Current Trends and Emerging Strategies in Friction Stir Spot Welding for Lightweight Structures: Innovations in Tool Design, Robotics, and Composite Reinforcement—A Review

Abstract

Friction stir spot welding (FSSW) is a solid-state joining technique increasingly favored in industries requiring high-quality, defect-free welds in lightweight and durable structures, such as the automotive, aerospace, and marine industries. This review examines the current advancements in FSSW, focusing on the relationships between microstructure, properties, and performance under load. FSSW offers numerous benefits over traditional welding, particularly for joining both similar and dissimilar materials. Key process parameters, including tool design, rotational speed, axial force, and dwell time, are discussed for their impact on weld quality. Innovations in robotics are enhancing FSSW’s accuracy and efficiency, while numerical simulations aid in optimizing process parameters and predicting material behavior. The addition of nano/microparticles, such as carbon nanotubes and graphene, has further improved weld strength and thermal stability. This review identifies areas for future research, including refining robotic programming, using artificial intelligence for autonomous welding, and exploring nano/microparticle reinforcement in FSSW composites. FSSW continues to advance solid-state joining technologies, providing critical insights for optimizing weld quality in sheet material applications.

1. Introduction

Friction stir spot welding (FSSW) is a novel transient sheet metal joining technique invented in 1991 that utilizes frictional heat generated by a rotating tool to soften and join the materials. This variant and innovative advancement of the friction stir welding (FSW) method was successfully implemented by Mazda Motor Corporation and Kawasaki Heavy Industry in 2003 [1]. FSSW has gained popularity in industries such as the automotive and aerospace industries for its ability to weld aluminum and magnesium alloys effectively, which are commonly used in these sectors for their high strength and lightweight properties [2]. Unlike FSW, there is no traverse movement after plunging a rotating tool into the workpiece to be welded with the FSSW. A cylindrical non-consumable rotating tool is used in the FSSW process, which is responsible for heating and plasticizing the base materials to be welded [3].

While both FSSW and Resistance Spot Welding (RSW) [4] are widely used for joining sheet metals, FSSW offers several advantages that make it a compelling alternative in many applications. For instance, FSSW offers improved weld quality, greater material compatibility, reduced distortion and residual stresses, enhanced energy efficiency, and reduced smoke and fumes, contributing to its environmental benefits. However, it suffers from lower welding speed, limited material thickness, and is susceptible to tool wear. Despite these drawbacks, the FSSW attracts the attention of researchers as well as industries, and its development is still ongoing to seek efficiency and perfection.

Principally, there are three main variants of the FSSW process, which are the conventional, swept, and refilled FSSW process (RFSSW). Each of these variants has a different level of complexity and diversity in terms of spot shear area, shear strength, the degree of control in motion, and time to complete the weld [5]. The conventional FSSW consists of three steps: (i) plunging of the rotating tool in the overlapped materials resulting in generating heat, (ii) holding of tool for a short period (dwell), to attain complete stirring, and (iii) retraction of the tool to finish the process as shown in Figure 1.

FSSW was applied in joining the bonnet and rear door of the sports cars in 2003 [1]. Then, it has been successfully integrated into the mass production of body frames, aluminum doors, and engine hoods in the automobile and aerospace industries. The FSSW process has benefited from advancements in robotics, leading to its use in mass manufacturing production across a variety of industries in metalworking. A bespoke C-frame gun or a computer numerical control (CNC) machine can also be used to achieve FSSW joints. Kawasaki Heavy Industries (KHI) has developed a pedestal C-frame gun for a robotic system as shown in Figure 2 [7].

In 2005, The Welding Institute Ltd. (TWI) introduced the Swept FSSW (SweFSSW) process, while GKSS in Germany invented the RFSSW process in 2007 [8]. Later, Hitachi developed Swing FSSW (SwiFSSW) [9] and Stitch FSSW (StiFSSW) [10]. Basically, the aforementioned FSSW variants are derived from the conventional FSSW technique. A further advancement of SweFSSW is carried out at Wichita State University (WSU), resulting in a variant known as Octospot [9]. The FSSW and SweFSSW processes share similar process features. However, SweFSSW is characterized by the circular movement of the tool between the plunging and retraction phases, resulting in an increase in the weld area. Since SweFSSW utilizes a closed-loop system, it takes into account additional process parameters compared to FSSW, such as tool traversal speed and tool tilt angle.

The RFSSW variant aims to improve the weld quality by eliminating the keyhole defect, which can potentially weaken the weld joint, typically produced at the end of the welding process (see Figure 1) [11]. The keyhole defect refers to the void or depression formed due to the absence of material in the center of the weld joint. In the RFSSW method, the sheet metals are joined by utilizing the relative motions of the tool pin and the shoulder to fill the keyhole (see Figure 3). By carefully monitoring the relative motions of the tool pin and shoulder, the RFSSW method allows for the displacement and refill of material, resulting in a more robust and complete weld joint.

SwiFSSW is also a variant of the conventional FSSW process (see Figure 4b,c). It has been developed by Okamoto et al. [9] based on the modification of the StiFSSW technique. In SwiFSSW, after the initial plunging of the tool into the workpiece, the tool is raised slightly, but this upward movement is considered negligible. The primary characteristic of SwiFSSW is the subsequent swing-like motion of the tool, which follows a large radius with a small angle. This swinging motion exposes the material that was squeezed and positioned at the end of the welding, increasing the actual area of the weld and the strength of the joints. The StiFSSW is developed by GKSS T.Y. [12]. Compared to the SwiFSSW method, the StiFSSW technique introduces a larger radius in the tool design, enabling a large welding area, resulting in joints with increased strength (see Figure 4d).

Figure 3. Schematic representation of the principal stages defining the RFSSW process. Reprinted from [13] under CC-BY 4.0 license.

Figure 3. Schematic representation of the principal stages defining the RFSSW process. Reprinted from [13] under CC-BY 4.0 license.

In StiFSSW, after the initial plunging of the tool, it travels a short distance (typically a few millimeters) along the joint line before retracting. This movement creates a series of closely spaced “stitches” or weld spots along the joint.

A comparative summary of the key differences among various FSSW variants is presented in

Table 1. Comparison of different friction stir spot welding (FSSW) variants based on tool movement, key features, weld quality, and application relevance.

FSSW Variant	Tool Movement	Key Feature	Weld Quality	Application Notes
Conventional FSSW	Vertical plunge and retract	Leaves keyhole	Good, keyhole present	Widely used; automotive panels
Swept FSSW (SweFSSW)	Tool swings in a circular path before retraction	Enlarged weld area	Improved over conventional	Used in aerospace and automotive
Refill FSSW (RFSSW)	Tool refills material after joining	Eliminates keyhole	Excellent	Aerospace-grade joints
Swing FSSW (SwiFSSW)	Swing-like tool motion post-plunge	Enlarges weld area	Enhanced	Improves bonding area and strength
Stitch FSSW (StiFSSW)	Tool traverses short distance	Creates multiple weld points	High	Stitch welds for load-bearing joints

. This table highlights the distinctive tool movement patterns, weld characteristics, and typical industrial applications associated with each variant, aiding in the understanding of their practical implementation and performance benefits.

The quality of the weld joint in the FSSW process is influenced by several factors, including tool rotational speed, tool plunge rate, shoulder plunge depth, dwell duration, tool pin length, pin profile, and tool shoulder features [15,16].

Figure 5 displays the fishbone diagram to represent the various process parameters involved in the FSSW technique. Tool rotational speed, tool plunge rate, and dwell time are key parameters to the conventional FSSW process. The main parameters affecting the FSSW process are the following:

• Tool design: The shape, size, and material of the tool, particularly the pin, play a crucial role in the FSSW process. Different designs and materials can affect heat generation, material flow, and overall weld quality [17].
• Tool rotation speed: The speed at which the tool rotates plays a crucial role in both heat generation and material plasticization during the welding process. It is imperative to fine-tune this parameter to strike a balance, ensuring adequate heat for welding without inducing any material defects [18,19].
• Plunge depth: The extent to which the pin penetrates the workpieces directly influences the degree of material plasticization. Achieving an appropriate insertion depth is essential to produce a robust and defect-free weld [20].
• Welding force: The axial force applied during the welding process affects the contact between the tool and the workpieces, as well as the material flow and joint formation [21]. It needs to be controlled to ensure adequate plastic deformation without excessive force that may lead to defects.
• Material properties: The properties of the materials being joined, such as their composition, thickness, and mechanical properties, can influence the FSSW process. Different materials may require adjustments in process parameters to achieve satisfactory weld quality.

Building upon the understanding of the FSSW process parameters, it is crucial to examine the resulting microstructure of the weld joint. This is because the interaction of heat and mechanical forces during FSSW creates distinct zones with unique microstructural characteristics. These zones influence the overall mechanical properties of the weld, such as strength and ductility. As depicted in Figure 6, the cross-section of the FSSW (see Figure 6a), the RFSSW (see Figure 6b), and SweFSSW (see Figure 6c), reveals five distinct zones, including the stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), parent material (PM), and the hook. All these zones have different microstructural characterization based on the heat input caused during the stirring action between the tool and base material. The SZ is the region that consists of well-refined and equiaxed grain formation due to dynamic recrystallization [22,23]. Because of the high heat generated by the rotating tool during plunging and the shoulder penetration, plastic deformation and dynamic recrystallization occur [24]. The HAZ is the region that experiences no plastic deformation and is only subjected to the thermal cycle [24]. TMAZ is the region influenced by both heat and severe plastic deformation, which leads to microstructure characterization.

Welding of lightweight alloys (e.g., Al, Mg alloys) is becoming increasingly important in the automotive industry [28,29]. Low-carbon steel components are progressively being replaced by lightweight materials, due to the growing demand for lightweight vehicles to reduce the carbon footprint. However, joining both similar and dissimilar materials presents significant challenges [30,31]. This limitation necessitates innovative joining techniques. FSSW emerges as a promising joining process that minimizes heat input and energy consumption, reduces distortion, and produces high-strength welds ideal for lightweight materials.

Notably, research has demonstrated that the mechanical strength of 5x series Al joints can be enhanced through the FSSW process by adjusting the traverse speed and rotational speed parameters of the tool [32,33]. Despite its advantages, it is essential to acknowledge that the FSSW process may lead to a reduction in weld joint properties, an aspect that requires careful consideration.

Seeking the improvement of FSSW joint properties, numerous studies have explored strategies focusing on incorporating reinforcement materials into welding joints [34,35,36]. For instance, aluminum joints reinforced with nanoparticles exhibit a reduction in grain size and improved tensile strength. In the case of FSSW of AA56061-T6 with SiC nanoparticles, the process resulted in grain refinement. Here, SiC nanoparticles act as nucleation sites, exerting a pinning effect on grain boundaries and contributing to increased hardness compared to joints without nanoparticle reinforcement [36].

