COLUMBUS, Ohio -- Researchers at Ohio State University have developed a fast new method to identify potential defects that may form in die-cast metal parts for cars, major appliances, toys and electrical components.
The new method analyzes part designs and locates spots where porosity or holes are likely to form during casting, weakening the material. It does so in seconds, unlike more complicated methods that may take hours or days.
R. Allen Miller, professor of industrial, welding and systems engineering, likened the new method to methods used to display computer-aided tomography (CAT) data in medicine.
“A CAT scan shows the difference in density of materials within a solid mass, like a person’s brain,” said Miller. “We used a similar principle to display open spaces -- the porosity that develops in a metal part during the casting process.”
Miller explained that when hot liquid metal flows into a die, the material closest to the outside cools first and forms a hard shell. Liquid metal shrinks as it solidifies, creating holes that weaken the final part. Most shrinkage holes form in the thickest sections of a part -- the sections that take the longest to cool and solidify.
In a recent issue of the journal Computer-Aided Design, Miller and his colleagues described how they built their new method into software that simulates die casting.
The software produces an image of what a part will look like after die casting, and color-codes bright red the areas that will take longest to solidify. If engineers see too much red, or see red in an area of the part where the strength of the material is critical, they’ll know to redesign the part before they cast it. The simulation takes only seconds.
Researchers call their new method of analysis a “binary voxel model” because it breaks three-dimensional pictures of metal parts into tiny cubic sections called voxels. The software counts how many voxels away from the surface an area of the part is, and uses this distance to depict roughly how long solidification will take.
If a part contains porosity, it may break easily. Or, if the pores are near holes where nuts and bolts secure the part, it could come loose.
Miller cited the example of an auto manufacturer who recently had to recall a vehicle because many of the die-cast transmission cases for that model contained pores that eventually leaked transmission fluid.
“We wanted to provide a quick evaluation method for people who design parts so that they won’t have to find out problems later on when they’re expensive to correct,” said Miller.
Miller said that in most companies someone just looks at each new set of computer aided design (CAD) models and tries to guess from experience whether there will be a problem with the part.
“It takes a lot of experience to do that right,” said Gary Kinzel, professor of mechanical engineering and co-investigator on the project. “We intended this software for someone who doesn’t have that much experience.”
Miller said that some companies use number-crunching computer programs to find potential flaws in parts, but that method is very complex and time-consuming.
“For a transmission case, numerical analysis would take anywhere from six weeks to two months to complete. That kind of method works very well in the hands of highly-skilled individuals, but an engineer can’t stop during the design stage to spend six weeks running a numerical simulation,” said Miller.
The binary voxel model performs essentially the same function as a numerical analysis, only with geometry. It’s based on the idea that regions of a part with more material in them will contain more heat, and take longer to cool.
“We tried to do the same thing with more complicated numerical analyses, and we got the same results. It just took longer,” said Kinzel.
“This way, we don’t get numerical results, but we get an immediate picture of the part with the problem areas clearly marked,” said Miller.
The North American Die Casting Association and the U.S. Department of Energy sponsored this research, along with several die casting companies.
Die casting has been around since the 1880s, but according to Miller, the auto industry’s need to make lighter vehicles has created new interest in the process.
“Die casting is typically used to make complex-shaped parts out of light metals like magnesium and aluminum,” said Miller, “and right now the auto companies are trying to make as many light parts as they can.”
Miller added that auto makers are now constructing many parts out of plastic, too, and in the future he and his colleagues are going to create a binary voxel model for plastic molding.
Contact:
Gary Kinzel, (614) 292-6884; Kinzel.1@osu.edu
Written by Pam Frost, (614) 292-9475; Frost.18@osu.edu
Journal
Computer-Aided Design