Keywords

1 Introduction

Friction Stir Welding (FSW) has gained popularity as it is an environment-friendly process. FSW is a solid-state welding process performed with the help of a rotating tool. The rotating tool generates frictional heat, that plasticizes the material below its melting point, forming a weldment. It has proven to be a technique capable of welding both thick and thin sheets. The main components of a conventional FSW tool are a shoulder and a pin, whereas a bobbin tool has an additional lower shoulder that eliminates the requirement of a backing plate. Friction Stir Spot Welding (FSSW) can be performed using a conventional tool, a pinless tool or a refill tool. The pinless variant of FSW tool consists of only a shoulder whereas the refill tool consists of a pin and a sleeve. Apart from welding parameters like rotational speed of the tool and plunge depth, tool designing plays an important role in producing a joint with higher mechanical properties. Critical design parameters include Pin Length (PL), Pin Diameter (PD), Shoulder Diameter (SD), pin and shoulder profiles and surface features. The designing of conventional tool, bobbin tool and friction stir spot welding tool has been discussed in this paper.

2 Conventional Friction Stir Welding Tool

A conventional tool is a rotating tool consisting of a pin and a shoulder. The role of the pin is to generate frictional heat and mix the softened molten material, whereas the shoulder is responsible for generating frictional heat on the surface of the workpiece and downward forging action that is required to constrain the heated softened material beneath it [1]. Conventional tool has specially designed pin and shoulder to achieve optimum characteristics for a weld. Pin profile, shoulder profile and dimensions of the tool affect the weld quality (Fig. 1).

Fig. 1
An illustration depicts the conventional friction stir welding process. The labels listed are, advancing side of weld, shoulder, joint, leading edge of the rotating tool, probe, retreating side of weld, and trailing edge of the rotating tool.

Conventional friction stir welding process [2]

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2.1 Conventional Tool Pin Profiles

The stirring pin profile affects the heat generation, micro-hardness, size of the deformation region, amount of plasticized material that is moved from advancing side to retreating side and the travel speed of the tool. Profiles like cylindrical, cylindrical taper, cylindrical threaded, triangular, square, pentagonal and hexagonal has been experimentally tested to obtain optimum weld quality. Threaded pins generate more heat and are efficient in moving heated material from advancing side to retreating side. Thread feature enhances the plastic material movement and accelerates the flow of plastic material by exerting a downward force [3]. Padmanaban et al. [4] concluded that the joints welded by threaded cylindrical pin are free from defects and exhibit higher tensile strength compared to other pin profiles. Pitch value of the threads determine whether the pin will act as a driller or a stirrer. The pin will act as a driller if the pitch of the threads is very high [2].

Deformation patterns and size of the deformation region depends on the pin profile. Pin profiles like square and hexagon reduce the size of the deformation region [5]. Features like flats on the pin are provided to increase the amount of material carried from advancing side to retreating side. Flat feature also helps in achieving higher temperatures in the weld zone. Pulsating action which is only achieved through providing flat features on the pin, refines the grain structure. Finer grains are obtained with square and hexagonal pin profiles [5,6,7]. Triangular pin profile has the least area of contact with the molten material which in turn generates least friction compared to square and hexagonal pins. Triangular profile pins sweep least amount of plasticized material due to its lowest static to dynamic volume ratio. Dynamic volume (swept volume) to static volume (pin volume), known as swept ratio quantifies the amount of material plasticized during the process [5, 7]. Less heat generation and amount of material plasticized by a triangular pin may result in lower strength and defects in a weldment. Taper feature provided to the pins help in achieving higher mechanical properties like hardness, elongation and ultimate tensile strength, defect-free joints and finer grain structure [2, 8] (Figs. 2 and 3).

Fig. 2
An illustration depicts 5 types of pin profiles. They are, conical pin, triangular pin, square pin, pentagonal pin, and hexagonal pin. A pin and a shoulder are indicated with arrows.

Pin profiles [5]

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Fig. 3
An illustration depicts 3 types of cylindrical tools. They are simple cylindrical, screw cylindrical, and taper cylindrical.

