Butt joints with square edges are optimal in terms of material flow. Also, lap welding joints can be done, but it is more difficult than butt welding because wider welds are necessary and, for aluminium alloys, oxide disruption at the sheet interface is more difficult, special tools are used, such as Flared-Triflute and A-Skew probes. These tools allow the dynamic to static volume ratio to be increased, improving weld quality. One other variant ща FSW is spot welding, developed for lap joints, that produces spot welds having higher mechanical strength than those produced by resistance spot welding.

No grooving is used in FSW because of of a risk of defect formation since the strategy is to press the workpiece butt edges against each other without a gap and thus compensate a separating force exerted by the FSW tool. However, our experiments showed that some small gap may be acceptable and did not result in any open channel defect. Grooving may be useful when using a consumable FSW pin.

Let me start by saying FSW/FSSW is based on establishing a sufficient frictional and deformational heat input require for intermingled material flow (metallurgical bonding) in the base materials. Thus, all joint types in which sufficient inter-material flow will be lacking are not desirable for FSSW/FSW.

With butt and overlap joint configurations, one can easily achieve a good tool-base material contact (meaning: adequate friction, through-thickness flow, bonding, etc) and with a simple fixture design. For instance, adequate contact and consequently sufficient material flow cannot be achieved in fillet welding and groove joint configurations with FSSW/FSW. As a result, I reckon that a good contact and the capacity of establishing intermixed flow (for metallurgical bonding) between the base materials are notable criteria for joint selection in FSW/FSSW.