Transitioning the focus to FSSW, similar benefits can be anticipated. FSSW is poised as a viable method for achieving flawless aluminum joints with reduced heat input. Research exploring the optimization of traverse speed and rotational speed in FSSW processes is crucial for enhancing mechanical strength, mirroring the efforts in FSSW. Moreover, investigating the incorporation of reinforcement materials, such as nanoparticles, holds promise for refining grain structure and strengthening the weld joints in the context of FSSW.

Researchers have started fabricating spot joints using the FSSW technique in recent years due to the enhanced properties of the composite welded joints produced by adding various reinforcements at the weld region of the plates. The addition of micro- and nano-sized ceramic particles in metallic materials tends to increase or change the properties of the base material. Several research works have been undertaken by adding ceramic particles to the base metal to produce composite joints. The different ceramic powders such as SiC, Al₂O₃, TiC, TiB₂, ZrB₂, ZrC, AlN, Si₃N₄, Al₃Ti, SiO₂, Al₄Mo, and Al₃Zr have been attempted. Figure 7 depicts the trend in the number of documents published on FSSW from 2011 to 2023, derived from the FSSW search process. Figure 7b illustrates patent records, wherein approximately 4000 patents were identified in the field of friction stir spot welding. The increasing trend of publications and patents signifies the growing significance of FSSW applications in various industries.

In the end, recent developments in the area of FSSW and FSSW process parameters, such as tool rotational speed, tool configuration, and material, along with their effects on weld properties such as tensile shear strength, microstructure, and the numerical simulation of this process, were examined and discussed in Section 2. The insights into the field of FSSW and its potential applications in diverse industries in terms of advancements and innovation were discussed in Section 3. It includes defect management and incorporation of nano/microparticles, including carbon nanotubes, graphene, ceramic nanoparticles, and metallic reinforcements, into the FSSW process. Future aspects and development in this area of research were disclosed in Section 4.

2. Progress in FSSW Process

There are several published studies and papers that discuss the effects of FSSW parameters on material characterization and the mechanical characteristics of welded joints, including both similar and dissimilar materials. Optimization of process parameters, such as rotation speed, dwell time, and plunge depth, continues to be a focal point in achieving high-quality welds with minimal defects [5,37,38]. Some developments have been made in different FSSW variants, primarily focusing on microstructural characterization, mechanical properties, and numerical simulations. Researchers have conducted systematic studies to identify the optimal combination of these parameters for specific materials and joint configurations. On the other hand, the tool design for FSSW has attracted the interest of researchers [39] to efficiently weld either similar or dissimilar materials [40], since the welding tool affects the heat production and material flow [41], which impacts the microstructure and mechanical properties of the joint. Therefore, proper design of the welding tool is essential to improving efficiency and enlarging the window of FSSW process parameters to achieve sound welding joints. The pin and the shoulder are the main features of FSSW. The tool shoulder and pin geometry, as active parts of the tool, have several design profiles, which vary depending on the material being welded and the desired welding parameters [42,43].

On the other hand, numerical simulation of FSSW has gained. This optimization is aimed at enhancing the mechanical properties and overall quality of the welds. Consequently, the design and optimization of the tool geometry are important considerations in achieving a successful and cost-effective FSSW process.

The work presented by Gao et al. [6] delves into a comprehensive microstructural analysis and prediction of dynamic recrystallization behavior, grain size alteration, and dislocation density changes in the context of FSSW of AA6082 alloy. Additionally, the study incorporates the prediction of the SZ’s shape formed during the welding process. The microstructure analysis reveals that the mechanism driving dynamic recrystallization in the SZ is identified as geometric dynamic recrystallization. The SZ exhibited a microstructure distinguished by extremely fine equiaxed grains as opposed to larger equiaxed grains [44]. Geometric Dynamical Recrystallization was fully completed within the SZ, with observable presence limited to the thermo-mechanically affected zone (TMAZ), as depicted in Figure 8.

Particle-stimulated nucleation appears to have minimal significance in FSSW, as reflected by the limited existing literature. Few papers identify it as a primary mechanism for altering the grain structure in the FSSW process. The prevailing observation suggests that particle-stimulated nucleation becomes dominant when particles exceed 1 μm in size. However, due to the breakup or dissolution of most particles and the suboptimal strain rates, this mechanism is proposed to have an insubstantial role in refining the grain structure in aluminum alloy welds [45,46].

The final grain structures in FSSW can therefore be expected to result from a combination of the initial grain sizes formed during the high-strain-rate deformation during welding, followed by grain growth during the rapid cooling when the tool is extracted. These two points have been studied in detail for conventional friction stir spot welding. The average deformable grain size in the SZ closely followed the values predicted by Equation (1) based on the Zener–Hollomon parameter.

$$d_s=a+b\log Z$$

(1)

where a, and b are empirical constants and Z is the Zener–Hollomon parameter.

The ultimate grain size in the SZ closely aligns with the standard grain growth model, as defined by Equation (2).

$$d_f^2-d_0^2=A\exp \left({\displaystyle \frac{-Q}{RT}}\right)t$$

(2)

where d_f is the final grain size, d₀ is the initial grain size, Q is the hot deformation activation energy, T is the temperature in Kelvin, t is time, and A is an empirical constant.

The study by Baudin et al. [47] investigates the local microstructure, texture gradient, and mechanical properties in upper and lower AA5182 aluminum sheets during FSSW. The upper sheet primarily comprises SZ and TMAZ, influenced by high deformation from tool rotation and shoulder download force. In contrast, the lower sheet exhibits SZ, TMAZ, HAZ, and BM. Texture changes transition from a shear-type texture at SZ to a plane strain compression deformation type at TMAZ, and then a recrystallization texture at HAZ and BM, as shown in Figure 9. To precisely locate BM in the upper sheet, additional EBSD measurements were conducted at x = 5.5 and 6 mm (on the left and right sides) from the keyhole center. The corresponding microstructures at x = −5.5 and 5.5 mm still exhibit characteristics of the HAZ region. The BM is ultimately attained at x = −6 and 6 mm from the keyhole center (see Figure 10).

Changing the parameters dynamically during welding offers a pathway to enhance weld quality. For instance, Badwelan et al. [48] introduced an approach utilizing Dynamic Welding Parameters (DWP) in FSSW by varying the welding parameters, such as spindle speed and feed rate, throughout the welding process using a stepwise variation function.

Their study revealed a significant improvement in weld strength, demonstrating a 12% to 21% increase compared to the static welding parameters approach, as shown in Figure 11. Dynamic adjustment of welding parameters presents a novel avenue for optimizing weld quality. The method proposed by Badwelan demonstrates the potential benefits of this approach, promising new possibilities for further exploration and advancements in FSSW techniques.

The dwell time in FSSW is a key factor influencing joint quality and performance. This parameter, representing the duration of tool-material interaction, significantly shapes the mechanical and metallurgical properties of the weld. Understanding and optimizing dwell time are crucial for achieving the desired outcome in terms of joint strength, microstructure, and defect formation. In essence, a nuanced exploration of dwell time is essential for enhancing the efficiency and reliability of FSSW across diverse materials and industrial applications.

The study by Ahmed et al. [32] explored the FSSW of 2 mm-thick AA5052-H32 aluminum alloy sheets to create 4 mm-thick lap joints. Emphasizing the importance of energy efficiency and productivity in industries like automotive, aerospace, and marine, this research explored the effects of low dwell times (1, 2, and 3 s) and a constant tool rotation speed (500 rpm) on the mechanical properties and microstructural evolution of the welded joints (see, Figure 12). This research offered concise insights into FSSW’s effectiveness for specific thicknesses and processing parameters. From the investigations, the authors revealed that longer dwell time makes defect-free lap joints with wider SZ, where increased hardness is witnessed due to grain refinement. According to the authors, a 2 s dwell time at 500 rpm is optimal, producing the highest tensile shear load.

Bozzi et al. [33] observed distinct trends in the tensile shear joint strength of FSSWed AA5081-O aluminum strip lap joints. The joints were processed at tool rotation speeds ranging from 1100 to 1500 rpm, with a 2.5 s dwell time. Interestingly, joint strength increased as the rotation speed elevated from 1100 to 1300 rpm. However, the increase in tool rotation speed showed a subsequent decrease in strength was noticed despite the expanded welded zone. A study by Ahmed et al. [49] underscored the pivotal role of the width of the weld in determining the properties of Friction Stir-Spot Welded (FSSWed) materials. This aspect is intricately linked to tool design and process parameters, particularly tool rotation speed and dwell time. For instance, Zhang et al. [50] delved into the metallurgical and tensile properties of FSSWed AA5052-H112 aluminum alloy sheets with a thickness of 1 mm. They explored the impact of two tool rotational speeds and three dwell times, revealing that joint strength is minimally affected by dwell time but decreases with higher rotational speeds. In a different approach, Uğurlu and Çakan [51] examined AA7075-T6 spot joints created at a constant dwell time of 20 s and varying tool rotation speeds (1040, 1320, and 1500 rpm). Their findings indicated an improvement in joint properties with higher tool rotational rates. The maximum tensile strength was achieved with a defect-free joint processed at the highest tool rotation speed.

The corrosion behavior of spot joints, particularly those created through FSSW, is a significant consideration in sectors like automobile, aerospace, and mechanical engineering. While FSSW is favored as an alternative to conventional joining processes, dissimilar aluminum metal joints often exhibit susceptibility to corrosion in saline water environments. To address this concern, this study delves into the corrosion characteristics of FSSWed joints involving AA2024 and AA7075 aluminum alloys. Electrochemical potentiodynamic tests were employed for quantitative corrosion behavior analysis, complemented by microstructural examinations for validation. Comparative results reveal a slightly higher corrosion rate in dissimilar welded joints, emphasizing the need for understanding and mitigating corrosion challenges in dissimilar metal joints. Nevertheless, the minimal variation in corrosion rates between similar and dissimilar joints suggests that FSSW may offer a promising solution to alleviate corrosion-related issues in dissimilar metal joints [52].

This becomes even more critical when dealing with magnesium (Mg) alloys due to the potential for accelerated mass loss [53]. In the case of Mg-alloy welds created through FSSW, an important observation is that regions with a fine grain size in the SZ near the shoulder exhibit increased nobility compared to the surrounding AZ31 alloy base material. Addressing the corrosion resistance of FSSW joints in magnesium alloys is essential for ensuring their long-term durability and performance [54].

2.1. FSSW of Similar Materials

FSSW excels in joining similar materials, particularly those with different hardness levels or temper conditions. This technique minimizes microstructural changes that can occur in fusion welding, leading to enhanced weld quality and improved material compatibility. Furthermore, the localized nature of the heat input in FSSW minimizes thermal distortion and residual stresses, making it ideal for joining thin or complex components where dimensional accuracy is paramount. Despite its advantages, FSSW still faces challenges. Tool wear, particularly under high welding forces and rotational speeds, requires careful monitoring and maintenance. Furthermore, achieving consistent weld quality requires precise control of process parameters, demanding advanced process control strategies and in situ monitoring techniques. Ongoing research focuses on addressing these challenges through optimized tool design, improved process control, and exploration of new applications for FSSW.