Simple cylindrical, screw threaded cylindrical, taper cylindrical [8]

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TWI has recently developed the WhorlTM and MX TrifluteTM tools. Frustum shaped probes in both the tools displace less material compared to cylindrical tool having the same root diameter. WhorlTM and MX TrifluteTM reduce the displaced volume by about 60% and about 70% respectively. Cylindrical probes require more effort to traverse through the plasticized material than a frustum shaped probe for a certain minimum probe tip diameter [9]. Re-entrant feature of the WhorlTM tool, helical ridge is responsible for providing a downward augering force, ensuring that the next ridge faces less interference and to enable a more effective material flow. The ridge having a non-uniform pitch, are placed at a distance greater than the thickness of the ridge itself [9]. These specially designed probes having re-entrant features have an increased surface area compared to featureless probes.

Re-entrant features on a triflute probe can be designed according to the material and joint geometry. They can be designed with any combination of left, neutral or right-handed flutes/ridges grooves. The inclined ridges move the fragmented oxides in an upward or a lower direction by deflecting the plasticized material. In order to reduce the volume of triflute tools, an additional helical feature is provided [9, 10]. These novel designs of the tools have very high swept rates compared to conventional cylindrical probes. The dynamic to static volume ratio for MX Triflute™ probe is 2.6:1, 1.8:1 for Whorl™ probe and 1:1 for a cylindrical probe [9] (Fig. 4).

Fig. 4
An illustration depicts M X triflute tool. The labels listed are, three flutes, left hand helix on outer diameter lands, example of helix showing well radiused corners, and not to scale.

Trade-marked MX triflute tool [9]

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2.2 Conventional Tool Shoulder Profiles

Flat, concave and convex type shoulder profiles are the most commonly used profiles. A flat shoulder, being the simplest design, does not trap plasticized material during the process, whereas a concave shoulder is highly used due to its material entrapment characteristic. A concave type shoulder has an inclination of about 6°–10° compared to a flat shoulder, which entraps the material and directs its flow towards the pin. Convex shaped shoulder has gained popularity due to its ability to maintain a constant contact with the base metal. Though it is considered that the influence of outer shape of shoulder on the weld quality is minimal, the outer surface of the shoulder can have a conical or cylindrical geometry [1, 11]. Galvão et al. [12] conducted experiments using a conical shaped shoulder and concluded that conical shape of the cavity will act as a reservoir for plasticized material and will direct the material flow towards pin (Fig. 5).

Fig. 5
An illustration depicts primary and secondary shoulder profiles. The shoulder outer surface is cylindrical and conical. The shoulder end surface is flat, concave, and convex. The end surface features listed are, featureless and featured. Featured are further branched as scrolls, ridges, knurling, grooves, and concentric circles.

Primary and secondary shoulder profiles [11]

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Re-entrant features like grooves, concentric circles, scrolls and ridges are often provided at the end surface of the shoulder to entrap the plasticized material and direct the flow towards the pin. These features prevent the molten material flow towards the periphery of the shoulder. The scroll feature is a spiral shaped channel that move the material from the periphery to the centre of the shoulder [1, 9]. A combination of convex shoulder and scroll feature can provide sound welds due to constant contact and entrapment of material throughout the process [1]. Although, a scroll feature is not suitable for the welding of thin sheets as the feature periodically deposits the material at the rear end of the tool and, increases the thickness of the material [12]. A. Scialpi et al. [13], experimentally proved that a combination of fillet and cavity can be used to improve the weld quality. It was also observed that though shoulder geometry has reduced effect on the micro-hardness and nugget grain structure, features like fillet, cavity and scroll improved the quality of weldment.

2.3 Conventional Tool Dimensions

For designing a conventional FSW tool, shoulder and probe diameter are critical design parameters. Zhang et al. [1], deduced empirical relationships for Shoulder Diameter (SD) and Sample Thickness, Probe Diameter (PD) and Sample Thickness and Shoulder Diameter (SD) and Probe Diameter (PD) from the available literature (Fig. 6).

Fig. 6
2 scatter plots depict shoulder and probe diameter in m m. Data are approximate. The lines in both graphs with labels, y equals 2.199 x plus 7.318 R 2 equals 0.637, y equals 0.834 x plus 2.224 R 2 equals 0.584, and y equals 2.097 x plus 4.760 R 2 equals 0.672 are of increasing trend and the plots are on either side of the trend line.

a Shoulder and probe diameter versus sample thickness and b shoulder diameter versus probe diameter [1]

