While everyone is preparing for an influx of aluminum in vehicles, there could be hybrid steel-aluminum structures on the way thanks to advances in cold metal transfer (CMT) welding.
The Wall Street Journal ran a lengthy article on Feb. 9 about a process being tested at Linz, Austria-based Voestalpine, Europe's third-largest steelmaker. It's a complex and pricey approach that uses argon gas and a zinc coating on the steel to bind the two metals together. The company hopes to reduce costs by a third so that it can be adopted by high-end automakers.
Audi has already been in touch, according to Voestalpine, and Honda is approaching the issue using a different technique to fuse steel and aluminum.
According to Jason Bartanen, director of industry technical relations at I-CAR, the CMT method has been around for a long time, and has been used for thin-gauge materials to avoid excessive heat and warping. "It's not really a fusion process, it's more of a bonding process," Bartanen says.
Joining the two metals this way was long considered impossible because of the differences in their chemical and physical properties. Attempts to weld them together led to brittle intermetallic phases, which impacted the tensile strength of the joint.
Currently, steel and aluminum structures can be combined, but with adhesives and mechanical fasteners. For the cold metal transfer process to be commercialized, Voestalpine would have to reduce costs through changing the alloy of the film between the metals, or making the process faster.
Jergen Bruckner of Fronius International, a company that partnered with Voestalpine on the process, presented research on the technology back in 2004 at the AWS Detroit Section's Sheeting Metal Welding Conference. At that point, he reported that the process showed good results for tensile strength, corrosion resistance, and in limiting fatigue strength. The process has been refined significantly since then, but challenges still remain. According to the Fronius website:
"The steel sheets must be galvanized (as is usual in automotive manufacturing), the aluminum sheets must be made from materials of the AW5xxx or 6xxx series, and AlSi3Mn1 must be used as the welding/brazing filler metal. The zinc layer on the steel sheet acts as a flux, wetting the steel. The crucial factor is the intermetallic phase (IMP). This should be as thin as possible, and no more than 10 µm. The strength of the joint is then so great that in tensile tests, the seam itself remains intact and the break takes place in the aluminum sheet."
According to Bartanen, Audi, BMW and other automakers have joined steel and aluminum components in their vehicles using rivet bonding technology.
"When or if we do see vehicles being produced with CMT technology used to combine steel and aluminum, I would envision that those repairs would be made using rivets or adhesive bonding," Bartanen says. "I don't see CMT welding begin done in a collision repair facility, definitely not in the next five years."
That's because CMT is expensive and complex, and requires skill and training that would be challenging for collision shops to deploy.
"I do think we'll see mixed materials growing, and we'll have hybrid vehicles with steel and aluminum, along with composites," Bartanen says. "From a collision repair standpoint, you have to know which materials you are working with, the reparability limits of each material, and the recommended procedures, whether that involves welding, rivet bonding, or other types of attachment methods. There will have to be a damage analysis process to understand what is repairable and what is not, and which tools you'll need."
Vehicles with aluminum and steel components will also pose a logistics challenge, regardless of how they are joined. OEM recommendations typically call for aluminum repairs to be conducted in a segregated section of the collision shop. "It will be hard to say what part of the shop you should repair that vehicle in," Bartanen says. "It will be a matter of staying up to speed on the trends and knowing what kinds of vehicles you are working on."
He adds that shops should evaluate the vehicle market in their area to determine what equipment and training to invest in. "Are you going to work on an aluminum once a year or once every week?" Bartanen says. "That will affect your plan for gearing up. You have to know what you will be working on, and have the skills to make those repairs."
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