As mentioned above, FSSW has been successfully applied in joining similar materials, particularly aluminum alloys, which are widely used in the automotive and aerospace industries [55]. This technique has also been extended to joining magnesium, copper, steel, and polymers. The demand for welding similar lightweight alloys has increased significantly, especially in automotive applications where steel components are being replaced by aluminum in critical parts such as pistons, cylinder heads, and brake calipers [56]. FSSW provides a reliable method to produce high-quality joints in similar lightweight materials with minimal heat input and energy consumption.

Research on joining different aluminum alloys using FSSW has shown that the microstructural and mechanical properties of joints can vary significantly based on process parameters [33,57,58,59]. During FSSW of aluminum alloys, a reasonable strain rate is typically accompanied by a slight temperature increase, causing variations in weld microstructure [60]. Shen et al. [61] investigated the welding of AA6061 T4 sheets and found that longer welding durations and higher tool rotational speeds are necessary to achieve desirable joint appearances and mechanical properties. The study revealed that optimizing these parameters leads to improved microstructural characteristics and enhanced joint strength. Tier et al. [62] highlighted that Alclad distribution in welds greatly influences the mechanical properties of aluminum alloy welds. The RFSSW process has been used to demonstrate effective welding joints for 5042 aluminum alloys. RFSSW is employed in automotive applications due to its ability to produce high-quality joints [63,64]. The process parameters, particularly plunge depth and tool rotational speed, significantly impact the microstructure and shear strength of the welds. Optimal parameters enhance the bonding ligament length and shear strength, while volumetric defects have a minor influence on mechanical performance. Lower rotational speeds improve joint strength by promoting horizontal material flow, while higher speeds reduce bonding ligament length and shear strength [33,59].

In addition to aluminum, FSSW has been successfully applied to other similar materials such as magnesium, copper, and steel. Magnesium alloys, known for their lightweight and high strength, are increasingly used in automotive and aerospace applications [65]. FSSW offers a solid-state joining method that maintains the integrity of magnesium alloys without the defects associated with melting. Shen et al. [66] found that raising the temperature during the FSSW process improved the fluidity of the magnesium alloy, reducing voids and enhancing the tensile shear load of the welded joint. Higher temperatures also enlarged the bonded region during welding.

Liu et al. [67] joined 1 mm-thick AZ31 magnesium alloy sheets using RFSSW and investigated the metallurgical features, microstructure, texture, and mechanical response of the joints. The effect of welding parameters on joint performance was evaluated, showing that the highest lap shear strength occurred at a rotational speed of 1500 rpm and a plunge depth of 1.4 mm. A fine-grain structure was observed in the SZ, with grain size decreasing as rotational speed and plunge depth lowered. The c-axes of grains were aligned with the normal direction at the spot-weld center and inclined toward the transverse direction at the shoulder edge, as shown in Figure 13.

It is well understood that severe hot deformation in hexagonal close-packed magnesium alloys generates a very strong crystallographic texture, which impacts their mechanical response [68]. Recently, Fu et al. [69] demonstrated an increase in lap shear strength (LSS) by modifying the texture through differential rotation RFSSW, highlighting the potential impact of texture changes on the performance of RFSSW magnesium alloy joints. However, the specific textural characteristics in these joints have not been thoroughly clarified. Additionally, the effects of texture evolution on the tensile-shear fracture mechanisms of RFSSW magnesium alloy joints remain unclear. Cast magnesium alloy welds produced by RFSSW typically show LSS and fail in the SZ shear mode due to heavily textured microstructures. To address this issue, a novel process called differential rotation RFSSW (DR-refill FSSW) has been developed [70]. This method induces discontinuous dynamic recrystallization (see Figure 14), creating a bimodal microstructure with weakened texture, which reduces deformation incompatibility between the SZ and the TMAZ. Consequently, DR-refill FSSW welds achieve 50% higher LSS than standard RFSSW welds and fail in the SZ pull-out mode instead of the shear mode. Microstructural analysis revealed that deformation incompatibility leads to shear bands and failure, accommodated by dislocation slip and repeated DRX, preventing shear band formation. DR-refill FSSW presents a promising strategy for improving spot weld performance in cast Mg alloys and has potential applications in other material combinations like Al/Al and Mg/steel.

Copper and steel, although less common in lightweight applications compared to aluminum and magnesium, benefit from the solid-state nature of FSSW. The process provides strong and defect-free joints, essential for applications requiring high electrical conductivity (in the case of copper) or high strength and toughness (in the case of steel). The welding of copper using the FSSW process has been relatively understudied. Sansui et al. [71] investigated the FSSW of commercially pure copper with a thickness of 3 mm, examining the microstructure and corrosion resistance of the joints. They experimented with three different tool rotation speeds and found a fully metallurgically bonded SZ with equiaxed, fine grains larger than those in the base metal. However, a hook defect indicated insufficient material flow during the process. EDS analysis detected residuals from the tool, potentially compromising corrosion resistance. The joint produced at 1200 rpm exhibited the lowest corrosion resistance, whereas joints made at 1600 rpm and 2000 rpm showed improved performance. The study concluded that corrosion resistance was enhanced by treating the joint with 3.5% NaCl.

According to Takashi Murakami et al. [72], FSSW of 1 mm SPCC steel plates was successfully performed using TiC_0.5N_0.5–72wt.% W cermet tools, enabling over 2000 defect-free welds—twice the durability of conventional Si₃N₄ tools. Joints remained crack-free across multiple cycles, and tensile shear tests confirmed bonding strength comparable to the base metal’s yield stress. Figure 15 displays EBSD-IPF images near the joint of SPCC steel plate specimens. The grain size was smallest near the joint center and increased with distance, with the center showing a grain size of approximately 5 μm, compared to 20–30 μm in untreated plates.

Additionally, FSSW has been extended to polymers, offering a method to join thermoplastic materials without the need for adhesives or fasteners. The heat generated by the rotating tool causes the polymer to soften and fuse, creating a strong bond upon cooling. This application is particularly useful in industries where lightweight and corrosion-resistant joints are required. Polycarbonate sheets were joined using the FSSW process, with research analyzing how different parameters, including rotational speed, dwell time, waiting time, preheating, and plunge rate, affected the mechanical properties of the welds [73]. In the case of polypropylene sheets, optimization of tool penetration depth was carried out to maximize tensile strength. Moreover, further investigations into welding polypropylene sheets highlighted the significant role of tool geometry [74]. Lambiase et al. [75] examined the impact of plunging force in FSSW of polycarbonate sheets on weld strength. Varying tool geometry and plunging force, mechanical tests showed that optimal plunging force improves weld strength by up to 37% without extra energy or increased production time. Improved strength results from reduced porosity at the weld interface. Excessive plunging force, however, weakens welds due to over-thinning. Optimal conditions led to weld shear strength of 34.5 MPa, comparable to the base material. Variations in tool geometry were found to influence the formation of the SZ and subsequently the strength of the weld joints [76].

Recently, Feizollahi et al. [77] investigated the effects of pin thickness and tool penetration depth in FSSW of AA6061 and AA5052 aluminum alloy plates. Their findings revealed that reducing the pin diameter and increasing tool penetration improved weld strength and microhardness. The reduced pin diameter increases the heat generation at the interface and thus, melting is improved. Moreover, it exerts less overall force, and hence warping and thermal distortion are avoided. This is a good strategy for welding thin parts [78]. Additionally, using a pinless tool resulted in greater microhardness in the weld nugget area. The placement of AA6061 aluminum alloy on top enhanced shear tensile strength, while increased grain size in the heat-affected zone correlated with decreased microhardness. The tool pin greatly influences the microstructure of the intermetallic layer, as shown in Figure 16, where reducing the tool pin diameter continuously decreases the thickness of the intermetallic layer, reaching its minimum value with a pinless tool, significantly impacting the weld’s mechanical properties.

In another recent study, researchers examined how the tool shoulder diameter affects friction stir spot welding of aluminum plates. They tested two tools with shoulder diameters of 20 mm and 25 mm at rotation speeds of 1000, 1600, and 2400 rpm. The findings revealed that a larger shoulder diameter improved the breaking force and tensile strength of the welds. There was an optimal rotation speed, where increasing speed initially enhanced strength, but eventually led to a decrease. Placing the AA2024-T3 aluminum plate on top also boosted strength. Most fractures occurred due to nugget pull-out, and the hardness varied across different weld areas, with the nugget region showing fine-grained structure due to heavy deformation [79].

2.2. FSSW of Dissimilar Materials

FSSW of dissimilar materials presents a significantly greater challenge compared to joining similar materials. The inherent differences in physical and metallurgical properties between dissimilar metals often lead to complexities such as intermetallic compound formation, differential thermal expansion, and variations in mechanical behavior. These factors can adversely affect the weld joint’s integrity and performance. Despite these challenges, FSSW has emerged as a promising technique for joining dissimilar materials due to its potential to mitigate some of the issues associated with traditional fusion welding processes. By carefully selecting process parameters and understanding the metallurgical interactions between the base materials, it is possible to achieve acceptable weld quality and performance in FSSW of dissimilar materials.

FSSW represents a cutting-edge approach to joining dissimilar materials, offering versatility in accommodating different alloys, metals, and even materials from distinct classes. This technique is particularly notable for its ability to weld materials at temperatures below their melting points, mitigating challenges associated with intermetallic formation that could lead to cracking. Joining dissimilar Al-alloys through FSSW poses challenges due to discontinuities in mechanical and technological properties across abutting surfaces. The inherent asymmetry in heat generation and material flow exacerbates the flow behavior asymmetry in dissimilar welding [80]. While difficulties persist, it is notably more feasible to implement FSSW for dissimilar Al-alloys compared to combinations with vastly differing properties, like Al-alloy to Mg-alloy [81], Al-alloy to steel [82], or Al-alloy to copper [83].

In a study conducted by Tozaki et al. [84], the application of FSSW was explored on 1 mm-thick sheets of AA2017-T6 and AA5052 aluminum alloys. The results, observed through optical microscopy, revealed delineated regions of intermixing. The distinct differences in etching rates between the alloys were evident, showcasing the effectiveness of FSSW in achieving intricate joints without resorting to melting.

The surface appearance and cross-section of the joint in the aluminum-copper (Al-Cu) FSSW process are intricately influenced by several key factors. These factors encompass the material composition of the plates, the lap configuration of the FSSW joint (encompassing both Al-Cu and Cu-Al arrangements), the specifications of the welding tool, and the precise welding parameters employed during the process. The interplay of these elements plays a pivotal role in determining the visual and structural characteristics of the resulting joint [85,86].

