Prehistoric man, accustomed to making fire from striking two rocks together, would have understood the general idea of friction welding. But today’s concept uses friction to make something less than fire—it creates plasticity in metals but keeps them several hundred degrees under their melting temperatures, then forges them together.
As a result, unlike other welding processes, friction welding retains the workpieces’ base metal properties. The process can join metals unweldable by other means and is controllable and repeatable. Friction welding, a solid-state process, includes several variations, some well-established and others developed more recently. All require machine tools, usually automated, instead of manual processes, and capital investment is higher than conventional welding.
Rotary friction welding (RFW), the oldest type of friction welding, was invented in the early 1950s and is widely used for round parts. Two variants, direct-drive and inertia welding, have been in use the longest. Today, advances in control methods and other features such as rotational orientation and tighter loss-of-length control allow wider use of the process, according to Tim Stotler, technology applications leader for EWI (Columbus, OH), an advanced manufacturing technologies organization.
RFW has made millions of production welds, from very small solid parts to very large diameter tubing, according to Mike Spodar, senior welding engineer for Coldwater Machine Co. (Coldwater, OH). “Process development is focused on materials, including more dissimilar material welding,” he said. “In automotive, RFW will be used for vehicle lightweighting as the transition from carbon steels to high-strength steels and nonferrous alternatives continues.”
Coldwater’s high-speed, direct-drive RFW process, called SpinMeld, focuses on welds with solid diameters below 1.5″ (38 mm), though it can weld hollow tubes larger than that. Dan Barry, vice president, sales & marketing for Coldwater, noted that projection welding has traditionally been used to attach studs, nuts and other fittings, but in many cases the stud or fastener must be oversized to achieve the desired weld strength. RFW offers superior strength so attachments need not be oversized, helping achieve lightweighting.
“One customer is using SpinMeld to develop a heat sink for LED lighting that is lighter and lower cost than its current cast heat sink,” said Barry. “We are also working on electrical satellite components that require a nickel/aluminum bimetal weld.”
As noted above, better control is helping to advance RFW. “We use a closed-loop process to control the upset, or loss of length of the part,” said Daniel Adams, president and CTO of solid-state joining solutions and services provider Manufacturing Technology Inc. (South Bend, IN). “The result is a Tri-Mode machine that can do direct-drive welding, inertia welding, or a combination of the two called hybrid RFW. The control also improves part concentricity.” He noted that MTI has built a Tri-Mode machine for the Manufacturing Technology Center (Coventry, England) and is building another for GE’s laboratory operations.
Linear friction welding (LFW), conceptualized in the late 1960s to weld noncircular, cross-section butt joints, is an established technology in several aerospace applications, such as bladed disk (blisk) assemblies, compressors and front fans. According to Jerry Gould, technology leader in resistance & solid-state processes for EWI, development areas for LFW include increasing oscillation frequencies and more rigid tooling and drive mechanisms.
“Higher frequencies offer potential for materials with higher thermal conductivity and smaller cross sections,” he said. “Higher tooling rigidity improves joint quality. Alternate drive mechanisms, while still being developed, include programmable cam and even vibrational systems.”
Gould noted that there is strong interest in using LFW for titanium fans in new-generation aircraft engines. Also, processes for adding bosses and hangers to titanium airframe structures via LFW are underway. “In the nonaerospace arena, applications are limited by economic considerations,” he said. “However, there has been a major emphasis on using LFW to assemble railroad rail strings.”
MTI’s Adams agreed that new LFW applications, such as near-net-shape aerospace frames and high-volume automotive parts, are on the way. “In aerospace, the factor driving adoption is a lower buy-to-fly ratio due to the lower amount of materials required for fabrication and the lower amount of material needed in finished components,” he said.
For example, in a long extrusion where 30% of the material is machined away to create mounting feet, the feet could instead be LFWed to the extrusion. “Other types of welds would not be strong enough, but a solid-state joint is as strong as if you had extruded it out of a single workpiece,” said Adams.
LFW equipment is expensive and, without a large volume of applications, is difficult to cost justify. However, airframe manufacturers are asking suppliers to invest in LFW to reduce the cost and the amount of material it takes to build aircraft. The supplier base is developing multiple applications to help justify equipment cost. “For our part, we are looking at how to reduce the cost of equipment,” said Adams.
