Next-generation vehicles depend on many types of lightweight materials. However, joining dissimilar materials has traditionally been a big challenge for engineers. A new “direct joining” method recently developed at the University of Michigan solves this issue. It can be used to attach a variety of plastic and metal components.

Traditional assembly methods, such as adhesive bonding and mechanical fastening, have significant limitations when joining dissimilar material combinations.

“The material characteristics of plastic and metal are quite different, because they involve unique chemistries and metallurgical properties,” says Pingsha Dong, Ph.D., a mechanical engineering professor at the University of Michigan. “Traditionally, joining plastic to metal involves time-consuming tasks that are not conducive to high-volume production environments. Joint designs are also limited and there are many quality concerns.

“Adhesive bonding requires surface preparation and long curing time, in addition to being susceptible to environmentally-induced degradation,” explains Dong, who is the director of the Welded Structures Laboratory at Michigan. “Mechanical fastening adds weight and process steps, and cannot effectively achieve hermetic sealing in some applications.

“Welding is about creating bonds between two materials at the molecular level,” notes Dong. “There is a growing demand for reliable plastic-to-metal joining processes that are suitable for high-volume production to meet today’s lightweighting challenges.

“The conventional wisdom for decades has been that plastic and metal are fundamentally incompatible, and there’s no reason to try welding them together,” says Dong. “However, we discovered that the right combination of heat and pressure in the right areas can cause the carbon and oxygen in plastic to bond with metal.”

Dong and his colleagues developed a robust technique for directly joining polymer to metal at speeds of up to 5 meters per minute. It works through the formation of chemical bonds at the joint interface by creating a localized pressure and temperature.

To apply heat and pressure, the engineers used a spindle tool adapted from an existing CNC machine. As it rotates at high speed and moves along a plate, the frictional tool generates heat that melts plastic to a bridging layer, creating a hybrid joint.

The joint configuration can be in the form of a spot or spatial curve. According to Dong, the strong chemical bond formed at the joint interface is the first viable method for welding plastic and metal directly together.

“We used an off-the-shelf machine that looks similar to a drill press with a cylindrical spinning head,” Dong points out. “The metal is placed on top of the plastic and the head is lowered onto the two materials.

“This creates heat and pressure, bonding the two materials together in either a spot weld or a linear weld,” says Dong. “A thin strip of a nylon-6 film provides the oxygen needed to force the formation of a nice, connected interface between the metal and plastic.

“Any metal can be bonded directly with any plastic that contains an adequate amount of oxygen-carbon compounds,” explains Dong. “The key is to calculate the ‘sweet spot’ of heat and pressure that will weld a given combination of materials. 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.”

A variety of plastics can be used, including nylon, polyetherimide, polyether ether ketone and polyethylene terephthalate. In addition to aluminum and steel, metals include copper, magnesium and titanium. Joining thicknesses for polymers range from 0.5 to 20 millimeters, while metal thicknesses vary from 0.5 to 5 millimeters.

The direct welding process produces consistent joints and is easy to automate, with fewer control parameters than alternative methods. For instance, there’s no need for special surface treatment for most polymer-metal combinations.

Potential applications for the patented process include a variety of automotive, aerospace, marine and rail vehicles. Bumpers and liftgates are a few auto parts that could be assembled with direct welding.

“Our new welding techniques could also improve EV battery packs and enclosures,” claims Dong. “Today, they’re multilayer structures that are usually held together with adhesives and mechanical fasteners. They’re very difficult to take apart for repair or recycling.

“Welded battery packs could be taken apart and reassembled much more easily,” explains Dong. “They could also be lighter, cheaper to manufacture and easier to keep cool.”

Dong is currently working with several Tier One suppliers and believes that direct welding will be commercially available within the next two years. “New designs will benefit the most from this process,” he points out. “It will also enable engineers to consider multimaterial designs that have been limited in the past.”