# A Contemporary Review on Friction Stir Welding of Circular Pipe Joints and the Influence of Fixtures on This Process

Authors: S. M. Senthil; M. Bhuvanesh Kumar; Milon Selvam Dennison
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

The permanent metal joining processes play a vital role in assembling the complex aluminium structures of the aerospace, automotive, marine, and oil industries. The invention of friction stir welding (FSW) redefined the mechanism of metal joining to produce high-quality welds by eliminating the consumption of electrodes and shielding gases, resulting in economical processes. Research works still lack to explore the application of FSW in complex geometries. In this view, the present work reports a critical review of various geometries welded by FSW, mainly circular geometries that are majorly used in the oil and gas industries. The various aspects of the FSW of pipes, such as process facilitation, process parameters, tool material selection, and tool design selection are discussed. The paper also presents insights into the parameters responsible for the weld quality, the feasibility of optimization techniques, and the status of various fixture arrangements. The facilitation requisites for FSW of pipes are also provided based on the reviews made. The review concludes by discussing the challenges associated with the FSW of pipes, economic aspects, and future research directions.


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## Body

## 1. Introduction

Metal joining processes are essential in most manufacturing industries since they can produce complex shapes with different material properties. Welding is an excellent permanent metal joining technique due to its superior joint properties compared to fastening and semijoining processes. Among the various advanced welding operations, friction stir welding (FSW) is an emerging and promising technique, that originated from the welding institute (TWI) UK in the year 1991 [1]. The basic principle of FSW is entirely different from other conventional welding processes. In the conventional welding process, the base material undergoes melting followed by solidification [2], whereas FSW produces metal joints by the solid-state deformation principle [3, 4]. Moreover, the metals Compared to other fusion welding processes, the FSW requires minimal power requirements, and preprocessing and postprocessing efforts [5]. The weld quality aspects such as joint strength, finishing, and metallurgical characteristics produced by FSW are far better than those of other fusion welding processes [6]. Due to the elimination of consumable electrodes and shielding gases, FSW is also highly advantageous in terms of economic and environmental considerations [7].FSW uses a nonconsumable tool that does not involve conventional electric arc generation and avoids harmful radiation [8]. Moreover, the unique principle of FSW operation enables it to join materials with different physical properties [9]. Dissimilar metal joints are highly beneficial to the automotive, aerospace, and marine industries. A few examples signifying the FSW applications in industrial structures are shown in Figures 1 and 2. Friction stir processing (FSP) is a derivative of the FSW technique, bearing the difference that the former is used to modify the surface properties of the metals [13, 14]. Though FSW proves advantageous, the process itself has inherent issues with the backing of weld zone, thinning of welds, and keyholes. These issues are also addressed by employing proper control strategies, these issues can be averted [15]. While FSW was invented to join metallic materials and recently its applicability has been extended to join various kinds of thermoplastics as given in the reference [16, 17]. In addition, research works have also explored the feasibility of making hybrid joints using dissimilar materials, such as metals [18, 19] with thermoplastics by FSW [20–23]. Notable works are carried out in the joining of metals with polymer matrix composites as well [24]. Previous researchers have proposed various auxiliary assistances in the FSW process to match the joint quality with industrial standards. A few examples are liquid nitrogen-assisted FSW [25], stationary shoulder-assisted FSW [26, 27], and ultrasonic assistance in FSW [28–30]. Also, FSW in recent times has taken new dimensions in the frontiers of extrusion, additive manufacturing, and grain deformation processes. FSW is employed in the extrusion process, called as friction stir extrusion has resulted in high-strength and ductile magnesium alloys [31]. FSW in additive manufacturing areas have proved to be one of the repairing tools which exhibited good performance in the repair aluminium alloy structures [32]. Similar to FSP, FSW has also been employed in the fabrication of ultra-refined grains of composites involving aluminium matrices [33]. These distinctive derivatives of FSW have been used in more extended applications, such as electronics, marine, railways, and aerospace [34]. NASA has fabricated a fuel tank with the help of the FSW process for a Space Shuttle [35]. The “iMac” panels of Apple have been welded using FSW [36]. The subframe at the front side of the “HONDA Accord” car consists of aluminium (Al), and steel material welded using FSW [37]. Al panels of rail commuters by Hitachi are fabricated with the help of FSW [38]. The desks of the cruise ship “The World” from Fosen Mek have been joined by FSW [39]. Based on this evidence, it may be envisaged that the FSW can be implemented in most of the industrial sectors in the near future. FSW is a “natural” technology because it does not create any hazardous materials that could cause environmental or human damage. Using FSW, various workpiece joints such as lap, butt, T, and fillet joints can be obtained. More importantly, the resulting surfaces do not require active cleaning, as FSW is a clean process compared to other conventional welding processes. The present study reviews the different geometrical shapes welded by FSW and their criticality in the process. The following sections discuss the principle of the FSW process, process parameters, and various geometries of FSW welded joints. Finally, this article is concluded by adding future scopes.Figure 1
Applications show dissimilar joints using Al and steel (a) aerospace, (b) automobile, (c) marine industries [10],and (d) space applications [11].Figure 2
Product specific applications of FSW [12]. (a) the eclipse 500 business jet, the first to use FSW, (b) 50 mm thick copper nuclear waste storage canisters, (c) dissimilar FSW of aluminium to steel in Honda front subframe, (d) deckhouse structure of littoral combat ship, (e) orbital FSW of steel pipes, and (f) floor panels of Shinkansen train.The geometrical nature of the majority of the surfaces joined by FSW is flat rather than curved or zigzag. The majority of research works were primarily focused on getting flat joints by the FSW process followed by analyzing weld geometry, mechanical characteristics, metallurgical characteristics, and tribological behaviors. But the research works concerning the applicability of FSW to complex and circular shaped joints are very few. This creates the necessity to critically review the current status of the FSW techniques, primarily concerning to the nonflat surfaces such as curved, italic, or zigzag and the fixtures associated with the process. Thus, the FSW process has a promising future towards the engineering of high-strength joints with improved performances.


## 2. Working Principle of FSW

The FSW uses the plastic deformation property of the materials for joining. The process is comparable to the forming process where the materials are deformed to the required shape by applying pressure and temperature utilizing the plasticity property of the base material. Usually, a vertical milling machine with a rotating spindle is used to mount and rotate the tool at the desired speed. The spindle is stationary, and the bed on which the workpiece is held gives feed motion [10]. The tool used for FSW is different in construction from other tools used in manufacturing. It has a pin of the required length and a shoulder, which attributes to the deformation of the work material during their interaction. When the rotating tool interacts with the work material, heat is generated due to friction. The frictional heat softens the work material; thus, it becomes more ductile near the interaction area [40]. There are four stages by which the work-tool interaction progresses and they are plunge-in, dwell, traverse, or welding, and retract [4]. The surfaces of workpieces that need to be joined are held together rigidly using fixtures to form a line of the weld. During the first stage of operation, the rotating tool plunges into the workpiece material at the joining line until the shoulder comes in contact with the work. At the dwell stage, the material flow due to plastic deformation is ensured at the same position.Further, the traverse stage is initiated, in which feed motion is given to the bed where the work is mounted. Due to tool rotation and feed movement, one side of the substrate will move along the direction of the tool rotation, and the other side forces in the opposite direction with respect to the tool rotation. The former is known as the advancing side (AS) and the latter is known as the retreating side (RS). The material transfer happens from AS to RS. The tool deforms the material and extrudes some from the AS, and is forged back to the RS. This process continuously happens till the end of tool travel. The material flow occurred in the ring form and was called onion rings [41]. The tool is retracted out at the end of the traverse motion, hence, producing a permanent metal joining process [42] (Figure 3).Figure 3
Schematic illustration of the FSW process depicting various stages of operation and process parameters.


### 2.1. FSW Parameters

The quality of the weld is majorly influenced by the parameters associated to the weld, tool profile, materials, and geometry of the weld. The following subsection details the effects of each parameter.


#### 2.1.1. Weld Parameters

The parameters of FSW attributes to the joining quality are tool rotation speed, weld/traverse speed, angle of tool tilt, and axial load (Figure4). The tool rotational speed (spindle speed) and the welding/traverse speed are the main parameters responsible for the generation of heat and achieving weld quality in terms of the strength of the weld joint. The tilt angle helps to ease the plunge; hence, the deformation and transportation of material are improved. The depth of the plunge will be the point at which the shoulder of the tool touches the surface of the workpiece, which ensures the heat generation and transfer of torque. The tool pin length is calculated based on the thickness of the workpiece. Thus, complete contact between the shoulder and work ensures the initiation of the weld.Figure 4
Schematic showing the FSW process parameters and tool profile.(1) Tool Rotational Speed. In FSW, a cylindrical tool has to rotate to produce heat and move to carry the heat along the joint. Due to this, the welding rate of the FSW process is relatively slow compared to other welding processes. Usually, the tool rotates in a speed range that varies from 300 to 3000 rotations per minute (rpm) [43–45].(2) Weld/Traverse Speed. This controls the duration of the welding process. This is also one of the major influencing factors for deciding the quality of the weld. The tool’s traverse speed or weld speed varies between 5 and 400 millimeters per minute (mm/min) along the joint length [46–48].(3) Tool Tilt Angle. Tilting the tool to some angle from the normal to the substrate is regarded as a tool tilting angle, which is claimed to affect the material flow and heat generation by many researchers. The resulting weld joints are approximately 2 to 4 degrees slightly lean or tilt position [49, 50] from normal to the surface. The tilt is given to prevent the effect of downward forces from damaging the joint. This parameter finds its significance during dissimilar welding.(4) Axial Load. The axial load has a strong influence on the frictional force generated. The load varies from material to material based on their physical properties [51]. For instance, steel may require a large axial load, whereas soft metals like Al may need less. The control automation of such axial loads is being attempted by researchers for work-specific applications [52].


#### 2.1.2. Tool Design Parameters

The successfulness of a weld in FSW depends on the elimination of defects in FSW [53]. The tool design parameters play a crucial part in achieving a defect-free weld [54]. The FSW tool (Figure 4) parts identified as parameters are:(i)
Pin: This is responsible for the stirring action during the welding process by excavating the molten material from the advancing side to the retreating side.(ii)
Shoulder: This part of the tool is responsible to exert the load over the stir zone and ensuring the creation of a friction force between the tool and workpiece. Thus, has more control over heat and torque generation.Different profiles of tool pins have been developed and experimentally investigated through various research works, as shown in Figure5. The most promising tool profiles in FSW are cylindrical, threaded, conical, square, etc. [57–59]. Compared to conical and cylindrical profiles, square pin produced an enhanced flow of material and ultimate tensile strength (UTS) at the weld joint [60]. Due to the presence of sharp edges in square pins, the pulses generated per second by sharp corners are higher than in other profiles, leading to a higher ratio between dynamic and static volume [61]. Spiral-shaped pins are considered after square pins due to the presence of ridges. The ridges create more downward movement of material during the tool work interface than the other tools [62]. Apart from all the conventional profiles of tools, a special design called “twin stir,” where two tools rotate in the opposite direction simultaneously. This design aims to improve the symmetrical nature of the material flow, minimize the clamping force, and reduce the rework (postweld works like removal of flashes, etc.) and defects of the weld [63]. Similarly, the effects of different shapes of shoulders have also been reported by previous works, such as convex, concave, etc. [64, 65]. Over the years, other configurations of tools have also been studied by various researchers, such as the two shoulders, bobbin tool, and self-retracted tool to conquer the formation of keyholes [66–70].Figure 5
Different tool designs profiles used in FSW process. (a) Shoulder designs [55], (b) tool pin profiles (CYL-cylindrical, CT-cylindrical tapered, SQ-square, CFT-cylindrical full threaded, TIF-threaded with three incremental flat faces) [56].Some of the experimental works conducted on different materials to find better parameter windows for successful FSW joints are shown in Table1. The various parameter levels and resulting mechanical properties, such as tensile strength and hardness values are also presented.Table 1
Different parameters are used to carry out the FSW process on various materials for good weld characteristics.
MaterialsTool pin profileThickness of plateRotational speed (rpm)Tool travel speed (mm/min)Physical appearance of jointMechanical propertiesRefTensile strength (MPa)Hardness (Hv)NiTi/Ti6Al4VCylindrical—47523.5Tunnel defects240710[71]AA6061 and Ti6Al4VTapered cylindrical31000100Good147.5—[72]Low carbon steel (St14)Cylindrical1.2–1.4800–140060Good350160[73]Stainless steel S32654Tapered cylindrical2.4300–400100Good895378[74]High-zinc brassThreaded cylindrical345016Good64.2—[75]AA6061-T6 and AA2014-T6Cylindrical—90024—175—[76]Az31B magnesium alloyCylindrical363063No visible defects20470[77]AA5083Taper cylindrical2.590016No visible defects103—[78]Square2.590016Good137—2219-T62 Al alloyTapered thread with 3-spiral flutes12300–50060–100Fine grains305105[79]AA6082-OTri-flute5120060Good77—[47]AA2219Threaded-cylindrical6160045.6Good22096[57]Square61500–170045.6Good24088[43](1) Tool Material. Tool material is selected based on the strength of the workpiece material. The tool material is required to be intact during the temperature rise and the frictional force generation that happens at the time of the welding process. Theoretically, the welding temperature will be below the melting point of the workpiece. The common tool materials used are tool steel, high speed steel, nickel alloys like Inconel, and polycrystalline cubic boron nitride (PCBN). At times, FSW tools are also coated with ceramic materials to increase their performance based on application requirements.


