1 Introduction

1.1 Friction Stir Welding (FSW)

Friction Stir Welding (FSW) is a very effective solid-state joining process. It does not need any filler material and shielding gas. Furthermore, this welding process is free from arc flash, fumes, and scatter [1, 2]. It exhibits certain advantages over traditional fusion welding methods. First, it avoids solidification related problems, such as shrinkage, porosity, and hydrogen solubility. Second, it can reduce Inter Metallic Compound (IMC) layer formation due to low heat input [3]. The schematic drawing of normal friction stir welding is shown in Fig. 1.

Fig. 1
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Schematic illustrations of friction stir welding [1]

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1.2 Evolution of Submerged Friction Stir Welding (SFSW)

The material undergoes plastic deformation at an elevated temperature during the FSW process, as a result of the creation of fine and equiaxed re-crystallized grains. This formation of grains is retained to achieve the good mechanical properties of friction stir welds. However, loss of the mechanical properties of normal friction stir welds of heat treatable aluminium alloys due to the effects of the welding thermal cycle [4].

In recent years, special interest is shown in the improvement of weld properties by controlling the temperature level. This is done by spraying onto the welding tool and the top surface of the weld sample during friction stir welding [5,6,7,8]. Fratini et al. [9] found some improvement in the properties of the joints that followed the water spraying procedure compared to the normal joints. Sakurada et al. [10] are the pioneers in the application of submersion in a rotary friction weld for aluminium alloys. Their study revealed the possibility of creating sufficient friction for welding despite the weld plates being submerged. Douglas et al. [11] used submerged friction stir processing for creating ultrafine-grained bulk materials through severe plastic deformation. In order take full advantage, the entire workpiece is immersed in the liquid during the welding which is called Submerged Friction Stir Welding (SFSW). Figure 2 illustrates a schematic view of underwater FSW.

Fig. 2
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Schematic view of underwater FSW [14]

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1.3 SFSW Process Parameters

Process parameter of the submerged friction stir welding process is the same as the friction stir welding process. Mechanical and microstructure properties are governed by choice of processing variables. These include rotational speed (rpm), welding speed, normal force, lateral force, shoulder plunge depth, water head, tool tilt angle, tool geometry and tool pin profile [12, 13]. It has been stated that the rotational speed and welding speed are main factors in submerged friction stir welding process.

The submerged friction stir welding process was carried out on different materials with varying plate thickness, tool material and tool geometry under the different process parameter conditions. The summary of experimentation was tabulated in Table 1.

Table 1 A Summary of experimentation
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1.4 Benefits of SFSW Over FSW

The benefits of SFSW over FSW are as follows [14],

  • SFSW prevents oxidation and provides a better surface finish than FSW.

  • Peak temperature of the SFSW is lower than in FSW, into limiting coarsening and precipitate dissolution.

  • SFSW presents a refined grain structure compared to FSW.

  • SFSW provides the better mechanical properties of the weld plate than FSW.

  • SFSW reduces residual stresses with distortion less than FSW.

2 Experimental Investigation of SFSW Process

2.1 Welding Characteristics

Welding characteristics such as axial forces, torque, power, coefficient of friction, temperature analysis and heat generation of submerged friction stir welding process of various alloys are discussed.

Papahn et al. [15] have investigated the effect of water on the applied force and temperature analysis during friction stir welding of the AA7075 alloy. They have reported the translational and axial forces and heat generation significantly increase in the underwater friction stir welding than the normal FSW. Bloodworth [16] has stated the torque values of normal and submerged friction stir welding as 16 Nm and 18.5 Nm respectively. The torque value was higher by 14.5% through SFSW when compared to Normal Friction Stir Welding (NFSW). It was anticipated that the generation of frictional heat would go into the heating of the water. Kishta and Darras [17] have made a detailed study on underwater friction stir welding of AA5083 marine grade alloy. The power consumption, void fractions, thermal histories, microhardness, and tensile properties of the welded were measured and analyzed. A PS3500 power data logger was used for observing the power consumption of the CNC machine during FSW and SFSW. This is shown in Fig. 3. They observed the minimum and maximum average power of 734 W and 961 W in submerged FSW at the rotational speed of 800 rpm and 1700 rpm respectively.

Fig. 3
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Power consumption for SFSW at different rotational speeds [17]

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Mofid et al. [5] have investigated the effect of water cooling during dissimilar FSW of Al alloy to Mg alloy at the constant travel speed of

50 mm/min and the rotational speed of 300 rpm. They obtained a lower peak temperature during submerged friction stir welding due to lower heat input and limited intermetallic compounds formation. Wang et al. [18] have conducted underwater welding experiments in three groups by changing rotational, or traverse speed of spray formed 7055 aluminium. The characteristic values of peak temperature and cooling rate of three groups are listed in Table 2. They have obtained the maximum peak temperature and cooling rate of 265 °C, 11 °C/s respectively, at the higher rotational and traverse speed of 3000 rpm and 155 mm/min.

