Friction Stir Welding (FSW): Solid-State Joining of Composites Materials

Abstract

Friction Stir Welding (FSW) has emerged as an effective method for joining composite materials, revolutionizing the field of composite welding. This chapter provides an in-depth exploration of FSW’s potential applications, advantages over conventional methods, and the associated challenges. By comparing FSW to existing welding techniques, current chapter demonstrate how it overcomes issues like porosity, distortion, and poor mechanical properties. Drawing from relevant literature, we delve into case studies of FSW-welded composite materials, investigating the weld joint quality and resulting material properties. The discussion extends to the identification of metal matrix composites that can be effectively joined using this innovative method, shedding light on its versatility. However, limitations are also considered to provide a comprehensive perspective. This chapter serves as a valuable resource for researchers, engineers, and practitioners in the field of materials science and engineering, offering insights into the promising future of FSW in the realm of composite material welding.

1. Introduction

The process of friction stir welding (FSW) was innovated in 1991 by The Welding Institute (TWI) located in Cambridge, United Kingdom [1]. It underwent further enhancements and received patent approval from The Welding Institute. The initial friction-stir welding machines, which became commercially available, were manufactured by ESAB1 Welding and Cutting Products at their equipment production facility in Laxa, Sweden. This innovative process marked a significant departure from traditional friction welding methods involving rotary motion and linear reciprocating techniques. It introduced a high degree of adaptability within the overall friction welding process group [2].

In 1993, NASA issued a challenge to Lockheed Martin Laboratories based in Baltimore, Maryland. The task was to devise a substitute for the aluminum alloy Al 2219, which was utilized in the original Space Shuttle External Tank, with a focus on achieving higher strength, lower density, and reduced weight. Lockheed Martin, in collaboration with Reynolds Aluminum and the laboratories at Marshall Space Flight Center in Huntsville, Alabama, successfully formulated a novel alloy named Aluminum Lithium (Al-Li 2195). This new alloy led to a substantial weight reduction of 7500 pounds (3402 kilograms) in the External Tank. Presently, the External Tank project utilizes this advanced alloy to construct the Shuttle’s Super Lightweight Tanks [3].

From 1995 onward in Europe, Friction Stir Welding has found application in various production scenarios. Initial use cases comprised the welding of extrusions to create panels for marine purposes [4]. Subsequently, this technique has undergone commercialization across diverse sectors, encompassing rail cars, automobiles, aerospace, heavy-duty trucks, and medical applications, among others. At present, the methodology is evolving towards the fabrication of intricate assemblies, resulting in noteworthy enhancements in both quality and cost-effectiveness. As the technique continues to mature, designers are capitalizing on its benefits by creating products tailored explicitly for the FSW process [5].

The lighter metals such as aluminum and magnesium alloys are frequently utilized in place of stainless steel within the automotive and aircraft sectors [6]. The metal aluminum is a base component in metal matrix composites, stands as a lightweight, high-performance material extensively employed across various industries, including aircraft, marine, defense, structural, automotive, and sports [7]. Key prerequisites for contemporary industrial applications include a high stability-to-weight ratio, robust weldability, and superior corrosion resistance. Recently, attention has shifted towards aluminum-magnesium and other alloys due to their remarkable toughness and excellent weldability [8]. Challenges arise in joining these alloys through fusion welding processes, primarily attributed to their low melting points. Notably, aluminum alloys of the 2000, 6000, and 7000 series present specific difficulties [9]. Fusion welding of aluminum alloys results in the occurrence of defects such as porosity, distortion, oxidation, and shrinkage [10, 11]. In response to these challenges, a pioneering solid-state welding technique, friction stir welding (FSW), has been developed. This method, recognized for its energy efficiency and environmental friendly attributes. Acknowledged as the most effective approach for joining similar materials, dissimilar materials, and metal matrix composites, FSW has been extensively researched. Previous studies showed the prime roles played by the type of parent material, tool material, tool pin profile, and the selection of appropriate process parameters in determining the quality of weld joints [12].