### 2.2. Materials and Joint Parameters

The weld quality and process efficiency also depend on the type of material used. The work material and the bed on which the material is mounted should possess low thermal conductivity to reduce the loss of energy in the form of heat. Unlike the other operating parameters, material-related parameters can be managed offline. In most cases, the profile of the tool is selected according to the type of material and thickness used. Also, based on the material type and its properties, the process parameters are optimized for obtaining sound weld joints [55, 56, 80–82].


### 2.3. Various Weld Geometries

The geometry of the parts to be welded plays a major role in defining the welding setup that is to be employed. The welding principle being the same, the fixture setups are designed to facilitate the particular weld geometry. The most common type of weld joint made is a butt joint, which is a staring seam resulting from flat plate geometry [10, 83–85]. Apart from butt joints, lap joints [86–88] and T-joints [89, 90] are also employed based on industrial needs. All these types of joints require only slight modifications in the fixture setup to facilitate the welding process. The examples of fixture setup used for producing FSW T-joints and circular joints are shown in Figures 6(a) and 6(b). But the sizeable untapped area in the FSW joining technology is its industrial employment to the joining of circular geometries i.e. pipes. The past decade is finding many research attempts in employing FSW to join pipes (detailed discussion in Section 3). The most challenging factor in facilitating FSW of pipes is the fixture design (detailed discussion in Section 4). The various other factors influencing the FSW of pipes and their fixture attributes must be examined carefully before taking FSW for industrial pipe joining applications.Figure 6
Fixture set up used to produce the FSW joints; (a) T-joint [91], (b) circular geometry [92], and (c) a typical rotary fixture for FSW of pipes [46].
(a)(b)(c)Hence, the FSW process parameters like tool rotational speed, weld speed, and tool geometries have a major influence over the weldment performance. The accomplishment of various geometries welding will have the aid of fixtures for its successful facilitation.


## 2.1. FSW Parameters

The quality of the weld is majorly influenced by the parameters associated to the weld, tool profile, materials, and geometry of the weld. The following subsection details the effects of each parameter.


### 2.1.1. Weld Parameters

The parameters of FSW attributes to the joining quality are tool rotation speed, weld/traverse speed, angle of tool tilt, and axial load (Figure4). The tool rotational speed (spindle speed) and the welding/traverse speed are the main parameters responsible for the generation of heat and achieving weld quality in terms of the strength of the weld joint. The tilt angle helps to ease the plunge; hence, the deformation and transportation of material are improved. The depth of the plunge will be the point at which the shoulder of the tool touches the surface of the workpiece, which ensures the heat generation and transfer of torque. The tool pin length is calculated based on the thickness of the workpiece. Thus, complete contact between the shoulder and work ensures the initiation of the weld.Figure 4
Schematic showing the FSW process parameters and tool profile.(1) Tool Rotational Speed. In FSW, a cylindrical tool has to rotate to produce heat and move to carry the heat along the joint. Due to this, the welding rate of the FSW process is relatively slow compared to other welding processes. Usually, the tool rotates in a speed range that varies from 300 to 3000 rotations per minute (rpm) [43–45].(2) Weld/Traverse Speed. This controls the duration of the welding process. This is also one of the major influencing factors for deciding the quality of the weld. The tool’s traverse speed or weld speed varies between 5 and 400 millimeters per minute (mm/min) along the joint length [46–48].(3) Tool Tilt Angle. Tilting the tool to some angle from the normal to the substrate is regarded as a tool tilting angle, which is claimed to affect the material flow and heat generation by many researchers. The resulting weld joints are approximately 2 to 4 degrees slightly lean or tilt position [49, 50] from normal to the surface. The tilt is given to prevent the effect of downward forces from damaging the joint. This parameter finds its significance during dissimilar welding.(4) Axial Load. The axial load has a strong influence on the frictional force generated. The load varies from material to material based on their physical properties [51]. For instance, steel may require a large axial load, whereas soft metals like Al may need less. The control automation of such axial loads is being attempted by researchers for work-specific applications [52].


### 2.1.2. Tool Design Parameters

The successfulness of a weld in FSW depends on the elimination of defects in FSW [53]. The tool design parameters play a crucial part in achieving a defect-free weld [54]. The FSW tool (Figure 4) parts identified as parameters are:(i)
Pin: This is responsible for the stirring action during the welding process by excavating the molten material from the advancing side to the retreating side.(ii)
Shoulder: This part of the tool is responsible to exert the load over the stir zone and ensuring the creation of a friction force between the tool and workpiece. Thus, has more control over heat and torque generation.Different profiles of tool pins have been developed and experimentally investigated through various research works, as shown in Figure5. The most promising tool profiles in FSW are cylindrical, threaded, conical, square, etc. [57–59]. Compared to conical and cylindrical profiles, square pin produced an enhanced flow of material and ultimate tensile strength (UTS) at the weld joint [60]. Due to the presence of sharp edges in square pins, the pulses generated per second by sharp corners are higher than in other profiles, leading to a higher ratio between dynamic and static volume [61]. Spiral-shaped pins are considered after square pins due to the presence of ridges. The ridges create more downward movement of material during the tool work interface than the other tools [62]. Apart from all the conventional profiles of tools, a special design called “twin stir,” where two tools rotate in the opposite direction simultaneously. This design aims to improve the symmetrical nature of the material flow, minimize the clamping force, and reduce the rework (postweld works like removal of flashes, etc.) and defects of the weld [63]. Similarly, the effects of different shapes of shoulders have also been reported by previous works, such as convex, concave, etc. [64, 65]. Over the years, other configurations of tools have also been studied by various researchers, such as the two shoulders, bobbin tool, and self-retracted tool to conquer the formation of keyholes [66–70].Figure 5
Different tool designs profiles used in FSW process. (a) Shoulder designs [55], (b) tool pin profiles (CYL-cylindrical, CT-cylindrical tapered, SQ-square, CFT-cylindrical full threaded, TIF-threaded with three incremental flat faces) [56].Some of the experimental works conducted on different materials to find better parameter windows for successful FSW joints are shown in Table1. The various parameter levels and resulting mechanical properties, such as tensile strength and hardness values are also presented.Table 1
Different parameters are used to carry out the FSW process on various materials for good weld characteristics.
MaterialsTool pin profileThickness of plateRotational speed (rpm)Tool travel speed (mm/min)Physical appearance of jointMechanical propertiesRefTensile strength (MPa)Hardness (Hv)NiTi/Ti6Al4VCylindrical—47523.5Tunnel defects240710[71]AA6061 and Ti6Al4VTapered cylindrical31000100Good147.5—[72]Low carbon steel (St14)Cylindrical1.2–1.4800–140060Good350160[73]Stainless steel S32654Tapered cylindrical2.4300–400100Good895378[74]High-zinc brassThreaded cylindrical345016Good64.2—[75]AA6061-T6 and AA2014-T6Cylindrical—90024—175—[76]Az31B magnesium alloyCylindrical363063No visible defects20470[77]AA5083Taper cylindrical2.590016No visible defects103—[78]Square2.590016Good137—2219-T62 Al alloyTapered thread with 3-spiral flutes12300–50060–100Fine grains305105[79]AA6082-OTri-flute5120060Good77—[47]AA2219Threaded-cylindrical6160045.6Good22096[57]Square61500–170045.6Good24088[43](1) Tool Material. Tool material is selected based on the strength of the workpiece material. The tool material is required to be intact during the temperature rise and the frictional force generation that happens at the time of the welding process. Theoretically, the welding temperature will be below the melting point of the workpiece. The common tool materials used are tool steel, high speed steel, nickel alloys like Inconel, and polycrystalline cubic boron nitride (PCBN). At times, FSW tools are also coated with ceramic materials to increase their performance based on application requirements.


## 2.1.1. Weld Parameters

The parameters of FSW attributes to the joining quality are tool rotation speed, weld/traverse speed, angle of tool tilt, and axial load (Figure4). The tool rotational speed (spindle speed) and the welding/traverse speed are the main parameters responsible for the generation of heat and achieving weld quality in terms of the strength of the weld joint. The tilt angle helps to ease the plunge; hence, the deformation and transportation of material are improved. The depth of the plunge will be the point at which the shoulder of the tool touches the surface of the workpiece, which ensures the heat generation and transfer of torque. The tool pin length is calculated based on the thickness of the workpiece. Thus, complete contact between the shoulder and work ensures the initiation of the weld.Figure 4
Schematic showing the FSW process parameters and tool profile.(1) Tool Rotational Speed. In FSW, a cylindrical tool has to rotate to produce heat and move to carry the heat along the joint. Due to this, the welding rate of the FSW process is relatively slow compared to other welding processes. Usually, the tool rotates in a speed range that varies from 300 to 3000 rotations per minute (rpm) [43–45].(2) Weld/Traverse Speed. This controls the duration of the welding process. This is also one of the major influencing factors for deciding the quality of the weld. The tool’s traverse speed or weld speed varies between 5 and 400 millimeters per minute (mm/min) along the joint length [46–48].(3) Tool Tilt Angle. Tilting the tool to some angle from the normal to the substrate is regarded as a tool tilting angle, which is claimed to affect the material flow and heat generation by many researchers. The resulting weld joints are approximately 2 to 4 degrees slightly lean or tilt position [49, 50] from normal to the surface. The tilt is given to prevent the effect of downward forces from damaging the joint. This parameter finds its significance during dissimilar welding.(4) Axial Load. The axial load has a strong influence on the frictional force generated. The load varies from material to material based on their physical properties [51]. For instance, steel may require a large axial load, whereas soft metals like Al may need less. The control automation of such axial loads is being attempted by researchers for work-specific applications [52].