Zhou et al. [40] examined the impact of different pin profiles in FSSW on the heat generation, microstructure, and mechanical properties of aluminum-copper joints. By using three tools with varying pin designs, the researchers analyzed the joints through microscopy and mechanical testing. The threaded pin tool produced increased heat and caused the formation of intermetallic compounds (between the red dash lines) at the joint interface. Although microhardness was similar across profiles, the tensile shear strength varied, peaking with the threaded pin joint. Fracture characteristics differed, with joints created using featureless pins exhibiting brittle fractures, while others showed mixed characteristics. Figure 17 shows the typical material flow pattern of Al/Cu FSSW joints and highlights how different pin profiles influence the microstructure of the welded joint. In this figure, samples 1, 2, and 3 were created with a featureless pin, threaded pin, and threaded pin with flutes, respectively. A notable contrast was observed when comparing the microstructure obtained using the three pin profiles. The threaded pin with flutes appears to promote more significant material mixing and heat generation, leading to a more complete layered structure at both interfaces. This is in contrast to the featureless pin, which shows less material flow and thinner IMC layers. The threaded pin without flutes falls somewhere in between. The authors reported that the CuAl₂-CuAl-Al₄Cu₉ IMC layered structure was found at the interface close to the keyhole in all samples. This indicates a good metallurgical bond between the aluminum and copper. The CuAl₂ layer is generally considered the strongest IMC, while CuAl and Al₄Cu₉ can be more brittle. However, for the interface away from the keyhole, CuAl₂-CuAl-Al₄Cu₉ layered structure was found in the sample using a threaded pin with flutes, while other samples were mainly composed of CuAl₂ and CuAl, owing to the insufficient heat input. This nuanced interplay between the pin design and material dynamics highlights the importance of tool configuration in influencing the distribution and behavior of IMC particles during the welding process of Al-Cu alloys. Furthermore, Gaohui et al. [86] studied 1060 aluminum–T2 copper dissimilar lap joints using FSSW and revealed that varying dwell times influenced IMC growth. Longer dwell times produced a continuous CuAl₂–CuAl–Al₄Cu₉ laminated layer, improving tensile properties. Fracture analysis highlighted fractures initiating at the CuAl₂–CuAl or CuAl₂–Al interface, emphasizing the role of dispersed IMC particles in alternative crack extension paths with strong metallurgical bonding at the hook interface.

Examining dissimilar lap joints of AA5052 Al/C11000 Cu using FSSW, with a 2 mm-thickness, Prasomthong et al. [87] explored the influence of various tool rotation speeds (2500, 3000, 3500, and 4000 rpm) and dwell times (2, 4, 6, and 8 s). Their results indicated that the weld zone hardness profile exhibited lower values compared to the base material (BM). Notably, the welding parameters of 3500 rpm rotation speed and 4 s holding time, at a 4 mm/min plunge rate, yielded the highest tensile shear strength, reaching 864 N.

In FSSW, a specialized rotational tool, featuring a distinct tip and shoulder, is introduced into the upper sheet before being retracted. The process addresses two primary challenges when dealing with aluminum/steel joints: firstly, the wear of the tool tip and the formation of the exit hole, both of which contribute to a decline in weld quality; and secondly, the emergence of interface Fe–Al intermetallic compounds (IMCs) [88,89].

The formation of IMCs poses a significant challenge in welding dissimilar metals and alloys, particularly at the interface of Al–Fe joints during FSSW. The presence of IMCs, arising from Al-rich or Fe-rich complexes, detrimentally affects the mechanical performance of such dissimilar joints. Among various factors influencing IMCs formation, heat input emerges as a critical determinant [90]. Efforts have been made to mitigate the peak temperature during the friction stir welding of aluminum/steel, such as incorporating interlayers into the process. Remarkably, research outcomes indicate a reduction in IMCs’ thickness and an enhancement in joint strength as a result of these measures. Saleh et al. [89] incorporated Zn and brass interlayers to enhance the load-bearing capacity of joints between AA6061 and DP600.

FSSW also demonstrates considerable promise in the creation of joints between aluminum (Al) and magnesium (Mg) materials. The successful bonding between these metals necessitates meticulous control over the intermixing and the thickness of intermetallic compounds, primarily Mg₁₇Al₁₂ and Al₃Mg₂ [91]. An alternative approach involves employing RFSSW to join Al/Mg [92]. This method offers improved control over the extent of material blending and the distribution of intermetallic compounds.

In the context of joining magnesium (Mg) with steel, especially galvanized DP600 steel, a novel solid-state spot-welding technique known as RFSSW was employed [93]. This method proved effective in creating defect-free welds with high strength across a broad parameter range, despite Mg/Fe being an immiscible alloy system. Microstructure analysis, examination of interfacial reactions, and assessment of mechanical properties unveiled the joining mechanism. The study revealed that the melting of the Zn coating, along with subsequent Mg-Zn reactions, led to the formation of Mg-Zn eutectic and intermetallic compounds within the welds (see Figure 18). Heterogeneous interfacial reactions were observed along the Mg/steel interface, and the correlation between interfacial structure and fracture behavior was explored.

Suhuddin et al. [92] conducted a study on the microstructure characteristics of dissimilar AA5754 aluminum and AZ31 magnesium alloy welds produced by Refill-FSSW. In their experiments, the welding process was intentionally halted during the dwell period using an emergency stop button. This allowed the researchers to create “as-quenched samples” by immersing them in a mixture of ice and water to rapidly cool and preserve the microstructure at various stages of the welding process. Their observations revealed the presence of eutectic phases (indicated by an arrow in Figure 19), which suggested that a liquid phase had formed during welding. This liquid phase facilitated rapid atomic diffusion, leading to the development of intermetallic compounds such as Al₁₂Mg₁₇ and Al₃Mg₂. Notably, no liquation cracks were detected in the as-welded joints.

In a related study, Dong et al. [94] examined the AA5083 aluminum and AZ31 magnesium alloy welds, discovering that a small amount of liquid eutectic phase formed within the first second of welding. Initially, at 1 s, the Al and Mg bonding interface was relatively flat with no apparent IMC layer (Figure 20a). When welding time was increased to 2 s, a noticeable IMC layer formed at the joint center (Figure 20b), closely connected to the Mg alloys and exhibiting an up-bending characteristic at the SAZ. IMC layers were also evident in joints welded for 4 and 6 s. An IMC layer thicker than 10 μm significantly impacts the mechanical properties of the joint as represented in Figure 21. These findings underscore the critical role of liquid phase formation and intermetallic compound development in the microstructure evolution of dissimilar aluminum-magnesium alloy welds.

2.3. Double-Side FSSW Technique

Double-sided FSSW is a relatively new and innovative approach to FSSW involving the simultaneous application of two rotating tools to opposite sides of the workpiece. This technique offers several potential advantages over conventional single-sided FSSW. By applying pressure and heat from both sides, it can achieve a more uniform and consistent weld microstructure. Additionally, double-sided FSSW can potentially enhance mechanical properties, leading to increased joint strength and ductility due to more symmetrical material deformation. Reduced process time is another potential benefit, as heat and pressure are applied simultaneously from both sides. Nevertheless, double-sided FSSW presents unique challenges, including tool synchronization, heat distribution, and the need for specialized tooling. Despite these challenges, the technique’s potential benefits have attracted significant research interest in recent years.

Double-sided FSSW techniques have been used in recent decades to enhance the weld strength [95,96,97], especially for magnesium alloys [98,99]. This double-sided FSSW method can achieve a high shear fracture load as compared to conventional FSSW. The schematic drawing of the double-sided FSSW process carried out in magnesium alloys is shown in Figure 22a.

X. Wang et al. [96] Fabricated high-quality ultra-high strength C-Si-Mn steel spot joints using double-sided FSSW with adjustable probes. They found that the severe vertical material flow and slip (see Figure 22b) in the sheet interface causes strong bonds. Furthermore, a large amount of martensite was formed around the interface of the tool and probe which resulted in the higher weld strength.

The mechanical properties of the welded joints increased with rotation speed, due to the increase in the hard phase and strengthened welding interface. In the surroundings of the shoulder/probe interface, a large amount of martensite was formed, resulting from the higher austenitizing temperature and the higher cooling rate. In addition, severe material flows vertically to the welding interface and strong relative slip along the interface conspicuously favored the fragmentation and dispersion of the oxides, and the driving force for grain boundary migration introduced by severe material flow further promoted the dispersion of the oxides. Consequently, a high-quality welding interface was fabricated, and a high-strength welded joint with a stable plug failure was obtained. Furthermore, we pointed out that the soundly welded high-strength interface of ultra-high-strength steel can be obtained by higher welding temperature and introducing strong material flow vertical to the interface (see Figure 23) [98].

The double-stage friction stir spot extrusion welding method is a modified version of the classical FSSW process. It involves the vibration of specimens normal to the tool movement direction, followed by coolant cooling at the joint [100]. This modified method has shown several advantages compared to the classical FSSW process. It results in a significant decrease in grain size in the SZ, enhancing grain refinement. The modified method also leads to increased shear strength, hardness, and corrosion resistance of the weld region. In addition, the double-stage friction stir spot extrusion welding method is effective in joining dissimilar materials with wide variations in their properties, such as steel and aluminum. Overall, the modified method offers improved mechanical properties and corrosion behavior compared to the classical FSSW process.

In another work by [101], the double-stage friction stir spot welding method, referred to as reversed friction stir spot welding (ReFSSW), was published. It resulted in smaller exit hole dimensions and higher mechanical properties compared to the classical FSSW. In the first stage of ReFSSW, a conventional FSSW process is performed using a larger pin and shoulder diameter. In the second stage, a smaller pin is used, and the shoulder diameter is designed to force the metal of the upper plate to flow toward the exit hole of the first stage, decreasing its dimensions. The authors of this research investigated the impact of tool dimension, tool rotational speed, and stir time on bond dimensions and weld strength of AA2024-T3 aluminum alloy. The metal flow in the second stage of ReFSSW was evaluated by examining the microstructure of the metal that filled the exit hole of the first stage. A thin SZ was found around the pin of the second stage, followed by a thermo-mechanically affected zone consisting of grains elongated in the vertical direction.

Additionally, in a recent study, Fan et al. [102] explored a new double-sided FSSW technique to enhance the joining of AA2198-T8 aluminum-lithium alloy plates. This method addresses the challenges of traditional single-sided welding, which often results in defects like hooks at the joint interface. By applying intense plastic deformation from both sides, the SDP-FSSW technique creates a wavy hook that improves metallurgical bonding and mechanical interlocking. The optimal process parameters included a rotation speed of 603 rpm, a dwell time of 3 s, and a plunge depth of 0.26 mm, leading to a joint strength of 13.0 kN—47% stronger than joints made with single-sided welding. The study highlighted that effectively controlling the wavy hook formation can significantly enhance joint quality and reliability, showcasing the potential of double-sided friction stir spot welding as a superior welding method.

