Growth of lightweight vehicle body and closure panel applications in the coming years, and particularly greater use of aluminum, is driving increased use of self-piercing rivets as a high-volume structural joining solution. Ford's aluminum-intensive F-Series program brought rivet-bonding into the mainstream, following lower-volume applications by Audi, BMW, GM (Corvette), JaguarLandRover and Tesla.
The combination of self-piercing rivets (SPR) and robotically-applied structural adhesives creates a robust joint with other benefits. The rivets clamp the adjacent metal panels together to allow the adhesive-bonding agents sufficient time to cure while the assembly goes through the body and paint shops. Without the rivets, a bonded-only assembly could not be produced at a line rate competitive with spot welding, according to experts.
“Because self-pierce riveting is a solution that can join aluminum and other multi-material designs, we see a significant increase in SPR usage worldwide over the next few years,” said Dr. Matthias Wissling, Manager of Fastener Engineering for Stanley Engineered Fastening (SEF), formerly Emhart Teknologies.
Wissling and other SEF officials recently sat down with Automotive Engineering at the company’s North America automotive headquarters in Troy, MI, to talk about SPRs and structural-joining innovation.
SPRs are well-entrenched on European vehicle applications after their 1990s debut on the aluminum-bodied Audi A8. As automakers in North America turn to aluminum sheet and other lightweight materials that can’t be joined by traditional welding techniques, SPRs are gaining momentum.
Use of aluminum sheet for vehicle body and closure parts is forecast to climb from less than 200 million pounds in 2012 to approximately 4 billion pounds by 2025, according to Ducker Worldwide’s 2015 North American Light Vehicle Aluminum Content Study. Of the total, 85% of the material will be used for hoods, 46% for doors, and 18% for complete bodies. By comparison less than 1% of bodies were aluminum-intensive in 2015, the study noted.
While spot welding remains the most common method for joining steel body-in-white (BIW) panels, advanced mechanical fastening is an increasingly popular option. The SPR process can join aluminum, steel, plastics, carbon-fiber-reinforced composites, and combinations of materials without the need for a pre-drilled hole.
SPRs don’t require heat to complete the assembly process, the experts noted, and the technique can be used to join all joints, such as aluminum-to-steel and steel-to-steel, on the same BIW. “This is important because each additional joining technology used for a BIW adds additional complexity and cost,” said Dr. Wissling.
SEF offers several different joining technologies, ranging from metal clips and inserts to studs and SPRs, as well as various fastening assembly systems, according to Dr. Sivakumar Ramasamy, SEF’s Vice President of Breakthrough Innovation.
“The 2025 CAFE [Corporate Average Fuel Economy] requirements are dictating a greater use of lightweight BIW materials, so that’s driven more types of fastening methodologies for the BIW and for attaching parts to the BIW,” said Dr. Ramasamy.
“My directive is to come up with innovative solutions--which could be products, processes, the way we do business--for whatever is coming next in the industry,” Ramasamy said. His newly-formed team of engineers and marketing specialists will begin their duties from a location near Giessen, Germany prior to 2017. The team’s sole focus is innovation.
“Could the autonomous vehicle be plastic-bodied? If so, what would be the fastening method for that vehicle with all of its body- and closure-attached sensors? It might be a plastic rivet or gluing that meets the requirements,” Dr. Ramasamy opined. “The innovations team needs to understand all of the emerging trends and come up with solutions.”
For more than 30 years, the company’s core product line was spot welding. As BIW materials have shifted, the fastening options have evolved.
The SEF innovation team’s double-digit percentage budget allocation is no accident. “We want to grow this business through innovation. Being able to fasten lightweight materials is one thing,” said Dr. Ramasamy, “But there are other things we can do. Because beyond 2025, something else is coming.”