## 2.1.2. Tool Design Parameters

The successfulness of a weld in FSW depends on the elimination of defects in FSW [53]. The tool design parameters play a crucial part in achieving a defect-free weld [54]. The FSW tool (Figure 4) parts identified as parameters are:(i)
Pin: This is responsible for the stirring action during the welding process by excavating the molten material from the advancing side to the retreating side.(ii)
Shoulder: This part of the tool is responsible to exert the load over the stir zone and ensuring the creation of a friction force between the tool and workpiece. Thus, has more control over heat and torque generation.Different profiles of tool pins have been developed and experimentally investigated through various research works, as shown in Figure5. The most promising tool profiles in FSW are cylindrical, threaded, conical, square, etc. [57–59]. Compared to conical and cylindrical profiles, square pin produced an enhanced flow of material and ultimate tensile strength (UTS) at the weld joint [60]. Due to the presence of sharp edges in square pins, the pulses generated per second by sharp corners are higher than in other profiles, leading to a higher ratio between dynamic and static volume [61]. Spiral-shaped pins are considered after square pins due to the presence of ridges. The ridges create more downward movement of material during the tool work interface than the other tools [62]. Apart from all the conventional profiles of tools, a special design called “twin stir,” where two tools rotate in the opposite direction simultaneously. This design aims to improve the symmetrical nature of the material flow, minimize the clamping force, and reduce the rework (postweld works like removal of flashes, etc.) and defects of the weld [63]. Similarly, the effects of different shapes of shoulders have also been reported by previous works, such as convex, concave, etc. [64, 65]. Over the years, other configurations of tools have also been studied by various researchers, such as the two shoulders, bobbin tool, and self-retracted tool to conquer the formation of keyholes [66–70].Figure 5
Different tool designs profiles used in FSW process. (a) Shoulder designs [55], (b) tool pin profiles (CYL-cylindrical, CT-cylindrical tapered, SQ-square, CFT-cylindrical full threaded, TIF-threaded with three incremental flat faces) [56].Some of the experimental works conducted on different materials to find better parameter windows for successful FSW joints are shown in Table1. The various parameter levels and resulting mechanical properties, such as tensile strength and hardness values are also presented.Table 1
Different parameters are used to carry out the FSW process on various materials for good weld characteristics.
MaterialsTool pin profileThickness of plateRotational speed (rpm)Tool travel speed (mm/min)Physical appearance of jointMechanical propertiesRefTensile strength (MPa)Hardness (Hv)NiTi/Ti6Al4VCylindrical—47523.5Tunnel defects240710[71]AA6061 and Ti6Al4VTapered cylindrical31000100Good147.5—[72]Low carbon steel (St14)Cylindrical1.2–1.4800–140060Good350160[73]Stainless steel S32654Tapered cylindrical2.4300–400100Good895378[74]High-zinc brassThreaded cylindrical345016Good64.2—[75]AA6061-T6 and AA2014-T6Cylindrical—90024—175—[76]Az31B magnesium alloyCylindrical363063No visible defects20470[77]AA5083Taper cylindrical2.590016No visible defects103—[78]Square2.590016Good137—2219-T62 Al alloyTapered thread with 3-spiral flutes12300–50060–100Fine grains305105[79]AA6082-OTri-flute5120060Good77—[47]AA2219Threaded-cylindrical6160045.6Good22096[57]Square61500–170045.6Good24088[43](1) Tool Material. Tool material is selected based on the strength of the workpiece material. The tool material is required to be intact during the temperature rise and the frictional force generation that happens at the time of the welding process. Theoretically, the welding temperature will be below the melting point of the workpiece. The common tool materials used are tool steel, high speed steel, nickel alloys like Inconel, and polycrystalline cubic boron nitride (PCBN). At times, FSW tools are also coated with ceramic materials to increase their performance based on application requirements.


## 2.2. Materials and Joint Parameters

The weld quality and process efficiency also depend on the type of material used. The work material and the bed on which the material is mounted should possess low thermal conductivity to reduce the loss of energy in the form of heat. Unlike the other operating parameters, material-related parameters can be managed offline. In most cases, the profile of the tool is selected according to the type of material and thickness used. Also, based on the material type and its properties, the process parameters are optimized for obtaining sound weld joints [55, 56, 80–82].


## 2.3. Various Weld Geometries

The geometry of the parts to be welded plays a major role in defining the welding setup that is to be employed. The welding principle being the same, the fixture setups are designed to facilitate the particular weld geometry. The most common type of weld joint made is a butt joint, which is a staring seam resulting from flat plate geometry [10, 83–85]. Apart from butt joints, lap joints [86–88] and T-joints [89, 90] are also employed based on industrial needs. All these types of joints require only slight modifications in the fixture setup to facilitate the welding process. The examples of fixture setup used for producing FSW T-joints and circular joints are shown in Figures 6(a) and 6(b). But the sizeable untapped area in the FSW joining technology is its industrial employment to the joining of circular geometries i.e. pipes. The past decade is finding many research attempts in employing FSW to join pipes (detailed discussion in Section 3). The most challenging factor in facilitating FSW of pipes is the fixture design (detailed discussion in Section 4). The various other factors influencing the FSW of pipes and their fixture attributes must be examined carefully before taking FSW for industrial pipe joining applications.Figure 6
Fixture set up used to produce the FSW joints; (a) T-joint [91], (b) circular geometry [92], and (c) a typical rotary fixture for FSW of pipes [46].
(a)(b)(c)Hence, the FSW process parameters like tool rotational speed, weld speed, and tool geometries have a major influence over the weldment performance. The accomplishment of various geometries welding will have the aid of fixtures for its successful facilitation.


## 3. Circular Geometries

Even though the usage of pipe dates back ages, the pipe welding process got intensified and went commercial only after the exploration of oils, as the pipelines were crucial components in the transportation systems. Quality pipe weld joints were the call of the day as the pipeline industry started booming. The commercialization of pipe welding is rooted in the filing of many patents. The first patent on the procedure involved in pipe welding was published by the Heating and Piping Contractors National Association in 1931. Subsequently, several patents were filed on the methods and apparatuses required for the welding of pipes [91–95]. The Welding Institute (TWI), since its inception in 1968, is constantly working in the area of welding pipes [96]. All the experimental studies related to the FSW of pipes invariably involved the development of their fixtures to facilitate the welding process. The reason being, the fixtures are dependent upon the dimensions of the pipe (diameter, thickness, and length) that were selected for the study (Figure 6(c)). Also, there are high chance that the design of the fixture influencing the process outcomes. The FSW process offers high mechanical strength at the welding joints, making it more advantageous than fusion welding techniques. The very low distortion in the FSW process makes it a suitable pipe joining technique, which is not the case with fusion welding techniques. The FSW process is also controlled by quite a number of parameters deciding the quality of weld like other welding processes. Most of the research work has been carried out in flat plate geometries for understating the outcome of process parameters on the weld joints. Tool design and tool speeds (rotational and traverse) are the primary factors for parameter optimization studies.Apart from friction welding [97–99], FSW is predominantly employed in joining Al and its alloy pipes. There are traces in the literature where copper (Cu) and steel pipes are also joined using the FSW process.


### 3.1. FSW of Al Alloy Pipes

The past decade’s history has traces of works involving attempts in employing FSW for joining of Al pipe. Lammlein et al. [100] used the FSW process to join 106.68 mm diameter Al alloy 6061-T6 pipes with a wall thickness of 5 mm and reported to achieve 70% strength that of the base metal. Ismail et al. [101, 102] studied the effect of tool rotational and weld speeds over the tensile strength of AA6063 pipe joints. They obtained high-strength joints of 126 MPa under optimum values of welding parameters (a rotational speed of 1500 rpm and a welding speed of 1.8 mm/s). In a similar approach, Ismail et al. [103] developed a dedicated fixture for joining pipes using the FSW process (Figure 7(a)). The fixture has a series of components that hold the pipe firmly while performing welding. It is also responsible for delivering rotary feed while welding. Welding of AA6063-T6 pipes was performed using this fixture by varying the weld speed and travel speed. This welding setup has produced a joint that has a smooth surface finish at the weld zone.Figure 7
(a) An orbital clamping unit used for pipe welding [103] (b) a portable machine for FSW of pipes and a model image of the pipe, comparison between the models (top view) (c) curved and (d) flat models, cross-section comparison between the (e) linear and the (f) curved modes [104].
(a)(b)(c)(d)(e)(f)Meyghani and Awang [104] performed a thermomechanical simulation and compared the FSW for flat and curved geometries (Figure 7(b)). They found that the temperature and stress behavior were approximately asymmetric on different sides of the tool. In the tool front and back sides for both the flat and the curved models an approximately different behavior was observed, while, there were some similarities in the temperature behavior at the advancing and the retreating sides. The temperature pattern in the flat model was lower at the front of the tool as compared to the trailing side (Figures 7(c) and 7(d)). Below the tool, the high heat flux between the outside of the shoulder radius was seen and the workpiece interface layer generated a “V” or ‘basin” shaped temperature gradient [104] (Figures 7(e) and 7(f)).During the FSW of Al alloy 6061-T6 pipes Doos and Bashar [105] found a tensile strength of 61.7% compared to that of base metal strength. Similarly,  Ismail et al. [101] obtained a void-free and high-strength pipe joint of AA6063 under optimum parameters (at 2.4 mm/s and 1500 rpm). Senthil and Parameshwaran [106] experimented with three different tool pin profiles: square, pentagon, and hexagon for FSW of AA 6063 pipes. It was seen that hexagonal tool pin profiles at 1600 rpm could produce maximum strength. Khourshid and Sabry [107] investigated the FSW of AA 6063 pipe joints. It was found that a higher tool rotational speed of 1400 rpm had produced pipe joints of high strength. Ismail et al. [108] have studied the temperature cure characteristics during the FSW of AA 6063-T6 pipes at the plunge stage. A dwell time of 54 s was required to complete the plunging process. It was found that the temperature on the AS was greater than the RS with variations of 5% to 25% from the weld centre. Maggiolini et al. [109] performed an analysis on the crack path and fracture modes in the Al alloy 6082-T6 friction stir welded tubes subjected to tension-torsion loading. 38 mm outer diameter (OD) tubes were welded using a specially fabricated fixture system. They noticed that 50% of the cracks were initiated from the AS of the weld joint, around 39% were from the RS, and 11% were from the remaining positions of the weld. A weld joint efficiency of 55% was achieved a similar observation when compared to flat welds. The major problem reported was the designing of a retractable tool to avoid the pin-hole formation. Automation was recommended for joining pipes using the FSW process.In 2009, Defalco [110] reported that FSW has a tremendously positive impact upon employing it to the pipeline industry in terms of performance and cost-saving. Tavassolimanesh and Alavi Nia [111] developed a new method to clad dissimilar pipes involving pure copper and AA 6061-T6 materials. The maximum shear strength has been produced at a tool rotational speed of 710 rpm and a weld speed of 60 mm/min. Senthil et al. [112, 113] performed a comparative study on the mechanical performances of friction stir weld produced by the same welding parameters for plate and pipe and reported around 10% variations between them (Figure 8). Ronevich et al. [115] analysed the hydrogen-accelerated fatigue crack growth of FSW X52 steel pipes (Figure 8(c)). Among tests in hydrogen, fatigue crack growth rates were modestly higher in the FSW than in the base metal (BM) and 15 mm off-center tests. Meyghani and Awang [104] compared the thermomechanical behavior of friction stir welded flat and curved surfaces using finite element analysis. They found that the temperature in the pin bottom area of the curved model is higher than the flat model. This is attributed to the difference in FSW zones between flat and curved models. The measured stress along the cross-section showed an M-shape pattern, in which the flat model had higher stress in the stir zone than the stir zone of the curved model. Senthil et al. [116] employed nondestructive testing techniques to analyze the defects present in the Al alloy pipes joined through FSW. Flexural and crashworthiness studies were also performed on FSWed aluminium alloy pipes [113]. A novel method of joining the pipes using hybrid friction stir welding has also been reported, which is addressed at eliminating root defects that occur during the welding process [117].Figure 8
FSW aluminium alloy pipes. (a) Dissimilar pipes [114], (b) crash test of FSWed AA 6063 pipes [113], and (c) steel pipes [115].
(a)(b)(c)


### 3.2. FSW of Other Material Pipes

Giorjão et al., [118] used the FSW process to join SAF 2507 super duplex stainless steel pipe with a 184.3 mm outside diameter and 8 mm thickness. A tool with a conical tip made of PCBN-WRe alloy was used to perform welding. Thermal simulations, performed to study the temperature effects, showed the different temperature levels at the upper and bottom sides of the pipe. This affected the uniform dynamic recrystallization of the stir zones. Experimental results revealed that the tensile and hardness values are comparable with plate-weld values. The tensile specimen failed at the low-hardness regions present in the base metal. The average hardness value at the stir zone was about 325 HV on the AS. Due to low thermal cycles, the root regions had intense grain refinement with some dispersion of elongated ferrite. Mosavvar et al. [119] developed a newly designed mechanism to join high-density polyethylene pipes using the FSW process. The fixture study revealed that having an internally expandable mandrel will favor a good welding process. The tool rotational speed, traverse speed, and tool offset were optimized for high-strength joints using the Taguchi method. It was found that tool rotational speed had more influence over joint strength. The considered process parameters were optimized at 2500 rpm of tool rotational speed, 110 mm/min of transverse speed, and 3.5 mm of tool offset. Chen et al. [114] tried to join AA 3003 and Cu pipes of small diameter (19 mm) using the FSW process under constant process parameter conditions (Figure 9). A tool offset study showed that large Cu particles were accumulated in the nugget zone and the bulk interface. The four weld regions of the pipe circumference, namely, the former (−40° to 90°), middle (90° to 180°), later (180° to 320°), and overlap (320° to 360°), had different responses to the mechanical properties of the weld joint due to their variation in temperature exposure. The overlap region resulted in defect-free joints due to the second pass. The latter region showed a high strength profile with a good yield curve, but the overlap region had the highest strength among all regions with a value of 213 MPa.Figure 9
The welding system: (a) the setting for pipe welding; a schematic diagram of the welding parameters showing O1 and O2. (b) Hardness distributions at the middle thickness along the horizontal dash lines in the cross-sections shown as insets for each region: overlap of all distributions; distribution for FR, MR, and LR. (c) Tensile curves of 4 regions; traverse cross-sections of fractured joints after tensile test showing two fracture modes. (PRS stands for pipe rotation rate; TRR for tool rotation rate; O1 stands for offset1; O2 stands for offset2; ID stands for insertion depth); FR: former region; MR: middle region; LR: later region; OR: overlap region. [114].


