ASSEMBLY Riveting Resistance Spot Riveting he need to reduce vehicle riveting equipment to accommodate A new joining method for weight has spawned myriad mixed-material assembly. Our RSR combines new technologies for assem- technology is simply a retrofit of existing bling aluminum, high-strength spot equipment. You just add a traditional riveting with and other materials. These feed system to the spot welding gun to new technologies include self- supply the rivets to the joint. resistance spot welding. Tpiercing rivets, flow-drilling and “The technology is totally backwards friction-stir spot welding. compatible. If you want to spot weld While all these joining technologies steel parts, you still can—with the same are effective, they share a major dis- equipment. You just don’t feed a rivet to advantage: They all that position.” require automotive This fastening technology can be OEMs to replace their used in a number of auto body assembly existing resistance applications, including frames, floors, spot welding equip- closeout panels, pillars, roofs and ment. Now, engineers intrusion beams. at Howmet Fastening Systems have devel- How It Works oped a new sheet- The RSR rivet is a small, mushroom- metal joining shaped made from low-carbon technology—resis- steel (electroplated with zinc for tance spot riveting corrosion protection) or aluminum. (RSR)—that com- Which material to choose depends on bines traditional riv- the bottom sheet—the steel rivet for eting with resistance steel sheets and the aluminum rivet for spot welding. The new aluminum sheets. The RSR rivet is a small, mushroom-shaped technology quickly and reliably joins The head of the steel rivet is fastener made from low-carbon steel or a wide range of dissimilar materials, 14.2 millimeters in diameter and aluminum. including steel, high-strength aluminum, 2.7 millimeters thick. The stem is 4 Photos courtesy Howmet Fastening Systems high-strength steel, ultra-high-strength millimeters in diameter. Three stem steel, magnesium and composites. And, lengths are available—3, 5.5 and 6.8 better still, RSR works with existing spot millimeters—for joining a top sheet (or welding equipment, enabling OEMs to sheets) with a total thickness of 0.5 to Reprinted with permission from weld or rivet joints without revamping 4 millimeters. A steel rivet with a low- ASSEMBLY© their body lines. profile head is also available. The head “One of the hurdles [to implementing is 11 millimeters in diameter and 1.8 www.assemblymag.com new] joining technologies for mixed- millimeters thick. Stem length is 5.5 material assemblies is that they millimeters. require a total overhaul of the user’s The stem of the aluminum rivet is 8 production facility,” says Graham millimeters in diameter and 3 millime- Musgrove, project engineer at Howmet ters long. Its head is 14.3 millimeters  By John Sprovieri Fastening Systems. “For example, an in diameter and 2.5 millimeters thick. Editor in Chief OEM that uses spot welding equipment It can join a top sheet (or sheets) with a [email protected] might have to rip it all out and replace it with total thickness of 0.5 to 2 millimeters. The rivet head is designed to match the shape of existing electrodes and to RSR works with existing spot welding equipment, enabling OEMs to weld or center the fastener on the weld spot. rivet joints without revamping their body lines.

