Welding Mixed Materials, Multiple Ways
As OEMs and suppliers seek lightweight solutions to meet higher fuel economy standards through multi-material structures, conventional welding techniques are beginning to give way to new solid-state joining methods better suited for creating strong bonds between dissimilar metals.
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“I don’t think one material is the solution for lighter-weight vehicles,” says Swamy Kotagiri, executive vice presi-dent of engineering and research and development at Magna (magna.com).
Kotagiri, along with many of his peers in the industry, are taking a broader view of material solutions to meet fast approaching fuel economy regulations.
“We are focused on multi-material solutions,” he says. “For example, if we’re making a column, maybe we want a steel enclosed section in one area but need an aluminum casting in another because it is an intricate shape. Now the challenge is how do we put these two materials together?”
Conventional fusion welding, where joints are formed when molten metal cools, doesn’t work very well when joining dissimilar metals like, aluminum and steel, so manufacturers are beginning to use solid-state welding techniques that require only pressure and friction to form robust bonds.
“There’s very little opportunity for fusion welding mixed materials,” says Matt Zaluzec, manager of materials and manufacturing research at Ford. “We’re looking at solid-state welding processes that are scalable.” While many of these processes have been used for low-production or even just lab work, it is necessary to achieve higher volume capability for them to become viable for use at places like Ford.
Here’s a rundown of a few of the welding methods Zaluzec, Kotagiri and others in the industry are exploring:
Friction stir welding (FSW): FSW was developed in the early 1990s at The Welding Institute in Cambridge, England, and has been used by NASA for space shuttle parts, Apple to bond the aluminum bodies of iMac computers, and Honda to join aluminum and steel in the 2013 Accord front subframe.
FSW involves a rotating tool pressing into the intersection of the two metals to be joined so that the friction heats the surrounding material to a plasticized state around the probe. As the tool rotates it stirs the materials so that a mechanical bond is formed. No melting occurs, leaving the weld in the same condition as the parent material(s).
This process has fewer elements to control than arc welding (e.g., purge gas, voltage and amperage, wire feed, travel speed, shield gas, and arc gap all must be regulated. For FSW, there are only three parameters to control: rotation speed, travel speed, and pressure.
What’s more, there is an energy-saving benefit. For the Accord subframe, Honda claims the FSW process uses 50% less electricity compared to conventional welding.
Friction bit joining (FBJ): FBJ involves cutting and friction to form a bond. Last year, researchers at Brigham Young University’s Friction Stir Research Laboratory (fsrl.byu.edu), working with the University of Ulsan in Korea and Korean automotive suppliers, joined a lightweight aluminum hub with a cast iron brake rotor by inserting a thin layer of steel between the two metals.
Michael Miles, a manufacturing engineering technology professor at BYU and co-developer of the process, explains how it works when joining aluminum to steel: “The bit rotates at a high speed”—around 2,500 rpm—“and cuts through the top layer of material, which in this case is an aluminum sheet.” When the consumable steel bit encounters the steel, via heat and pressure, it melts and forms a metal-lurgical bond, essentially a friction weld.
Miles says FBJ could be used to weld a high-strength steel A- or B-pillar and aluminum roof, or to join lighter-weight materials into parts of the car door.
“Our process is a technical success, now we need to go forward to make it commercially viable,” he adds. Miles and his team continue to research FBJ with assistance from stir welding machine-maker MegaStir (megastir.com) and Oak Ridge National Laboratory (ornl.gov).
Magnetic pulse welding: What happens when you drive one metal workpiece (e.g., a 4-in. diameter aluminum tube) into another (e.g., a steel shaft) with a massive magnetic pulse (>1-million amps through the coil in 100 msec) at 900 mph? If the setup is right, then the coalesce of the two materials, a metal to metal bond. That’s essentially magnetic pulse welding (MPW), which is a “cold” process.
Dana Corporation (dana.com) has used MPW for more than 10 years to weld aluminum and steel driveshaft components. Although it might seem like a lot of energy is used, according to Dana, it uses 100 times less energy than the equivalent metal inert gas (MIG) weld. Before Dana adopted the MPW process, conventional driveshafts required bolting a CV joint to a companion flange from the axle or transmission because it was impossible to weld a heat-treated bearing element. A thermal welding process
would distort the joint. By adopting the “cold,” magnetic approach, driveshafts design options increased.
“Our MPW process allows us to join steel and aluminum components to create a wide variety of innovative driveshaft designs,” says Jim Duggan, a chief engineer at Dana. “The result is a bond that outperforms conventional MIG welding and other metallurgical processes.”
The process also works for joining non-metallic materials such as composites, polymers, and rubbers.
Cold Metal Transfer
Another viable method for welding dissimilar materials, especially ultra-thin (0.3 to 0.8 mm) sheets, is an arc welding process known as “cold metal transfer” (CMT). What happens is that there is a digitally controlled wire feeder. As soon as an arc is detected as the wire reaches the weld pool, the weld wire is retracted and the weld current is lower, so that there is a droplet deposited. This has been described as a “hot-cold, hot-cold” process because the wire retracts—applies heat and then removes it—up to 90 times per second.
Compared to conventional welding, CMT requires less current for the same amount of material deposition, which means there is comparatively low heat input. Fronius (fronius.com), which makes CMT welding systems, describes this process as the “most stable weld process in the world.” Advantages include a low thermal input and a stable arc, which doesn’t produce spatter because the short circuit is controlled and the current is kept low.