### 3.3. Employment of Optimization

After establishing the FSW process for pipes, researchers have attempted to optimize the process parameters to obtain sound weld joints (Table2). Both traditional and nontraditional optimization techniques have been used to optimize the process parameters. El-Kassas and Sabry [120] employed a hybrid RSM-fuzzy model to optimize the parameters of underwater FSW to join AA 1050 pipes. The process parameters, namely, tool rotation speed, traverse speed, and tool shoulder diameters, were varied, and their optimal conditions were found for superior tensile strength of the pipe joint. These variable process parameters were optimized at 980 rpm, 200 mm/min, and 20 mm, respectively, for this welding process considered. Akbari and Asadi [121] used the FSW process to weld A356 pipes. They used the Taguchi method to optimize the mechanical properties of the FSWed AA356 pipes. The process parameters viz., tool pin shape (threaded, triangular, square), tool rotational speed, and traverse speed were optimized for optimal combinations under three levels. The responses considered were Si particle size, hardness, and tensile strength. The 3D FEM analysis method was employed to simulate the process using a constant shear friction model as follows:(1)τf=μσn,where τf, μ and σn represent the frictional stress, friction factor, and shear yield stress, respectively. The simulation results showed that the 0.5 mm plunge depth produced a defect-free weld. The results of the simulation were appended to the experimental methodologies, and welds were created as per the Taguchi method’s L9 array. The optimal conditions have arrived as a tool rotational speed of 1600 rpm, traverse speed of 80 mm/min, and a square tool pin shape.Table 2
Consolidation of notable research works pertaining to the FSW of pipes.
AuthorPipe materialPipe outside diameter (mm)Pipe thickness (mm)Type of pipe fixtureMandrel typeParameters studiedProperties studiedLammlein et al. [100]AA 6061-T6106.885Job rotatingWedge(i) Weld speed(i) Tensile strengthIsmail et al. [101]AA 6063895Job rotatingFixed(i) Tool rotational speed(i) Tensile strength(ii) Weld speedIsmail et al. [102]AA 6063895Job rotatingFixed(i) Tool rotational speed(i) Hardness(ii) Weld speedIsmail et al. [103]AA 6063-T61005Job rotatingFixed(i) Weld speed(i) Tensile strength(ii) Travel speedDoos and Bashar [105]AA 6061-T6——Job rotatingFixed(i) Tool rotational speed(i) Tensile strength(ii) Weld speed(ii) HardnessSenthil and Parameshwaran [106]AA 6063503Job rotatingFixed(i) Tool pin profile(i) Tensile strength(ii) Tool rotational speedChen et al. [114]Cu–AA 3003191–1.5Job rotatingExpanding mandrelConstant(i) Tensile strengthKhourshid and Sabry [107]AA 6063——Job rotatingFixed(i) Tool rotational speed(i) Tensile strengthIsmail et al. [108]AA 6063-T6895Job rotatingFixed(i) Constant(i) Temperature during plungeMaggiolini et al. [109]AA 6082-T6383.5Job rotatingWedge(i) Retractable tool pin(i) Tension-torsion loadingGiorjão et al. [118]Super duplex stainless steel184.38——(i) Tool rotational speed(i) Tensile strength(ii) Weld speed(ii) Hardness(iii) Axial loadMosavvar et al. [119]High-density polyethylene (HDPE)635.8Job rotatingWedge(i) Tool rotational speed(i) Tensile strength(ii) Weld speed(iii) Tool offsetAliha et al. [122] joined AA6063 Al alloy cylinders of 140 mm outside diameter using the FSW process. During this study, the effect of tool rotational speed and traverse speed on the mechanical properties of the weld was studied. Bend tests were performed for the longitudinal (L) and transverse (T) orientations of the welded joint specimen. Higher welding speeds (1250 rpm) produced high-strength welds. The average hardness of the FSW zones was found to be greater than the base metal hardness. The fracture load and energy of the FSW specimens were found to be significantly higher than the base metal. El-Kassas [56] optimized the FSW of AA 6061 pipes using multicriteria decision-making techniques. They used the Technique for Order of Preference by Similarity to an Ideal Solution (TOPSIS) and grey relational analysis (GRA) to optimize the process parameters for attaining maximum tensile properties. A tool pin diameter of 5 mm, a tool rotational speed of 1800 rpm, and welding speed of 10 mm/min were found to be the optimal process parameters. Khoushrid et al. [123] employed regression analysis to optimize the process parameters for joining aluminium alloy 6061 pipes using the FSW process. The regression equations, which were the results of their investigation, are as follows:(2)Tensilestrength=160+0.0364x−7.05y−1.44z,Elongation%=5−0.001x+0.542y+0.067z,HardnessVH=62+0.0099x−3.67y−0.604z,where x = tool rotational speed in rpm, y = workpiece thickness in mm, and z = tool traverse speed in mm/min. Senthil et al. [46] employed an RSM-based desirability function approach to optimize the welding speed and the tool rotational speed for joining AA 6063-T6 50 mm OD pipes. The RSM technique was used to populate the design matrix, based on which experiments were conducted. The desirability function was employed to carry out the optimization process, as shown in Figure 10. The optimized parameters for obtaining more than 70% of the base metal strength were a 1986 rpm tool rotational speed and a 0.65 rpm (102 mm/min) weld speed.Figure 10
Optimized process parameters for high values of yield strength (YS), ultimate tensile strength (UTS), and percentage of elongation (%E) during the FSW of AA pipes [46].


### 3.4. Closure of Exit-Hole

One of the most shortcomings in joining pipes using the FSW process is that the exit-hole forms at the end of the process [124]. A few nascent attempts at closing this exit-hole are found in the literature. Ghavimi et al. [125] proposed a “filling FSW” method to fill the exit-hole that results during the FSW of pipes. They compared the semiconsumable similar pin with the semiconsumable dissimilar pin for filling the FSW joint between AA 5456 plates and pipes. The optimized filling process parameters of 800 rpm pin rotation, 50 mm/min plunge velocity, with a similar pin of 8° cone angle were proposed. Also, similar filling FSW has also been reported by Han et al. [126]. Hattingh et al., [127] joined 38 mm OD Al alloy 6082-T6 pipes using the FSW process and compared them with plate welds. A retractable tool was used for performing the welding process to avoid the left-over hole. The results from both tubular and microtensile specimens had joint efficiency of 0.55. A firm shear texture was observed in the central part of the stir zone. Behmand et al. [128] have used consumable pins to fill the exit-hole of FSW joints. The best results were obtained with appropriate process parameters, such as rotational speed and plunging time, as shown in Figure 11. New FSW derivatives-likeself-support FSW has shown their efficiency in addressing keyhole issues of AA materials [129].Figure 11
The cross-sectional microscopic view of exit-holes filled with consumable pin at (a) 100 rpm & 30 sec, (b) 300 rpm & 30 sec, (c) 500 rpm & 30 sec, and (d) 700 rpm & 30 sec [128].Thus, apart from straight geometry studies, circular geometry studies have taken the FSW process closer to industrial applications. During FSW of circular geometries, the facilitation of rotary fixtures has played a major role in establishing the welding process.


## 3.1. FSW of Al Alloy Pipes

The past decade’s history has traces of works involving attempts in employing FSW for joining of Al pipe. Lammlein et al. [100] used the FSW process to join 106.68 mm diameter Al alloy 6061-T6 pipes with a wall thickness of 5 mm and reported to achieve 70% strength that of the base metal. Ismail et al. [101, 102] studied the effect of tool rotational and weld speeds over the tensile strength of AA6063 pipe joints. They obtained high-strength joints of 126 MPa under optimum values of welding parameters (a rotational speed of 1500 rpm and a welding speed of 1.8 mm/s). In a similar approach, Ismail et al. [103] developed a dedicated fixture for joining pipes using the FSW process (Figure 7(a)). The fixture has a series of components that hold the pipe firmly while performing welding. It is also responsible for delivering rotary feed while welding. Welding of AA6063-T6 pipes was performed using this fixture by varying the weld speed and travel speed. This welding setup has produced a joint that has a smooth surface finish at the weld zone.Figure 7
(a) An orbital clamping unit used for pipe welding [103] (b) a portable machine for FSW of pipes and a model image of the pipe, comparison between the models (top view) (c) curved and (d) flat models, cross-section comparison between the (e) linear and the (f) curved modes [104].
(a)(b)(c)(d)(e)(f)Meyghani and Awang [104] performed a thermomechanical simulation and compared the FSW for flat and curved geometries (Figure 7(b)). They found that the temperature and stress behavior were approximately asymmetric on different sides of the tool. In the tool front and back sides for both the flat and the curved models an approximately different behavior was observed, while, there were some similarities in the temperature behavior at the advancing and the retreating sides. The temperature pattern in the flat model was lower at the front of the tool as compared to the trailing side (Figures 7(c) and 7(d)). Below the tool, the high heat flux between the outside of the shoulder radius was seen and the workpiece interface layer generated a “V” or ‘basin” shaped temperature gradient [104] (Figures 7(e) and 7(f)).During the FSW of Al alloy 6061-T6 pipes Doos and Bashar [105] found a tensile strength of 61.7% compared to that of base metal strength. Similarly,  Ismail et al. [101] obtained a void-free and high-strength pipe joint of AA6063 under optimum parameters (at 2.4 mm/s and 1500 rpm). Senthil and Parameshwaran [106] experimented with three different tool pin profiles: square, pentagon, and hexagon for FSW of AA 6063 pipes. It was seen that hexagonal tool pin profiles at 1600 rpm could produce maximum strength. Khourshid and Sabry [107] investigated the FSW of AA 6063 pipe joints. It was found that a higher tool rotational speed of 1400 rpm had produced pipe joints of high strength. Ismail et al. [108] have studied the temperature cure characteristics during the FSW of AA 6063-T6 pipes at the plunge stage. A dwell time of 54 s was required to complete the plunging process. It was found that the temperature on the AS was greater than the RS with variations of 5% to 25% from the weld centre. Maggiolini et al. [109] performed an analysis on the crack path and fracture modes in the Al alloy 6082-T6 friction stir welded tubes subjected to tension-torsion loading. 38 mm outer diameter (OD) tubes were welded using a specially fabricated fixture system. They noticed that 50% of the cracks were initiated from the AS of the weld joint, around 39% were from the RS, and 11% were from the remaining positions of the weld. A weld joint efficiency of 55% was achieved a similar observation when compared to flat welds. The major problem reported was the designing of a retractable tool to avoid the pin-hole formation. Automation was recommended for joining pipes using the FSW process.In 2009, Defalco [110] reported that FSW has a tremendously positive impact upon employing it to the pipeline industry in terms of performance and cost-saving. Tavassolimanesh and Alavi Nia [111] developed a new method to clad dissimilar pipes involving pure copper and AA 6061-T6 materials. The maximum shear strength has been produced at a tool rotational speed of 710 rpm and a weld speed of 60 mm/min. Senthil et al. [112, 113] performed a comparative study on the mechanical performances of friction stir weld produced by the same welding parameters for plate and pipe and reported around 10% variations between them (Figure 8). Ronevich et al. [115] analysed the hydrogen-accelerated fatigue crack growth of FSW X52 steel pipes (Figure 8(c)). Among tests in hydrogen, fatigue crack growth rates were modestly higher in the FSW than in the base metal (BM) and 15 mm off-center tests. Meyghani and Awang [104] compared the thermomechanical behavior of friction stir welded flat and curved surfaces using finite element analysis. They found that the temperature in the pin bottom area of the curved model is higher than the flat model. This is attributed to the difference in FSW zones between flat and curved models. The measured stress along the cross-section showed an M-shape pattern, in which the flat model had higher stress in the stir zone than the stir zone of the curved model. Senthil et al. [116] employed nondestructive testing techniques to analyze the defects present in the Al alloy pipes joined through FSW. Flexural and crashworthiness studies were also performed on FSWed aluminium alloy pipes [113]. A novel method of joining the pipes using hybrid friction stir welding has also been reported, which is addressed at eliminating root defects that occur during the welding process [117].Figure 8
FSW aluminium alloy pipes. (a) Dissimilar pipes [114], (b) crash test of FSWed AA 6063 pipes [113], and (c) steel pipes [115].
(a)(b)(c)