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$${\text{SD}}\left( {{\text{mm}}} \right) = 2.2*{\text{Sample}}\,{\text{thickness}}\left( {{\text{mm}}} \right) + 7.3\,{\text{mm}}$$
(1)
$${\text{PD}}\left( {{\text{mm}}} \right) = 0.8*{\text{Sample}}\,{\text{thickness}}\left( {{\text{mm}}} \right) + 2.2\,{\text{mm}}$$
(2)
$${\text{SD}}\left( {{\text{mm}}} \right) = 2.1*{\text{PD}}\left( {{\text{mm}}} \right) + 4.8\,{\text{mm}}$$
(3)

3 Bobbin Tool Friction Stir Welding

Self-reacting friction stir welding, also known as bobbin tool friction stir welding has a specially designed tool having two shoulders and a pin. During the process, both shoulders generate frictional heat. The reactive forces generated during the process, are cancelled out by the two shoulders resulting in a very low axial force [14, 15]. During a bobbin tool friction stir welding process, no backing plate is required, as the lower shoulder supports the base metal. Fixed gap bobbin tool, floating bobbin tool, adaptive or double driven bobbin tool and pinless bobbin tool are major variants of this technique. The gap between the shoulders remains constant throughout the process for a fixed gap bobbin tool. The movement of fixed gap tool about z-axis is restrained [14]. A floating bobbin tool, is designed as a fixed gap bobbin tool and is allowed to float in the axial direction. The novel design of floating bobbin tool adjusts its position according to the base metal in such a way that the axial force reduces drastically [16] (Fig. 7).

Fig. 7
An illustration depicts 3 types of bobbin tools. They are fixed gap, floating and adaptive bobbin tools. The labels listed are, work piece, tool holder, and fixed bobbin tool.

Fixed gap bobbin tool, floating bobbin tool, adaptive bobbin tool [16, 17]

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An adaptive tool or a double driven bobbin tool, is driven from both the ends, providing an enhanced flow path. Double driven tool is capable of altering the distance between both the shoulders during the process, and is allowed to oscillate in the axial (z-axis) direction. In a double driven tool, the aspect ratio of the probe can be altered [14, 17].

3.1 Bobbin Tool Pin Profiles

Bobbin Tool Friction Stir Welding (BT-FSW) process has been employed with various pin profiles like cylindrical taper, trigonal, hexagonal, outward conical and inward conical. Out of all these pin profiles, cylindrical tapered pin with flats is a commonly used profile [18]. Mohammad Hosein Mirzaei et al. [19] conducted double shoulder friction stir welding using trigonal, square, hexagonal, inward conical and outward conical pins and concluded that the heat generated by un-edged pins is more than edged pin profiles. Trigonal and outward conical pins result into joints having lower mechanical properties whereas hexagonal and square pin profiles yielded joints having superior mechanical properties like ultimate tensile strength (UTS) and elongation. As the number of edges increase from square to hexagonal profile, the material boxes moved by the pins decrease in size but increase in number. Such a material movement enhances the material coalescence, improving the joint properties [19]. Pin profiles like outward conical and trigonal should be avoided as a trigonal profile results into undesired flow of material and an outward conical profile pin reduces the mechanical properties by excessive heating [19] (Fig. 8).

Fig. 8
An illustration depicts 6 types of pin profiles of a bobbin tool.

Various pin profiles of a bobbin tool [19]

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Re-entrant features like flats, threads, grooves and flutes can be added to the tool pin to enhance the horizontal and vertical movement of plasticized material. Thread feature is responsible for the vertical movement of the molten material and flat feature enhances the horizontal movement [14]. While designing the tool pin, grooves shall be carefully added as they trap the material between them and resist the vertical flow. Excessive heat is generated for the materials trapped between grooves as a groove feature has a tendency to only generate heat and not axial flow [19]. A pin having three to four flat features and having threads will generate a sound weld having high mechanical properties due to combined horizontal and vertical material flow [14, 19, 20]. Wu et al. [21], designed and experimented a novel adjustable-gap bobbin tool on Al-Cu aluminium alloy. The adjustable-gap tool pin, having half-right and half-left feature enhanced material mixing in the vertical direction of the weldment. Specially designed half-right hand and half-left hand thread filled the voids by converging material flow towards the centre (Fig. 9).

Fig. 9
An illustration depicts a novel adjustable gap bobbin tool pin with left and right handed threads.