2.4. Advancements in FSSW Tool Design

Tool design is a critical factor influencing FSSW process quality and efficiency. Recent advancements focus on pinless tool designs to simplify processes and reduce wear, refill tool designs to address keyhole formation, and material selection for improved wear resistance, thermal conductivity, and strength. Optimizing tool geometry through modifications to shoulder and pin profiles, as well as exploring novel shapes, enhances weld quality and process efficiency. Additionally, tool coating technologies are being developed to improve wear resistance, reduce friction, and enhance heat transfer, leading to extended tool life and superior weld quality. These advancements collectively aim to develop optimized FSSW tools for various industrial applications.

The geometry of the FSSW tool plays a crucial role in determining heat generation, material flow, and joint strength. Various tool designs with different shoulder shapes, pin profiles, and tool lengths were attempted to optimize the welding process. The aim of this research was to enhance heat dissipation, reduce tool wear, and achieve better weld quality. The generation of heat in FSSW is contingent upon the frictional interaction between the non-consumable rotating tool and the workpiece [103]. Consequently, the geometry of the tool coupled with the tool material plays a pivotal role in controlling heat generation, alongside key process parameters such as rotational speed, dwell time, plunge rate, etc. [42,104]. Notably, the lower section of the shoulder significantly contributes to heat generation as well as heat distribution as compared to the pin. As the pin predominantly governs the movement of plasticized materials during the welding process, meticulous design of the shoulder and pin is important to ensure a robust weld outcome [105].

The objectives of tool design in linear FSW and FSSW differ significantly. In FSW, the focus is on horizontal mixing, ensuring material transfer between workpieces in the welding plane, causing material flow across the vertical interface line. Some FSW tool designs may not be suitable for lap configuration joining due to excessive thinning of the upper sheet. This factor highlights the need for an optimization strategy before applying FSW tools in FSSW [106]. Particularly, FSSW tools have evolved into new technologies, including pinless and RFSSW tools [107,108]. In recent years, there have been several advancements in FSSW tool designs aiming at improving the welding process and thereby achieving better weld quality and tool life. Specific advancements, such as tool materials, tool geometry, active cooling systems, sensor integration, tool coatings, and multi-tool systems, are implemented depending on the industry and application requirements.

Bolouri et al. [104] employed a static-shoulder design in FSSW for joining aluminum alloys and carbon fiber-reinforced polymer (CFRP) composites. In comparison to the conventional FSSW setup, this static-shoulder friction welding design streamlines the manufacturing process for producing joints between aluminum alloy and CFRP. Their noteworthy findings include the substantial influence of pin rotational speed against weld temperature and the moderate effect of pin plunge depth and feed rate against weld quality. Cross-sectional analysis revealed the embedding of CFRP into undulations on the deformed aluminum alloy surface, with variations in pin design affecting macro-mechanical interlocking (see Figure 24). The use of a fluted pin, in particular, demonstrates enhanced interlocking, resulting in a significant improvement in joint performance.

Illustrated in Figure 25 are snapshots capturing the momentary material dynamics at various time points throughout the tool plunging stage for the conventional tool. The material movement around the pin periphery is directed upwards. In proximity to the shoulder, two distinct flow patterns emerge. Closer to the centerline, the material experiences a downward push, influenced by the force exerted from the shoulder face. On the shoulder periphery, however, the material exhibits an upward and outward flow, attributed to the extrusion resulting from the shoulder plunge.

2.5. Numerical Simulation of FSSW

Numerical simulation is a valuable tool for understanding and optimizing the FSSW process. Finite element analysis models thermal, mechanical, and metallurgical phenomena during welding, aiding in process parameter optimization, predictive capabilities, and cost reduction. By simulating microstructure evolution and its impact on mechanical properties, researchers can enhance FSSW process comprehension. However, challenges in accurately modeling material behavior at high temperatures and complex boundary conditions persist, necessitating ongoing research for improved simulation accuracy and broader applicability.

In general, it is well recognized that experimental research in FSW and FSSW imposes a very high rate of time and cost. In this context, advanced numerical simulation and modeling methods serve as invaluable tools, facilitating a deeper, more cost-effective, and expedited understanding of the process [110]. However, simulating the FSSW process presents significant challenges due to the need to integrate multiple interacting phenomena, including heat transfer, material flow, and microstructural evolution. The FSSW process generates considerable heat through friction between the rotating tool and the workpiece material, making accurate modeling of heat generation and dissipation crucial for predicting temperature distribution and its impact on microstructural transformations and mechanical properties [111]. The plastic deformation and flow of material around the tool require sophisticated constitutive models to describe the non-linear, temperature-dependent flow stress of the material, alongside accurately representing tool geometry, rotational speed, and traverse speed interactions. Thermal and mechanical processes in FSSW induce significant microstructural changes such as phase transformations, grain growth, and precipitation, necessitating phase transformation kinetics and grain growth models to predict microstructure evolution. Additionally, tool-material interaction, including tool wear and its impact on process efficiency and joint quality, also plays a crucial role and needs to be integrated into numerical models.

While the simulation of FSW is extensively reported [112], the simulation of the FSSW process remains briefly addressed in the literature, particularly concerning the joining of both similar and dissimilar materials. These simulations have focused on material flow and temperature distribution, with thermal modeling being a major area of study. Various commercial software, such as ABAQUS, ANSYS, and FLUENT, along with user-defined codes, are employed for these simulations.

Numerical simulations of FSSW predominantly utilize the Finite Element Method (FEM) for thermo-mechanical coupling, capturing heat transfer, material flow, and stress distribution [113,114]. Computational Fluid Dynamics (CFD) methods [115,116], particularly the Coupled Eulerian–Lagrangian (CEL) approach, are employed to model material flow [117], while Smoothed Particle Hydrodynamics (SPH) handles large deformations without mesh issues [118]. The Arbitrary Lagrangian–Eulerian (ALE) method offers adaptive mesh refinement [119,120].

Hannachi et al. [121] developed a three-dimensional (3D) thermomechanical finite element (FE) model for FSSW using Abaqus/Explicit. They compared the Arbitrary Lagrangian–Eulerian (ALE) and Coupled Eulerian–Lagrangian (CEL) methods in simulating the FSSW process of AA6082-T6 aluminum alloy. Their results indicated that the CEL method was not only more accurate but also simpler and more computationally efficient than the ALE method. Further, Hannachi et al. [111] evaluated different friction models for their suitability in simulating the FSSW process of AA6082-T6 aluminum with a CEL approach. They assessed the conventional Coulomb model, a modified Coulomb model, and a model based on a temperature-dependent friction coefficient. They concluded that the temperature-dependent friction coefficient model was the most accurate for predicting real-time temperature changes during welding, as it accounts for material softening at higher temperatures, aligning well with the actual temperature evolution during the welding process.

According to Jo et al. [122], FEA was performed to simulate the FSSW process using the CEL method in ABAQUS software. As the FEA results alone could not directly confirm bonding, parameters such as contact pressure, flow stress under specific temperature, strain, and strain rate conditions, as well as the velocity of a selected node, were extracted and evaluated using a bonding criterion. Figure 26 shows the FEA results for two different sets of welding parameters. At a higher rotational speed (1200 rpm), greater plunge depth (2.4 mm), and longer dwell time (10 s), sufficient frictional heat was generated, resulting in successful bonding between the upper and lower sheets. In contrast, at a lower rotational speed (800 rpm), reduced plunge depth (2.2 mm), and shorter dwell time (6 s), the generated heat was insufficient, and no bonding occurred.

Chen et al. [123] used the CEL simulation method to examine the interfacial material flow dynamics between steel and aluminum during FSSW. They analyzed material distribution patterns at different plunging depths of the tool. They elucidated the mechanism of hook formation, which is a critical microstructure feature developed in dissimilar FSSW. Moreover, Shobri et al. [124] investigated the plunging phase of FSSW between copper and aluminum using a 3D finite element simulation model integrating CEL formulation. They examined the effect of tool rotation speed and plunge depth on the welding process, finding that higher rotation speeds significantly enhance plastic deformation and heat generation. Furthermore, they identified an optimal plunge depth that maximizes these benefits while maintaining joint integrity. The simulation results correlated closely with experimental data.

Salloomi and Al-Sumaidae [125] investigated using the CEL method, the effect of rotational speed on the thermal and residual stress environments generated during the FSW of dissimilar aluminum alloys AA2024-T3 and AA6061-T6. Across simulations, they found that the increase in the tool rotational speed will cause an increase in the amount of heat generated due to the friction contact between the stirring tool and the welded sample. As the tool pin begins contact with the dissimilar alloy sample, the temperature starts rising in the area below and around the tool pin to a high value, causing a circular pattern of temperature contours to be spread away on the sample surface. Figure 27 displays temperature distribution profiles at maximum tool plunge for two values of tool rotational speed.

Berger et al. [126] developed a (2D) two-dimensional axisymmetric thermo-mechanical model for RFSSW of AA7075-T6 aluminum alloy sheet. Throughout this numerical simulation, the authors aimed to accurately predict welding temperatures, material flow, and defect formation during the RFSSW process. They validated the simulation model predictions by comparing measured temperatures within the weld nugget, showing accuracy within 10% of the measured values. Janga et al. [127] developed a 3D numerical model to simulate and analyze the effects of tool rotational speeds (RS) and plunge rate (PR) on RFSSW of AA7075-T6 sheets. Validation was achieved by comparing the temperatures recorded in the model with those from previous experimental studies, showing a peak temperature error of 2.2% at the weld center, as shown in Figure 28. The findings demonstrated that increasing RS resulted in higher weld temperatures, effective strains, and time-averaged material flow velocities. Conversely, increasing PR led to reduced temperatures and effective strains. Material movement within the SZ improved with higher RS, while higher PR enhanced material flow in the top sheet but reduced it in the bottom sheet. This study provided a detailed understanding of how RS and PR influence joint strength by correlating thermal cycles and material flow velocity from the numerical model.

Thus, numerical models for FSSW, which integrate these multi-physics phenomena, not only enhance the understanding of fundamental mechanisms but also aid in optimizing tool design and process parameters, leading to improved joint quality and performance. Advancements in computational capabilities promise further improvements in accuracy and predictive power for industrial applications.

3. Emerging FSSW Techniques and Variations

3.1. Submerged Friction Welding

Submerged friction stir welding (SFSW), also known as underwater friction stir welding (UFSW) or immersed friction stir welding (IFSW), is a variant of FSW where the welding process takes place beneath a liquid medium, typically water or brine. This innovative technique was initially developed to address the excessive heat generation often encountered in conventional FSW, which can lead to the formation of brittle intermetallic compounds. By submerging the welding process, SFSW offers several potential advantages, including reduced heat input due to the liquid acting as a heat sink, improved weld quality by mitigating the formation of defects like porosity and cracks, and potential environmental benefits through reduced fume and smoke emissions. However, SFSW presents unique challenges, such as equipment design, process control, and the impact of water on weld properties. Despite these complexities, the potential benefits of SFSW have attracted significant research interest, particularly in industries such as marine and offshore construction.

