The rise of composite-to-metal research in automotive applications

The main focus of modern automotive lightweighting research is to develop hybrid material automobiles – where different material components are selected on a functional basis to exploit the best characteristics of each material. In order to develop the next generation of multimaterial automobiles, research laboratories like the Laboratory for Materials and Joining Technology (LWF) at the University of Paderborn in Germany are working with the auto industry to solve the technical barriers in joining dissimilar lightweight materials in high-volume, cost-effective ways for future automotive production.

“This is the first time composites are being used as structural automotive structures, previously only seen in motorsport or mainly for use in the outer skin or nonstructural components,” says Dr. Gerson Meschut, PhD, director of the LWF. “Now with composites moving into serial production cars, automotive suppliers need to think about suitable joining technologies. Our main challenge is there are no technologies available to join these materials as easily as sheet metal.” Composites will be a key material moving forward and advance hybridjoining techniques just might be the key to address the technical challenge of joining large composite structures to other lightweight metals. Dr. Meschut has been researching joining methods in automotive applications for more than 15 years and has worked for and with various automotive companies around the world to develop efficient joining methods. Meschut predicts hybrid joining – a combination of adhesive bonding, mechanical joining and welding – and similar technologies will lead the way for faster and cheaper methods. Several laboratories around the world, including his lab in Germany, have recently started the practice of investigating new hybrid techniques for faster production times.

Addressing technical barriers

Kim Hamner, engineering director at Continental Structural Plastics, has been involved with the automotive composites industry since exterior sheetmolded compound (SMC) body components were becoming viable options on cars other than the Corvette over 40 years ago. She has been developing applicationsfor composites to reduce vehicle weight ever since. “Basic joining methods have not changed significantly since the architecture of vehicles has not changed over the last 20 years,” says the composite manufacturer. “To connect to the structural parts of the vehicle, we will need to develop structural adhesives and as well as assembly techniques to take advantage of the lighter weight composites.”

Example of adhesive joining technology by Dow Automotive Systems.

Example of adhesive joining technology by Dow Automotive Systems.

Hamner is referring to the techniques most automotive manufacturers are using today – typically mechanical joining, adhesive bonding or a combination of the two – to join composites to other materials. Each method is selected depending on the application of the part and has a unique set of benefits and limitations. For example, mechanical joining is historically used to join steel parts, which requires drilling or cutting holes into the composite piece effectively limiting stress redistribution in composite components.

Adhesive bonding is the most common method of composite-to-composite joining, but the technology takes time to cure. “Slow processing time is the main reason automotive companies prefer mechanical joining,” says Dr. Meschut. “Additionally, some manufacturers trust adhesive joining, while others feel more comfortable with mechanical joining.” To be absolutely safe, automotive companies have been adding additional mechanical bonds to the adhesive joint – which is why a new method, known as hybrid or fusion joining, is a popular research topic. The theory is that even if the adhesive fails, the joint will work.