Novel adjustable-gap bobbin tool pin having right and left-handed threads [21]

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3.2 Bobbin Tool Shoulder Profiles

Similar to a conventional friction stir welding tool, the primary features of bobbin tool shoulder include flat, concave and convex [18]. A flat shoulder, in spite of being the simplest design, does not trap plasticized material and produces flash. Concave profile of a shoulder entraps the molten material and feeds it to the pin by acting as a reservoir. A convex shoulder acts in a unique way by remaining in a constant contact with base metal plate of varying thickness [1, 15, 18]. The outer shape of the shoulder can be cylindrical or tapered. Tapered shoulders have successfully welded flash-less joints [16]. Scroll feature is the most used secondary feature on the shoulder as it promotes the flow of plasticized material in a horizontal direction towards pin. Okamoto et al. [20] deduced that scroll feature will enhance the vertical flow and dispersion of oxide film when provided on convex shoulder instead of a flat shoulder (Fig. 10).

Fig. 10
2 illustration depicts the outer shape of the shoulders. They are in a cylindrical and tapered shape.

Cylindrical and tapered shape of shoulders [11]

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3.3 Bobbin Tool Dimension and Size

Pin Diameter (PD), Pin Length (PL), Upper Shoulder Diameter (USD), Lower Shoulder Diameter (LSD) and gap between upper and lower shoulder are critical parameters while tool designing. The upper and lower shoulder are generally taken to be of equal diameters [15, 20, 22,23,24,25,26,27,28,29,30,31,32,33], but it is recommended that the lower shoulder should have a smaller diameter than upper shoulder. Lesser diameter of lower shoulder results into less bending moment and torque on the pin, as well as less generation of frictional heat on the lower surface due to less contact with the plate [34,35,36]. The lower shoulder will provide insufficient support to the plate from the vibration if a very small value of diameter is chosen. Also, a reduced area of contact may result into premature solidification of weld zone.

Shoulder gap being an important design parameter, enhances material stirring and vertical flow. An optimum compression effect and frictional heat is produced if the shoulder gap is determined [18]. The pinching gap of the shoulders generate dragging force for the material to flow around. An optimal combination of pinching gap at the centre and periphery are required to produce sound welds [20]. In the literature available, no definite method has been suggested for the selection of shoulder and pinching gap and have been designed using a trial and error. Pin length usually kept the same as base metal thickness, can also be taken as 0.9 times the thickness of the base metal [14]. Figure provided below summarizes experiments conducted with bobbin tool and empirical relations have been obtained using least square approximation method [15, 20, 22,23,24,25,26,27,28,29,30,31,32,33, 37,38,39,40] (Fig. 11).

Fig. 11
2 scatter plots depict U S D and P D versus P T, and U S D versus P D. Data are approximate. The lines in both the graphs with labels, y equals 1.867 x plus 9.0028 R 2 equals 0.6668, y equals 0.8451 x plus 3.3096 R 2 equals 0.743, y equals 1.802 x plus 4.995 R 2 equals 0.597 are of increasing trend and plots lie on either side of the trend line.

a Upper shoulder diameter/pin diameter versus plate thickness, b upper shoulder diameter versu pin diameter

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$${\text{USD}}\left( {{\text{mm}}} \right) = 1.87*{\text{PT}}\left( {{\text{mm}}} \right) + 9\,{\text{mm}}$$
(4)
$${\text{PD}}\left( {{\text{mm}}} \right) = 0.85*{\text{PT}}\left( {{\text{mm}}} \right) + 3.31\,{\text{mm}}$$
(5)
$${\text{USD}}\left( {{\text{mm}}} \right) = 1.8*{\text{PD}}\left( {{\text{mm}}} \right) + 5\,{\text{mm}}$$
(6)

4 Friction Stir Spot Welding (FSSW)

A friction stir spot welding involves the rotation, plunging and retraction of the tool. The rotational speed of the tool and its plunge depth are predetermined. A conventional FSSW tool has a shoulder-pin setup, where the length of the pin is fixed. A conventional tool has a tendency to move the material in a downward direction, away from the tool shoulder. As the tool pin has a fixed length, a plate having a constant thickness can be welded. The tool is responsible for heating the base metal, inducing material flow, confine the plasticized material and prevent it from overflowing [41]. The drawback of a conventional FSSW tool is that, it can cause thinning of the top sheet due to excessive tool penetration [42]. Due to the presence of pin, a keyhole defect at the end of welding process is observed (Fig. 12).