## 3.2. FSW of Other Material Pipes

Giorjão et al., [118] used the FSW process to join SAF 2507 super duplex stainless steel pipe with a 184.3 mm outside diameter and 8 mm thickness. A tool with a conical tip made of PCBN-WRe alloy was used to perform welding. Thermal simulations, performed to study the temperature effects, showed the different temperature levels at the upper and bottom sides of the pipe. This affected the uniform dynamic recrystallization of the stir zones. Experimental results revealed that the tensile and hardness values are comparable with plate-weld values. The tensile specimen failed at the low-hardness regions present in the base metal. The average hardness value at the stir zone was about 325 HV on the AS. Due to low thermal cycles, the root regions had intense grain refinement with some dispersion of elongated ferrite. Mosavvar et al. [119] developed a newly designed mechanism to join high-density polyethylene pipes using the FSW process. The fixture study revealed that having an internally expandable mandrel will favor a good welding process. The tool rotational speed, traverse speed, and tool offset were optimized for high-strength joints using the Taguchi method. It was found that tool rotational speed had more influence over joint strength. The considered process parameters were optimized at 2500 rpm of tool rotational speed, 110 mm/min of transverse speed, and 3.5 mm of tool offset. Chen et al. [114] tried to join AA 3003 and Cu pipes of small diameter (19 mm) using the FSW process under constant process parameter conditions (Figure 9). A tool offset study showed that large Cu particles were accumulated in the nugget zone and the bulk interface. The four weld regions of the pipe circumference, namely, the former (−40° to 90°), middle (90° to 180°), later (180° to 320°), and overlap (320° to 360°), had different responses to the mechanical properties of the weld joint due to their variation in temperature exposure. The overlap region resulted in defect-free joints due to the second pass. The latter region showed a high strength profile with a good yield curve, but the overlap region had the highest strength among all regions with a value of 213 MPa.Figure 9
The welding system: (a) the setting for pipe welding; a schematic diagram of the welding parameters showing O1 and O2. (b) Hardness distributions at the middle thickness along the horizontal dash lines in the cross-sections shown as insets for each region: overlap of all distributions; distribution for FR, MR, and LR. (c) Tensile curves of 4 regions; traverse cross-sections of fractured joints after tensile test showing two fracture modes. (PRS stands for pipe rotation rate; TRR for tool rotation rate; O1 stands for offset1; O2 stands for offset2; ID stands for insertion depth); FR: former region; MR: middle region; LR: later region; OR: overlap region. [114].


## 3.3. Employment of Optimization

After establishing the FSW process for pipes, researchers have attempted to optimize the process parameters to obtain sound weld joints (Table2). Both traditional and nontraditional optimization techniques have been used to optimize the process parameters. El-Kassas and Sabry [120] employed a hybrid RSM-fuzzy model to optimize the parameters of underwater FSW to join AA 1050 pipes. The process parameters, namely, tool rotation speed, traverse speed, and tool shoulder diameters, were varied, and their optimal conditions were found for superior tensile strength of the pipe joint. These variable process parameters were optimized at 980 rpm, 200 mm/min, and 20 mm, respectively, for this welding process considered. Akbari and Asadi [121] used the FSW process to weld A356 pipes. They used the Taguchi method to optimize the mechanical properties of the FSWed AA356 pipes. The process parameters viz., tool pin shape (threaded, triangular, square), tool rotational speed, and traverse speed were optimized for optimal combinations under three levels. The responses considered were Si particle size, hardness, and tensile strength. The 3D FEM analysis method was employed to simulate the process using a constant shear friction model as follows:(1)τf=μσn,where τf, μ and σn represent the frictional stress, friction factor, and shear yield stress, respectively. The simulation results showed that the 0.5 mm plunge depth produced a defect-free weld. The results of the simulation were appended to the experimental methodologies, and welds were created as per the Taguchi method’s L9 array. The optimal conditions have arrived as a tool rotational speed of 1600 rpm, traverse speed of 80 mm/min, and a square tool pin shape.Table 2
Consolidation of notable research works pertaining to the FSW of pipes.
AuthorPipe materialPipe outside diameter (mm)Pipe thickness (mm)Type of pipe fixtureMandrel typeParameters studiedProperties studiedLammlein et al. [100]AA 6061-T6106.885Job rotatingWedge(i) Weld speed(i) Tensile strengthIsmail et al. [101]AA 6063895Job rotatingFixed(i) Tool rotational speed(i) Tensile strength(ii) Weld speedIsmail et al. [102]AA 6063895Job rotatingFixed(i) Tool rotational speed(i) Hardness(ii) Weld speedIsmail et al. [103]AA 6063-T61005Job rotatingFixed(i) Weld speed(i) Tensile strength(ii) Travel speedDoos and Bashar [105]AA 6061-T6——Job rotatingFixed(i) Tool rotational speed(i) Tensile strength(ii) Weld speed(ii) HardnessSenthil and Parameshwaran [106]AA 6063503Job rotatingFixed(i) Tool pin profile(i) Tensile strength(ii) Tool rotational speedChen et al. [114]Cu–AA 3003191–1.5Job rotatingExpanding mandrelConstant(i) Tensile strengthKhourshid and Sabry [107]AA 6063——Job rotatingFixed(i) Tool rotational speed(i) Tensile strengthIsmail et al. [108]AA 6063-T6895Job rotatingFixed(i) Constant(i) Temperature during plungeMaggiolini et al. [109]AA 6082-T6383.5Job rotatingWedge(i) Retractable tool pin(i) Tension-torsion loadingGiorjão et al. [118]Super duplex stainless steel184.38——(i) Tool rotational speed(i) Tensile strength(ii) Weld speed(ii) Hardness(iii) Axial loadMosavvar et al. [119]High-density polyethylene (HDPE)635.8Job rotatingWedge(i) Tool rotational speed(i) Tensile strength(ii) Weld speed(iii) Tool offsetAliha et al. [122] joined AA6063 Al alloy cylinders of 140 mm outside diameter using the FSW process. During this study, the effect of tool rotational speed and traverse speed on the mechanical properties of the weld was studied. Bend tests were performed for the longitudinal (L) and transverse (T) orientations of the welded joint specimen. Higher welding speeds (1250 rpm) produced high-strength welds. The average hardness of the FSW zones was found to be greater than the base metal hardness. The fracture load and energy of the FSW specimens were found to be significantly higher than the base metal. El-Kassas [56] optimized the FSW of AA 6061 pipes using multicriteria decision-making techniques. They used the Technique for Order of Preference by Similarity to an Ideal Solution (TOPSIS) and grey relational analysis (GRA) to optimize the process parameters for attaining maximum tensile properties. A tool pin diameter of 5 mm, a tool rotational speed of 1800 rpm, and welding speed of 10 mm/min were found to be the optimal process parameters. Khoushrid et al. [123] employed regression analysis to optimize the process parameters for joining aluminium alloy 6061 pipes using the FSW process. The regression equations, which were the results of their investigation, are as follows:(2)Tensilestrength=160+0.0364x−7.05y−1.44z,Elongation%=5−0.001x+0.542y+0.067z,HardnessVH=62+0.0099x−3.67y−0.604z,where x = tool rotational speed in rpm, y = workpiece thickness in mm, and z = tool traverse speed in mm/min. Senthil et al. [46] employed an RSM-based desirability function approach to optimize the welding speed and the tool rotational speed for joining AA 6063-T6 50 mm OD pipes. The RSM technique was used to populate the design matrix, based on which experiments were conducted. The desirability function was employed to carry out the optimization process, as shown in Figure 10. The optimized parameters for obtaining more than 70% of the base metal strength were a 1986 rpm tool rotational speed and a 0.65 rpm (102 mm/min) weld speed.Figure 10
Optimized process parameters for high values of yield strength (YS), ultimate tensile strength (UTS), and percentage of elongation (%E) during the FSW of AA pipes [46].


## 3.4. Closure of Exit-Hole

One of the most shortcomings in joining pipes using the FSW process is that the exit-hole forms at the end of the process [124]. A few nascent attempts at closing this exit-hole are found in the literature. Ghavimi et al. [125] proposed a “filling FSW” method to fill the exit-hole that results during the FSW of pipes. They compared the semiconsumable similar pin with the semiconsumable dissimilar pin for filling the FSW joint between AA 5456 plates and pipes. The optimized filling process parameters of 800 rpm pin rotation, 50 mm/min plunge velocity, with a similar pin of 8° cone angle were proposed. Also, similar filling FSW has also been reported by Han et al. [126]. Hattingh et al., [127] joined 38 mm OD Al alloy 6082-T6 pipes using the FSW process and compared them with plate welds. A retractable tool was used for performing the welding process to avoid the left-over hole. The results from both tubular and microtensile specimens had joint efficiency of 0.55. A firm shear texture was observed in the central part of the stir zone. Behmand et al. [128] have used consumable pins to fill the exit-hole of FSW joints. The best results were obtained with appropriate process parameters, such as rotational speed and plunging time, as shown in Figure 11. New FSW derivatives-likeself-support FSW has shown their efficiency in addressing keyhole issues of AA materials [129].Figure 11
The cross-sectional microscopic view of exit-holes filled with consumable pin at (a) 100 rpm & 30 sec, (b) 300 rpm & 30 sec, (c) 500 rpm & 30 sec, and (d) 700 rpm & 30 sec [128].Thus, apart from straight geometry studies, circular geometry studies have taken the FSW process closer to industrial applications. During FSW of circular geometries, the facilitation of rotary fixtures has played a major role in establishing the welding process.


## 4. Importance of Fixtures in the Welding Process

Most of the automated manufacturing, inspection, and assembly operations require fixtures to achieve cost and time effectiveness. Fixtures are used to locate and hold the workpiece and facilitate the particular industrial process. Also, such a holding must be consistent throughout the operation and must constrain the workpiece in a secured position till that special operation completes. Fixtures are usually designed and manufactured for a particular workpiece individually. The three major structural components of a fixture are locators, clamps, and supports. The various factors that influence the fixture design are the overall dimensions of the part, condition of a part material, degree of accuracy required, number of pieces to be made, loading and clamping surfaces, and finally, type and size of a machine tool. Fixtures are widely used in machining, welding, assembly, inspection, and in testing processes. More gaps exist that are to be filled in welding fixture research areas [130]. Fixtures are employed in a welding process to obtain repeatability along with the achievement of the required targets. Residual stresses (both tensile and compressive) and distortions are the major problems associated with all welding processes. Stress corrosion cracks are caused by the tensile residual stresses reducing the fatigue life of the weldment [131, 132]. Compressive residual stresses are induced when there is compressive loading, which results in a decline of buckling strength [133]. The distortion may cause mismatching of joints and deviations in the targeted physical parameters [134]. Most of the studies on the weld quality and optimization of welding techniques (involving residual stresses and distortion) were parameterized only based on the metallurgical factors and physical effects of tools and workpieces [135]. But fixture effects were not incorporated as they too influenced the quality of the weld and productivity [136, 137]. However, few researchers have started to consider the effect of fixtures into account during their work, which is the primary attention caught up by this paper. Figure 12 assembles the various fixtures used in the FSW of pipes. Figure 12(a) depicts the fully automated fixture for FSW of similar pipes, and the semiautomatics fixture is shown in Figure 12(b). A completely automatic fixture used to join dissimilar pipes (Al-Cu) is shown in Figure 12(c). This employs an expanding mandrel to facilitate the stir zone support. Fixtures with bottom rollers is used to reduce the bending moment created during pipe welding due to axial load (Figure 12(d)). Fixture for the lap joint of pipes is also employed, as shown in Figure 12(e).Figure 12
Various fixtures employed for friction stir welding of pipes (a) fully automated fixture for similar welding [100], (b) semiautomated fixture for similar welding [46], (c) fully automated fixture for dissimilar welding [114], (d) fixture with bottom roller supports [138], and (e) fixture for lap joint of pipes [111].


### 4.1. Impact of Fixtures

Figure13 shows the primary functions of a typical fixture along with its possible parameter controls. The desired fixture in a welding process must be the one that brings the distortion to zero and also, at the same time, must facilitate a stress-free weldment with optimal fixture elemental factors involved. Also, such a fixture must also be able to take care of the after-effects of the welding process like cooling. The elemental factors of a fixture may include the number of clamps and locators, the location of clamps and locators, and the amount of clamping forces. Locators play an important role in orienting the workpieces with respect to the welding directions. But recent studies [137, 139] reveal that the traditional “3-2-1” locating scheme is no longer valid in meeting the latest desired needs of the fixture [140]. This calls for more researches related to computer-aided welding fixture design [130]. A welding fixture thus developed should help in the reduction of deformation in the workpiece due to thermal expansion, hence avoiding the dimensional variation.Figure 13
Primary functions of a typical welding fixture.


