Next Shift

Friction Stir Welding Joins Metals That Were Never Supposed to Meet

For decades the rule was simple: you do not weld aluminum to steel. Friction stir welding is breaking that rule, and the machine doing the work is starting to look a lot like a machining center.

In September 2012, Honda announced that the front subframe of the North American 2013 Accord would join steel and aluminum with friction stir welding. Honda called it the world's first use on the frame of a mass-production car. Its figures: a subframe 25 percent lighter than an all-steel one, about 50 percent less electricity in the welding process, and 20 percent more rigidity at the suspension mounting points.

No arc. No filler wire. No melting. A spinning tool and a lot of force. Here is how it works, why it matters for metals that would not weld before, and why the machine on your floor might be part of it.

Welding Without Melting

TWI, the British joining institute, says friction stir welding was invented by Wayne Thomas at TWI in 1991. A non-consumable tool, a shoulder with a probe sticking out of it, spins and plunges into the joint line, then travels along it. Friction heats the metal until it goes soft. The probe stirs the softened metal from both sides together, and the shoulder forges it down behind the tool.

The metal never melts. Research cited in a 2015 review in Jurnal Teknologi puts peak temperatures at roughly 70 to 90 percent of the base metal's melting point. TWI says the process makes butt welds from 0.3 mm to 75 mm thick in a single pass, with no filler metal and no shielding gas.

That one fact, no melting, is what changes the rules.

Why Aluminum and Steel Never Got Along

Melt aluminum and steel together and they form intermetallic compounds at the joint. A 2023 review in the journal Metals is blunt about them: they are brittle and prone to catastrophic failure. A fusion weld between the two makes a lot of them.

Friction stir welding does not stop them forming, but it keeps them thin. Because the metal stays solid and the heat input is controlled, the same review reports intermetallic layers under 2 micrometers in some aluminum-to-steel joints, and describes one study where water cooling cut a layer from 7 micrometers to 1. TWI lists aluminum to steel among the dissimilar pairs FSW can join.

The same logic applies to aluminum and copper, the pair that carries current in electric vehicles. That is where the newest demand is coming from.

It Started in Rockets

Friction stir welding earned its reputation on parts that could not fail. According to a 2002 NASA technical report, Lockheed Martin had been developing FSW for the Space Shuttle external tank's longitudinal welds since 1995. SpaceNews reported that tank ET-134, which flew in 2010, was the first with friction stir welds on all four hydrogen barrels and the oxygen barrel.

In 2014, NASA unveiled the Vertical Assembly Center at its Michoud plant in New Orleans to friction stir weld the core stage of the Space Launch System. NASA called the 170-foot tower the world's largest spacecraft welding tool. TWI says its FSW technology was used to build the SpaceX Crew Dragon flown on the Demo-2 mission and in Falcon Heavy.

It spread down to Earth, too. Hitachi built aluminum rail car bodies with it in the early 2000s. In 2007, Eclipse Aviation dedicated a 50,000 square foot FSW plant in Albuquerque and said FSW replaced rivets in more than 60 percent of its jet's structure, as much as 20 times faster than hand riveting, by the company's own figures.

Then It Moved Into Cars

Honda's 2012 subframe was the proof that FSW could join dissimilar metals at automotive volume. Electric vehicles are pushing it much further.

Battery trays and cooling plates are long aluminum welds that have to seal. In June 2024, electrive reported that KUKA won an order for 23 FSW robot cells for battery trays and cooling plates from an unnamed automaker, which KUKA called its largest single order in the field. In July 2025, KUKA announced a follow-up order for 12 more cells from a major automaker in the U.S.

The copper side is moving too. At the Battery Show Europe in June 2024, the UK firm Samuel Taylor showed aluminum-to-copper EV busbars joined by friction stir spot welding. And Bond Technologies of Elkhart, Indiana, which builds FSW and refill spot welding equipment for battery trays and chill plates, claims refill spot welding can use as little as 2.5 percent of the energy of resistance spot welding. That figure is the company's own.

The Welder Looks Like a Machining Center

Here is the part most machinists never think about: a friction stir welder is a spindle, a rigid frame and a CNC that controls position and force. That is most of a machining center.

Mazak made the connection official. In December 2018, Cutting Tool Engineering reported, Mazak bought MegaStir, an FSW tooling company in Provo, Utah, to grow its hybrid machines. Mazak builds a 5-axis VTC-800 FSW and a VTC-300C FSW, the friction stir version of the traveling column machine in our Meet the Machine on the Mazak VTC-300C. In 2023, Heller announced FSW on all of its horizontal machining center series, using adaptive control in the Siemens 840D sl.

The other route is a head that bolts into an existing spindle. Stirweld, a French company, sells one and publishes what a machine needs to carry it:

Stirweld says its head takes the welding force so only torque reaches the spindle, keeps force under 80 percent of the machine's maximum, and needs about a third of the torque of a typical cut. Those are vendor claims. The number most shops will not know off the top of their head is the Z-axis thrust rating. It is on the builder's spec sheet, and it decides whether a machine is a candidate.

What It Will Not Do

FSW is not a drop-in replacement for a MIG gun. TWI lists the tradeoffs: the down force is high, so parts need much heavier clamping than for arc welding; the tool leaves an exit hole where it pulls out; and with no filler metal, the joint gap has to be tightly controlled.

Aluminum, copper and magnesium are the easy metals. Steel and titanium need tools made of exotic materials. TWI reported a 6-meter continuous weld in 6 mm steel and tool life beyond 45 meters with cubic boron nitride tools, which shows it can be done and how much tooling it takes. Cycle time can also be long. In a 2022 case study, KUKA described battery-housing cells that took 20 minutes per part, cut to 15 by automating the hydraulic clamping.

The Labor Angle

In July 2024, the American Welding Society said the U.S. would need more than 330,000 welders by 2028. Friction stir welding does not need a welder's hand on the joint. It is a programmed, mechanized process run from a CNC or a robot controller. Operators still need training, and there is an international qualification standard for FSW of aluminum, but the skill shifts from the torch to the setup, the fixture and the program. That is our read, not a forecast.

Market researchers expect steady growth, not a boom. One forecast, from Market Glass in December 2025, puts the FSW equipment market at about US$237.5 million in 2024 growing to about US$309.2 million by 2030. That is a forecast, and other reports put the number higher.

Resell CNC Take

For most shops, friction stir welding will not arrive as a new welding department. It will arrive as a question about a machine they already own or are about to buy: does it have the Z thrust, the rigidity and the table to push a spinning tool into aluminum all day? A heavy, rigid used machining center can be a cheaper starting point than a dedicated welder. Read the thrust rating before you read the price.

What It Means for Your Floor

Know your Z thrust. Pull the spec sheet for every mill on the floor and find the Z-axis thrust rating. It is the first gate for FSW.

Look at your aluminum assemblies. Long seams in aluminum plate, extrusions and cooling plates are FSW's home ground.

Think about fixtures first. FSW clamping is heavier than machining clamping. The fixture is a bigger project than the head.

Watch the EV supply chain. Battery trays, cooling plates and busbars are where dissimilar-metal joining is growing fastest.

Buy rigid when you buy used. A machine with high thrust and a heavy column keeps more future work open, including this one. Our Next Shift on Done In One makes the same case for doing more on one machine.

The Next Shift

For a century, joining two different metals meant bolts, rivets or a compromise. A spinning tool that never melts the metal is changing that, one seam at a time, and the machine holding it is starting to look like the one already on your floor. The weld of the future may come out of a spindle.

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