The Friction Stir Welding process is evidently a recent welding technique. The schematic of the FSW process is illustrated in Figure 1, and proves to be an effective method, particularly for welding metal component. The traditional rotary friction welding procedure necessitates the rotation of at least one of the components to be joined [13]. It faces practical limitations in connecting regularly shaped elements, preferably circular in cross-section, and constrained in length. For instance, short tubes or round bars with equivalent diameters serve as suitable examples.

Figure 1.

Schematic of friction stir welding process.

Welding composites using conventional techniques poses inherent challenges due to the diverse nature of composite materials, often comprising different matrices and reinforcing fibers. The complexities arise from the need to balance the melting points and thermal expansion coefficients of these varied components. In contrast, Friction Stir Welding (FSW) offers a distinct advantage by eliminating the challenges associated with melting and solidifying dissimilar materials [14]. Its solid-state nature allows for a more straightforward and effective joining process, ensuring greater ease of welding for composites, a feat that conventional methods struggle to achieve. This simplification in the welding process through FSW contributes significantly to enhanced efficiency and precision in composite fabrication. Apart from this, numerous potential merits of Friction Stir Welding (FSW) in contrast to traditional fusion-welding methodologies have been recognized.

One of the very important advantage of the process is that it inculcates sound mechanical characteristics in the joined component. Enhanced safety ensures from the nonexistence of harmful emissions or the splattering of liquefied substances. There are no consumables used during the process, the usual tools are, the threaded pin crafted from a customary tool alloy that possesses the capability to weld over considerable length and no additional filling material or gas shielding is mandated for aluminum welding. The process is readily adaptable to uncomplicated milling apparatus. One can automate the FSW process effectively across a variety of positions (e.g., horizontal, vertical). Generally, the welding exhibits a pleasing appearance with minimal disparities in thickness, mitigating the necessity for costly post-weld machining. The utilization of thinner materials is feasible without compromising joint strength. This approach boasts a minimal environmental footprint.

2. Process parameters

The process parameters of the Friction Stir Welding (FSW) play a crucial role in achieving structurally sound joints (ref. Figure 2). The strength of the weld joints relies on the careful selection of these parameters [15]. Within the FSW process, critical factors such as the rotational speed of the tool, transverse speed of the tool (TTS), axial load, and tilt angle significantly influence the microstructure and consequently, the mechanical properties of the joints. Numerous researchers have explored the impact of these process parameters on the microstructure of FSW joints [16, 17, 18, 19, 20, 21, 22, 23, 24].

Figure 2.

The schematic of the various process parameters involved in the FSW process.

In Friction Stir Welding (FSW) of composite materials, the tool geometry exert a profound influence on various aspects of the welding process. The shoulder of the tool, which is responsible for applying axial force and generating heat through friction, plays a critical role. The diameter of the shoulder significantly affects heat input, with larger diameters leading to increased contact area and potentially higher heat, while smaller diameters may result in localized heating. The pin, positioned at the center of the tool, determines penetration depth and mixing of the metal to be joined. Different pin geometries, such as cylindrical, tapered, or threaded designs, impact the stirring and mixing of composite materials during welding [25]. Proper material flow is essential for achieving defect-free welds and ensuring uniform distribution of reinforcing fibers in composite joints. The generated heat, influenced by the tool geometry, has implications for temperature distribution and, consequently, the thermal degradation of the composite matrix. Additionally, the joint quality and mechanical properties are directly affected by the combination of tool shoulder and pin geometry. Optimizing these parameters is crucial for enhancing joint strength, fatigue resistance, and overall performance in composite welding. The choice of tool material, with an emphasis on wear resistance and thermal conductivity, further contributes to the effectiveness and longevity of the FSW tool during the welding process. In essence, meticulous consideration and optimization of tool geometry are essential for achieving the desired mechanical properties and overall success in FSW of composite materials.

The rotational speed of the FSW tool also plays a crucial role in achieving an enhanced microstructure of the joints. In FSW of composite materials, the rotational speed of the tool significantly influences the process by affecting heat generation, material flow, and the quality of the resulting weld; higher rotational speeds often lead to increased heat input, greater material mixing, and potential improvements in joint properties, while lower speeds may result in inadequate material softening and insufficient mixing, impacting the overall welding performance and final weld quality [25].

