Friction stir spot welding (FSSW) is a very useful variant of the conventional friction stir welding (FSW), which shows great potential to be a replacement of single-point joining processes like resistance spot welding and riveting. There have been many reports and some industrial applications about FSSW. Based on the open literatures, the process features and variants, macro- and microstructural characteristics, and mechanical properties of the resultant joints and numerical simulations of the FSSW process were summarized. In addition, some applications of FSSW in aerospace, aviation, and automobile industries were also reviewed. Finally, the current problems and issues that existed in FSSW were indicated.
Recently, lightweight metals such as aluminium alloys are increasingly used, especially in aerospace and automotive industries, where weight saving is extremely important [
Friction stir welding (FSW) was developed by TWI in 1991 [
As a variant of FSW, friction stir spot welding (FSSW) has been proposed to realise a spot weld. It shows great potential to be a replacement of single-point joining processes like resistance spot welding and riveting, and it has wider applications in aerospace, aviation, and automobile fields [
Conventional FSSW was invented by Mazda Motor Corporation in 1993 [
Illustration of the FSSW process: (a) plunging, (b) stirring, and (c) retracting [
In order to eliminate the keyhole or increase the strength of joints, several processes have been proposed, such as the refill FSSW, pinless FSSW, and swing FSSW [
The refill FSSW was developed and patented by Helmholtz-Zentrum Geesthacht, Germany [
Schematic diagram of the refill FSSW process: (a) friction, (b) first extrusion, (c) second extrusion, and (d) pull-out stage [
There are fewer applications about this process, because of complicated procedures, long dwell time, and high cost. However, the keyhole could be eliminated, and the weld strength is improved.
The pinless FSSW was invented by Tazokai. In this process, the tool without a probe but with a scroll groove on its shoulder surface has been proposed in 2009 [
Illustration of the pinless FSSW process: (a) plunging, (b) stirring, and (c) retracting [
The swing FSSW was developed by TWI, UK. In this process, the tool moves along a preset path after plunging (Figure
Illustration of weld path of swing FSSW [
To obtain a weld joint without a keyhole, Sun et al. [
Schematic illustration of the novel FSSW [
Wang and Lee [
Three distinct regions were revealed in FSSW weld joint: the stir zone (SZ), the thermomechanically affected zone (TMAZ), and heat affected zone (HAZ) [
A typical micrograph of the cross-section of a friction stir spot weld [
Macroscopic appearance of a FSSW joint with refilled probe hole: (a) top view of weld zone and (b) cross-section of weld zone [
Cross-sectional macrostructure of the joints at different dwell time was observed by Fujimoto et al. [
It is known that macroscopic appearance of FSSW joint is influenced by temperature and material plastic deformation. Moreover, welding parameters (mainly include rotation speed, dwell time, plunge depth, and plunge rate) decide the friction heat during welding. Hence, macroscopic appearance changes with the change of welding parameters. Yuan et al. [
Feng et al. [
Schematic illustration of the joint interface: (a) ZAM and GI, (b) AS and GA [
The common metallurgical zones on the cross-sections of FSSW weld are hooking, partial bonding, and bonding ligament (Figure
OM macrograph of a typical FSSW joint cross-section [
Another common defect that could be seen in weld is void. For the refill FSSW joint of 6061-T4 alloy, Shen et al. [
Under friction heat and stirring, SZ presented fine equiaxed grains due to recrystallisation [
TMAZ experienced both frictional heat and deformation which resulted in highly deformed grains [
The HAZ only experienced a welding thermal cycle, which caused the coarser grains [
For the new FSSW technique used by Sun et al. [
Yin et al. [
In tensile-shear tests, shims of the same material and thickness as the sample were used when clamping the samples to induce pure shear [
For the cross-tension sample, the tensile-shear strength was affected by rotation speed, while dwell time had less influence on strength. The weld strength reached the maximum 902.1N [
Tozaki et al. [
Morphology of the tool has influence on the weld. Badarinarayan et al. [
For FSSW weld of dissimilar materials, the variations of weld strength depended on the material positioned on the upper side of the specimen configuration [
In tensile-shear tests, there are usually three different separation modes: interfacial shear separation, nugget pull-out separation, and upper or lower sheet fracture separation. The joint with nugget fracture separation had higher strength [
Prakash and Muthukumaran [
Zhang et al. [
The fatigue cracks were observed to propagate through the tip of hooking [
For A6061 and low carbon steel sheets dissimilar lap-shear welds, fatigue fracture modes were dependent on fatigue load level. Shear fracture through the interface occurred at high load levels, and a fatigue crack grew through the upper sheet at low load levels [
(a) Schematic plots of a spot friction weld in a lap-shear specimen under applied resultant loads (shown as the bold arrows), (b) a schematic plot of the cross-section along the symmetry plane of a 5754-7075 weld in a lap-shear specimen, and (c) failure modes of the 5754-7075 welds in lap-shear specimens under quasistatic and cyclic loading conditions [
For cross-tension specimen, Lin et al. [
Schematics of (a) the top view of a cross-tension specimen, (b) the cross-sections along the horizontal (marked by H) and transverse (marked by T) symmetry planes of the spot friction weld, respectively, and (c) the failure modes of spot friction welds under different loading conditions [
Hassanifard et al. [
Uematsu et al. [
To optimise the process parameters and develop FSSW new tools, it is important to understand the physics of this complex process that involves fatigue life, temperature gradient, and strength by numerical simulation [
For the friction heat, Awang and Mucino [
In order to optimize welding parameters of FSSW and increase strength of welds, Atharifar [
Kim et al. [
For the refill FSSW, Muci-Küchler et al. [
At present, the FSSW process has become one of the most optimal processes in substituting the conventional resistance spot welding and riveting in joining lightweight structural metals, such as aluminum and magnesium alloys, in the automotive and aerospace industries.
FSSW could be classified into four types: conventional FSSW, refill FSSW, pinless FSSW, and swing FSSW. Normally, three distinct regions are observed in the FSSW joints: the stir zone (SZ), the thermomechanically affected zone (TMAZ), and the heat affected zone (HAZ). In tensile-shear tests, there are usually three different failure modes: interfacial shear separation, nugget pull-out separation, and upper or lower sheet fracture separation. The fatigue cracks usually propagate through the tip of hooking. However, there are still no mature theory and abundant database for applications of FSSW. Reliability of joints has not been understood totally. To the authors’ knowledge, there are still important issues that need to be revealed. FSSW techniques without keyhole defect (refill FSSW, pinless FSSW, and other new processes) should be paid more attention. The materials used in FSSW should be enlarged. Besides aluminum and magnesium alloys, engineering plastics and other materials also need to be introduced into the research scope. The flexible, multipurpose, and reliable FSSW equipment should be developed for better applications in industrial production.
The authors declare that they do not have a direct relation with any commercial identities that might lead to a conflict of interests for any of them.
The authors would like to appreciate financial supports from the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (131052), the Fundamental Research Fund of NPU (JC201233), and the 111 Project (B08040).