The Future of Welding Aluminum to Steel

Today’s cars and trucks are very different than those produced 40 or 50 years ago, and with some ambitious mile-per-gallon mandates on the horizon, we’re bound to see more changes in the coming years. Specifically, by 2025, vehicles will be expected to average 52 mpg. In order to meet that standard, manufacturers are looking for every opportunity to cut down on weight without compromising safety. Aluminum is a big part of the answer, and combining this lighter material with stronger steel is a big hurdle. Here’s a look at the techniques used to join these important metals today and those in development for the future.

Current Techniques
Today, dissimilar metals are often joined using rivets. Tools like the PNP90 self-piercing rivet gun use high levels of force to drive rivets through sheet metal, and can be used to join aluminum and steel. Other methods include specially designed glues and epoxies. So why are we using these methods instead of just welding aluminum and steel? These two metals famously don’t work well together. They have different melting points, and welding them together produces brittle, inter metallic compounds that can make cars unsafe, so traditional methods just don’t work. The car industry hasn’t settled on an affordable, standard solution to this problem, but various groups are exploring different methods. Two big ones:

Friction Stir Welding
Friction stir welding (FSW) is quite different from traditional welding methods, and some manufacturers are already using it to combine steel and aluminum parts. When you dig into it, the name is pretty explanatory. Basically, a rotating tip causes friction, heating the two overlapping or butted-together metals without melting them completely. Once they’re soft, the rotating tip stirs them together without sparks, shielding gases or fumes. Friction + stirring = friction stir welding. Car and Driver has a great illustration of the method and reports that manufacturers like Honda and Audi have used the process in their vehicles.

One big barrier to this type of welding? The cost of the machinery. That’s part of the reason we haven’t seen widespread adoption.

Cold Metal Transfer                                                                                                                        This method, in development from the Austria-based-company Voestalpine AG, recently got some coverage in The Wall Street Journal. The process heats a zinc-coated piece of steel and a piece of aluminum to a temp high enough to melt the aluminum but cool enough to keep the steel intact. It requires special solder and torches, and because of the complexity of the method, it’s still very expensive. The company developing the technique is aiming to reduce costs by at least a third, and even then it’s still aiming at high-end vehicles, not main-stream cars.

So cost is a big factor when it comes to joining aluminum and steel. Even so, it could very well be the way of the future. The need for lighter, stronger cars isn’t going away anytime soon, and like it always does, the auto industry will find new and better technologies and methods to meet demand.

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