says CSIRO research program leader Robert O'Donnell. “The technology takes a different approach to traditional high-pressure die casting (HPDC) using an innovative runner design to modify the melt consistency prior to entering the die.”According to O'Donnell, the approach eliminates anywhere from a few ounces of metal in small castings to three or more pounds in large castings by using a smaller runner mass. (Runners are the tunnels along which molten metal flows into the die.) “Some castings have runners that represent the thickest — and therefore the slowest cooling — section of the shot,” he says. “Where the solidification of the runner dictates cycle time, smaller runner masses will reduce cycle time. In addition, the new technique makes stronger parts than traditional HPDC. Some parts are heat-treatable, so the runner technology permits the redesign of components for thinner sections, which cuts part weight and further reduces metal usage.” “The feed system goes against conventional design wisdom because it includes a constriction in the runner with a smaller cross-sectional area than the gate,” he says. (Gates are the relatively narrow openings to the die cavity.) “The metal accelerates through this constriction, undergoing extensive shear during its passage. This results in the shearing of cold flakes, gas bubbles, pre-solidified grains, and any other entrained entities into much finer fragments. The metal thus flows into the cavity better, and produces castings with finer microstructure and greater integrity.” Additionally, in many castings, the technique eliminates the need for overflow openings on the exit side of the die, which further reduces shot weight. “For existing dies, we typically design the smaller ATM runner to fit inside the existing runner. In fact, die casters can try ATM with a simply fabricated, removable constriction that fits neatly in an existing runner, producing a number of shots that won’t interrupt production by more than minutes at a time,” says O'Donnell.