Another important parameter, that is the transverse travel speed (TTS) also impact as heat generation, material flow, and interfacial bonding, potentially leading to changes in joint strength, quality, and microstructural characteristics [26]. Moreover, the axial load applied to the tool exert a significant influence on various aspects of the welding operation. This force, acting in the direction of the rotation of the tool, that plays a crucial role in heat generation as increased axial load results in elevated friction between the tool and the composite material. This heightened friction contributes to localized softening of the material, facilitating better mixing and penetration of the rotating tool into the composite layers. While an optimal axial load is essential for achieving a defect-free joint with proper consolidation, excessive pressure may lead to undesirable outcomes such as voids or porosity, affecting joint quality [27]. In addition to this, the axial load impacts tool wear, with excessive force accelerating wear and potentially affecting tool longevity. The temperature distribution within the welding zone, influenced by the axial load, has implications for the microstructure and mechanical properties of the composites. Additionally, the axial load influences residual stresses and the overall structural integrity of the composite joint. [28, 29]. Therefore, careful consideration and optimization of the axial load are imperative in FSW of composite materials to ensure an effective welding process and the attainment of high-quality joints with desirable mechanical characteristics [30, 31].

3. Brief overview of composite welding by FSW

Metal matrix composites (MMCs) hold immense significance in engineering applications owing to their unique combination of properties, which result from reinforcing a metallic matrix with other materials [32]. Among the various types of reinforcements employed, Al₂O₃, B₄C, SiC, graphite, and similar materials play pivotal roles in tailoring the performance characteristics of MMCs. Al₂O₃ (alumina) contributes to enhanced hardness and wear resistance, while B₄C (boron carbide) provides excellent strength and stiffness. SiC (silicon carbide) reinforcement enhances thermal conductivity and wear resistance, making it particularly valuable in high-temperature applications [33]. Graphite, with its low density and high thermal conductivity, is suitable for lightweight and thermally conductive applications. The significance of MMCs lies in their ability to offer a desirable combination of properties that traditional materials often struggle to achieve individually. However, despite their advantages, welding these composite materials poses a significant challenge. The difficulty arises from the diverse nature of the materials involved and their distinct thermal properties, making conventional fusion welding processes less effective.

MMCs are engineered materials that consist of a metal matrix reinforced with ceramic particles, fibers, or other materials. The properties of MMCs, such as high hardness, wear resistance, and low thermal conductivity, can make them difficult to weld using traditional welding methods. The dissimilarity in thermal expansion coefficients between the metal matrix and the reinforcing phase (ceramic) can lead to residual stresses and cracking during the welding process. This can be more pronounced in MMCs compared to homogeneous metals. Ceramic reinforcements in MMCs are often much harder than the metal matrix. Traditional welding processes may struggle to accommodate these hardness differences, leading to difficulties in achieving a uniform weld. The low thermal conductivity of many ceramic reinforcements can result in uneven heating during welding. This can lead to thermal gradients and gradients in mechanical properties, affecting the integrity of the weld. More importantly, achieving uniform distribution of ceramic particles in the metal matrix is essential for maintaining the desired properties of MMCs. Welding processes may alter this distribution, affecting the overall performance of the material. Overcoming these challenges is crucial to unlocking the full potential of MMCs in various industries. Developing effective welding techniques for MMCs is imperative to harness their unique properties and ensure their successful integration into advanced engineering applications.