#### 4.1.1. Fixtures in the FSW Process

The effect of the fixture on the FSW process is much higher than in any other welding process. The fixturing elements involved in FSW are shown in Figure14. In FSW, nearly half of the total mechanical energy is converted into heat energy, increasing the workpiece’s temperature, and some part of the remaining energy causes deformation in the fixture elements [143]. Researchers are considering the clamping forces majorly controlling the distortion that happens during the welding process and the residual stresses developed due to the fixturing systems. The initial and working clamping forces in an FSW process were studied by Richter-Trummer et al. [144] for butt welding of AA2198-T851. They designed a special fixture device that controlled the initial clamping force and measured the evolution of the clamping force during welding. A stereo-based vision system measured the distortion, and load cells measured the clamping force. Mechanical tests and microstructural analysis reports revealed that the clamping forces did not influence the tensile properties; however, dispelling of tensile properties occurred under higher clamping forces. Distortion and residual stress measurements showed that the lower residual stress was present in highly distorted workpieces and vice versa. A higher clamping force of 2500 N led to superior weldment properties in their work. The effect of the fixture (before and after release) on the orthogonal residual stress distributions was reported by Chen and Kovacevic [145] during their study on the FSW process. Before releasing the fixture, the lateral stress with a maximum value of 316 MPa was found to be higher than that of the longitudinal stresses. But both the longitudinal stress and the lateral stress decreased significantly when the fixture was released. Thus, both longitudinal and lateral stresses were influenced by the fixture release. Hence, fixture control during and after welding is required to control workpiece deformation and inducement of residual stresses. Finally, they concluded that the fixturing release will affect the weld’s stress distribution. Also, they remarked that further development of the FSW requires an assessment of the fixturing condition to control the stress distribution. However, the fixture system employed was not reported.Figure 14
(a) Fixture elements in FSW [141] and (b) tool supplements in the FSW process study [142].
(a)(b)The effect of fixture release alone on the residual stress was studied by Zhu and Chao [143]. They modeled the fixture release effect during the cooling of the weldment area. It was observed that the residual stresses in the longitudinal directions decreased notably after releasing the fixture than that of releasing before. Hence, reports were made that the fixture release must be considered in the analytical simulations also for the residual stresses determination in the FSW process. The same was cited by Baghel in his survey [146]. Also, Baghel and Siddiquee, in another study [147], designed and developed a fixturing system involving a fixture, clamps, and a key using mild steel for the FSW process that is to be mounted on a vertical milling machine. This developed fixture is best suited for a robust vertical milling machine and proved its flexibility in welding stainless steel 304 plates of various thicknesses. A fixture setup used in the FSW process by Soundararajan et al. [148] consisted of a clamp to constrain the sides of the workpieces and a backing plate to constrain the bottom portion. The amount of clamping force used was not provided. It was found that due to fixture constraints on the sides of the workpiece, high compressive stresses were found at the welded areas because of thermal expansion. Another type of fixture was employed by Hwang et al. [149] during their study on the FSW process, but its effects on the welding process were not examined. Patil and Soman [47] carried out the FSW with the help of a specially designed fixture on a CNC vertical milling machine. They reported that the fixture was used to secure the plates to obtain the initial joint configuration. However, its effects and influence on the welding process were not reported. Salloomi et al. [150] have shown that the effect between the tool and its surrounding area coupled with the effect of clamps will lead to variable compression stress along the weld direction with compression near the tool location. They observed that due to high clamping, the longitudinal and transverse residual stresses reach maximum values at the edges. It was also concluded that clamping constraints and locations affect the stress components through significant localized effects beyond the heat-affected zone. The analysis model of Buffa et al. [151] for an FSW welding process was assured with clamping and supporting conditions. Only the effect of fixture release (unclamping state) was included. However, the fixture considered and its effects were not presented in detail. A specially designed rotating clamping fixture was used by Doos and Bashar [105] to clamp and hold the two segments of the 6061-T6 Al pipe together for butt joint welding through the FSW process. This clamping fixture consisted of a gearbox with a rotating clamp, a fixing mandrel, and internal and external anvils. Their report on the fixturing effect was limited only in preventing the workpiece assembly from any movement during the welding operation.


#### 4.1.2. Fixtures in Other Welding Processes

Other welding processes like arc welding and laser welding are also affected by the fixture systems, but the studies on their effects are only limited. Arc welding processes also experience distortions like the FSW process [152]. Researchers observed that the welding fixtures employed for the arc welding process influenced both weld pool geometry and residual stress distribution. The fixture developed by Kohandehghan and Serajzadeh [153] made the depth of the weld pool reduce by 21% and the transverse residual stress by 76%. Sikstrӧm et al. [154] performed an experimental and modeling approach to study the clamping forces' influence on distorting the workpiece during gas tungsten arc welding. They used the specially designed fixture setup to investigate the influence of fixture clamping forces on the structural integrity of the welded workpiece. The designed fixture consisted of screw holes for stiff fastening of the workpiece at one side and a cylinder to maintain the variable fixture force through the base support, block, and the yoke. The replaceable plates ensured the same friction for all welding facilitating zero slip. The workpiece was made to experience three different levels of clamping force, namely, stiff, medium, and loose clamping with force values of 295 N, 84 N, and 0 N, respectively. The experiments showed that the welding with loose clamping produced minimized residual deformation of 0.30 mm when compared to 0.40 mm and 0.53 mm with medium and stiff clamping, respectively.Welding fixtures were also examined to prevent distortions during the cooling process. Vural et al. [155] examined the effect of weld fixtures in preventing the distortions during cooling of the workpiece after arc welding. They used a special welding fixture and analyzed it with two conditions, namely, cooling on fixture and cooling outside fixture. The designed welded steel structure had a U-shaped configuration, which consists of part1 constrained by attachments in the x and y directions and by pressure force in the z-direction. Part1 and attachments constrained the y and z directions of part2 and part3. Their x-direction was constrained by pressure force. Therefore, part 2 and part 3 were used in measuring the distortions along the x-axis. For each experiment, two specimens were welded; one was cooled on the fixture and the other outside the fixture. Results showed that the distortion increased with an increase in heat input. Also, minimum distortions were developed while cooling was on the fixture, compared to maximum distortions if the cooling was outside the fixture. In a laser welding process, Liu et al. [156] performed a measurement of clamping force variation. They used preset clamping forces in different experiments on different workpieces with thickness, namely, 1 mm, 1.2 mm, and 1.5 mm. The measurement of clamping forces revealed that the local material expansion resulted in variations in the clamping forces during the welding process. It was found that the established HAZ (heat affected zone) induced a thermal expansion, therefore increasing the clamping forces [144]. It was also proposed that the weld breaking load and weld strength can be improved by choosing optimal clamping forces. The effect of the fixture in the various welding process is depicted in Table 3.Table 3
Notable researches related to fixture effect in various welding processes.
Welding processClamping aloneFixturing effects accountedReferencesResidual stressDistortionCooling rateDeformationFSW✓[143]FSW✓✓[144]FSW✓[145]FSW✓[148]FSW✓[47]FSW✓[150]FSW✓[151]FSW✓[105]GTAW✓[153]Fusion welding✓✓[154]GMAW✓✓[155]Laser welding✓[157]Pulsed GMAW, DC GMAW, cold metal transfer, laser hybrid welding✓✓✓[158]Fixturing element optimizedLocators✓✓[159]


### 4.2. Fixture Development for FSW of Pipes

In the FSW of pipes, the fixture plays a very crucial role. The fixture development for FSW of pipes will require two approaches, as presented in Figure15. In the first approach, the pipes are mounted onto a rotary fixture and rotate during welding. This will constitute the welding speed parameter. The second approach being the pipes can be held stationary and the entire tool head can be rotated. The former is the type that all the laboratory scale studies follow, as this facilitation is easy to establish. All the researches discussed in Section 3 holds the same state as the first approach. Whereas the latter is a difficult one, as it requires a complex machine setup for performing FSW [160]. In the case of pipe rotation, the setup intends to aid the welding speed (rotational speed of pipe) that is independent of the machine, which controls the tool rotational speed and tool plunge depth. Hence, this particular parameter becomes unanimous during the welding process. When the entire tool head rotates, it will also make the spindle head to which the tool is fixed also to rotate, which also requires a guiding rail. Also, the axial load also needs to be enforced along the tool rotating axis, which is continuously moving. As many factors are included, this will lead to a decrease in the quality of the weldment produced.Figure 15
Approaches for FSW of pipes. (a) Pipe rotating approach and (b) tool rotating approach.
(a)(b)


#### 4.2.1. Fixture Desirability for FSW of Pipes

The fixture development is an integral part of the FSW of pipes, and the first approach remains the common choice due to its relative simplicity compared to the second approach. However, the developed fixture using the first approach has to satisfy the following requisites for successful facilitation of the pipe welding process for Figure16 an industrial application.(i)
Holding of the pipe firmly without any slippage: The pipes are to be clamped firmly to the rotation fixture (along the mandrel) so that zero slippage is ensured throughout the welding process [161]. Also, care is to be taken that the clamping forces are not affecting the weld property.(ii)
Withstanding the axial load exerted by the tool: During the welding process, the tool is plunged into the weld zone with an axial load calculated according to the material mounted. This axial load may be low for AAs, whereas high for steel. The action of the tool load will make the fixture appear to be in a fixed beam condition with a point load at the centre, which is a perceived diagram of the pipe fixture (D–pipe diameter, 1/2 L–pipe specimen length). This will induce the bending moments to be generated at the clamped points (fixed locations) as well as bending at the weld zone (centre of the beam) as per the bending in the following equations: (3)Bendingmomentatendsupports,M=FyL8,(4)Deflection at the weld point∆centre=FyL3192EI.WhereFy is the tool load acting in a downward direction (kN), E is Young’s modulus of the pipe material (MPa), and I is the area moment of inertia (mm4). This effect may cause the pipes to bend at the joints, which will result in poor cause root defects. As per (4), this bending will increase as the length of the pipe between the end-supports the increase. In contrast, the bending decreases when thickness (T) and diameter (D) increase proportionately (since I=TD3/12). This factor if underestimated may result in root defects, which is a result of poor integrity at the toot side. Studies have also been reported where these root defects are addressed by novel hybrid welding processes [117].(iii)
Support to hold the integrity of the weld zone viz., mandrel: Backing the weld zone is a major facilitation for holding its integrity. In the case of plate welding, backing plates are used [162]. For pipes, a mandrel is used as a backing structure. This mandrel is to possess favorable thermal properties like a high melting point and low thermal expansion when compared to that of the pipe material considered. The mounting and removal of pipe on the mandrel must be made accessible. Expanding mandrels would be the apt option for this case [114]. Mostly, mandrels are made for a specific diameter of pipe. Hence, each diameter of pipe will require a separate mandrel to be fabricated and employed.(iv)
Rotary motion, with variable speed options: This is one of the basic requisites of any pipe fixture for delivering rotary motion of pipe, which is parametrized as weld/traverse speed. This was achieved using an external motor with a suitable gearbox. The motor’s torque is to have enough capability to overcome the frictional shear forces at the tool-pipe junction [121].Figure 16
Pipe fixture–perceived structure with forces involved.The second approach will have more complexity as the entire tool head has to rotate, ensuring the uniform tool load. Technological advancements may pave the way for the selection of approaches based on the application requirements. Figure17 shows the facilitation chart for FSW of pipes for obtaining sound pipe joints.Figure 17
FSW of pipes–process facilitation chart as per the first approach.Thus, the impact of fixtures on the quality of the weld is very high when it pertains to weld geometries other than flat. The circular geometries like pipes have critical compensations from the fixture and required perfect facilitations in order to achieve sound weld joints.


## 4.1. Impact of Fixtures

Figure13 shows the primary functions of a typical fixture along with its possible parameter controls. The desired fixture in a welding process must be the one that brings the distortion to zero and also, at the same time, must facilitate a stress-free weldment with optimal fixture elemental factors involved. Also, such a fixture must also be able to take care of the after-effects of the welding process like cooling. The elemental factors of a fixture may include the number of clamps and locators, the location of clamps and locators, and the amount of clamping forces. Locators play an important role in orienting the workpieces with respect to the welding directions. But recent studies [137, 139] reveal that the traditional “3-2-1” locating scheme is no longer valid in meeting the latest desired needs of the fixture [140]. This calls for more researches related to computer-aided welding fixture design [130]. A welding fixture thus developed should help in the reduction of deformation in the workpiece due to thermal expansion, hence avoiding the dimensional variation.Figure 13
Primary functions of a typical welding fixture.


