New equipment, processes for friction-stir welding

Features - Cover Story

Mazak’s acquisition of MegaStir adds solid-state welding to machining centers as researchers develop new ways to use the technology.

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April 9, 2019

A Megastir friction-stir welding (FSW) tool joins two copper plates without melting the base material. Florence, Kentucky-based Mazak Corp. acquired Utah-based Megastir late last year to add FSW technology to its Hybrid Multi-Tasking machine line.

Friction-stir welding (FSW) has garnered a lot of attention in recent years because of the technology’s ability to join dissimilar metals – primarily steel to aluminum. However, much of the development has been driven by low-volume needs that have resulted in large, custom-built machines.

“Historically, the initial investment was associated with large-scale, custom, one-off solutions that only had one application,” says Dale Fleck, general manager of Mazak MegaStir, a division of the machine tool builder formed late last year when Mazak acquired FSW technology company MegaStir. “Adding FSW technology to multi-function machining centers really changes that investment picture.”

Mazak has added FSW technology to two vertical machining centers (VMCs) in its Hybrid Multi-Tasking lineup – a grouping that also includes VMCs with additive manufacturing, automated gear cutting, or hot wire laser-deposition technologies. Fleck says adding FSW to the hybrid lineup will allow more shops to experiment with the technology without having to invest in single-purpose machinery.

Friction-stir technology

FSW plunges a spinning tool into the top layer of two metal sheets to be joined. The tool generates friction that heats the material, making it pliable enough to grab, then stirs it into material from the bottom layer, creating a solid-state joint without melting the metals. Staying cooler than the melting point protects material properties that can change in laser or torch-based welding systems.

“If you take aluminum and try to join it to steel or stainless steel, you’re going to have galvanic corrosion,” Fleck explains. Adhesives offer one solution, but without an intermetallic bond, joints aren’t as strong and require rivets or other mechanical fasteners. With FSW, users get the intermetallic bond without melting metals, avoiding material weakening at the melt point and the formation of dendrites, tree-like crystalline structures that can form as molten metal cools.

He adds that the two biggest barriers to FSW adoption have been equipment costs for custom equipment and costs from short-lived, expensive tools. Prior to joining Mazak, MegaStir was part of a Schlumberger division that made artificial diamonds, and it got into FSW research with the encouragement of the U.S. Defense Advanced Research Projects Agency (DARPA), which wanted to develop new military applications for the joining technology.

“We figured out that the optimal material for tooling was not artificial diamonds, it was PCBN (polycrystalline cubic boron nitride),” Fleck says. “It works and is great for high temperatures, and diamond is good for low temperatures. So, now we had two materials that covered the full gamut of refractory metals.”

He adds that better tooling eliminates the one barrier to FSW adoption, Mazak offering multi-tasking systems that cost less than big custom rigs eliminates the other. So, he expects the popularity of the technology to grow rapidly as shops become familiar with processes.

Mazak Megastir’s friction-stir tool operates off of a machine’s main spindle, mechanically joining metals in a solid-state bond.

Mainstream adoption

Mazak FSW Product Manager Joe Wilker says Mazak began working with MegaStir in 2017 because a semiconductor manufacturer asked Mazak for a VMC that could machine and friction weld.

“MegaStir had the tools, so we developed a closed-loop friction-stir process control and programming software, and it went off from there,” Wilker says. The Mazak VTC-800 FSW VMC was the first machine with the technology, and the VTC-300C FSW soon followed, debuting at IMTS 2018.

For the machine tool maker, Wilker says FSW fits nicely into Mazak’s Hybrid Multi-Tasking lineup strategy – add functions that give owners the ability to perform one more function on Mazak machines, reducing the need of specialty, single-purpose equipment.

“We have several projects in the works,” Wilker says of the early response to Mazak’s FSW technology. “There’s a lot of automotive battery work where this applies. Battery cells use a lot of copper/aluminum alloys, so FSW makes a lot of sense for those.”

Conceptually, FSW fits milling centers better than laser or heat-based welding, he adds. Milling and FSW both use spinning tools that can be controlled by a computer numeric control (CNC). Mazak engineers added FSW programs to the Mazatrol SmoothG CNC control to manage force of the thrust of the tool into material, the depth of the plunge, the geometric pattern to be welded, and removing the tool from its plunge.

Most milling tools spin clockwise, and FSW spins counterclockwise, but that wasn’t a difficult difference to overcome, Wilker adds. He also says that Mazak should be able to add the technology to any of its machines after studying the properties of the application.

“The only thing we have to look at is the application and how much thrust they want to put onto the structure of that machine, how much thrust you’re putting into the Z-axis,” Wilker says. Smaller machines might not be capable of thick-gauge FSW without warping machine structures, but if Mazak receives an inquiry for something that can handle higher forces, it could add FSW to one of its bridge-style milling machines.

Mazak Megastir equipment friction welds 19mm (0.75") steel plates at 100mm/m (4ipm) using a polycrystalline cubic boron nitride (PCBN) tool in a single pass.
Photos courtesy of Mazak Corp.

Applications

“As cars keep getting more modular components, you’re going to see FSW getting used more in the larger units,” Wilker says. Lightweighting trends will encourage automakers to continue mixing steel with aluminum, and FSW can weld smaller aluminum pieces to modular steel structures.

Fleck adds that the technology plays well with modern design tools that are allowing engineers to imagine multi-material parts with custom properties.

“I want to use low-cost steel for a component but add some Inconel so I can have some strength and anti-corrosion benefits in one spot,” Fleck says. “You can do all of that in a hybrid environment where we’re cutting and joining in the same machine.”

FSW equipment welds stainless steel to steel, creating a minimal heat-affected zone at the bimetallic weld point for both metals.

Large molds, such as the ones used for commercial vehicle tires, can also benefit from the ability to add material to specific spots on smaller blanks instead of having to cut patterns on much larger blocks of steel.

Because the system runs on familiar CNC equipment, Fleck says machinists with no welding experience should be able to use it. With most manufacturers having a tough time recruiting skilled workers, Mazak FSW machines could give some shops new capabilities to make their current employees more productive.

“Most machining facilities have machining right next to welding, but they’re different disciplines with different personnel,” Fleck says. “When you can integrate those two and have one person do both on one platform, it can provide a real use case for the facility to consider this technology.”

Mazak Corp.
https://www.mazakusa.com