FSW is a solid-state welding process, meaning it does not involve melting the materials. Instead, it relies on the mechanical action of a rotating, non-consumable tool to stir and forge the materials together. This characteristic is beneficial for MMCs because it avoids the problems associated with the melting and solidification of dissimilar materials with different thermal properties. FSW is a solid-state welding process, meaning it does not involve melting the materials. Instead, it relies on the mechanical action of a rotating, non-consumable tool to stir and forge the materials together. This characteristic is beneficial for MMCs because it avoids the problems associated with the melting and solidification of dissimilar materials with different thermal properties. FSW generates heat through friction between the rotating tool and the workpieces. The heat is localized around the tool, leading to more uniform heating compared to traditional welding methods. This helps in minimizing thermal gradients and preventing localized overheating of the MMC. FSW generates heat through friction between the rotating tool and the workpieces. The heat is localized around the tool, leading to more uniform heating compared to traditional welding methods. This helps in minimizing thermal gradients and preventing localized overheating of the MMC.

3.1 Weldability of aluminum alloys and AMCs

The strength effectiveness of unalloyed aluminum proves insufficient for structural applications. To overcome this limitation, it undergoes alloying with diverse metals like copper, manganese, magnesium, zinc, and silicon [34]. It is quite possible to achieve different mechanical properties by adjusting the amounts of alloying elements and using heat treatments. Owing to aluminum cubic crystal structure (face-centered cubic), it known for superior malleability and ductility. Aluminum alloys are accessible in wrought and cast configurations. The former is fabricable through rolling (either hot or cold), extrusion, and forging, whereas the latter can be shaped through sand casting, lost wax casting, permanent steel mold casting, and die-casting. Wrought aluminum is categorized into two groups based on the primary alloying components. Weldable non-heat-treatable aluminum alloys, encompassing AA1xxx, AA3xxx, and AA5xxx series, derive strength from cold working. Conversely, heat-treatable non-weldable alloys such as AA2xxx, AA6xxx, and AA7xxx series attain strength through precipitation hardening [35].

Moreover, careful consideration is imperative during the welding of aluminum and its alloys, particularly for the non-weldable alloys. Challenges may arise, including strength loss and defect formation in fusion welding processes. Trapped porosity might manifest in the coalescence cross-section, attributed to the dissolution of shielding gases (oxygen, nitrogen, and hydrogen) or moisture in the electrode and flux in molten metal, as depicted in Figure 3a. Additionally, the formidable melting temperature of stable aluminum oxide on the surface, reaching up to 2060°C, contributes to issues like lack of fusion. Centre-line or solidification cracking is a significant concern in the fusion welding of aluminum alloys, as illustrated in Figure 3b. This failure results from stresses induced by metal contraction during cooling, arising from the disparity in solidification temperatures between pure metal and alloying elements. The fluctuating heating and cooling cycles in the Heat Affected Zone (HAZ) typically lead to reduced joint strength in non-weldable alloys [36].

Figure 3.

(a) Trapped porosity and (b) solidification cracking defects in conventional liquid state welded parts.

Beyond the aforementioned challenges associated with welding aluminum and its alloys, additional complexities emerge when Aluminum Matrix Composites (AMCs) undergo fusion welding processes. These include: (a) incomplete blending between filler and base metal, (b) the excess formation of eutectic structures, (c) the presence of large-sized porosity exceeding 100 μm in the fusion zone, and (d) reactions between molten metals and reinforcements resulting in undesirable phases such as Al₄C₃ [36, 37].

Despite the challenges associated with welding aluminum and its alloys, additional complexities arise when dealing with Aluminum Matrix Composites (AMCs) during fusion welding processes.

4. Macrostructure and microstructure of FSW joints in AMCs

Friction Stir Welding (FSW) produces well-defined macrostructures and microstructures in Aluminum Matrix Composites (AMCs), enhancing the overall strength and integrity of the joints. This welding technique ensures precise and uniform bonding for robust composite materials. It is understood that the metal undergoes thermal cycles and intense plastic deformation at elevated temperatures during the welding process. This may be either due to excessive or inadequate heat input within the welding area. The issues like tunnel defects and improper bonds may arise in the weld [32]. Additionally, notable variation may occur in the form and composition at the welding zone. Henceforth, appropriate welding parameters must be chosen to prevent these issues and to obtain better macrostructure and microstructure in FSW joints. The severe distortion induced during the FSW process affect the local microstructure. The microstructural changes at the friction stir welded can be divided in to four different zones as shown in Figure 4. The first one is the base metal zone, typically referred to the microstructure of the base metal which has not affected by the process. Then it is followed by the heat affected zone, thermo mechanically affected zone (TMAZ) and nugget zone (NZ).