Table 2 Characteristic value of peak temperature and cooling rate
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Mofid et al. [5] joined the plates of AZ31 and AA5083 H34 by friction stir welding in different environments like air, water, and liquid nitrogen at 400 rpm and 50 mm/min. They have investigated the temperature profile, microstructure, tensile strength, and hardness. Temperature profiles of FSW under the various medium are shown in Fig. 4. The temperature profiles showed the generation of the largest amount of temperature under liquid nitrogen welded plate, which was performed at the lowest temperature.

Fig. 4
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Temperature profiles of FSW in the air, water, and liquid nitrogen [5]

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Zhang et al. [4] conducted FSW experiments under two conditions, namely, air and water. The temperature of the weld samples was measured using K-type thermocouples during FSW. The locations of the thermal couples are shown in Fig. 5.

Fig. 5
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Locations of thermal couples (unit: mm) [4]

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They obtained the temperature history of normal and underwater joints in different zones including weld nugget zone, thermal mechanically affected zone and heat affected zone, as shown in Fig. 6a–c. They have concluded that the peak temperature of underwater FSW was lower than the normal FSW in all weld zones.

Fig. 6
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a–c Thermal histories of different zones [4]

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2.2 Tensile Properties

The evaluation of mechanical properties including tensile strength and microhardness is very importance for submerged friction stir welded samples.

Fratini et al. [8] investigated the possibility for enhancing the FSW joint performances of 7075-T6 aluminium alloy through in process heat treatments. FSW was conducted under three different test conditions, i.e., free air, forced air, and with water flowing on the surface of the welds. An improvement was observed in the ultimate tensile strength of the developed welds, using an external cooling during the FSW process. This improvement was found to be higher than the forced air welded joint. Fu et al. [19] performed normal and underwater FSW on high-strength 7050 aluminium alloys with a thickness of 5.8 mm. The quality of the underwater welded joints was better than the normal welded joints due to the higher cooling rates around the weld zones. Xue et al. [20] conducted friction stir welding with rapid water cooling on the pure copper plate at a constant welding speed of 50 mm/min with the different rotational speed of 400 and 800 rpm. They achieved strength almost equal to the parent metal at the rotational speed of 400 rpm and a welding speed of 50 mm/min via additional rapid cooling. Liu et al. [21] carried out the underwater friction stir welding of 2219 aluminium alloy. The homogeneity of mechanical properties of the joint was examined by dividing the joint into three layers. The results showed improvement in the tensile strength of the underwater FSW in all three layers. They have found this as an intrinsic reason for the strength improvement through underwater FSW. Fu et al. [22], during their submerged friction stir welding of 7050 high strength aluminium alloy in hot water determined the ratio of the ultimate tensile strength and elongation of the joint welded to the base metal. They found the ratio of the ultimate tensile strength and elongation of the joint welded to the base metal to be 92% and 150% respectively. The fracture regions of FSW samples at different ambient conditions are shown in Fig. 7a, b. The grain size and population of the precipitates are increased in the air welded joints.

Fig. 7
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a, b TEM for FSW samples at different ambient conditions [22]

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The tensile strength of the welded joint was up to 152.3 MPa which was 63.5% AZ31 Mg alloy strength. At the rotational speed of 1200 rpm and the welding speed of 80 mm/min, the sound welded joints with good mechanical properties were obtained using underwater FSW [23]. Sabari et al. [24] have found that the joint made by taper threaded pin tool with underwater cooling medium achieved higher joint efficiency of 76% and tensile properties of 345 MPa which was 5% higher than underwater friction stir welded joints with straight threaded cylindrical pin profile. The enhancement of weld strength and efficiency is attributed to the grain boundary strengthening and precipitation hardening. Xu et al. [25] have investigated the tensile strength and strain hardening behaviour of a friction stir welded AA2219 aluminium alloy under air and water cooling conditions. The yield strength was lower in the friction stir welded joints than in the base metal, the ultimate tensile strength of the friction stir weld joints with water cooling condition reached almost to the base metal. Sree Sabari et al. [26] have studied the effect of rotational speed on the tensile properties of friction sir, and underwater friction stir welded 2519-T87 aluminium alloy. The tensile specimens of the joints all fractured were at the HAZ adjacent to TMAZ on the AS when the welding speed increased to 100 and 150 mm/min.

The tensile specimens of the joints were fractured in the HAZ adjacent to the TMAZ on the RS (Retreating Side) at lower welding speed of 50 mm/min. Figure 8 shows the fracture locations of the welded samples [27]. Zhao et al. [28] did joining by friction stir welding in immersed and normal conditions of 6013 aluminium and AZ31 magnesium alloy. Figure 9a, b shows the SEM fracture morphology of normal and underwater FSW. The normal direction to the fracture surface of the welded sample was observed. Both the joints of air and underwater failed due to the brittle fracture. The fracture location of the tensile samples was in the center of the weld.

