When Ford Motor Co. moved from steel to aluminum on the F-150 pickup in 2015, the automaker switched from holding the truck’s body together with thousands of spot welds to using rivets, flow-drilled screws, and adhesives. The problem, engineers say, is aluminum riveting is slower and more difficult than steel spot-welding.
“We had to buy fasteners we’d never bought before in very high volumes, and we needed new types of equipment. There was a huge learning curve,” says Shawn M. Morgan, global vehicle architecture manager for Ford Motor Co. “When you switch from spot-welding, it’s difficult. The speeds are slower, so we had to add more line points. The rivet heads are large, so we had to rethink access to the joint. It wasn’t a simple one-for-one trade between rivet robots and weld systems.”
At the 4th Lightweight Vehicle Manufacturing Summit in Detroit, Michigan, automakers and fastener producers discussed the challenges being brought about by the need to cut vehicle weights. Replacing well-understood steel processes with aluminum technology has been taking place, but automaker engineers complain that there’s been some overpromising from suppliers, and joining experts say changing material options and design demands require new product development.
Pete Edwards, joining leader for Honda Engineering North America Inc. in Marysville, Ohio, says he likes the idea of using rivets conceptually, but in practice, there are several barriers to adoption.
“The way Honda is set up, it would be nearly impossible to swap out welding for riveting. We don’t have the space to add more line spots,” Edwards says. Another concern he notes is the lack of flexibility. Honda and many other automakers build several different models on the same assembly lines, something that spot-weld robots can handle with little trouble.
“You can program a welding robot to recognize a lot of different geometries, and the equipment fits. The rivet-driver heads are much larger, and that creates a lot of challenges,” Edwards says. “Our priority is finding the ideal joint for the product and for the most efficient plant-floor layout.”
Michael Danyo, aluminum technology leader at Ford, says there’s a simple way to solve those problems – but it’s not the answer engineers like to present to managers.
“To keep the same productivity in the same shop footprint, we had to invest heavily. There’s no way around the fact that you’re making a major change to the basic structure of the build,” Danyo says.
Edwards says companies may be willing to spend more on joining equipment if they were confident that the systems would be adaptable to changing technologies, materials, and build processes.
“If you know that your joining technologies are going to continue changing, a solution that only works with one grade of aluminum or one joining process is not going to work,” Edwards says. “We need to plan for the future, for multiple products on the same line using different joining technologies. Some tools are not perfect solutions to any specific challenge, but they can set us up for further changes in the future.”
Experts from fastener producers and companies that make rivet-driving equipment say they’re working on the speed and adaptability issues that create problems for automakers, but they’re dealing with an evolving landscape as well. Simultaneously, automakers are asking for rivets that can tackle specific joining challenges and trying to reduce the number of special-purpose parts that they have in every plant.
“Our customers are telling us that they want to be able to use the same rivets for different joining challenges,” says Viral Varshney, director of engineering for fastener engineering company Rifast Systems LLC. “That creates a challenge for our engineering department.”
Using one set of fasteners for multiple joining techniques typically means using heavier-duty rivets and screws for lighter joints. The upside is ordering simplicity – one set of parts for multiple joints. The downside is that oversized rivets go against other industry trends – thinner metal gauges and thinner part geometries. Varshney says companies are working to solve the contradictory challenges and that automakers could help by relaxing part tolerances.
“We have to get to the point where we’re not overdesigning everything. That’s when we can get a lot more weight out of joints,” Varshney says, adding that fastener companies continue to innovate.
Rifast recently redesigned its LBM lightweight nut, moving to a 10-rib design from an 8-rib option, providing more grip without increasing surface area. The redesign should allow some customers to use a smaller LBM nut shared by other assembly components, rather than upsizing.
Another issue for rivet producers comes from high-tech materials. Matt Smith, new business development director for self-piercing rivets (SPR) and stud-welding equipment (SWS) at Stanley Engineered Fastening, says ultra-high-strength steel alloys favored for structural components are great at reducing vehicle weight, but they are engineered to resist piercing and deforming stresses. That’s a problem because rivets are supposed to pierce and deform metal in controlled ways to create joints.
“A lot more punch force is needed to get the rivets to pierce, and the rivet heads create gaps with the sheet metal. People were reporting higher fail rates on rivets,” Smith explains.
Stanley engineers have redesigned SPR geometries, working with new materials so newer rivet models are able to join extremely tough frame elements. But he adds there’s still a tradeoff – engineered fasteners for a specific task vs. commodity rivets that can be used for multiple applications.
Using rivets and other fasteners is not new in motor vehicle manufacturing. Companies have been using such systems on structural steel components for decades. However, the push to cut more weight from vehicles means more aluminum and composites in future designs, and those materials will require new fastening techniques.
Automotive engineers say fastener suppliers have been very responsive to changing needs, and that flexibility will be tested in the coming years. Welding will remain the dominant joining technique at most facilities as it is highly adaptable, fast, compact, and cost effective. However, mechanical fastening is mounting a big challenge to welding’s dominance, so expect continuing developments in the near future.
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or firstname.lastname@example.org.