Resistance spot riveting - New Joining Technology That Can Retrofit Existing Equipment
The need to reduce vehicle weight has spawned myriad new technologies for assembling aluminum, high-strength steel and other materials. These new technologies include self-piercing rivets, flow-drilling screws and friction-stir spot welding.
While all these joining technologies are effective, they share a major disadvantage: They all require automotive OEMs to replace their existing resistance spot welding equipment. Now, engineers at Howmet Fastening Systems have developed a new sheet-metal joining technology—resistance spot riveting (RSR)—that combines traditional riveting with resistance spot welding.
The new technology quickly and reliably joins a wide range of dissimilar materials, including steel, high-strength aluminum, high-strength steel, ultra-high-strength steel, magnesium and composites. And, better still, RSR works with existing spot welding equipment, enabling OEMs to weld or rivet joints without revamping their body lines.
“One of the hurdles [to implementing new] joining technologies for mixed-material assemblies is that they require a total overhaul of the user’s production facility,” says Graham Musgrove, project engineer at Howmet Fastening Systems. “For example, an OEM that uses spot welding equipment might have to rip it all out and replace it with riveting equipment to accommodate mixed-material assembly. Our RSR technology is simply a retrofit of existing spot welding equipment. You just add a feed system to the spot welding gun to supply the rivets to the joint.
“The technology is totally backwards compatible. If you want to spot weld steel parts, you still can—with the same equipment. You just don’t feed a rivet to that position.”
This fastening technology can be used in a number of auto body assembly applications, including frames, floors, closeout panels, pillars, roofs and intrusion beams.
How It Works
The RSR rivet is a small, mushroom-shaped fastener made from low-carbon steel (electroplated with zinc for corrosion protection) or aluminum. Which material to choose depends on the bottom sheet—the steel rivet for steel sheets and the aluminum rivet for aluminum sheets.
The head of the steel rivet is 14.2 millimeters in diameter and 2.7 millimeters thick. The stem is 4 millimeters in diameter. Three stem lengths are available—3, 5.5 and 6.8 millimeters—for joining a top sheet (or sheets) with a total thickness of 0.5 to 4 millimeters. A steel rivet with a low-profile head is also available. The head is 11 millimeters in diameter and 1.8 millimeters thick. Stem length is 5.5 millimeters.
The stem of the aluminum rivet is 8 millimeters in diameter and 3 millime-ters long. Its head is 14.3 millimeters in diameter and 2.5 millimeters thick. It can join a top sheet (or sheets) with a total thickness of 0.5 to 2 millimeters.
“Those four fasteners cover the full range of automotive materials that we’ve been contacted about so far. They have a pretty good grip range,” says Musgrove. “To get the head and body diameters, we used the joint strength of standard steel-to-steel and aluminum-to-aluminum spot welds as our benchmarks. …That said, the new process delivers a better nugget diameter than traditional spot welding, so we get better joint strength.”
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 special compact feed unit delivers a rivet beneath the upper electrode. When the weld cycle is initiated, the feeder retracts, and the electrodes apply force and current to the rivet and the bottom sheet, creating a weld. Total cycle time is 3 to 5 seconds, which is comparable to a traditional spot weld.
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. As a result, the top sheets can be any material: similar or dissimilar, painted or unpainted, conductive or nonconductive. As long as the total thickness of the top sheets matches the grip range of the fastener, the sheets can be any combination of steel, aluminum, carbon fiber, glass-reinforced compos-ite or plastic.
There’s no limitation on the thickness of the bottom sheet. “Obviously, there are practical limitations,” says Musgrove. “You’re not welding a thin sheet to a 1-inch thick plate. But in general, the process is not sensitive to the thickness of the bottom sheet, because you’re not passing through the bottom sheet. You’re only contacting it, making a weld on the top side.”
The rivet head is designed to match the shape of existing electrodes and to center the fastener on the weld spot. The spot welder has a standard radiused electrode on the top side and a truncated electrode on the bottom side to provide a flat, aesthetic joint. “The key parameters are force and current, just like with any spot welding process, so the weld schedules are very similar,” says Musgrove. “If your spot welding system is capable of welding the joint now, it can handle RSR. It does not require any additional force for current capabilities.
“In fact, aluminum RSR actually requires less current than a standard aluminum-to-aluminum spot weld, which saves on power consumption,” he adds. “Traditionally, an aluminum-to-aluminum spot weld would take a lot more current than a steel-to-steel weld. But, because there’s a fastener in the joint, it focuses the current, allowing you to make the welds at levels lower than those for traditional spot welding of aluminum.”
Reprinted with permission of Assembly Magazine.