Friction stir spot welding (FSSW) is a solid-state joining process, which is a derivative of friction stir welding (FSW); however, during FSSW, no linear movement of the tool occurs. This results in a localised ‘spot’ weld. Although one continuous process, FSSW occurs over three stages: plunging, stirring and extraction, as shown in Figure 1.A). The key industrial drivers for this technology are to replace mass-adding fastening processes, such as riveting, and to join a range of dissimilar materials, which are currently difficult to join using fusion welding processes. Despite the interest in the technology among a range of sectors, there has been limited industrial implementation, which is partly due to concerns about the small weld area and exit hole produced.
The Refill FSSW process has two variants, shoulder-plunge and probe-plunge, which depend on the plunging component.
The principle of the Refill FSSW process is described below and its variants are shown in Figure 3 and Figure 4:
Stage 1 - As the weld cycle begins, the three components move to the surface of the top sheet and dwell for a certain amount of time to produce initial frictional pre-heating.
Stage 2 – For the shoulder-plunge variant, the shoulder is the plunging component to a set depth into the base material. At the same time, the probe retracts to create a chamber for the displaced material to flow into. During the plunge stage, the friction on the shoulder causes the material to heat and soften. For the probe plunge variant, the principle is similar but the probe is the plunging component.
Stage 3 - The rotating components return to the surface of the top sheet whilst rotating and the previously retracted component consolidates the weld material by offsetting the positions from stage 2.
Stage 4 – The weld cycle finishes by extracting the tool from the surface of the material.