“Those four cover the the feeder retracts, and the electrodes there are practical limitations,” says full range of automotive materials that apply force and current to the rivet and Musgrove. “You’re not welding a thin we’ve been contacted about so far. They the bottom sheet, creating a weld. Total sheet to a 1-inch thick plate. But in gen- have a pretty good grip range,” says cycle time is 3 to 5 seconds, which is eral, the process is not sensitive to the Musgrove. “To get the head and body comparable to a traditional spot weld. thickness of the bottom sheet, because diameters, we used the joint strength of The weld is between the fastener and you’re not passing through the bottom standard steel-to-steel and aluminum- the bottom sheet. The top sheets are not sheet. You’re only contacting it, making to-aluminum spot welds as our bench- electrically involved. Rather, they are a weld on the top side.” marks. …That said, the new process mechanically captured by the head of The rivet head is designed to match delivers a better nugget diameter than the fastener. As a result, the top sheets the shape of existing electrodes and to traditional spot welding, so we get bet- can be any material: similar or dissimi- center the fastener on the weld spot. ter joint strength.” lar, painted or unpainted, conductive The spot welder has a standard radiused To create a joint, two or more sheets or nonconductive. As long as the total electrode on the top side and a truncated are placed between the electrodes of thickness of the top sheets matches the electrode on the bottom side to provide a standard spot welding system. The grip range of the fastener, the sheets can a flat, aesthetic joint. upper sheets have holes for the rivet. be any combination of steel, aluminum, “The key parameters are force and The bottom sheet does not. At the start carbon fiber, glass-reinforced compos- current, just like with any spot welding of the cycle, a special compact feed unit ite or plastic. process, so the weld schedules are very delivers a rivet beneath the upper elec- There’s no limitation on the thick- similar,” says Musgrove. “If your spot trode. When the weld cycle is initiated, ness of the bottom sheet. “Obviously, welding system is capable of welding

To create a joint, two or more sheets are placed between the electrodes of a standard spot welding system. The upper sheets have holes for the rivet. The bottom sheet does not. At the start of the cycle, a feeder delivers a The RSR process exceeds the strength of competing technologies for rivet beneath the upper electrode. Next, the electrodes apply force and current and future automotive materials, such as high-strength current to the rivet and bottom sheet, creating a weld. aluminum and ultra-high-strength steel. Riveting the joint now, it can handle sheets. The fastener welds to RSR. It does not require any the top steel sheet first and additional force or current then welds to the bottom capabilities. sheet.” “In fact, aluminum RSR actually requires less current Coming Soon than a standard aluminum-to- Howmet Fastening Systems aluminum spot weld, which has been developing the saves on power consumption,” RSR process for the past two he adds. “Traditionally, an to three years. Formerly a aluminum-to-aluminum spot part of aluminum producer weld would take a lot more Alcoa Inc., Howmet is current than a steel-to-steel currently working with weld. But, because there’s a several automotive OEMs to fastener in the joint, it focuses determine how and where the the current, allowing you to process can be used in make the welds at levels lower vehicle assembly. than those for traditional spot The RSR process was developed as a way to join mixed-material Musgrove says his com- assemblies in auto body applications, such as aluminum to steel. This welding of aluminum.” graph shows the tensile strengths created when a steel rivet is used pany has been welding As for joint design, the to attach various aluminum alloys to a steel sheet. thousands of test coupons on same considerations that various materials to apply to resistance spot weld- determine joint strengths ing apply to RSR. Design rules for edge [not having a hole] should be more of a and process parameters. “We’re distance and spot-to-spot spacing are fallback plan, because it does reduce grip doing a lot of validation right now,” similar for both processes. “Any joint range and increase cycle time. If the rivet he says. “We’re still learning a lot strength that you can get from a spot pilots through the top sheet, the cycle time about the capabilities of the technol- weld, we can get from RSR,” says Mus- is about twice the time required than with ogy. In hindsight, we had a rather grove. a hole.” limited initial view of what this pro- In most cases, the top sheets must In most cases, the RSR process cess was capable of, but based on have holes for the fastener. However, requires access to both sides of the customer feedback…we’re learning if the bottom sheet is steel and the top assembly. However, it is possible to that the technology can do far more sheet is aluminum, then the top sheet create joints with access to only one than we first imagined.” does not necessarily require a hole. side, as when welding a sheet to a tube. Musgrove anticipates that the “The steel RSR will actually extrude It’s also possible to weld three or more technology will be fully commercially its way through the aluminum top sheet sheets together. “For example, in one available available in one to two and still make a good weld,” explains of our test stack-ups, an aluminum years. “The fastener is 90 percent Musgrove. “I would say, though, that top sheet is joined to two steel bottom mature at this point,” he says. “The first generation of the feed system is ready and is being used for all our validation work. However, based on lessons learned, we’re working on a second-generation feed system that will be more compact, more adaptable, and less expensive.” While the technology was initially aimed at the automotive industry, Howmet is beginning to see interest from other industries, too, including aerospace and heavy trucks. “This is a sheet-metal joining technology, so I don’t see it moving into applications like railcar assembly, which use heavier-gauge materials,” concedes Musgrove. In RSR, the weld is between the fastener and the bottom sheet. The top sheets are not electrically involved. Rather, they are mechanically captured by the head of the fastener. For more information about RSR, call 254-751-5505 or visit www.hfsindustrial.com. A