Figure 4.

SEM image showing microstructure of friction stir welded composite [38].

The microstructure and macrostructure of Friction Stir Welding (FSW) joints in Aluminum Matrix Composites (AMCs) exhibit distinct characteristics:

Wake Effect: The primary characteristic is the designated wake effect formed at the surface of plates due to frictional action between the tool shoulder and horizontal motion on the plate during FSW.

Nugget Zone (NZ) Configuration: The configuration of the Nugget Zone (NZ) is a key trait, classified into basin and elliptical shapes. The shape is influenced by the tool rotation speed, with higher speeds resulting in basin-shaped NZ and lower speeds leading to elliptical shapes as depicted in Figure 5.

Figure 5.

(a) Nugget shape, and (b) basin and elliptical shapes [38].

Onion Ring Structure: It is one of the features characterizing the NZ. The appearance of an onion ring structure in a swirl pattern (ref. Figure 6), influenced by the flow behavior of softened metals and differences in dislocation density during the joining process.

Figure 6.

Onion ring pattern formed in the nugget zone [38].

Tunnel Defect: One of the major problems associated with the friction stir welding, specifically for the composite materials is the macroscopic defects. The example image of the macrostructural examination of friction stir welded composite (ref. Figure 7) which revealed a tunnel defect. The frequency of occurrence of such defect is dependent on the speed of tool rotation. The higher rotation speeds result in defect-free joints, which is attributed to the sufficient energy input and plastic flow, minimizing tunnel defects (Figure 8).

Figure 7.

The optical microscopic images showing the pin hole or tunnel defect in FSW [37].

Figure 8.

(a, b) surface morphologies of aluminum and aluminum matrix composites (c) the microstructure taken at cross section of the weld joint, and (d) cross-weld macrostructure [37].

4.1 Challenges in microstructure and macrostructure of FSW joints in AMCs: microstructure challenges

Dynamic Recrystallization: New equiaxed grains form in the nugget zone due to dynamic recrystallization, leading to smaller grains than the base composite. This change in the microstructure lead to altered material properties, and in some cases, the mechanical properties may not align with the requirements of a specific application. In addition to this the dynamic recrystallization process during friction stir welding can introduce complex residual stresses in the material, which may affect mechanical properties. The interaction between the refined grain structure and residual stresses can be challenging to predict and control.

On the other hand, the formation of new equiaxed grains due to dynamic recrystallization also work in the benefit of the enhancement in mechanical properties. Because, smaller equiaxed grains can contribute to improved toughness and ductility in the material. This is because the refined grain structure can hinder the propagation of cracks, making the material less prone to brittle failure. Also, the refined grain structure result in higher strength due to grain boundary strengthening mechanisms.

PWHT Influence: Post-weld heat treatment (PWHT) results in grain growth in the nugget zone, increasing grain size. Larger grains generally result in reduced toughness and ductility. This is because the larger grain boundaries may act as initiation sites for cracks, making the material more susceptible to brittle fracture. In applications where toughness and ductility are critical, an increase in grain size could be detrimental.

Pin Profile Impact: The choice of pin profile influences microstructure, with square pins suggested for smaller and finer grain structures. Achieving uniform and consistent microstructures in composite materials with square pins can be challenging. The interaction between the square pin and the composite constituents may lead to variations in heat generation, material flow, and cooling rates, impacting the uniformity of the microstructure.

Moreover, the composite materials often contain reinforcing fibers, and the use of square pins may cause disruption or damage to these fibers during the welding process. This can compromise the mechanical properties of the composite, such as its strength and stiffness.

