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Determining if your application could benefit from rotary friction welding

It used to be that the word welding evoked visions of a welder in a mask with flying sparks being emitted from hand-held torches – a natural reaction since resistance welding, invented in 1877, and metal-arc welding, introduced in the 1940s, account for the largest volume of welding today.

Since then, many new welding processes have been and are continuing to be developed. In particular, processes such as friction welding are receiving considerable attention thanks to their ability to assist in the development of lightweighting technologies and multi-material joining.

Rotary friction welding (RFW) was developed in the 1950s and is the oldest type of friction welding. RFW joins a spinning workpiece to a stationary one by rotational friction and externally applied force (see Figure 1).

The heat produced by the spinning process is of a high enough temperature for the materials to reach a plastic state at the joint interface. Because the material never really melts, but just softens enough to join the two materials together in a solid state, the resulting weld quality is often superior to that of an arc weld. Solid-state welds also have less distortion than MIG and TIG welding, which is a major benefit of the process (see Figure 2).

Additionally, RFW is extremely fast, making it an appropriate solution for high-volume production environments. And, RFW machines can be automated, enabling integration of the process directly into a production cell, helping further reduce production time.

Although its use is generally restricted to cylindrical or circular parts – creating a 100 percent bond of the contact area around the diameter – a major benefit of RFW is that it is an excellent technology for the joining of dissimilar materials. In fact, historical usage shows that friction welding of dissimilar metals accounts for nearly half of the welds made by the process.

There are two variants of RFW – direct drive and inertia. In the direct-drive version, a motor keeps the part spinning at a constant rpm whereas the inertia version uses energy stored in a flywheel to keep one part spinning. Although both produce similar quality welds, the direct-drive process offers slightly better control.

A clear choice

Coldwater began offering RFW systems and services about 12 years ago, focusing on the high-speed, direct-drive RFW process under the SpinMeld brand. Coldwater views the technology as an alternative for companies currently using a more traditional drawn arc stud or projection weld or MIG or TIG welding for thicker components.

Figure 2. In RFW, the two materials soften and fuse together.

Although these approaches are competitive to SpinMeld in terms of time and quality, the RFW process offers some definite advantages in today’s environment where the focus on lightweighting has taken center stage. It is particularly well-suited for airbag inflators, drive shafts, camshafts, hollow engine valves, axles, shaft bracket assemblies, suspension links and retainers, to name a few.

Traditional thinking looks at projection welding as the tried-and-true solution for attaching studs, nuts and other fittings. However, with this technology, the stud or fastener has to be oversized to achieve the desired weld strength. The RFW process creates joints of forged quality, resulting in a higher strength bond than that created with fusion processes such as MIG and TIG welding. This superior weld strength means that attachments do not need to be oversized, helping to lighten weight.

SpinMeld technology can join steels, aluminum, cast or sintered metals, magnesium, brass, carbon fiber composites, ceramics with mixed metal connections and dissimilar combinations of these materials. While many companies offer friction welding systems for the joining of large components, Coldwater has purposely focused on parts with solid diameters below 1.5 in. (38 mm), though it can weld hollow tubes at larger sizes.

In addition to its ability to join dissimilar and lightweight materials, other RFW benefits include high-quality joints with a small heat-affected zone and consistency in weld duplication. It’s also environmentally cleaner and safer with no filler material, spatter, smoke, radiation or shield gases.

As compared to alternative joining processes, it’s clear that RFW offers some significant advantages. It is quick, inexpensive, eliminates the need for consumables, can accommodate in-process monitoring for quality assurance, and due to its solid-state nature allows the parts being joined to retain properties close to those of the parent materials.

Is RFW for you?

It’s human nature to become comfortable with the way you’ve always done things. But, just because a process isn’t “broke,” doesn’t mean there aren’t improvements to be gained by new approaches. For example, although MIG and TIG welding are reliable processes, companies need to examine each application to see if RFW might be a better solution, and not just for joining of dissimilar materials. In many applications, there are also productivity gains.

