Refill Friction Stir Spot Welding

Job Knowledge 150

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). 

To address these concerns, a new variant of the process - refill friction stir spot welding (Refill FSSW) 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.

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. 

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 (SZ_S) and by the probe (SZ_P). 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 microstructural composition of these zones are dependent on the material and processing conditions (parameters and tool design, for example). 

Applications

Refill FSSW has been mainly used to join heat treatable aluminium alloys, without and with surface protection, for example anodized or coated. Dissimilar combinations between aluminium and other engineering materials, such as magnesium, steel, titanium, copper and carbon fibre composites, have also been explored with successful results. Welding of polymers and polymer composites have also proven possible.

As a single-spot welding process, Refill FSSW is an alternative solution to replace mass-adding fastening processes, such as rivets, and to address challenging material combinations for fusion spot welding processes. Although this technology is only currently being used for research and development applications, possible industrial sectors for this technology include:

• Shipbuilding and Marine 
  • panels for decks, sides, bulkheads and floors
• Aerospace 
  • Skin/stringer applications 
  • Fuselage and wing structures
• Railway industry
  • Rail stock vehicle floor, side and roof panels, namely for high-speed trains
  • railway tankers
• Automotive 
  • engine chassis cradles
  • car body structures

Advantages

Refill friction stir welding offers many advantages over current spot joining processes, especially when joining aluminium alloys:

• Remaining in the solid-state, avoiding many of the defects associated with melting and solidification during fusion welding, such as pores and solidification cracks. 
• The peak temperatures are lower, allowing a reduction in distortion and shrinkage.
• No filler material or third body is added to the weld.
• No exit hole and the roughness of the spot weld is lower than the base material.
• Being able to join many ‘non-weldable’ aluminium alloys, namely from the 2xxx and 7xxx series.
• No fumes, porosity or spatter are generated.
• Easily automated, making the process suitable for production.
• Energy efficient.

Summary

TWI Ltd has developed experience and knowledge in the Refill FSSW process across various industry sectors. If you would like to know more about the process, please contact friction@twi.co.uk