Refill Friction Stir Spot Welding (Refill FSSW)
Refill Friction Stir Spot Welding (RFSSW), or “refill spot welding” for short, is a method of joining sheets of aluminum that improves upon the drawbacks of resistance spot welding or rivets.
RFSSW was invented at Helmholtz-Zentrum Geesthacht (HZG) in Germany. It is an emerging technology for both aerospace and automotive applications. Bond investigated the capabilities and potential opportunities for this process, and developed a machine that provides the performance required to meet industry objectives.
Overlapped aluminum sheets are commonly used in airplane fuselages and wings, automotive & truck body panels, and many other products requiring a high structural strength-to-weight ratio. Commonly used techniques include rivets (aerospace), resistance spot welding, and self-piercing rivets (automotive).
RFSSW has been shown to produce excellent tensile strength and fatigue strength, in many cases superior to current joining methods. Aluminum alloys in the 2XXX, 5XXX, 6XXX, and 7XXX series families have been demonstrated. Cycle times as low as 600ms per weld are possible.
In order to create the best-performing equipment for RFSSW, Bond worked collaboratively with researchers and industry members to identify key features that would be required for success. Capabilities that distinguish Bond’s RFSSW head from others include higher torque, faster traverse speed, and an advanced control system. Bond’s deep understanding of solid-state processes translates into superior equipment design. Bond’s RFSSW head can be used in a fixed mount stationary configuration, or as an end-of-arm tool for robotic applications.
A Twist on Rivets and Spot Welding for Aluminum
The benefits over previous technologies such as rivets or resistance spot welding include:
- Plastic deformation refines the grain structure and stays below the melt point, an improvement over melt-phase processes such as resistance spot welding.
- Friction and plastic deformation generate all of the heat needed for the process, greatly improving energy efficiency.
- No consumable is required aside from periodic tool replacement.
- Top sheet thickness can vary for a given toolset by simply choosing different program parameters, unlike rivets.
- The refined microstructure has high resistance to crack propagation, allowing for good fatigue strength.
How RFSSW Works
RFSSW is similar to conventional Friction Stir Welding in the way that heat and deformation are produced. The process relies on friction and plastic work produced by relative motion between a non-consumable tool set and a workpiece. As a result, it is undesirable to liquefy the workpiece, since the source of heating depends on resistance to relative motion. The solid-state nature of the process produces all of its desirable characteristics.
In the RFSSW process, three concentric tooling components are used to plasticize and mix two (or more) overlapped sheets of metal. To date, aluminum is the most practical material of choice.
An outer clamp ring is non-rotating, as it presses down to secure the top sheet to its substrate. A shoulder, which is also a ring, and a probe, which is a cylinder within the shoulder, rotate together and move down flush with the surface. The shoulder and probe spin and push to generate frictional heat, whereupon the shoulder descends into the sheet while the probe retracts.
This combined vertical motion allows plasticized material to be displaced from the shoulder area into the volume created by the probe. Once the shoulder is at full depth, which approximates the top sheet thickness, the top and bottom sheet materials become mixed together. Then, the process reverses the shoulder, and the probe returns to a position flush with the top surface. What results is a top and bottom sheet that are joined by a circle the size of the shoulder outer diameter, and a smooth top surface.