In automotive, lightweighting and the need for reduced cycle times are driving adoption, he noted. “Even though the cost of LFW is traditionally higher than fusion welding, because it is so much faster we can displace three fully automated fusion welding cells with one LFW cell,” said Adams. “Also, fusion welding requires additional welding material to compensate for a weaker weld, so LFW helps on lightweighting.”
One of LFW’s limitations is the high friction and forging forces required. However, if the rate at which energy going into the part is increased, LFW process forces and machine size can be decreased. “MTI has developed low-force friction welding for both linear and rotary processes,” said Adams. “The process is faster, uses less force and reduces cycle time compared to traditional friction welding.”
MTI has built a low-force rotary machine for an automotive customer and is building another for its application support center. A full-scale commercial launch is planned for early 2017. Also, LIFT (Lightweighting Innovations for Tomorrow, Detroit), an industry-led, government funded consortium, includes a focus on LFW. To enable production of large parts using traditional LFW, MTI is building a 75-ton (68-t) LFW machine for LIFT and members will be able to use it to create parts ranging from prototypes to full production runs. The machine will be delivered in spring 2018.
Friction Stir Welding (FSW) was invented in 1991 by Wayne Thomas at The Welding Institute (Cambridge, UK) and produces very-high-quality welds. In FSW, a rotating, nonconsumable tool spins to create friction with stationary parts. The patents covering the basic process and methods of enhancing material flow recently expired, making it less costly to use FSW.
The most common FSW applications are in aerospace and rail car manufacturing, according to Stotler of EWI. While FSW has been used primarily to weld aluminum, its use in hard metals, such as steel and titanium, are being explored. Improving fracture toughness in high-strength steels will open new application areas for FSW, he said.
FSW can join alloys unweldable by other means in large sections with low distortion. “More recently, dissimilar applications, such as aluminum to steel, are in use or showing real promise,” said EWI’s Stotler. “This offers a solid-state joining process for weld lengths not before possible.”
FSW can be used on materials with thicknesses below 1 mm, as well as on workpieces up to 75-mm thick, according to Peter Kjällström, product director, automation and handling equipment, for welding equipment supplier ESAB (Annapolis Junction, MD and Gothenburg, Sweden). He noted that the use of FSW is growing in aerospace as more countries go into space; welds in a rocket fuel tank must be absolutely defect-free.
“Also, fuel tanks must be lightweight and FSW promotes this as well,” said Kjällström. “FSW’s productivity gains are of minor interest in this segment since relatively few components are produced.” (To see a video of ESAB FSW technology being used to build a 130′ [39.6-m) rocket fuel tank at NASA’s Marshall Space Center, go to youtube.com and search for "Done in 60 seconds.”)
For automakers, railcar manufacturers and other transportation equipment makers, the quality of FSW welds is, of course, a key factor, but productivity gains are also important. Kjällström noted that battery enclosures for Tesla’s electric cars are FSWed and there are also applications using copper in the electrical component industry.
To widen FSW’s scope, attempts have been made to join carbon steel. "The challenge is finding a tool durable enough to last in carbon steel,” Kjällström said. "A tool for aluminum or copper may last for two miles of welding while the same tool in carbon steel would last maybe 10′ [3.0 m]. We are still looking for the right tool material.”
Michael Skinner, vice president, corporate business development for MTI, agreed that FSW is the dominant welding process in the North American space launch business, and said that other regions are moving in that direction. For example, Ariane 6, the latest European launch vehicle, uses FSW components. Since an FSW joint is much stronger than a fusion welded joint, less material must be used, and for every pound removed from the rocket, another pound of cargo can be added.
Despite this progress, FSW took its biggest step backward when, in the mid-2000s, Airbus moved from a metallic fuselage to a composite one for its development aircraft, according to Skinner. “Airbus had spent 10 years qualifying FSW for use in fuselage parts, and even though FSW designs were 20% lighter than composite designs, the trend to composites was underway; 90% of the research funding for airframe manufacturing went from metals into composites.”
