Friction-stir welding handles applications almost unweldable with fusion-welding techniques.

Leslie Gordon, associate editor

Friction-stir welding creates a solid-state bond with excellent mechanical properties.

In friction-stir welding, the tool spins and travels forward, generating a plasticized zone around the pin. Pin pressure crushes or "stirs" this plasticized material, while the tool shoulder consolidates the metal into the joint to create the bond.

With its new machine, EWI will handle complex curvatures on large parts such as airplane wings, frames, and even full-sized vehicles.

Friction-stir welding may seem too good to be true because it consistently produces solid-state, low-distortion welds in several traditionally difficult applications. But for certain automotive, aerospace, and heavy manufacturing jobs, the process reduces weight, lowers costs, and produces higher-quality welds as compared to fusion-welding techniques such as TIG and pulsed-arc MIG.

Friction-stir welding handles butt, overlap, T-section, fillet, and corner welds on products including automotive-structural components, space-shuttle fuel tanks, and ferry decks. These products are made from materials such as aluminum alloys, copper, lead, and magnesium — some of which are almost unweldable with fusion-welding techniques.

"Friction-stir welding is a revolutionary solid-state joining technology, which has been one of the most critical emerging technologies over the past 10 years," says Harvey Castner, director of the Edison Welding Institute (EWI) government programs office. EWI, based in Columbus, Ohio, is an engineering and technology organization dedicated to welding and materials joining.

Friction, not fusion
In a solid-state welding process, joining occurs without the need to melt the work material. Materials to be joined are heated to their plastic state, typically 60% to 80% of their melting point.

In friction-stir welding, friction creates the heat. Although materials are still thermally effected, they don't melt. The resulting welds are therefore left in a fine-grained state and are not prone to cracking or gas porosity.

Friction-stir welding, invented and patented in 1991 by The Welding Institute (TWI), Cambridge, England, closely resembles milling. The process employs a specially shaped, nonconsumable, rotating tool that features a pin, or probe, protruding from the tool's larger-diameter shoulder. This tool is plunged slowly into the joint between two rigidly held pieces to be welded, until the shoulder touches the material and the pin pierces a small hole. The process accommodates a gap of up to 10% of the material thickness.

As the tool spins and travels forward, it generates frictional heat, which creates a plasticized zone around the pin. Pin pressure crushes or "stirs" this plasticized material, while the tool shoulder consolidates the metal into the joint, creating a solid-state bond with excellent mechanical properties.

Since friction-stir welding involves no melting, the process eliminates the problems associated with fusion welding such as fumes, arc glare, spatter, solidification cracking, shrinkage, and solidification stresses. And because friction-stir welding doesn't require filler materials or certified welding personnel, it eliminates the costs associated with conventional welding. Also, in mass production, friction-stir welding often does away with the need for post-weld operations such as grinding, brushing, or pickling. However, there are a few disadvantages, including high capital equipment and tooling costs.

Stirring saves money
Friction-stir-welding material and process substitution can save shops money in other ways too. For example, it lets the automotive industry replace steel parts with lighter-weight aluminum. Therefore, engineers can add vehicle features without increasing overall weight. Also, the lack of distortion ensures flat and straight weldments, reducing the time required for post-weld machining.

Shops can perform friction-stir welding on readily existing, simple, and energy-efficient machine tools by buying a machine system from one of several vendors, or converting existing equipment. For example, shops can convert old milling machines for simple operations (Note: a fabricator must first buy a process license from TWI). The machine tool used must deliver consistent spindle rotations and travel rates as well as provide sufficient rigidity. The machine must also be able to withstand the forces imposed on the its frame. Proper weld parameters, such as spindle rpm, feedrate, tool geometry, and position of the material must be established.

Other considerations include tool-material selection. Steel tools can be used to weld aluminum or magnesium. But, special high-temperature tool materials and water cooling are required to join copper, steel, stainless steel, or titanium.

Working with EWI
Another option is working with EWI, a membership-based organization with extensive experience in friction-stir welding and other mate-rial-joining processes. EWI supports about 3,000 government and industrial organizations globally that produce everything from raw material to final product.

EWI is working with the U.S. Army and industry to define and develop friction-stir welding applications for the Army's Future Combat System (FCS). Tim Stotler, senior engineer at EWI, explains, "Our organization has several large projects with the Army and was selected because of its welding capabilities. The main focus is how to join one alloy to another and how to solve some of the Army's joining problems."

Stotler continues, "For friction-stir welding, EWI has several machines, including a vertical-horizontal mill and a converted horizontal planer. These two machines operate only in the X-Y plane. A third machine provided by the Army is a five-axis machine that can join complex curvatures. An additional machine is being manufactured by the General Tool Co., Cincinnati, to support welding large military and commercial applications."

Big machine tackles large parts
The funding for the new machine comes from a $22 million, five-year agreement with the U.S. Army Research Laboratory, Aberdeen, Md. With this machine, EWI will handle complex curvatures on large parts such as airplane wings, frames, and even full-sized vehicles.

The machine features a load capability of up to 35,000 lb, with complex curvature capabilities on its seven axes. It will be the largest gantry-type machine available in North America with a working envelope 10 1026 ft. A rotary/tilt table will be integrated onto one end of the machine, so it can weld round parts. Stotler adds, "At 10 ft, the Z axis has the most clearance available under the spindle of any current friction-stir welding machine. It will have all the standard features of most friction-stir machines, as well as complex weld-path planning downloaded from CAD/CAM software. EWI will also be adding special clamping features. And the machine has high-speed machining capabilities for applications requiring pre or post-weld operations."

To date, most friction-stir welding is performed on aluminum, which can be extensively deformed without recrystallization. However, work has been done with higher-temperature alloys such as steel, titanium, Inconel, and nickel alloy.

Contact information:

• EWI Columbus Phone: 614.688.5000 ewi.org
• TWI Cambridge, U.K. twi.co.uk