### 4.1.1. Fixtures in the FSW Process

The effect of the fixture on the FSW process is much higher than in any other welding process. The fixturing elements involved in FSW are shown in Figure14. In FSW, nearly half of the total mechanical energy is converted into heat energy, increasing the workpiece’s temperature, and some part of the remaining energy causes deformation in the fixture elements [143]. Researchers are considering the clamping forces majorly controlling the distortion that happens during the welding process and the residual stresses developed due to the fixturing systems. The initial and working clamping forces in an FSW process were studied by Richter-Trummer et al. [144] for butt welding of AA2198-T851. They designed a special fixture device that controlled the initial clamping force and measured the evolution of the clamping force during welding. A stereo-based vision system measured the distortion, and load cells measured the clamping force. Mechanical tests and microstructural analysis reports revealed that the clamping forces did not influence the tensile properties; however, dispelling of tensile properties occurred under higher clamping forces. Distortion and residual stress measurements showed that the lower residual stress was present in highly distorted workpieces and vice versa. A higher clamping force of 2500 N led to superior weldment properties in their work. The effect of the fixture (before and after release) on the orthogonal residual stress distributions was reported by Chen and Kovacevic [145] during their study on the FSW process. Before releasing the fixture, the lateral stress with a maximum value of 316 MPa was found to be higher than that of the longitudinal stresses. But both the longitudinal stress and the lateral stress decreased significantly when the fixture was released. Thus, both longitudinal and lateral stresses were influenced by the fixture release. Hence, fixture control during and after welding is required to control workpiece deformation and inducement of residual stresses. Finally, they concluded that the fixturing release will affect the weld’s stress distribution. Also, they remarked that further development of the FSW requires an assessment of the fixturing condition to control the stress distribution. However, the fixture system employed was not reported.Figure 14
(a) Fixture elements in FSW [141] and (b) tool supplements in the FSW process study [142].
(a)(b)The effect of fixture release alone on the residual stress was studied by Zhu and Chao [143]. They modeled the fixture release effect during the cooling of the weldment area. It was observed that the residual stresses in the longitudinal directions decreased notably after releasing the fixture than that of releasing before. Hence, reports were made that the fixture release must be considered in the analytical simulations also for the residual stresses determination in the FSW process. The same was cited by Baghel in his survey [146]. Also, Baghel and Siddiquee, in another study [147], designed and developed a fixturing system involving a fixture, clamps, and a key using mild steel for the FSW process that is to be mounted on a vertical milling machine. This developed fixture is best suited for a robust vertical milling machine and proved its flexibility in welding stainless steel 304 plates of various thicknesses. A fixture setup used in the FSW process by Soundararajan et al. [148] consisted of a clamp to constrain the sides of the workpieces and a backing plate to constrain the bottom portion. The amount of clamping force used was not provided. It was found that due to fixture constraints on the sides of the workpiece, high compressive stresses were found at the welded areas because of thermal expansion. Another type of fixture was employed by Hwang et al. [149] during their study on the FSW process, but its effects on the welding process were not examined. Patil and Soman [47] carried out the FSW with the help of a specially designed fixture on a CNC vertical milling machine. They reported that the fixture was used to secure the plates to obtain the initial joint configuration. However, its effects and influence on the welding process were not reported. Salloomi et al. [150] have shown that the effect between the tool and its surrounding area coupled with the effect of clamps will lead to variable compression stress along the weld direction with compression near the tool location. They observed that due to high clamping, the longitudinal and transverse residual stresses reach maximum values at the edges. It was also concluded that clamping constraints and locations affect the stress components through significant localized effects beyond the heat-affected zone. The analysis model of Buffa et al. [151] for an FSW welding process was assured with clamping and supporting conditions. Only the effect of fixture release (unclamping state) was included. However, the fixture considered and its effects were not presented in detail. A specially designed rotating clamping fixture was used by Doos and Bashar [105] to clamp and hold the two segments of the 6061-T6 Al pipe together for butt joint welding through the FSW process. This clamping fixture consisted of a gearbox with a rotating clamp, a fixing mandrel, and internal and external anvils. Their report on the fixturing effect was limited only in preventing the workpiece assembly from any movement during the welding operation.


### 4.1.2. Fixtures in Other Welding Processes

Other welding processes like arc welding and laser welding are also affected by the fixture systems, but the studies on their effects are only limited. Arc welding processes also experience distortions like the FSW process [152]. Researchers observed that the welding fixtures employed for the arc welding process influenced both weld pool geometry and residual stress distribution. The fixture developed by Kohandehghan and Serajzadeh [153] made the depth of the weld pool reduce by 21% and the transverse residual stress by 76%. Sikstrӧm et al. [154] performed an experimental and modeling approach to study the clamping forces' influence on distorting the workpiece during gas tungsten arc welding. They used the specially designed fixture setup to investigate the influence of fixture clamping forces on the structural integrity of the welded workpiece. The designed fixture consisted of screw holes for stiff fastening of the workpiece at one side and a cylinder to maintain the variable fixture force through the base support, block, and the yoke. The replaceable plates ensured the same friction for all welding facilitating zero slip. The workpiece was made to experience three different levels of clamping force, namely, stiff, medium, and loose clamping with force values of 295 N, 84 N, and 0 N, respectively. The experiments showed that the welding with loose clamping produced minimized residual deformation of 0.30 mm when compared to 0.40 mm and 0.53 mm with medium and stiff clamping, respectively.Welding fixtures were also examined to prevent distortions during the cooling process. Vural et al. [155] examined the effect of weld fixtures in preventing the distortions during cooling of the workpiece after arc welding. They used a special welding fixture and analyzed it with two conditions, namely, cooling on fixture and cooling outside fixture. The designed welded steel structure had a U-shaped configuration, which consists of part1 constrained by attachments in the x and y directions and by pressure force in the z-direction. Part1 and attachments constrained the y and z directions of part2 and part3. Their x-direction was constrained by pressure force. Therefore, part 2 and part 3 were used in measuring the distortions along the x-axis. For each experiment, two specimens were welded; one was cooled on the fixture and the other outside the fixture. Results showed that the distortion increased with an increase in heat input. Also, minimum distortions were developed while cooling was on the fixture, compared to maximum distortions if the cooling was outside the fixture. In a laser welding process, Liu et al. [156] performed a measurement of clamping force variation. They used preset clamping forces in different experiments on different workpieces with thickness, namely, 1 mm, 1.2 mm, and 1.5 mm. The measurement of clamping forces revealed that the local material expansion resulted in variations in the clamping forces during the welding process. It was found that the established HAZ (heat affected zone) induced a thermal expansion, therefore increasing the clamping forces [144]. It was also proposed that the weld breaking load and weld strength can be improved by choosing optimal clamping forces. The effect of the fixture in the various welding process is depicted in Table 3.Table 3
Notable researches related to fixture effect in various welding processes.
Welding processClamping aloneFixturing effects accountedReferencesResidual stressDistortionCooling rateDeformationFSW✓[143]FSW✓✓[144]FSW✓[145]FSW✓[148]FSW✓[47]FSW✓[150]FSW✓[151]FSW✓[105]GTAW✓[153]Fusion welding✓✓[154]GMAW✓✓[155]Laser welding✓[157]Pulsed GMAW, DC GMAW, cold metal transfer, laser hybrid welding✓✓✓[158]Fixturing element optimizedLocators✓✓[159]


## 4.1.1. Fixtures in the FSW Process

The effect of the fixture on the FSW process is much higher than in any other welding process. The fixturing elements involved in FSW are shown in Figure14. In FSW, nearly half of the total mechanical energy is converted into heat energy, increasing the workpiece’s temperature, and some part of the remaining energy causes deformation in the fixture elements [143]. Researchers are considering the clamping forces majorly controlling the distortion that happens during the welding process and the residual stresses developed due to the fixturing systems. The initial and working clamping forces in an FSW process were studied by Richter-Trummer et al. [144] for butt welding of AA2198-T851. They designed a special fixture device that controlled the initial clamping force and measured the evolution of the clamping force during welding. A stereo-based vision system measured the distortion, and load cells measured the clamping force. Mechanical tests and microstructural analysis reports revealed that the clamping forces did not influence the tensile properties; however, dispelling of tensile properties occurred under higher clamping forces. Distortion and residual stress measurements showed that the lower residual stress was present in highly distorted workpieces and vice versa. A higher clamping force of 2500 N led to superior weldment properties in their work. The effect of the fixture (before and after release) on the orthogonal residual stress distributions was reported by Chen and Kovacevic [145] during their study on the FSW process. Before releasing the fixture, the lateral stress with a maximum value of 316 MPa was found to be higher than that of the longitudinal stresses. But both the longitudinal stress and the lateral stress decreased significantly when the fixture was released. Thus, both longitudinal and lateral stresses were influenced by the fixture release. Hence, fixture control during and after welding is required to control workpiece deformation and inducement of residual stresses. Finally, they concluded that the fixturing release will affect the weld’s stress distribution. Also, they remarked that further development of the FSW requires an assessment of the fixturing condition to control the stress distribution. However, the fixture system employed was not reported.Figure 14
(a) Fixture elements in FSW [141] and (b) tool supplements in the FSW process study [142].
(a)(b)The effect of fixture release alone on the residual stress was studied by Zhu and Chao [143]. They modeled the fixture release effect during the cooling of the weldment area. It was observed that the residual stresses in the longitudinal directions decreased notably after releasing the fixture than that of releasing before. Hence, reports were made that the fixture release must be considered in the analytical simulations also for the residual stresses determination in the FSW process. The same was cited by Baghel in his survey [146]. Also, Baghel and Siddiquee, in another study [147], designed and developed a fixturing system involving a fixture, clamps, and a key using mild steel for the FSW process that is to be mounted on a vertical milling machine. This developed fixture is best suited for a robust vertical milling machine and proved its flexibility in welding stainless steel 304 plates of various thicknesses. A fixture setup used in the FSW process by Soundararajan et al. [148] consisted of a clamp to constrain the sides of the workpieces and a backing plate to constrain the bottom portion. The amount of clamping force used was not provided. It was found that due to fixture constraints on the sides of the workpiece, high compressive stresses were found at the welded areas because of thermal expansion. Another type of fixture was employed by Hwang et al. [149] during their study on the FSW process, but its effects on the welding process were not examined. Patil and Soman [47] carried out the FSW with the help of a specially designed fixture on a CNC vertical milling machine. They reported that the fixture was used to secure the plates to obtain the initial joint configuration. However, its effects and influence on the welding process were not reported. Salloomi et al. [150] have shown that the effect between the tool and its surrounding area coupled with the effect of clamps will lead to variable compression stress along the weld direction with compression near the tool location. They observed that due to high clamping, the longitudinal and transverse residual stresses reach maximum values at the edges. It was also concluded that clamping constraints and locations affect the stress components through significant localized effects beyond the heat-affected zone. The analysis model of Buffa et al. [151] for an FSW welding process was assured with clamping and supporting conditions. Only the effect of fixture release (unclamping state) was included. However, the fixture considered and its effects were not presented in detail. A specially designed rotating clamping fixture was used by Doos and Bashar [105] to clamp and hold the two segments of the 6061-T6 Al pipe together for butt joint welding through the FSW process. This clamping fixture consisted of a gearbox with a rotating clamp, a fixing mandrel, and internal and external anvils. Their report on the fixturing effect was limited only in preventing the workpiece assembly from any movement during the welding operation.