It is important to highlight that the application of submerged friction welding, conducted in a welding environment with water or other rapid-cooling media to regulate precipitate transformations, has not undergone assessment. While this technique has demonstrated promise in mitigating heat-affected zone (HAZ) deterioration in linear friction stir welding (FSW) joints, implementing localized cooling through liquid media in a spot-welding process presents significant challenges [128].

A study by Basak et al. [129] presents gas pocket-assisted friction stir spot welding (GA-FSSW) as an innovative underwater wet welding technique for the sub-sea industry. Utilizing continuous gas flow for stabilization, GA-FSSW effectively addresses challenges associated with underwater conditions. Microstructure analysis reveals unique precipitation patterns, finer precipitates near the SZ’s pinhole, and a higher surface microhardness region compared to conventional underwater and air-based friction stir spot welding (UFSSW and FSSW). The study evaluates GA-FSSW on a commercially available aluminum alloy in a simulated seawater solution, showcasing its potential for improved microstructural effects in underwater welding applications.

3.2. Friction Stir Spot Vibration Welding

Friction Stir Spot Vibration Welding (FSSVW) is a specialized welding technique that combines the principles of FSSW with the application of vibration to the welding tool. In FSSW, a rotating tool is used to join materials through heat and pressure without melting them. FSSVW takes this concept a step further by introducing oscillatory motion (vibration) to the welding tool during the process. The primary objective of incorporating vibration is to enhance material mixing, reduce defects, and potentially improve the mechanical properties of the weld joint. By inducing vibrational energy into the system, FSSVW aims to break down material structures, facilitate better interdiffusion, and refine the microstructure of the weld zone. However, the exact mechanisms by which vibration influences the welding process are still under investigation, and the optimal parameters for achieving desired results are yet to be fully established. FSSVW is a relatively new technique, and its full potential and limitations are still being explored. While promising results have been reported in some cases, further research is necessary to fully understand the benefits and challenges associated with this welding method.

FSSVW technique leverages the benefits of both friction stir welding and ultrasonic or mechanical vibrations to improve material flow, reduce defects, and strengthen the welded joints [130].

In FSSVW, a non-consumable rotating tool is plunged into the materials to be joined, similar to the conventional FSSW process. However, in addition to the rotational and axial forces applied by the tool, vibrational energy is introduced. This vibrational energy can be ultrasonic or mechanical, depending on the specific setup and requirements of the welding process [131]. The vibrations assist in breaking up oxides and other contaminants at the faying surfaces, promoting better bonding between the materials [132]. They also enhance the plasticization of the material around the tool, leading to improved mixing and a more uniform microstructure in the weld zone. The synergistic effect of frictional heat and vibrational energy results in a more efficient welding process, potentially reducing the required tool forces and energy input [133].

Ji et al. [134] developed the Ultrasonic FSSW technique for welding dissimilar AZ31 and AA6061 aluminum alloys. Their study showed that ultrasonic vibration facilitated the upward movement of the bottom plate, creating a robust joint. Moreover, they found that ultrasonic vibration expanded the SZ width and resulted in finer grains within the SZ. Rostamiyan et al. [132] combined FSSW with ultrasonic welding to enhance weld quality. In their approach, ultrasonic vibration was applied to the tool during the FSSW process. They investigated the influence of various process factors, including vibration, tool rotational speed, tool plunge depth, and dwell time, on lap-shear force and hardness. Their findings indicated that the incorporation of vibration significantly increased both lap-shear force and hardness.

Bagheri et al. [135] investigated the effects of vibration during FSSW on the microstructure and mechanical properties of AA5083 aluminum alloy. The innovative technique, termed FSSVW, was used to introduce vibration to the workpiece during the welding process. Both experimental methods and finite element simulations were employed to analyze the outcomes. The results demonstrated that incorporating vibration during FSSW led to significant grain refinement in the SZ, with grain size decreasing by approximately 25%. This refinement improved the mechanical properties, notably increasing tensile shear strength by about 20% and enhancing hardness. The study also found that higher vibration frequencies further improved these properties. It is observed that the SZ grains in the FSSVWed specimens are smaller than those in the FSS welded specimens. Another study by Bagheri et al. [136] demonstrated that applying vibration during FSSW of AA5083 significantly refines the microstructure and enhances the thermal properties of the weld, as evidenced by both experimental and numerical analyses. Rostamiyan et al. [132] conducted experimental studies on ultrasonically assisted FSSW of AA6061, finding that the introduction of ultrasonic vibration significantly improves lap shear force and hardness, with process factors such as tool rotary speed, plunge depth, and dwell time also playing crucial roles in enhancing mechanical properties.

3.3. Robotic Assisted FSSW

Robotic-assisted FSSW involves the integration of robotic systems into the FSSW process (see Figure 29). This automation enhances precision, consistency, and efficiency compared to manual operations. By employing robotic manipulators, the welding tool can be accurately positioned and controlled, resulting in improved joint quality and reduced process variability. Additionally, robots can handle complex welding patterns and perform tasks in hazardous environments, making them ideal for large-scale production and applications where human intervention is limited.

The integration of welding robots in industrial applications has revolutionized various welding processes by mitigating human-related challenges. This paradigm shift brings forth numerous advantages, prominently enhancing welding quality, productivity, and overall cost-effectiveness. Among the array of robotic welding applications, spot welding has historically stood out, particularly within the automotive industry. The cost-effectiveness and ease of programming of modern robotic systems have facilitated their expanded use beyond traditional domains, making significant inroads into industries like shipbuilding, offshore operations, construction, and job shops. This broader implementation underscores the transformative impact of robotic welding systems, offering potential benefits across diverse industrial sectors [137].

Robotic-assisted friction stir spot welding (R-FSSW) has emerged as a cutting-edge approach, combining the precision of robotic systems with the efficiency of the FSSW process. This integration holds a substantial promise for advancing the state-of-the-art in spot welding technologies, offering improvements in terms of accuracy, repeatability, and overall process optimization. R-FSSW is a technology that allows for precise control over the welding force and spot position, enabling point-to-point normal welding and supporting, and it can be used for various thicknesses of metals, ranging from 0.3 to 30 mm. It has advantages over traditional welding machines, including high precision, sustainability, flexibility, time and cost-effectiveness, and a small operation area. To support and guide the robot during FSW, a supporting and guiding device is used, which avoids vibration and reduces precision issues caused by the robot’s high flexibility. In addition, a probe in the tool unit can increase welding speed and stirring efficiency by forming a flat surface or groove on the tip of the probe. Integration with robotic systems facilitates the incorporation of in-situ monitoring technologies. Researchers have explored the use of sensors and vision systems mounted on robotic arms to provide real-time feedback on the welding process, enabling immediate adjustments for optimal joint formation [138].

Integration with robotic systems facilitates the incorporation of in-situ monitoring technologies. Researchers have explored the use of sensors and vision systems mounted on robotic arms to provide real-time feedback on the welding process, enabling immediate adjustments for optimal joint formation. R-FSSW mirror image welding system and method that aims to achieve accurate control over welding force and spot position in the field of welding. Figure 29 shows the industrial robotic-assisted FSSW system for RFSSW. The system consists of a control system, a welding robot, and a supporting robot, with a spot-welding head on the welding robot and a supporting head on the supporting robot [139]. Balasubramaniam et al. [140] utilize a robotic RFSSW system (see Figure 30) that is designed and developed by Kawasaki Heavy Industries (KHI) [141,142] to fabricate multi-spot-welded AA7075-T6 panels.

Figure 29. Industrial robotic-assisted RFSSW system [143].

Figure 29. Industrial robotic-assisted RFSSW system [143].

3.4. Reinforced Weld Joints

In light of advancements in FSSW technology, this section explores and presents the particle reinforcement into weld joints, which may be referred to as composite spot joints. The reinforcement fillers are often in the form of ceramic or metallic in macro or nano size. Many researchers have carried out research so far in FSW/FSP techniques by introducing different hard ceramic particles into the base material through a slot/groove [35,144,145,146,147]. The incorporation of particles in FSW joints leads to better grain refinement, which consequently improves weld mechanical properties and metallurgical characteristics [148,149,150]. Nano-size fillers showed improved joint properties compared to the micro-size particles. Nanoparticles, characterized by their extremely small size and high surface area, have gained attention as potential reinforcements in FSSW. Their unique properties, such as enhanced reactivity and improved dispersion within the weld zone, make them promising candidates for strengthening joint interfaces. Previous research, such as the research conducted by Suresh et al. [151], has demonstrated the positive effects of incorporating nanoparticles into the FSSW process, resulting in increased joint strength and improved mechanical properties.

Numerous research studies have explored the diverse impacts of reinforcements like SiC, Al₂O₃, TiC B₄C, etc., in concurrence with various parameters such as tool rotation speed, plunge rate, dwell time, etc., on both the mechanical and microstructural properties of FSSW. SiC, in particular, has garnered significant attention due to its favorable characteristics, including low thermal expansion and high melting point. A plethora of investigations have delved into FSSW applications involving a range of materials, including aluminum alloys [152], magnesium alloys [153], copper [154], and other alloys reinforced with silicon carbide during the welding process.

The integration of particle reinforcements into the FSSW process requires careful consideration of the welding parameters and the interaction between the particles and the rotating tool. The particles may be introduced into the weld zone through various methods, such as pre-placement of fillers on the material through a supplementary feed mechanism. For instance, a groove over the base material to add filler particles is shown in Figure 31 and Figure 32. The size of the groove has been changed according to the volume of reinforced materials to be added or packed. Mohd Isa et al. [155] employed a method of introducing cast graphene, prepared by dispersing it in ethanol and drying it, as illustrated in Figure 33. This was performed to incorporate secondary particles into the weld area before FSSW.

Table 2. Research summary of FSSW concerning nanoparticle reinforcement and process parameters. TRS: Tool rotational speed, DT: dwell time, VF: volume fraction, GHD: Guide Hole Diameter, TTS: Tool Traverse Speed, TPD: tool plunge depth, TPR: tool plunge rate.

