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Process Capability Study for Friction Stir Spot Welding

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TWI Technical Literature Review 23973

By Khalid Nor and Dwight Burford

Background

This technical review provides an overview of FSSW principles and practices based on a literature and industry survey.

Friction stir spot welding (FSSW) is a solid state welding process in which the baseline enabling technology is friction stir welding (FSW). In conventional FSSW, the non-consumable FSW tool is used without any linear movement thereby producing a localised joint where the tool was plunged into the material. This single-point joint is similar to a resistance spot and rivet joint. Friction stir spot welding is presently being used by transportation industries to form small aspect ratio (SAR) discrete lap joints between sheets of malleable materials, typically in aluminium alloys, magnesium alloys and zinc coated steel. It is also used to join sheets to supporting structures like frames and beams.

With conventional FSSW, the small welded area and exit hole left after the tool is retracted has caused concerns in terms of structural integrity, especially in safety critical applications. Many variants have emerged to address the concerns including: swept, stitch, swing, refill, probeless and other miscellaneous variants. The different FSSW techniques for forming discrete joints are individually described and weld property data presented. Comparisons made are also made between process variants.

In the literature search it was found that on-going research is looking at FSSW for applications in the automotive and aerospace sectors, with a view to replacing the current conventional techniques used. Research work is on-going to identify the requirements to reduce or eliminate the barriers to implement FSSW, especially in the aerospace sector. For aerospace applications, FSSW without an exit hole would be desirable, although swept FSSW has shown its potential against riveted panels. Also in the aerospace sector, accepted and recognized standardized joint design values/data for strength properties of materials are required. In the automotive industry, the main push for FSSW is to minimise the welding time when compared against the RSW and (SPR) processes. Although much work on basic FSSW has been done, the barrier is still is the longer welding time for automotive. The probeless FSSW process is tipped to be a solution for aluminium to steel joints for automotive.

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