A variant of friction stir welding, where the probe and shoulder rotate separately, is being investigated by TWI. It is called dual-rotation friction stir welding and the welding tool features a central probe and an outer shoulder which rotate independently, with respect to both direction and speed. The welding head allows a range of different rotational speeds to be pre-selected, or even varied automatically by in-process control, to suit the required welding conditions.
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. An optimised combination of rotational speeds for both probe and shoulder can be selected. In traditional FSW 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. A double sided butt weld using non-optimised conditions was made to demonstrate that dual-rotation stir welding is practicable for certain applications.
To date 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. However further work is needed to characterise the performance fully and verify acceptability of the dual stir technique.
Work will continue at TWI 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.
The results from the dual-rotation show promise, but much more work will be required to develop and perfect the technique.
For more information about Friction Stir Welding, please email contactus@twi.co.uk