Base Material	Plate Thick	Reinforced Particle and Their Size	FSSW	Process Parameters	Ref.
Aluminum-copper	3 mm	50 nm and 250 nm SiC particles	FSSW		[157]
AA5083 + HD-PE	2 mm	Al2O3, TiO2, SiO2	FSSW	TRS, DT, VF	[158]
AA6061-T6	2 mm	SiC—50 nm	SweFSSW	GHD, TRS, TTS	[159]
AA7075-T6	2 mm	Al2O3—30 nm	FSSW	TRS	[34]
Interstitial free (IF) steel	0.7 mm	TiO2—35 nm	FSSW	TRS, TPD, DT and VF	[160]
AA6061-T6	2 mm	Al2O3—30 nm	SweFSSW	GHD, TRS, TTS	[156]
AA6061-T6	2 mm	Al2O3—30 nm	FSSW	TRS, TPR and DT	[161]
AA6061-T6	2 mm	SiC—50 nm	FSSW	TRS, VF and DT	[36]
AA6061-T6	3 mm	45 to 65 nm	FSSW	TRS, VF and DT	[152]
AZ31-B	2 mm	SiC—40 nm	FSSW	TRS, TPR and DT	[162]
AA5052-H32/AA6061-T4	3 mm	graphite powder—10 µm	FSSW	TRS, TPR, TPD and DT	[163]
AA5083-H321	3 mm	Fe3O4—20 nm	FSSW	TRS and DT	[164]
Interstitial free (IF) steel	0.7/1 mm	SiC—25 nm	FSSW	TRS, DT	[165]
AA2024-T3	1.6 mm	SiC—30 nm	FSSW	TRS, DT	[166]
AA5052-H32	3 mm	Graphite (15 µm)	FSSW	TRS, TPD	[167]
AA2024 + Aluminum/pure Copper	2 mm	SiC	FSSW	TRS, DT, VF	[168]
AA6061-T6	3 mm	SiC—45 µm	FSSW	TRS, DT and GHD	[169]

presents a compilation of such research, highlighting essential process parameters considered in FSSW studies concerning particle reinforcements. Investigations into the FSSW of aluminum alloys, magnesium alloys, and copper, each reinforced with silicon carbide, have yielded valuable data on the interplay between reinforcement type, process parameters, and resultant joint properties. The multifaceted exploration of reinforcements and associated parameters in FSSW, particularly with SiC nanoparticles, represents a dynamic and evolving field.

3.4.1. Effect of Nanoparticles on Microstructure Characteristics

The incorporation of nanoparticles into FSSW can significantly alter microstructural characteristics, influencing grain refinement, phase transformation, dislocation density, and particle distribution within the weld zone. Nanoparticles act as nucleation sites, promoting finer grains and enhancing mechanical properties. Additionally, they can influence phase composition and dislocation behavior, impacting overall material properties. Achieving uniform nanoparticle distribution is crucial for maximizing benefits, as agglomeration can lead to localized property variations. Understanding these microstructural changes is vital for optimizing nanoparticle-reinforced FSSW joints.

The study by Bagheri et al. [170] investigated how SiC nanoparticle size (50 nm and 250 nm) affects the microstructure and mechanical properties of aluminum-copper composites during FSSW. Results showed that larger particles increase temperature and grain size, while finer particles enhance shear strength and stress due to improved distribution. Their findings have provided valuable insights into the relationship between SiC nanoparticle characteristics and FSSW-processed composite properties. Hassanifardet et al. [171] reported the influence of Al₂O₃ particle introduction into the nugget zone during FSSW of AA7075-T6 aluminum alloy joints to evaluate the fatigue strength of the joints. Results reported that adding 1 wt.% alumina particles improves tensile strength slightly and significantly enhances fatigue lives (up to 2.5 times) compared to as-welded joints. However, introducing 2.5 wt.% alumina particles did not improve mechanical properties. Various cracking modes were observed at different load levels, with 1 wt.% alumina leading to circumferential cracks and pull-out fracture modes, while 2.5 wt.% alumina resulted in annular cracks and lap-shear failure modes. This study underscores the nuanced effects of alumina particle concentration on FSSW joint fatigue properties. The incorporation of nanoparticles into the FSSW process introduces significant changes in the microstructure of the weld zone, ultimately influencing the grain size reduction and crystalline structure of the joints. One of the primary effects of nanoparticles in FSSW is grain refinement within the weld zone. As the rotating tool stirs the base material and the nanoparticles, the intense local heating and mechanical deformation lead to dynamic recrystallization. This process results in a finer, more uniform grain structure within the SZ.

The incorporation of nanoparticles into the FSSW process introduces significant changes in the microstructure of the weld zone, ultimately influencing the grain size reduction and crystalline structure of the joints. One of the primary effects of nanoparticles in FSSW is grain refinement within the weld zone. As the rotating tool stirs the base material and the nanoparticles, the intense local heating and mechanical deformation lead to dynamic recrystallization. This process results in a finer, more uniform grain structure within the SZ.

The study by Abdollahzadeh et al. [157] successfully achieved a joint of aluminum to copper sheets with a SiC nanoparticle interlayer using FSSW. The introduction of SiC nanoparticles during FSSW refined the microstructure and enhanced the interface characteristics of the joint. Continuous phases of CuAl₂, CuAl, and Al₄Cu₉ developed at the interface, with a significant reduction in the thickness of the IMC layer. Mechanical properties, such as strength and hardness, were improved in the FSSW-ed samples with SiC particles compared to conventional FSSW-ed specimens. Specifically, the grain size in the SZ was significantly reduced, and the IMC layer thickness decreased from 81 to 17.01 μm. The addition of SiC particles increased joint strength from 6.1 to 8.4 kN and elongation from 1.82 to 2.24 mm. The study recommended applying FSSW with SiC nanoparticles for welding Al-Cu joints in industries like automotive and aerospace.

Suresh et al. [172] has focused on addressing the persistent challenges in joining dissimilar lightweight metals, particularly aluminum (AA6061) and magnesium (AZ31B) alloys, which are essential for aerospace and automotive applications. In this study, the role of SiC nanoparticles in FSSW was investigated as a mean to refine microstructure and suppress the formation of brittle Al–Mg intermetallic compounds. By introducing SiC through a pre-drilled hole and varying the tool’s rotational speed, the research demonstrates that nanoparticle addition leads to a more homogeneous weld structure and a significant increase in joint strength. The findings accentuate the potential of SiC reinforcement in improving the mechanical performance of dissimilar Al/Mg joints. The high-energy balling effect of nanoparticles during FSSW promotes nucleation sites for new grains. Consequently, the grain size in the SZ is significantly smaller compared to the base material. Finer grains offer improved mechanical properties, such as increased strength and hardness, due to enhanced grain boundaries that obstruct dislocation movement. In Figure 34, the SZ of the AA6061-T6/SiC FSSW joint reveals the appearance of reinforced particles within the grain boundaries. This observation highlights the distribution and presence of reinforcing particles, providing insight into their localization within the welded zone [159]. Achieving a uniform dispersion of reinforced fillers in the weld area is crucial for harnessing their beneficial effects. Proper distribution ensures that nanoparticles contribute evenly to the microstructural characteristics. However, the dispersion of nanoparticles can be influenced by various factors, including tool design, rotational speed, and feed rate. The fracture surface fractography of the bare and SiC-reinforced weld samples is displayed in Figure 35. Higher tensile strength is the result of the SiC-added sample’s strong coherency between SiC nanoparticles and the aluminum matrix. The sample with SiC added has fewer, shallower dimples on its fracture surface than the sample without SiC, which is covered in larger, deeper dimples.

Researchers have explored different methods to ensure uniform particle distribution. The use of specially designed tool pins, featuring grooves or channels for nanoparticle placement, has proven effective in promoting even distribution. Nanoparticles in FSSW also influence material flow during the welding process. The presence of nanoparticles can alter the flow patterns, leading to a more uniform mixing of material at the weld interface. This improved material flow contributes to a more homogenous microstructure, minimizing the formation of defects like voids or inclusions [165].

In their study, Abdollahzadeh et al. [157] focus on the successful fabrication of an aluminum-to-copper joint using FSSW with the incorporation of a nanoparticle interlayer, specifically SiC. The research investigates the impact of SiC reinforcing particles on both the microstructure and mechanical properties of the joints. The introduction of nanoparticles during FSSW was found to lead to microstructure refinement within the joint area, enhancing the characteristics of the interface. Notably, continuous phases of CuAl₂, CuAl, and Al₄Cu₉ were observed at the interface joint, and the thickness of the intermetallic compound (IMCs) layer in the bond interface decreased with the presence of nanoparticles [173].

The fabrication of particle-reinforced FSSW joints presents challenges, including the potential for particle agglomeration, tool wear, and uneven distribution. Naturally, joint strength may be affected by agglomeration or coarsening of reinforced particles during FSW and FSP [174,175]. One challenge encountered when using nanoparticles as reinforcement in welding is the issue of agglomeration within the weld region, leading to a reduction in joint strength. The propensity of nanoparticles to aggregate is linked to their elevated surface area, leading to an expansion of the interparticle space and ultimately giving rise to pore formation [176,177]. In efforts to mitigate this concern, experiments are underway using the FSSW process, involving alterations to tool geometries and sizes, as well as limiting the volume addition of particles along the joint to minimize agglomeration. Studies have indicated that the agglomeration of nanoparticles in the weld can be reduced by limiting the volume addition of particles, varying the tool geometry, and increasing the tool rotational speed. In addition to addressing agglomeration, the selection of nanoparticles also presents challenges in the FSSW process. Research has demonstrated that the choice of nanoparticles influences joint properties; for instance, friction stir spot welding of AA6061-T6 aluminum alloy using Al₂O₃ nanoparticles results in joint formations with high microhardness with an agglomeration of Al₂O₃ nanoparticles in the SZ (see Figure 36) which led to the uneven distribution of the reinforced particles [156]. It is crucial to note that the quantity of nanoparticle reinforcement significantly affects weld joint formations. Additionally, optimizing the FSSW process parameters can aid in preventing agglomeration or clustering of nanoparticles, which can lead to localized variations in microstructure and properties.

3.4.2. Effect of Nanoparticles on Mechanical Properties

The incorporation of nanoparticles into FSSW can significantly influence the mechanical properties of the weld joint. By acting as nucleation sites, nanoparticles can refine the grain structure, leading to enhanced strength and ductility. Additionally, nanoparticles can interact with dislocations, affecting their movement and distribution, further impacting mechanical properties. The distribution of nanoparticles within the weld zone is critical, as uniform dispersion maximizes reinforcement benefits, while agglomeration can lead to localized property variations. Understanding these microstructural changes induced by nanoparticles is crucial for optimizing the mechanical performance of FSSW joints.

