Rotary friction welding (RFW) is a solid-state joining process which works by rotating one workpiece relative to another while under a compressive axial force. The friction between the surfaces produces heat, causing the interface material to plasticise. The compressive force displaces the plasticised material from the interface, expelling the original surface oxide layer and other contaminants and promoting metallurgical and/or surface interlocking joining mechanisms. This deformation process forms a flash collar and causes the workpieces to shorten in the direction of the compressive force. Once the required shortening has been achieved (known as burn-off distance) the rotation movement is ceased and a forging force is often held, or increased, for a period of time to help consolidate the weld. There are two primary mechanisms for delivering the energy to the weld interface:
- Direct drive - the rotating part is continuously driven by the equipment spindle motor (Figure 1).
- Inertia - the rotating part is connected to a flywheel which is disconnected from the drive motor once a desired rotating speed is achieved. The workpieces then engage and the flywheel supplies the energy to the interface. During this approach the rotation speed gradually decreases to a stop (see Figure 2).
RFW is widely implemented across the manufacturing sector and has been used for numerous applications, including:
- Turbine shafts
- Automotive parts including steel truck axels and casings
- Monel-to-steel marine fittings
- Piston rods
- Copper-aluminium electrical connections
- Cutting tools
- Tubular transition joints combining dissimilar metals (Aluminium-Titanium and Aluminium-Stainless steel, for example)
Microstructure and Mechanical Properties
Rotary friction welds are similar in appearance in that they have several distinct zones: a weld centre zone (WCZ), a thermo-mechanically affected zone (TMAZ) and a heat affected zone (HAZ). The extent and microstructural composition of these zones are dependent on the material and processing conditions used. The weld region is surrounded by a flash collar. A typical example of a weld is shown in Figure 6.
When optimum processing conditions are used, RFW can produce joints that are superior or similar in strength to the parent material. This is true for many similar and dissimilar material combinations.
Rotary friction welding offers many advantages over competing manufacturing processes, for example:
- The weld remains in the solid-state, avoiding many of the defects associated with melting and solidification during fusion welding, such as pores and solidification cracks. The distortion of the welded component is also reduced.
- The process has lower peak temperatures than fusion welding, reducing intermetallic formation and allowing for a range of dissimilar materials to be joined.
- The process does not require a filler metal, flux and shielding gas.
- The process is easily automated, making the process highly repeatable and not dependant on human influence, resulting in very low defect rates.
- When used to fabricate preforms, the material usage and manufacturing costs are reduced when compared to subtractive techniques (eg machining from ingots and forgings).
Rotary friction welding offers many advantages to the manufacturing sector for a wide range of applications. TWI Ltd has developed extensive knowledge of RFW over many years of research and development. If you would like to know more about the process, please contact us.