Friction stir spot welding is a version of friction stir welding. This process crucially works below the melting point of the target materials. This differentiates it from other welding processes which work at much higher temperatures. There are advantages and disadvantages of this process over alternatives.

What is friction stir spot welding (FSSW)?

Friction stir spot welding (FSSW) creates single spot welds using a rotating tool, which generates large forces and heat from the friction and high pressure, changing the composition of the two materials to be welded (plasticising). The tip of the pin plunges into the joint area between the materials and stirs-up the oxides. The shoulder applies a high forging pressure, which bonds the components without melting the materials. A spot weld can be made approximately every 5 seconds.

The pin is the part of the tool that penetrates and the shoulder may be profiled to push the plasticised material in a particular direction and to efficiently break-up and disperse the oxide skins. The rotating tool is surrounded by a non-rotating clamping ring with which the workpieces are pressed firmly against each other.

The crucial parameters are the rotating speed and contact pressure. This is the plunge feed rate. Modern spot welding guns can be used either via position control or force control or via a product-specific programmed force-displacement control. Position control is used and then switched to force control during the dwell time. 

Welding guns

Friction stir spot welding process advantages

No damage or distortion to the material, unlike in alternative processes caused by the extreme heat, such as through other joining processes such as laser or arc welding. In certain aluminium alloys, the strength in the weld seam and the heat-affected zone is much higher than in other welding methods. Other advantages include;

• A solid-state material reduces the likelihood of defects associated with melting and solidification e.g. pores
• Lower temperatures reduce the probability of shrinkage
• The ability to join certain traditionally difficult alloys of aluminium
• No fumes or spatter
• Automation is viable and hence this process is suitable for production

Stir spot welding applications

The high strength joins makes this processes suitable for parts that are exposed to particularly high loads. In addition to automotive, the aerospace industry is evolving the process. Aluminium and copper can be friction stir spot welded.

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