A variant of friction stir welding, where the probe and shoulder rotate separately, is being investigated by TWI.
It's been dubbed dual-rotation friction stir welding.
And as the name suggests, the welding tool features a central probe and an outer shoulder which rotate independently, with respect to both direction and speed.
Fig.1. Dual-rotation friction stir welding in the same direction
In conventional FSW, the relative velocity of the tool increases from zero at the centre to a maximum at the shoulder's periphery.
The dual-rotation technique allows the relative rotational speeds of the probe and the shoulder to be varied.
For example the shoulder speed can be varied from about 30% slower than the probe speed in the same direction, to about 25% slower in the opposite direction.
So a high probe rotational speed can be achieved without a corresponding increase in shoulder speed. In other words an optimised combination of rotational speeds for both probe and shoulder can be selected.
It's been found in traditional FSW that overheating or melting along the shoulder contact side can occur, and this is dependent upon the material and process conditions used.
Melting can create fusion-related defects along the shoulder contact side weld surface.
However the dual-rotation technique can be used to lower the shoulder rotational speed and consequently reduce this tendency.
Fig.2. Macrosection of a dual-rotation stir double-sided butt weld in 16mm thick 5083-H111 aluminium alloy, at a welding speed of 3mm/sec (180mm/min), using 584 rpm for the probe and 219 rpm for the shoulder. Double-sided weld passes were made in the opposite direction, with the first pass shown on top
A double sided butt weld using non-optimised conditions was made to demonstrate that dual-rotation stir welding is practicable for certain applications.
Figure 2 shows the macro-structural features provided by dual-rotation stir welding using a Flared-triflute TM type probe.
Guide bend testing demonstrated freedom from gross defects as shown in Fig.3.
Fig.3. Guided side bend test, carried out on a double-sided butt weld, in 16mm thick 5083-H111 aluminium alloy achieved 180°
Although these are early days in its development the dual-rotation technique has demonstrated that it has the following advantages over conventional FSW.
- It provides more idealised process conditions to suit dissimilar clad plate. So the shoulder rotational speed is selected to suit the cladding, and the probe rotational speed is selected to suit the core material.
- It requires less reactive torque to clamp plates together when opposed rotation is used.
- It reduces any tendency towards overheating or melting associated with the shoulder side weld surface.
- The dual rotation technique could aid material processing and subsequent plate bending where the outer surface condition is particularly important.
Early investigations into dual rotation stir welding have revealed that the technique certainly works for butt welding 16mm thick 5083-H111 condition aluminium alloy.
The results from the dual-rotation show promise, but work will continue to investigate the use of dual-rotation on spot, butt, and lap welds. Trials will also be undertaken to achieve improvements in traverse rate, and investigate tool tilt angle. Further work will also be undertaken to study the use of the contra-rotation variant (see Figure 4).
Fig.4. Principle of contra-dual-rotation friction stir welding
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