Fig. 12
An illustration depicts, a rotation of tool, plunging of tool, dwell period, and drawing out of the tool.

a Rotation of tool, b plunging of tool, c dwell period, d drawing out of tool [41]

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A pinless tool, as the name suggests, is a tool only having a shoulder. The pinless tool performs welding process by rotation, plunging and retraction from the base material. This tool eliminates the occurrence of keyhole defect at the end of the welding process [43]. Features like scroll and flutes can be provided to enhance the quality of the weld (Fig. 13).

Fig. 13
An illustration depicts a pinless F S S W tool. A depicts tool rotation with pin, sleeve, and clamp. B depicts sleeve plunging and pin retraction, C depicts sleeve and pin retraction, and D depicts retrieval of the F S S W refill tool.

Pinless FSSW tool with scroll feature, sleeve plunge variant of FSSW [41]

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A refill FSSW tool comprises of a sleeve, pin and a clamp. The clamp is responsible for holding the sheets together whereas the sleeve and pin are responsible for spot welding the sheets. Pin plunge and sleeve plunge are the two variants of this process. For a sleeve plunge method, once the clamp has held the sheets together tightly, the sleeve plunges into the sheets till a predetermined depth and simultaneously the pin moves upwards. Once the predetermined depth has been achieved, the sleeve retracts to its original position and the pin moves towards the sheets. In this process, the hole is completely filled with the molten material, eliminating the keyhole defect. For a pin plunge method, the pin is responsible for the penetration in the sheets, whereas the sleeve retracts. A sleeve plunged joint has a larger welded area compared to a pin plunged joint [44].

4.1 Pin Profiles

Pin profiles like triangular, square, hexagonal, spherical, inverse-tapered and other off-centre have been employed for friction stir spot welding process [45,46,47,48,49]. A high forging force is exerted by a cylindrical pin, but is a commonly used pin profile in a conventional FSSW process [48, 49]. It has been experimentally proven that a tapered pin forms a smaller hook compared to a conventional cylindrical pin [49]. A triangular pin generated more heat compared to a cylindrical pin due to the presence of flat features. Joints having higher mechanical strength and larger bonding area formed using triangular pins. Inspite of a triangular pin having above mentioned advantages, it is recommended that, it should not be used to weld steels, as the sharp flat features will worn away after being used for some time [41, 48, 49, 50]. For an inverse tapered pin, a hook feature will be extended up to the top surface, resulting a joint has low mechanical properties [45] (Fig. 14).

Fig. 14
An illustration depicts 4 F S S W tool pin profiles. They are cylindrical, tapered, inverse tapered, and triangular.

FSSW tool pin profiles—cylindrical, tapered, inverse tapered, triangular [41]

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Secondary features such as threads, flutes and flats increase the area of contact and increase the heat generation. Such features have given improved results for lightweight materials, but for materials like steel, these features get worn out after a few spot welds. Hence, it is recommended that complex secondary features shall not be provided for materials like steel [41, 51]. Although, features like stepped-spiral and coarse thread can be provided on polycrystalline cubic boron nitride (PBCN) tools [52, 53]. A cylindrical pin is the most used pin profile for refill FSW as it is designed to move in axial direction [41].

4.2 Shoulder Profiles

The primary shoulder profiles in a conventional FSSW are the same as conventional FSW and Bobbin Tool FSW. The primary profiles include flat, concave and convex [48, 54]. A concave shoulder, the most commonly used profile produces a weld sound by the formation of a sharp hook. Concave shoulder, which acts as a reservoir for the plasticized material results into a weld joint having higher mechanical properties compared to a flat or convex shaped shoulders [45, 48, 54]. The joint properties exhibited by a convex shoulder is inferior to those exhibited by a flat shoulder, making the flat shoulder an intermediary choice. A convex shoulder welds a joint having lowest mechanical properties due to thinning of the top sheet and insufficient mixing of molten material [48, 49]. Like conventional FSW tool and bobbin tool, secondary features like scroll and concentric circles can be provided at the end surface of concave and convex shoulders (Fig. 15).

Fig. 15
An illustration depicts 3 shoulder profiles. They are concave, flat, and convex.

Concave, flat and convex shoulder profiles [41]

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Re-entrant features like long and short wiper flutes, scroll flutes and proud wiper are the most important features for a pinless tool to produce a joint of good quality. Welds exhibit higher mechanical properties with use of re-entrant features, due to an enhanced flow of molten material and deeper penetration of the plastic zone [55]. Uematsu et al. [56] experimentally obtained joints having higher tensile-shear strength using a scroll grooved tool compared to a conventional tool having concave shoulder and a probe (Fig. 16).