Heat-affected Zones (HAZ): TMAZ and nugget zone experience higher heat, leading to stress, particle rearrangement, and changes in reinforcement particle areas. This heat can lead to the softening of the matrix material and potentially degrade the reinforcing fibers if any. The nugget zone, where the material is mechanically mixed, also experiences high temperatures. Excessive heat can affect the properties of both the matrix and the reinforcement, leading to changes in their microstructure and mechanical properties. Moreover, it is also been observed that the high heat in the TMAZ can induce thermal stresses in the composite material. Additionally, the thermal cycle may cause the rearrangement of reinforcement particles or fibers, that alter their distribution and orientation in the matrix.

4.2 Macrostructure challenges

Incomplete Blending: Fusion welding can result in incomplete blending between filler and base metal. Composite materials are typically composed of different phases, such as fibers and a matrix. These components can have different melting temperatures and thermal conductivities. During fusion welding, achieving uniform melting and blending between the filler and base metal can be challenging due to the material mismatch. In addition to this, the reinforcements in the in composite materials possesses different thermal conductivity than the matrix material. This can result in uneven heat distribution during welding, and the insufficient heat in certain areas may lead to inhomogeneity in the reinforcement distribution.

Eutectic Structure Formation: Excessive eutectic structure formation poses a challenge.

Large-sized Porosity: Porosity exceeding 100 μm in the fusion zone is observed.

Undesirable Phases: Reactions between molten metals and reinforcements lead to the formation of undesirable phases like Al4C3.

Despite these challenges, Friction Stir Welding (FSW) proves effective for Aluminum Matrix Composites (AMCs). FSW addresses microstructural issues, optimizing grain structures, and enhances macrostructural integrity by ensuring proper blending and minimizing porosity. This method offers a promising solution for achieving robust FSW joints in AMCs, balancing and improving their mechanical properties.

5. Mechanical properties of FSW joints in AMCs

Extensive research has primarily focused on the friction stir welding (FSW) of composite materials, particularly metals, with a particular emphasis on studying their mechanical properties. The examination of metal matrix composites, incorporating reinforcements like SiC, reveals distinct characteristics in the joints produced through FSW. Assessing weld quality often involves scrutinizing ultimate tensile strength (UTS), yielding stress (YS), and elongation to failure (% elongation). Variations in ductility and tensile strength relative to the base metal are common observations in FSW joints of metal matrix composites.

The mechanical properties of these joints are intricately linked to key welding parameters such as tool rotation speed, welding speed, and heat input. Microstructural changes post-FSW, including the arrangement of reinforcing particles and the presence of nugget zones, heat-affected zones (HAZ), and base metal zones, significantly impact the overall strength and integrity of the weld.

Exploration into the impact of diverse FSW parameters on the mechanical aspects of composite materials is a continual focus, aiming to optimize joint efficiency and elongation. Additionally, investigations into fracture characteristics provide insights into the modes and locations of fractures, offering valuable information about the structural robustness of the welded components.

In a related study by Omar S. Salih et al. [37] the microstructure and mechanical properties of friction stir welded AA6092/SiC metal matrix composite were explored. The study highlights the critical impact of welding parameters on FSW joint performance and fracture characteristics. The results are depicted in Figure 9a, which shows the stress-strain curves for base metal, cross-weld, and longitudinal weld. The bar graph in Figure 9 compares ultimate tensile strength (UTS), yielding stress (YS), and percentage of elongation (% El). Cross-weld samples displayed higher ductility but lower tensile strength compared to BM. The mechanical properties of FSW joints, significantly influenced by welding parameters, reveal optimal joint efficiency (75%) and elongation (5%) at the lowest heat input, contrasting with the least favorable results at the highest heat input. Longitudinal welded samples show a smooth curve with 86% joint efficiency, attributed to a homogeneous fine grain structure [35].

Figure 9.

The tensile characteristics of the base metal (BM), cross-weld specimens, and longitudinal weld specimen were analyzed through stress–strain curves (a), bar graphs illustrating strength and percentage of elongation (b), and identification of fracture locations (c and d) [37].

Wear Properties:

• Addition of reinforcement particles enhances wear resistance by improving hardness FSWed materials.

• Various studies reveal a reduction in wear rate with increased volume of reinforcing particles [24, 25, 36, 37].

• Plastic deformation during joining diminishes with higher particle volume.

