General Motors Corp.’s research arm has developed a resistance spot welding technology it claims is the first for the auto industry: it is based on a patented electrode design that is more reliable for welding aluminium. The importance of this is that it will allow the auto-maker to make greater use of aluminium in automotive bodies (e.g., hoods, lift gates, doors), which will lower vehicle weight and thereby improve fuel economy.
It’s the second recent development for welding automotive aluminium. Earlier this month, Honda announced a friction-stir welding development it is using to weld aluminium to steel for automotive subframes.
By using its own new process, GM stated, it expects to eliminate nearly two pounds of rivets from aluminium body parts. It said it has already adopted the process in production of the hood of the Cadillac CTS-V and the lift gate of the hybrid versions of Chevrolet Tahoe and GMC Yukon. It said it will implement the process more extensively in 2013.
Resistance spot welding is a joining process in which contacting metal surfaces are joined by the heat that develops from resistance to electric current flow. Work pieces are held together under pressure from electrodes. Typically, the process uses two shaped copper alloy electrodes to concentrate welding current into a “spot,” and simultaneously to clamp the metal pieces together. Forcing a current through the spot melts the metal and forms the weld.
RSW is used widely in automotive manufacturing because it is able to join pieces quickly without heating the metal excessively. It’s also inexpensive, and reliable.
A new resistance spot welding process that General Motors developed uses a proprietary multi-ring domed electrode.
As explained by the auto-maker the new process developed by GM Research & Development uses a patented multi-ring domed electrode that is able to weld aluminium to aluminium, which standard smooth electrodes cannot do reliably: aluminium oxide minimizes the ability of the electrode to achieve the heat that forms the weld.
GM said its new technique works on sheet, extruded, or cast aluminium because its proprietary multi-ring domed electrode head disrupts the oxide on aluminium’s surface, so the current achieves a stronger weld.
“The ability to weld aluminium body structures and closures in such a robust fashion will give GM a unique manufacturing advantage,” stated GM chief technology officer Jon Lauckner, who also is vice president of Global R&D for the auto-maker. “This new technology solves the long-standing problem of spot welding aluminium, which is how all manufacturers have welded steel parts together for decades,” Lauckner said. “It is an important step forward that will grow in importance as we increase the use of aluminium in our cars, trucks and crossovers over the next several years.”
Typically, auto-makers use self-piercing rivets to join aluminium body parts, because of variability in production with conventional resistance spot welding. However, rivets add cost and riveting guns have a limited range of joint configurations. In addition, end-of-life recycling of aluminium parts containing rivets is more complex.
“No other auto-maker is spot-welding aluminium body structures to the extent we are planning to, and this technology will allow us to do so at low cost,” stated Blair Carlson, GM manufacturing systems research lab group manager. “We also intend to consider licensing the technology for non-GM production in automotive, heavy truck, rail and aerospace applications.”
GM cited automotive market research group Ducker Worldwide to note that aluminium use in vehicles is expected to double by 2025. In addition to being lighter than steel, which helps to improve vehicles’ fuel economy, it is corrosion-resistant.