Several studies have been reported on the incorporation of nanoparticles that increase tensile strength, hardness, and fatigue resistance in FSSW joints. The presence of nanoparticles not only refines the grain structure but also contributes to grain boundary strengthening. These effects collectively lead to improved mechanical performance. For instance, the addition of SiC nanoparticles to magnesium alloys has been shown to enhance lap shear strength and microhardness due to grain boundary strengthening. Moreover, the increased hardness and strength often result in a more stable plug failure mode during tensile testing, reducing the likelihood of premature joint failure. For instance, Wu et al. (2017) [162] conducted the FSSW experiment on AZ31 magnesium alloy by adding SiC particles in the SZs. They reported that both the lap shear strength and microhardness of welds were found to increase by grain boundary strengthening. Tebyani et al. (2015) [165] used SiC nanoparticles of 25 nm in size to produce an FSSW joint of interstitial-free (IF) steel and reported that the hardness and joint strength were improved significantly compared to SiC-free joints. They also found that the welds with the reinforced nanoparticles were subjected to a combination of peel and shear modes of fracture. Hong et al. (2017) [163] used a nanoparticle deposition system (NPDS) to fabricate a carbon/aluminum MMC joint using FSSW. They reported that the carbon reinforcement enhanced the strength and toughness of the joint. They also suggested that the combined effect of FSSW and NPDS provides an effective method to improve the mechanical performance of spot joints without changing the material or increasing the number of joints. Hardness testing revealed a discernible increase in the weld zone hardness where particles were reinforced in FSSW. The heightened hardness can be correlated with the refined microstructure resulting from particle reinforcement. Interestingly, the hardness profile exhibited, with increasing volume, an addition of reinforcements, indicating the effectiveness of the chosen particle type in augmenting the local mechanical properties.

In a study conducted by Suresh et al. [36], an experiment was carried out using FSSW on AA6061-T6, incorporating varying volume percentages of SiC nanoparticles. It was reported that better deformation behavior (see Figure 37) and hardness were observed up to a particular increase in the volume addition of SiC. However, beyond that point, there was a reduction in both tensile strength and hardness. The researchers found that the average microhardness of the heat-affected zone (HZ) in specimens with 29%, 22%, and 14% SiC additions was measured as 93.27 HV, 85.92 HV, and 82.62 HV, respectively. It was observed that the microhardness of the heat-affected zone (HAZ) increased proportionally with the volume percentage of SiC added. This notable enhancement in hardness can be attributed to the presence of the hard ceramic SiC, which influences the grain size and consequently impacts the welding properties of the aluminum matrix.

Suresh et al. (2020) [34] investigated the FSSW of AA7075-T6 with reinforcing Al₂O₃ nanoparticles, and they found that the reinforcement considerably influences the ultimate tensile strength and hardness of the joint. It was also shown that the presence of SiC in the grain boundaries prevents the growth of grains. Moreover, the increase in tool rotational speed makes uniform dispersion of nanoparticles in the weld region.

Few research studies have investigated the effects of nano-particle reinforcements in aluminum alloy using the swept-FSSW technique. For instance, Suresh et al. [156,159] employed the swept-FSSW technique to join AA6061-T6 with the addition of Al₂O₃ and SiC nanoparticles in the SZ in their respective works. Various amounts of addition of nanoparticles and tool rotational speed were considered for the investigation. Figure 38 illustrates the impact of nanoparticle reinforcements on tensile shear strength in comparison to unreinforced joints, as derived from various research studies.

Additionally, the configuration of the tool pin also plays a great role in the dispersion of reinforced particles and material flow. Balamurugan et al. [178] conducted a study on FSSW using AA5052 and AA6061 aluminum alloys, where they utilized a taper tool pin with varying pin end tapers ranging from 0 to 4 mm, as shown in Figure 39. Their findings indicated that the use of a taper pin with a 5 mm pin end diameter, combined with Mg particles, resulted in defect-free joint formation and prevented particle agglomeration when compared to other tools. On the other hand, tool pins with lower tapers exhibited a lower percentage of elongation due to the formation of coarse grains. This decrease in strength could be attributed to inadequate heat generation between the tool and base material. Consequently, the geometrical shapes of the tool pin in particle-reinforced joints have diverse effects on the properties of the joint.

Nanoparticles, such as ceramics or carbides, enhance the wear resistance of FSSW-welded materials. They act as protective barriers, reducing material loss and preventing excessive wear on joint surfaces. This is particularly beneficial in applications with abrasive conditions, ensuring improved durability and performance. This increased wear resistance is particularly advantageous in industries such as automotive and aerospace, where components may experience friction and abrasion during operation. Liu et al. [179] reported improvements in the shearing strength and tribological behavior of the AA5754/AA5083 joint through the utilization of B₄C ceramics. These enhancements were attributed to the dislocation-strengthening mechanism, which led to the formation of new nucleation sites by the embedded fine B₄C particles. This, in turn, reduced grain growth and increased resistance against corrosive environments. Paidar et al. [166] investigated AA2024-T3/SiC hybrid aluminum composites in terms of wear and microstructural properties. Figure 40 displays FESEM images comparing worn surfaces of weld samples with and without Al₂O₃ nanoparticles. The nanoparticle-free sample exhibits severe plastic deformation and surface layer removal, suggesting severe adhesive wear [156]. Conversely, the sample with Al₂O₃ nanoparticles demonstrates the prevention of plastic deformation, reduced material removal, and smooth, uniform wear morphology, emphasizing the efficacy of Al₂O₃ nanoparticles in mitigating wear.

In conclusion, reinforced weld joints showed higher strength, stiffness, and fatigue strength compared to unreinforced FSSW joints. Silicon carbide and aluminum oxide have been mostly used as reinforcement for aluminum alloy MMC, and boron carbide is used for copper MMC. As there are plenty of ceramic materials available, readers may pursue further research in this area and investigate the effect of other materials on reinforcement.

3.5. Mitigation of FSSW Defects

Mitigation of FSSW defects involves strategies to minimize or eliminate imperfections in FSSWs. Common defects include voids, cracks, incomplete welds, keyholes, and hook defects. Techniques such as process parameter optimization, tool design refinements, and material pre-treatment can help reduce defect formation. Additionally, implementing quality control measures, including non-destructive testing methods, is relevant for identifying and addressing defects early in the manufacturing process. By effectively mitigating defects, the quality and reliability of FSSW joints can be significantly improved.

Researchers strive to minimize these defects through adjustments in process parameters and tool design. This involves not only achieving defect-free welds but also ensuring consistency in quality across a range of material thicknesses and compositions.

A significant challenge encountered in FSSW is the formation of keyholes at the center of the spot post-welding, which are considered defects [180]. In recent years, only a limited number of researchers have undertaken the development, characterization, and validation of potential methods aimed at eliminating keyhole defects in FSSW. Their research efforts have been specifically on achieving void-free FSSW. The hooking defect poses a critical challenge in the thermo-mechanically affected zone (TMAZ) of lap joints during FSSW, affecting both similar and dissimilar materials [181,182]. Figure 41 represents typical FSSW joints with hook defects and the effect of this on the bonding.

Contributing factors to hooking encompass suboptimal tool design, incorrect tilt angle, low welding speed, high rotational speed, and an improperly fitted workpiece fixture [184,185]. Notably, recent investigations delve into addressing these factors for improved welding outcomes [186,187]. Lunetto et al. [188] explored the pre-hole FSSW for joining DP590 and DP780 steels that are widely used in the automotive industry. The study focused on addressing the hook defect commonly associated with FSSW joints formed using traditional truncated conical tools. Their research focused on a comprehensive exploration of the welding processes through a thorough comparative analysis, considering conventional FSSW, pinless FSSW techniques, and Resistance Spot Welding (RSW). The authors proposed an innovative approach utilizing a stationary shoulder to generate substantial forging force during FSSW. This technique aimed to counteract the factors contributing to hooking, providing an alternative strategy for defect prevention [189].

Another notable research strategy is the incorporation of an interlayer of graphene nanoplatelets at the weld interface. This approach has eliminated hooking defects and shown promising enhanced strength of FSSW lap joints [190]. These recent developments signify a concerted effort within the research community to not only understand the mechanisms underlying the hooking defect in FSSW but also to implement effective strategies for its rectification. As researchers continue to explore novel techniques and refine existing methodologies, the goal is to advance the reliability and quality of friction stir spot welding, particularly in lap joint configurations.

4. Future Outlook

The current status of FSSW is reviewed based on a thorough literature survey. Some points are recognized as emerging technologies in the FSSW field for prospective advancements in the future.

Artificial intelligence integration: Intelligent FSSW systems using AI and machine learning can enable real-time monitoring, adaptive control, and autonomous decision-making for process optimization.

Reinforcement strategies: future work should focus on optimizing reinforcement deposition techniques and mitigating nanoparticle agglomeration, especially with carbonaceous reinforcements like CNTs and graphene.

Advanced tool materials and coatings: Exploration of new tool materials with improved wear resistance and thermal stability is essential for consistent weld quality in high-strength alloys.

Expansion to dissimilar and hybrid materials: More studies are needed on dissimilar combinations such as Al/Cu, Al/Mg, Mg/Steel, and their interfacial behavior under FSSW, particularly using hybrid or submerged techniques.

Emerging variants: Techniques such as vibration-assisted FSSW, double-sided FSSW, and Reversed-FSSW should be further explored for enhanced joint strength and process efficiency.

Sustainability and lifecycle assessment: Comprehensive environmental and energy assessments are needed to validate FSSW’s sustainability benefits across industries.

Overall, FSSW holds immense potential for lightweight structural applications in the next generation of manufacturing, especially as automation, reinforcement strategies, and smart welding systems become more integrated.

5. Conclusions

This comprehensive review has explored the advancements in FSSW technology, highlighting its significant potential across various industries. Recent research and technological developments demonstrate that FSSW offers a compelling alternative to traditional joining methods, providing numerous advantages in terms of weld quality, material compatibility, and energy efficiency. The study emphasizes the critical role of process parameters, including tool design and microstructure-property relationships, as well as the importance of numerical simulations in optimizing the welding process to achieve sound FSSW weld joints. The need for further research in process control, material selection, and automation is also highlighted. Key conclusions from this review include the following:

• FSSW shows transformative potential in solid-state joining, offering high-quality welds efficiently and reliably across diverse industrial applications.
• Process parameters such as tool design, rotational speed, axial force, and dwell time are crucial for weld quality and performance. Innovations like friction stir spot vibration welding and robotic-assisted FSSW enhance material flow, reduce defects, and improve mechanical properties.
• Numerical simulations of FSSW have significantly improved understanding and optimization, accurately predicting heat transfer, material flow, and mechanical behavior to enhance joint quality and performance.
• Integrating nano/microparticles like carbon nanotubes, graphene, ceramic nanoparticles, and metallic reinforcements is a pivotal advancement, improving weld strength, hardness, wear resistance, and corrosion properties, though challenges in dispersion and particle agglomeration remain.
• FSSW effectively joins both similar and dissimilar materials, enabling new multi-material designs and applications. Robotic-assisted FSSW enhances precision, efficiency, and complex weld geometries, expanding its scope in high-performance industries.
• Future research is vital to address challenges, particularly in refining robotic programming and leveraging artificial intelligence for autonomous decision-making in welding processes. Further exploration of nano/microparticles in composite FSSW joints is a promising area for future investigation.