• Adhesive and abrasive wear behaviors are observed, with the weld zone generally exhibiting superior wear resistance compared to base composites [39, 40, 41].

Corrosion Properties:

• Investigation of friction stir-welded aluminum SiC-Gr hybrid composite indicates that corrosion resistance increases with higher welding or transverse speed.

• Conversely, corrosion resistance decreases with an increase in rotational speed.

• The study emphasizes the importance of controlling welding and transverse speeds for enhancing corrosion resistance in friction stir-welded hybrid composites.

The addition of reinforcement particles improves wear resistance in FSWed composites. Optimal wear resistance is achieved with a square pin profile and specific combinations of rotational and welding speeds, showcasing the importance of process parameters. Furthermore, corrosion resistance in friction stir welded aluminum SiC-Gr hybrid composite is influenced by welding and transverse speeds, emphasizing the need for careful parameter control to enhance material properties.

6. Applications of FSW

The initial commercial application of FSW pertained to the production of hollow aluminum panels for deep freezing of fish on fishing vessels in November 1996 at Sapa in Finspang (Sweden). These panels are crafted from friction stir joined aluminum extrusions. Pre-assembled broad aluminum panels for high-velocity ferry vessels, cruise ships, and offshore oil platforms can be generated via friction stir welding and are readily accessible commercially. The panels are formed by connecting extrusions, which can be manufactured in standard size extrusion presses. Numerous panels with an overall weld length of numerous hundreds of kilometers have been created and dispatched by Marine Aluminum in Haugesund (Norway) since 1996. Post-welding, the panels can be rolled for overland transport, as they are rigid solely in the longitudinal direction. If conveyed by vessel, they can be piled atop each other. These panels are also commonly deployed for constructing hermetically sealed helicopter platforms of oil-rigs, preventing incendiary aircraft fuel from descending into the living quarters of offshore oil platforms after a helicopter crash landing. Friction stir welded panels are presently utilized globally for high-speed ferries, hovercraft, and cruise ships. Large premanufactured aluminum modules can be hoisted by hoist into vessels or oil platforms, to economize time during the ultimate assembly.

The capability to manage microstructure and retain microstructures not readily acquired through alternative pathways implies that comprehending the correlation between FSW circumstances and microstructure is crucial for effectively customizing weld characteristics. In conjunction with this, friction-stir processing (FSP) has the potential to produce nearly surface-level microstructures not effortlessly accomplished through alternative means. Consequently, FSW and FSP are actively utilized throughout a diverse array of industrial implementations, as demonstrated in Figure 10a–f.

Figure 10.

Instances of the industrial implementation of friction stir welding and processing: (a) the eclipse 500 business aircraft, the initial application of FSW, (b) 50 mm dense copper nuclear waste containers, (c) floor components of Shinkansen train, (d) dissimilar FSW of aluminum to steel in Honda front subframe, (e) automobile piston. (f) Orbital FSW of steel [42].

7. Conclusions

Since its inception in 1991, friction stir welding (FSW) has transcended its initial aerospace applications and found widespread use in commercial sectors such as automotive, rail, and medical industries. Particularly well-suited for joining lightweight materials, FSW’s success lies in the meticulous control of critical process parameters, including rotational speed, transverse speed, axial load, and tilt angle. This precision is crucial for determining the microstructure and mechanical properties of FSW joints. The method’s applicability extends to welding composite materials, especially metal matrix composites (MMCs), where its solid-state nature simplifies the joining process, ensuring sound mechanical characteristics, enhanced safety, and adaptability to milling apparatus with minimal environmental impact. Despite challenges encountered in Aluminum Matrix Composites (AMCs), such as dynamic recrystallization and post-weld heat treatment (PWHT) influence, FSW stands out for optimizing grain structures, overcoming macrostructure and microstructure issues, and ensuring robust joint integrity. The mechanical properties of FSW-welded metal matrix composites are intricately linked to welding parameters and resulting microstructural changes, showcasing its potential to significantly enhance wear and corrosion resistance through careful control of process parameters.

Conflict of interest

The authors declare no conflict of interest.