28 JOB KNOWLEDGE REFILL FRICTION STIR INTRODUCTION
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. A general FSSW weld cross-section is shown in Figure 1.B).
The RFSSW process has two variants, shoulder-plunge and probe-plunge, which depend on the plunging component. The principle of the RFSSW process is described below and its variants are shown in Figures 3 and Figure 4 below:
• 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.
Figure 1. A) Schematic drawing of the conventional FSSW process and B) correspondent cross-section.
To address these concerns, a new variant of the process – refill friction stir spot welding (RFSSW) or friction spot welding (FSpW) was patented in 2004. The process uses a non-consumable tool comprised of two rotating components - probe and shoulder – assembled concentrically with a static clamp ring. The major improvement to this process, in comparison with the other friction stir spot processes, is that the rotating elements have independent vertical movement. This allows the production of spot welds without an exit hole after the tool extraction, as shown in Figure 2.
Figure 2: (Left) A weld top surface, and (Below) RFSSW equipment at TWI Cabridge
• 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.
Figure 3. RFSSW process shoulder-plunge variant
Figure 4. RFSSW process probe-plunge variant
MICROSTRUCTURE PROPERTIES
Refill friction stir spot welds typically exhibit three main microstructural regions, as shown in Figure 5. The stirred zone (SZ) is the dynamically recrystallised volume of material in the centre of the weld. Two different regions can be identified in this area, corresponding to the volume of material displaced by the shoulder (SZS) and by the probe (SZP). The thermo-mechanically affected zone (TMAZ) is an area that experiences less deformation than the SZ, therefore dynamic recrystallisation does not occur. The heat-affected zone (HAZ) is characterised by increased grain size due to local increase of temperature. The extent and
WELDING WORLD MAGAZINE | ISSUE 04 | AUGUST 2018
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