Think friction welding and the image that most likely comes to mind is a rotary process for butt-joining cylindrical or tubular materials under an axial load. For many years, the impression of friction welding was that of an expensive, custom process suitable only for certain high-volume applications. The process is well-known in automotive, truck, and heavy-equipment circles, where almost every axle or driveshaft has some friction-welded component. Yet practitioners such as Joel Donohue, general manager of American Friction Welding (AFW), a full-service job shop in Brookfield, WI, describes the process as greatly underutilized by industry. Shops such as his can efficiently provide five-piece orders if that's what the customer requires.

With patents going back to the late 19th Century, rotary friction welding remains the most common commercial friction welding process. Modern friction welding in the US began moving forward in the early 1960s, mainly at Caterpillar, Rockwell International, and AMF. According to Dietmar Spindler, general manager of Manufacturing Technology Inc. (MTI, South Bend, IN), the first two modern friction-welding machines sold were by AMF for steering worm shafts and by Caterpillar for turbochargers. Both companies used flywheels as a capacitor to store all or part of the energy for welding. Caterpillar called their system inertia friction welding, while AMF called theirs flywheel friction welding. Rockwell built four machines for its own use to weld spindles to truck differential housings, and never sold friction welding equipment to outside customers. In 1975, MTI purchased the patents, rights, and know-how from Caterpillar and AMF for the flywheel-based inertia friction welding process. Ten years later, MTI also purchased the rights to the New Britain line of friction welding equipment.

Friction welding basically works by producing heat by rubbing two components together under load. Once they reach the required temperature and material deformation, the action stops and the load is maintained or increased to produce a solid-phase bond. According to Dave Nicholas of Cambridge, UK-based The Welding Institute (TWI), friction is ideal for welding dissimilar metals with very different melting temperatures and physical properties, such as copper to aluminum, titanium to copper, and nickel alloys to other steels. As a rule of thumb, forgeable metallic engineering materials are friction-weldable, as well as castings and powdered metals. MTI's Spindler adds that most metals can be welded and post heat-treated to 100% base metal strength. And every weld should be tested in a setup that mimics its intended purpose rather than relying on standard mechanical test results. For example, a hardened SAE 10B35 or 1045 weld would be the equal of any forging in a drive shaft used to transmit torque, but can be as much as 30% weaker in a push-pull operation.

Every friction weld should be tested in a setup that mimics its intended purpose.

As a job shop specializing in friction welding, Joel Donohue describes AFW as fairly unique. "We have very few competitors, and we cover many industries across North America," he says. "We weld pump shafts, electric motor shafts, printing press rollers, and a lot of automotive and construction equipment applications such as tie rods and axles." In conjunction with their full-service machine shop next door, American Friction Welding supplies turnkey projects as well as welding-only services for customers. The company uses direct-drive rotary friction welders, where motors rather than flywheels or inertia-type momentum provide the welding energy. "There's a little different heat-affected zone [HAZ] and grain flow, but basically the results are similar between the two," he says.

Friction welding is both a conventional and growing process for Donohue's company. "We get some offshoots of axle or driveshaft work on a regular basis," he says, "small production runs the customer doesn't want to tool up for. On the other hand, I get at least one phone call a day form people wanting more information on the process, looking for a part sample, or asking if they can pay us a visit. I would say unequivocally that friction welding is under-utilized in manufacturing today."

How does Donohue evaluate whether a customer's request is right for friction welding? "The first thing is to get a visual on what type of parts need welding and their configuration," he answers. "If the part's not cylindrical or tubular, which is the most common case for rotary friction welding, you have to tool up for it. Basically the parts need to spin and butt-up at the weld joint. If there's not a natural axis of symmetry, the tooling needs to create it."

The next consideration is materials. As mentioned earlier, friction welding is ideal for welding dissimilar metals with different melting temperatures and physical properties, but that's not to say the process is ideal for all metals. "There are a few materials we need to stay away from, namely free-machining and resulfurized steels," Donohue says. "That extra lead or sulfur in such steels creates nonmetallic inclusions for breaking chips. These materials, particularly at the weld joint, tend to counteract what we're trying to do, which is create friction and heat."

Another phase in evaluating if a part is right for friction welding is asking how it would be joined otherwise. "Many people realize they can mate dissimilar materials, but that doesn't mean they've fully evaluated the alternatives," adds Donohue. "We often find early on that we can replace castings or forgings, which can be expensive to tool. For example, we can use friction welding and pre-machined bar stock to create a cast-like blank. This helps keep material costs and machining time down, for relatively low up-front tooling cost. And friction weldng is the preferred process in such cases, as conventional MIG or TIG arc welding would represent a weak point in the part."

Post-weld testing also is an integral part of the process. "We test for many conditions depending on the part, material, and customer requirements," Donohue explains. "Typical test results might be for torsional or bend moments, hardness evaluation of the heat-affected zone, and a tremendous amount of ultrasonic inspection to evaluate the weld."

The last item in evaluating a part for friction welding is volume. According to Donohue, high volumes are a no-brainer, as the process is speedy and can turn out hundreds of parts per hour depending on the application. "But given the right attention, prototype and short-run parts can be cost-effective as well," he says. "A customer came to us for a 2.5" [63.5 mm] Hastalloy shaft for a highly corrosive pumping environment. After looking at the part, it turns out two-thirds of the shaft stays in an enclosed gear case. We were able to replace two-thirds of the original Hastalloy shaft with stainless steel by friction welding. Not only did we save about 75% in material costs per shaft, total machining time per part went from eight hours to two hours."

Friction welding is ideal for joining similar materials, but there are some metals unsuitable for the process.

While rotary welding remains the most common friction welding process so far, others exist and represent alternative friction welding solutions for particular applications. One is friction stir welding. A friction stir weld is formed by plunging a rotating shouldered pin tool with a pin length slightly less than the required weld depth into the faces of the materials to be joined until the tool shoulder is in contact with the work surface. The rotating pin within the workpiece causes friction, heating the metal and producing a plasticized tubular shaft of metal around the pin. As the pin moves in the welding direction, the leading face of the pin, assisted by a special profile, forces plasticized material to the back of the pin while applying a substantial forging force to consolidate the weld.

According to TWI, further developments within friction stir welding demonstrate that plate thicknesses to 75 mm in 6082 aluminum alloy can be welded in two passes. Other metals including copper, lead, magnesium, and titanium have been successfully stir-welded, but more studies are needed.

Friction welding's benefits extend beyond joining dissimilar materials in that the process is environmentally friendly, producing little smoke or slag and requiring no flux, gases, or filler material. Friction welding is also energy-efficient, using between 25 to 100 watts per square centimeter of weld area under normal operating conditions.

The fact that friction welding is a simple, machine-controllable process helps. According to MTI's Dietmar Spindler, many manufacturers rely on in-process monitoring for weld quality control.

Standard friction-welding process controls monitor and feed back pressure and slide control in a closed loop to improve accuracy and cycle time. A standard operator interface would show part number and weld cycle, last weld report, and a real-time view of speed, position, weld time, and dwell time. Control options include the ability to select and save weld parameters by part number and weld options, such as one or two-stage welds.

"In the last two years, we have re-controlled most of our machines to better monitor pressure, time, rpm, and other weld parameters," says Joel Donohue of American Friction Welding. "We do work to ISO and QS9000 requirements for customers, so the process is certifiable. Last year, having better control of the process was prompted by the customer, this year it's by us."

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