General Motors is preparing to significantly increase its use of aluminum in vehicle body structures with a new twist on an old joining technology: resistance spot welding. The automaker is expanding use of what GM engineers claim is an industry-first aluminum spot-welding process that features a new type of electrode developed and patented by GM R&D.
The technology is designed for much higher production rates than are currently employed in automotive aluminum-structures manufacturing. It centers on a new electrode-tip design that will enable GM’s global body shops to spot-weld virtually any combination of aluminum sheet, extrusions, and castings, according to Blair Carlson, Lab Group Manager, Lightweight Materials Processing Group, GM R&D.
“No other automaker is spot-welding aluminum body structures to the extent we are planning to, and this technology will allow us to do so at low cost,” he said.
By increasing its use of aluminum spot welds per vehicle, GM expects to eliminate nearly 2 lb (0.9 kg) of self-piercing rivets from aluminum body assemblies including doors, hoods, and liftgates.
Using rivets to join aluminum pieces adds up-front cost, while complicating end-of-life recycling efforts. Rivet-gun operating limitations also restrict the joint configurations that can be employed in a structure.
GM also aims to commercialize the welding technology. “We’ve got a good handle on it in our internal production, and we’ve licensed it to the GM suppliers for upcoming programs,” Carlson told AEI. “Now we’re taking the next step to license it externally for non-GM production” which he expects to include heavy truck, railroad, and aerospace applications.
GM owns a suite of intellectual property around three concepts: the electrode design, the controls for the electrical current, and the technology for dressing (cleaning) the electrode tip intermittently, Carlson said.
The resistance-welding technology has been in use on select hood (Cadillac CTS-V) and liftgate (hybrid versions of Chevrolet Tahoe and GMC Yukon) applications since 2008. GM’s invention is the unique design of the electrode tip. Its concentric domed rings (see accompanying image) break through the aluminum oxide layer contained on all aluminum parts.
“That layer is the bane of aluminum welding,” Carlson explained. “The rings allow the electrode to engage the surface of the material so that current passes more easily and generates a weld nugget in the middle, centered between the two parts,” he said.
The process is not affected by material gauge and has demonstrated improvements in process consistency and electrode life since it entered volume production. Carlson recalled GM’s implementation: “Basically the ME [manufacturing engineering] guys wanted us to do due diligence, so we took the recommended practices of The Aluminum Association for weld schedules and electrodes and did process windows vs. our technology. We published this in an earlier sheet-metal conference paper.”
The new electrode tip design gives a larger and more robust weld process window, with much tighter consistency than with conventional aluminum spot welding, Carlson noted. “We don’t have traditional issues such as the sheet metal sticking to the electrode, which usually means the welding cell will stop and the operator has to go in and check it out. We avoid all of those interruptions in production.” GM uses MFDC (mid-frequency direct current) in its aluminum fabrication operations.
Jon Lauckner, GM’s Chief Technology Officer, views the technology as a strategic asset. “The ability to weld aluminum body structures and closures in such a robust fashion will give GM a unique manufacturing advantage,” he said in a statement. “It is an important step forward that will grow in importance as we increase the use of aluminum in our cars, trucks, and crossovers over the next several years.”