However, since then there has been a major R&D funding shift back into metals because fewer composite airframes are being built and interest in advanced aluminum alloys is growing, according to Skinner. Airbus and Boeing have qualified FSW for fuselage, wing ribs, and other critical structures. Advanced aluminum alloys, such as AIRWARE from aluminum supplier Constellium (The Netherlands), are on the market and research data are being developed. The low-density alloy combines weight reduction with lower assembly and maintenance costs and, combined with advanced welding practices and aerostructure redesign, offers up to 25% weight reduction, according to Constellium.
Other FSW challenges are being worked on as well. One difficulty is that the process starts off cold, but within a few feet pin tool temperatures begin to increase. With a long structure (some over 50′ [15.2-[15.2-m] pin temperature at the end of the process can be significantly different than at the beginning if energy is added at a constant rate, causing distortion and bending.
To combat this problem, MTI developed IntelliStir, a closed-loop system that varies energy input (including speed and torque) based on pin temperature. The system maintains consistent weld quality and develops machine data to document it, eliminating the need for destructive testing. And, since high-temperature pin tools are typically brittle and breakage prone, controlling pin temperature extends tool life, according to MTI.
In addition to temperature control, tool material choice for FSW is critical. For example, 7000 series aluminum can be FSWed with pin tools made from H13 tool steel or MP159, a cobalt-based tool steel, because the temperature needed to make the material flow is not excessively high, according to Russell Steel, director of business development, MegaStir Technologies LLC (Provo, UT), a unit of oil and gas services firm Schlumberger. However, steels, stainless and nickel-based alloys begin to flow at higher temperatures, creating wear issues with tool steels, and may require polycrystalline cubic boron nitride (PCBN) tools, which even at 1200°C are stable and with a 3600 Vickers hardness.
For example, MegaStir has done development work with the Office of Naval Research (ONR) on the manufacturing of deck panels. “They can arc weld ¼” [6.35-[6.35-mm] [15.2-[15.2-m]s quickly, but it can take eight hours of straightening to get them flat again,” said Steel. “We clamped the plate and welded it with a PCBN tool in one pass with no finishing required.”
Orvilon, a fabrication unit of Holtec International (Jupiter, FL), used FSW to join Metamic-HT, a nanoparticle reinforced metal matrix composite that serves both as the neutron absorber and as the structural material of the fuel baskets in Holtec’s canisters and transportation casks, which are used to store spent nuclear fuel. The process replaced a metal inert gas (MIG) welding process. Using a PCBN tool from MegaStir, Orvilon welded the honeycomb structure, which is 14′ (4.3-m) tall and 16′ (4.9 m) in diameter.
Another promising application is pipeline welding; FSW requires just 10% of the manpower used for conventional welding, according to Steel. However, few industrial codes, such as the API 1104 specification for girth welds on pipelines, cover the process, so suppliers and oil and gas companies are working on FSW specs. While the cost/benefit ratio for pipeline welding is low for FSW in flat, easily accessible areas, it increases considerably for hard-to-access areas such as parts of Brazil or the Arctic, he added. And, since it requires much less equipment than conventional welding, FSW has less impact on the pipeline right-of-way—an important consideration in environmentally sensitive areas.
FSW is also growing in automotive as more companies move to aluminum from steel for lightweighting and encounter more dissimilar material joining, according to Coldwater’s Barry. For example, the company’s SpotMeld process uses a three-piece tool with a non-rotating clamp ring, a rotating sleeve and a rotating pin tool, and is based on refill friction stir spot weld (RFSSW) technology (see diagram).
“We are working with an electronics company to join thin-sheet aluminum and stainless using SpotMeld, and there is interest from companies in the electric vehicle market that are using aluminum and magnesium,” said Barry.
SpotMeld is being tested in production environments and includes a cleaning station and quick-change station where the tool can be changed automatically while production continues, according to James Schweiterman, project manager/mechanical engineer for Coldwater. “Our process doesn’t leave a hole, offers limited marking on the back side of the weld, and we can process tougher-to-weld material such as 2000 and 7000 series aluminums,” he said.
With multiple new applications, interest in friction welding is growing. And, as applications change, its advantages become clearer. As MegaStir’s Russell said, “The easy oil is gone. Drillers are having to go deeper and hotter, and with the materials they are using, conventional welding can’t get the job one. Friction welding can.”
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