One of the ways that OEMs can lower the overall weight of their vehicles is by using lighter materials. Like aluminum. According to The Aluminum Association (aluminum.org), “Replacing iron and steel with auto aluminum creates a weight savings of 45-50%, while increasing vehicle performance and fuel economy without sacrificing automotive safety.” While that may seem self-evident (lighter car = lighter material), it is easier said than done, particularly when it comes to joining the material through welding.
It’s not as though there aren’t automotive aluminum spot welding applications being performed right now, but what’s typically the case is that the welding is being supplemented by mechanical fasteners, such as self-piercing rivets. Not only are those rivets a secondary application, they can add weight to the components being welded, which subtracts from the weight benefit that can be realized by using aluminum. General Motors estimates that almost two pounds of rivets could be used on an aluminum-intensive car.
As Blair Carlson, GM Manufacturing Research Lab group manager, explains, the problem for spot welding aluminum—and recognize that when it comes to spot welding steel, this is pretty much the proverbial slam-dunk, as this is an operation that has been performed in plants around the world pretty much since the Dodge brothers hooked up with the E.G. Budd Manufacturing Co. back in 1914—is that there is an oxide on the surface of the material. This aluminum oxide, he explains, causes resistance between the surfaces to be welded and the electrodes, which results in a greater power draw, greater wear on the electrodes, and, consequently, reduced electrode life.
So what they’ve done is to create an electrode with multiple concentric rings on its tip. Carlson says the ridges on the electrode stretch the oxide layer to the point of fracture (remember we are talking about a micro-scale layer, not a crust). “Then you have intimate contact of the electrode with the base aluminum, which is really what you want. This allows us to weld with higher process stability and with higher quality,” he says.
He explains that the same electrode and the same electrode dresser used for steel spot welding are used. It is primarily a matter of putting the concentric rings on the surface of the electrodes, which means there are slightly different cutting blades used in the dresser. “Everything else is the same,” Carlson says.
“What this allows us to do is reuse more of our existing infrastructure as we transition to aluminum,” he says, noting that tip dressing is a standard, global approach in GM assembly plants. “In certain cases, you might have to change out the transformer, but the cost of that is minimal compared to a completely new cell.”
In addition to sheet aluminum, the process also allows the welding of aluminum extrusions and castings, which is a change from conventional practice. “You’ll see next year as we launch a new vehicle that this has gone beyond closures to structures,” Carlson says.
Speaking of this development, Jon Lauckner, GM chief technology officer and vice president of Global R&D, said, “The ability to weld aluminum body structures and closures in such a robust fashion will give GM a unique manufacturing advantage.”
The process has been proven in production, as it is being used on the hood of the Cadillac CTS-V and the hybrid versions of the Chevrolet Tahoe and GMC Yukon. While these are arguably low-volume vehicles, GM’s intent seems to be to expand the use of the material, presumably in efforts to increase the fuel efficiency of its vehicles through mass reduction.
As most assembly plants are set up for welding steel, Carlson says that they are looking at the possibility of using the multi-ring domed electrodes on both steel and aluminum parts. This would allow alternating between the two materials, adding production flexibility: for example, steel doors, then aluminum doors. “We need to validate this before we put that into production, but we are getting close to that point,” Carlson says