Coldwater’s SpinMeld RFW system.

For instance, someone welding a steel rod measuring 0.5 in. in diameter would typically use a MIG or TIG process. SpinMeld, however, can join the materials quickly using a speed of 20,000 rpm with a force of about 3,000 lbs. to accomplish the weld in approximately 6 sec. (see Figure 3).

Conversely, the TIG or MIG process is going to be slow and have a lot of heat input that will cause distortion issues. With SpinMeld, the weld integrity is better and the perpendicularity of the rod is going to be close to perfect. Additionally, there are no fillers or fumes. 

Although RFW does offer many benefits, there are considerations to be taken prior to moving to the technology. The biggest one is there needs to be a sufficient level of production to achieve acceptable ROI on the machine investment. For SpinMeld in particular, well-suited applications include:

  • Components that are round, hex or symmetric in shape, such as standard nuts, flange nuts, studs or flange studs
  • Components at least 0.1 in. thick and up to 4 in. to 5 in. long
  • Diameter of 1.5 in. or smaller for solid pieces and up to 2 in. for hollow tubes
  • Average part to part cycle times of about 7 sec., but it can be less depending upon the component
  • Maximum weld area of 1.77 in.2 (1,140 mm2)
  • Full-strength bond with no additional weight

Transitioning to RFW

From Coldwater’s experience, engineers often have a limited view of the technologies available for what they need to accomplish. Typically, the customer has a process already in place, requiring a Coldwater representative to go in and prove that a weld can be produced of equal or superior quality using Coldwater equipment. However, it isn’t a simple apples-to-apples change. It often requires assistance from the welding system supplier.

In most cases, once the customer is presented the technology, they understand its benefits. But, then they are left to figure out how to apply it in their particular process.

Fortunately, Coldwater has already done the background work to help companies make the transition. For example, if they’ve been using a typical M12 stud weld, Coldwater can match them up with a new fastener design that would enable the implementation, as well as the integration, of a SpinMeld machine into their manufacturing environment.

One example highlights a customer that had invested in RFW machines for joining an aluminum nut to an aluminum box member. The customer was originally designing their own nuts for the process. The shape of the nut required very specific alignment from the weld head, which is spinning extremely fast as it descends to complete the joining process. The spinning head kept knocking the nut out of the holder causing a misalignment and creating a weld fault. This was devastating the cycle times and producing excessive scrap.

Coldwater intervened and took control of the nut head design, running more than 1,500 spin nuts through multiple design iterations. Coldwater simplified the nut design and increased the thread engagement, thereby removing the features that were causing the misalignment.

With the initial new nut design, the customer experienced about 20 percent fallout of successfully picking up the nut. Therefore, the process was re-examined leading to a change in tool, which improved fallout to only 2 percent. The new tool design, which accommodated an emboss feature on M6 spin nuts, provided three times the tool life as well as being three times less expensive than the original design.

Today, this RFW process results in less than 1 percent scrap for the customer. The solution has allowed them to achieve their quoted cycle times as well as reduce scrap and downtime.

The lesson here is that new technologies can reap large improvements. If you’re currently manufacturing anything with a cylindrical component – be it tube-tube, tube-bar, bar-bar, bar-plate or tube-plate – then RFW delivers some obvious benefits:

  • Joins dissimilar metals
  • Airtight welds absent of voids
  • Higher weld strength
  • Minimal heat-affected zone
  • Consistent weld duplication
  • No consumables or fillers
  • Process monitoring of weld parameters for statistical process control

Additionally, in the case of a SpinMeld system, there is an opportunity for increasing production rates through fully automated part load/unload and inspection capability. Plus, the servo-driven feature brings critical parameters under control with in-process monitoring to improve part quality and eliminate post-weld inspection.

But, the bottom line is that you just don’t buy a machine off the shelf and expect to push the “go” button with a perfect outcome. Work with an experienced supplier that already has the foundational base addressing your particular application to help you realize all the benefits that RFW technology can offer.

Coldwater Machine Co.

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