All-steel or all-aluminum vehicle structures currently dominate the auto industry. Manufacturing the structures out of both metal and plastic could make vehicles lighter, more efficient, stronger, and safer. But quickly and cheaply joining those two types of materials to create a hybrid structure is a long-standing problem. Engineers at the University of Michigan are pushing the concept toward reality with their novel welding method.

“Computer models show us we can make the structures of cars and light trucks as much as 40% lighter by building them with a combination of metals and plastics,” said Pingsha Dong, engineering professor, in a news release. “Gas-powered vehicles could see better fuel economy, while electric vehicles can get more range.”

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The chemical makeup of plastic makes it difficult to fuse with metal without the use of fasteners or adhesives. The new process eliminates the need for these joiners.

The engineers found that the right combination of heat and pressure in the right locations causes the carbon and oxygen in the plastic to bond with metal. They used an off-the-shelf machine that resembles a drill press to create heat and pressure by rotating at high speeds. As it moves, it causes the metal to deform and fuse with the plastic to create a strong hybrid material joint.

The team says any plastic that contains an adequate amount of carbon and oxygen can be bonded to any metal with this process.

“The key is to calculate the ‘sweet spot’ of heat and pressure that will weld a given combination of materials, and we can work with manufacturers to determine that,” Dong said. “For plastics that don’t have enough oxygen-carbon compounds, like polypropylene, we can put an inexpensive plastic film between the two materials to ‘seed’ the bond with oxygen and carbon.”

Other applications

The new techniques also could improve EV battery packs and their enclosures. Current models usually are made with adhesives and fasteners that make the units difficult to take apart for repair or recycling. Hybrid welded battery packs could be taken apart and reassembled more easily, in addition to making the packs lighter and easier to cool.

The engineers also developed a process that could allow aluminum to be 3D printed onto steel. Prior work on this concept resulted in an expensive process that created a brittle point where the two metals joined. The heat and pressure used in the team’s new process prevent brittleness and present a new way to combine lightweight aluminum with strong steel.

What’s next

The engineers already have patented their welding process, which they say is affordable and scalable. They’re working with equipment manufacturers to develop commercial equipment that performs this welding method and can be licensed to automakers and other manufacturers. Dong expects the technology to be in industrial settings within two years.

Photo: Ph.D. student Yuning Zhang prepares to weld a sample of plastic and metal. (Credit: Brenda Ahearn / University of Michigan College of Engineering)