Fig. 16
An illustration depicts 6 types of re entrant features for the pinless tool.

Re-entrant features for pinless tool [41]

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Shude Ji et al. [57] studied sleeve features like grooves on the wall and concentric circles and grooves at the bottom surface of the sleeve. It was observed that, groove width is negatively proportional to flow velocity. Provision of threaded grooves on sleeve walls alters the behavior of material flow. Experiments proved that scrolled grooves on the bottom surface of sleeve are more efficient than concentric circles (Fig. 17).

Fig. 17
An illustration depicts 7 secondary features of a refill F S S W sleeve. The labels listed are, pin, sleeve, 0.6 m m, 12 m m, 0.3 m m, right threaded groove, and concentric circles.

Secondary features of a refill FSSW sleeve [58]

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4.3 Tool Dimensions

Shoulder Diameter (SD), Pin Diameter (PD), Pin Length (PL) and angle of concave shoulder are critical design parameters that need to be determined in a correct way to obtain a sound weld. Heat generation depends on the pin and shoulder diameter. Larger diameters result into more frictional heat [1, 58]. Pins having larger diameters have proven to bond larger area, but produces excessive flash [45, 59]. Design of concave shoulder for a conventional FSSW tool must not have an extreme angle. Extreme angles for a concave shoulder result into insufficient axial force, decreasing the weld quality (mechanical strength) of the joint [58]. Zhikang Shen et al. [41] graphically represented the linear increase of pin length with the plate thickness. It was also observed that the shoulder diameter linearly increased with the plate thickness. For a conventional FSSW tool, a pin diameter of 4 mm can be employed, irrespective of the plate thickness [5]. The pin length for thin workpiece has less importance and the tool can be considered to be pinless [42]. From the available literature for pinless tool, it can be observed that a shoulder diameter of 10 mm is the most used diameter [42, 43, 56, 60, 61] (Fig. 18).

Fig. 18
2 scatter plots depict base metal thickness versus shoulder pin and diameter, and pin length in m m. Data are approximate. The plots in both graphs follow an increase in trend, and the plots lie on either side of the trend line.

a Shoulder and pin diameter versus base metal thickness b pin length versus base metal thickness [41]

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External and internal sleeve diameter, pin diameter and groove width on sleeve are important dimensions for designing a refill friction stir welding tool. The outer diameter and groove width influence the material flow velocity whereas threaded grooves on the inner wall affect the bonding ligament thickness [57]. From the literature available, it can be deduced that, a diameter of 9 mm for sleeve and 5–6.4 mm for a pin diameter is commonly employed in a refill friction stir spot welding process [41].

5 Conclusion

In this paper, pin profiles, shoulder profiles and tool dimensions for conventional tool, bobbin tool and friction stir spot welding tools were reviewed.

For a conventional tool, pin profiles like cylindrical, cylindrical tapered, triangular, square, pentagonal and hexagonal have been used for friction stir welding. Similarly, pin profiles like cylindrical, trigonal, hexagonal, inward and outward conical have been used for bobbin tool friction stir welding. In a refill friction stir spot welding tool, cylindrical and triangular pin profiles result into joints having superior mechanical properties. Secondary features like flats, flutes, threads and grooves can be provided to increase the contact area between plate and pin, increase heat generation and to acquire joints with superior qualities. Although, for friction stir spot welding of steels it is recommended that complex secondary features shall not be provided as it decreases the tool life.

Flat, concave and convex profiles are common to conventional, bobbin and friction stir spot welding tools. Re-entrant features like scroll, concentric circles and grooves can be provided on the bottom surface of shoulder to improve the weld quality.

Empirical relationship between Pin Diameter (PD), Shoulder Diameter (SD) and sample thickness have been mentioned for a conventional tool. Similarly, empirical relationship between Upper Shoulder Diameter (USD), Pin Diameter (PD) and Plate thickness (PT) have been established using least square approximation method. From the available literature, it has been concluded that a pin with 4 mm diameter can be used for conventional spot welding process irrespective of the plate thickness. For refill spot welding tool, a sleeve diameter of 9 mm and a pin diameter of 5–6.4 mm can be used. For a pinless spot welding tool, a shoulder diameter of 10 mm can be used to obtain defect-free joints.