## 4.1.2. Fixtures in Other Welding Processes

Other welding processes like arc welding and laser welding are also affected by the fixture systems, but the studies on their effects are only limited. Arc welding processes also experience distortions like the FSW process [152]. Researchers observed that the welding fixtures employed for the arc welding process influenced both weld pool geometry and residual stress distribution. The fixture developed by Kohandehghan and Serajzadeh [153] made the depth of the weld pool reduce by 21% and the transverse residual stress by 76%. Sikstrӧm et al. [154] performed an experimental and modeling approach to study the clamping forces' influence on distorting the workpiece during gas tungsten arc welding. They used the specially designed fixture setup to investigate the influence of fixture clamping forces on the structural integrity of the welded workpiece. The designed fixture consisted of screw holes for stiff fastening of the workpiece at one side and a cylinder to maintain the variable fixture force through the base support, block, and the yoke. The replaceable plates ensured the same friction for all welding facilitating zero slip. The workpiece was made to experience three different levels of clamping force, namely, stiff, medium, and loose clamping with force values of 295 N, 84 N, and 0 N, respectively. The experiments showed that the welding with loose clamping produced minimized residual deformation of 0.30 mm when compared to 0.40 mm and 0.53 mm with medium and stiff clamping, respectively.Welding fixtures were also examined to prevent distortions during the cooling process. Vural et al. [155] examined the effect of weld fixtures in preventing the distortions during cooling of the workpiece after arc welding. They used a special welding fixture and analyzed it with two conditions, namely, cooling on fixture and cooling outside fixture. The designed welded steel structure had a U-shaped configuration, which consists of part1 constrained by attachments in the x and y directions and by pressure force in the z-direction. Part1 and attachments constrained the y and z directions of part2 and part3. Their x-direction was constrained by pressure force. Therefore, part 2 and part 3 were used in measuring the distortions along the x-axis. For each experiment, two specimens were welded; one was cooled on the fixture and the other outside the fixture. Results showed that the distortion increased with an increase in heat input. Also, minimum distortions were developed while cooling was on the fixture, compared to maximum distortions if the cooling was outside the fixture. In a laser welding process, Liu et al. [156] performed a measurement of clamping force variation. They used preset clamping forces in different experiments on different workpieces with thickness, namely, 1 mm, 1.2 mm, and 1.5 mm. The measurement of clamping forces revealed that the local material expansion resulted in variations in the clamping forces during the welding process. It was found that the established HAZ (heat affected zone) induced a thermal expansion, therefore increasing the clamping forces [144]. It was also proposed that the weld breaking load and weld strength can be improved by choosing optimal clamping forces. The effect of the fixture in the various welding process is depicted in Table 3.Table 3
Notable researches related to fixture effect in various welding processes.
Welding processClamping aloneFixturing effects accountedReferencesResidual stressDistortionCooling rateDeformationFSW✓[143]FSW✓✓[144]FSW✓[145]FSW✓[148]FSW✓[47]FSW✓[150]FSW✓[151]FSW✓[105]GTAW✓[153]Fusion welding✓✓[154]GMAW✓✓[155]Laser welding✓[157]Pulsed GMAW, DC GMAW, cold metal transfer, laser hybrid welding✓✓✓[158]Fixturing element optimizedLocators✓✓[159]


## 4.2. Fixture Development for FSW of Pipes

In the FSW of pipes, the fixture plays a very crucial role. The fixture development for FSW of pipes will require two approaches, as presented in Figure15. In the first approach, the pipes are mounted onto a rotary fixture and rotate during welding. This will constitute the welding speed parameter. The second approach being the pipes can be held stationary and the entire tool head can be rotated. The former is the type that all the laboratory scale studies follow, as this facilitation is easy to establish. All the researches discussed in Section 3 holds the same state as the first approach. Whereas the latter is a difficult one, as it requires a complex machine setup for performing FSW [160]. In the case of pipe rotation, the setup intends to aid the welding speed (rotational speed of pipe) that is independent of the machine, which controls the tool rotational speed and tool plunge depth. Hence, this particular parameter becomes unanimous during the welding process. When the entire tool head rotates, it will also make the spindle head to which the tool is fixed also to rotate, which also requires a guiding rail. Also, the axial load also needs to be enforced along the tool rotating axis, which is continuously moving. As many factors are included, this will lead to a decrease in the quality of the weldment produced.Figure 15
Approaches for FSW of pipes. (a) Pipe rotating approach and (b) tool rotating approach.
(a)(b)


### 4.2.1. Fixture Desirability for FSW of Pipes

The fixture development is an integral part of the FSW of pipes, and the first approach remains the common choice due to its relative simplicity compared to the second approach. However, the developed fixture using the first approach has to satisfy the following requisites for successful facilitation of the pipe welding process for Figure16 an industrial application.(i)
Holding of the pipe firmly without any slippage: The pipes are to be clamped firmly to the rotation fixture (along the mandrel) so that zero slippage is ensured throughout the welding process [161]. Also, care is to be taken that the clamping forces are not affecting the weld property.(ii)
Withstanding the axial load exerted by the tool: During the welding process, the tool is plunged into the weld zone with an axial load calculated according to the material mounted. This axial load may be low for AAs, whereas high for steel. The action of the tool load will make the fixture appear to be in a fixed beam condition with a point load at the centre, which is a perceived diagram of the pipe fixture (D–pipe diameter, 1/2 L–pipe specimen length). This will induce the bending moments to be generated at the clamped points (fixed locations) as well as bending at the weld zone (centre of the beam) as per the bending in the following equations: (3)Bendingmomentatendsupports,M=FyL8,(4)Deflection at the weld point∆centre=FyL3192EI.WhereFy is the tool load acting in a downward direction (kN), E is Young’s modulus of the pipe material (MPa), and I is the area moment of inertia (mm4). This effect may cause the pipes to bend at the joints, which will result in poor cause root defects. As per (4), this bending will increase as the length of the pipe between the end-supports the increase. In contrast, the bending decreases when thickness (T) and diameter (D) increase proportionately (since I=TD3/12). This factor if underestimated may result in root defects, which is a result of poor integrity at the toot side. Studies have also been reported where these root defects are addressed by novel hybrid welding processes [117].(iii)
Support to hold the integrity of the weld zone viz., mandrel: Backing the weld zone is a major facilitation for holding its integrity. In the case of plate welding, backing plates are used [162]. For pipes, a mandrel is used as a backing structure. This mandrel is to possess favorable thermal properties like a high melting point and low thermal expansion when compared to that of the pipe material considered. The mounting and removal of pipe on the mandrel must be made accessible. Expanding mandrels would be the apt option for this case [114]. Mostly, mandrels are made for a specific diameter of pipe. Hence, each diameter of pipe will require a separate mandrel to be fabricated and employed.(iv)
Rotary motion, with variable speed options: This is one of the basic requisites of any pipe fixture for delivering rotary motion of pipe, which is parametrized as weld/traverse speed. This was achieved using an external motor with a suitable gearbox. The motor’s torque is to have enough capability to overcome the frictional shear forces at the tool-pipe junction [121].Figure 16
Pipe fixture–perceived structure with forces involved.The second approach will have more complexity as the entire tool head has to rotate, ensuring the uniform tool load. Technological advancements may pave the way for the selection of approaches based on the application requirements. Figure17 shows the facilitation chart for FSW of pipes for obtaining sound pipe joints.Figure 17
FSW of pipes–process facilitation chart as per the first approach.Thus, the impact of fixtures on the quality of the weld is very high when it pertains to weld geometries other than flat. The circular geometries like pipes have critical compensations from the fixture and required perfect facilitations in order to achieve sound weld joints.


## 4.2.1. Fixture Desirability for FSW of Pipes

The fixture development is an integral part of the FSW of pipes, and the first approach remains the common choice due to its relative simplicity compared to the second approach. However, the developed fixture using the first approach has to satisfy the following requisites for successful facilitation of the pipe welding process for Figure16 an industrial application.(i)
Holding of the pipe firmly without any slippage: The pipes are to be clamped firmly to the rotation fixture (along the mandrel) so that zero slippage is ensured throughout the welding process [161]. Also, care is to be taken that the clamping forces are not affecting the weld property.(ii)
Withstanding the axial load exerted by the tool: During the welding process, the tool is plunged into the weld zone with an axial load calculated according to the material mounted. This axial load may be low for AAs, whereas high for steel. The action of the tool load will make the fixture appear to be in a fixed beam condition with a point load at the centre, which is a perceived diagram of the pipe fixture (D–pipe diameter, 1/2 L–pipe specimen length). This will induce the bending moments to be generated at the clamped points (fixed locations) as well as bending at the weld zone (centre of the beam) as per the bending in the following equations: (3)Bendingmomentatendsupports,M=FyL8,(4)Deflection at the weld point∆centre=FyL3192EI.WhereFy is the tool load acting in a downward direction (kN), E is Young’s modulus of the pipe material (MPa), and I is the area moment of inertia (mm4). This effect may cause the pipes to bend at the joints, which will result in poor cause root defects. As per (4), this bending will increase as the length of the pipe between the end-supports the increase. In contrast, the bending decreases when thickness (T) and diameter (D) increase proportionately (since I=TD3/12). This factor if underestimated may result in root defects, which is a result of poor integrity at the toot side. Studies have also been reported where these root defects are addressed by novel hybrid welding processes [117].(iii)
Support to hold the integrity of the weld zone viz., mandrel: Backing the weld zone is a major facilitation for holding its integrity. In the case of plate welding, backing plates are used [162]. For pipes, a mandrel is used as a backing structure. This mandrel is to possess favorable thermal properties like a high melting point and low thermal expansion when compared to that of the pipe material considered. The mounting and removal of pipe on the mandrel must be made accessible. Expanding mandrels would be the apt option for this case [114]. Mostly, mandrels are made for a specific diameter of pipe. Hence, each diameter of pipe will require a separate mandrel to be fabricated and employed.(iv)
Rotary motion, with variable speed options: This is one of the basic requisites of any pipe fixture for delivering rotary motion of pipe, which is parametrized as weld/traverse speed. This was achieved using an external motor with a suitable gearbox. The motor’s torque is to have enough capability to overcome the frictional shear forces at the tool-pipe junction [121].Figure 16
Pipe fixture–perceived structure with forces involved.The second approach will have more complexity as the entire tool head has to rotate, ensuring the uniform tool load. Technological advancements may pave the way for the selection of approaches based on the application requirements. Figure17 shows the facilitation chart for FSW of pipes for obtaining sound pipe joints.Figure 17
FSW of pipes–process facilitation chart as per the first approach.Thus, the impact of fixtures on the quality of the weld is very high when it pertains to weld geometries other than flat. The circular geometries like pipes have critical compensations from the fixture and required perfect facilitations in order to achieve sound weld joints.


## 5. Conclusions

The FSW of pipes has started attracting researchers and the industrial community in reaching industrial standard applications. All the studies that had been reported so far in FSW of pipes have their own fixture being developed by the first approach. But the fixture setup will vary from one study to another. Thus, FSW of pipes would require a thorough study of the fixtures to obtain sound weld joints. As the process parameters are dependent on the fixture, the optimization of the fixture elements also needs to be incorporated. One more challenge for FSW to hit the market is the unavailability of dedicated machines. Since the rotary pipe fixture is used as an external unit, the control mechanisms do not become interdependent. Also, the controls of fixturing elements were not mechanized. The optimization works on the fixturing elements that can provide relationship characteristics to help creating controller mechanisms for the rotary pipe fixtures. For this purpose, dedicated machines can be designed and developed for FSW of pipes in future studies. This will pave the way for computer-aided control over the process and fixture parameters, thus increasing the efficiency of the pipe welding process. Concerning the material choice, Al pipes are mostly exploited by the FSW process due to its economic tool selection options. The FSW tool is still considered to be costly compared to conventional Al welding tools. But the economical joining of steel pipes using FSW will be challenging unless the development of a tool for joining steel is made comparatively cheaper.


### 5.1. Future Research Directions

The studies related to the development of fixtures for FSW are only count to be few. The research directions that the future work can focus will be:(i)
Joining of thermoplastic pipes using FSW is one of the areas which researchers can explore for all possible outcomes.(ii)
Development of a generic fixture that can hold to perform FSW on a variety of joints can be attempted.(iii)
Development of intelligent control systems is another area to automate the FSW for different materials and different geometrical requirements with good physical characteristics.(iv)
Future studies can focus on integrating different conventional and nonconventional welding processes with FSW to eliminate the defects and extensive requirement of rigid fixturing elements. This can extend the choice of materials.


## 5.1. Future Research Directions

The studies related to the development of fixtures for FSW are only count to be few. The research directions that the future work can focus will be:(i)
Joining of thermoplastic pipes using FSW is one of the areas which researchers can explore for all possible outcomes.(ii)
Development of a generic fixture that can hold to perform FSW on a variety of joints can be attempted.(iii)
Development of intelligent control systems is another area to automate the FSW for different materials and different geometrical requirements with good physical characteristics.(iv)
Future studies can focus on integrating different conventional and nonconventional welding processes with FSW to eliminate the defects and extensive requirement of rigid fixturing elements. This can extend the choice of materials.



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*Source: 1311292-2022-12-28.xml*