Fig. 8
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Fracture locations at the different welding speeds [27]

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Fig. 9
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SEM fracture morphology a normal, b underwater FSW [28]

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2.3 Hardness

Azarafza et al. [29] performed normal and underwater friction stir welding on aluminium 2024 alloy. They have found the maximum hardness at a rotational speed of 1000 rpm and traverse speed of 100 mm/min. Increase in the rotational speed caused a decrease in the hardness due to increase in the size of coaxial grains. The hardness and tensile strength of the underwater FSW joint showed improvement through the controlling of thermal cycles by the water cooling effect rather than normal FSW joint [30]. Zhao et al. [23] have carried out underwater friction stir welding on spray formed 7055 aluminum alloy. The result shows of hardness of the underwater welds are higher than that of normal welds. Figure 10 shows the microhardness profiles of the air and underwater FSW joints.

Fig. 10
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Microhardness profiles of the air and underwater FSW joints [23]

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Huijie and Li [31] carried out underwater friction stir welding of 2219 aluminium alloy. SFSW joints exhibited lower hardness in the WNZ and higher in the TMAZ and HAZ compared with the normal FSW joint. ‟W” and “U” type profile of the layers have obtained in the normal and underwater joint. Lin et al. [32] have studied the microstructure and mechanical properties of friction stir welded AA7055 alloy under air and water cooling. They have identified that the hardness of air and water cooled sample is 134 HV and 142 HV respectively. The maximum hardness value was observed in the water cooled sample due to reduced softening. Figure 11 shows the microhardness distribution of the air and water cooled FSW.

Fig. 11
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Hardness profiles of the air and water cooled sample [32]

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Zhang et al. [33] exhibited submerged friction stir weld on 2219-T6 aluminium alloy at a fixed welding speed of 100 mm/min and different rotational speeds of 600, 800, 1000, 1200 and 1400 rpm respectively. The minimum hardness value was obtained in the SFSW at a rotational speed of too low and too high conditions. Rathinasuriyan and Senthil Kumar [34] have found that the microhardness of SFSW samples is smaller when compared to the normal FSW samples. Bloodworth [16] performed normal and immersed FSW of AA6061-T6 at the spindle speeds from 60 to 3300 rpm and traverse speeds from 0 to 14.8 mm/s. They found an average weld hardness of 73 Hv for normal FSW and 81 Hv for Immersed FSW respectively.

2.4 Corrosion Analysis

Corrosion is defined as the destruction or deterioration of a material because of its reaction with environment. It causes the equipment to fail, because of perforation with only a small percent weight loss of the entire structure [35].

Wang et al. [36] have studied the corrosion behavior of spray formed aluminum alloy welded by normal and underwater friction stir welding process. The intergranular corrosion and exfoliation corrosion tests were carried out for finding the change of corrosion resistance of those joints. Appearance of intergranular corrosion (IGC) samples and measurement for corrosion depths are shown in Fig. 12. Normal joint has suffered intergranular corrosion with corrosion pits. In underwater joint, intergranular corrosion was deeper with some pits and tended into the inner grain. They have identified from the Tafer curves, underwater FSW joint possessed more negative corrosion potential & lower corrosion current density than the base metal and normal FSW joint.

Fig. 12
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Appearance of IGC samples and measurement for corrosion depths a base metal, b normal joint and c underwater FSW joint [36]

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Rathinasuriyan et al. [37] have investigated the quality and corrosion behavior of a submerged friction stir welded sample of AA 6061-T6 alloy using the various process parameters such as rotational speed (rpm), welding speed (mm/min) and water level (mm). A high corrosion rate of 4.63 mm/year was found at 800 rpm, 30 mm/min and water level 75 mm. Clark [38] has studied the microstructure and corrosion resistance of underwater friction stir welded 304 l stainless steel. They have found that higher sensitization in Underwater Friction Stir Welding (UFSW) compared to the ambient FSW.

2.5 Macro and Microstructural Analysis

After the welding process, the sectioned specimen is polished using various grids and etched with Tucker’s reagent by an optical microscopy for macro analysis. In microstructural analysis the cross section of welded sample were polished with a diamond paste, etched with Keller’s reagent and observed on Scanning Electron Microscope (SEM).

The surface finish of the underwater FSW as extremely dark compared to the surface finishes produced by Normal FSW, as shown in Fig. 13. This is most probable due to an oxidation reaction happening between the hot plate surface and the water medium [38].

Fig. 13
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Surface finishes of welded samples [38]

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Chai et al. [39] conducted friction stir and submerged friction stir processing of AZ91 magnesium alloy with a thickness of 6 mm at a rotational speed of 600 rpm and a welding speed of 60 mm/min. The top surface appearance of Normal Friction Stir Processing (NFSP) and Submerged Friction Stir Processing (SFSP) are shown in Fig. 14. It shows the NFSP sample creating excessive flash, especially on the retreating side.

Fig. 14
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Macroscopic features [39]

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Cao et al. [40] investigated the macrographs, microstructure, and mechanical properties of Mg–Nd–Y alloys during normal and submerged friction stir processing. Figure 15 shows the cross-sectional macrographs of the NFSP and SFSP samples, in which onion-ring patterns were observed in the nugget zone. The average width of nugget zone measured at the middle height of the sample was 6.73 mm and 6.59 mm for normal and submerged FSP, respectively. The processing area was smaller for submerged FSP compare with normal FSP due to the superior cooling effect.

Fig. 15
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Macrographs of FSP-ed samples: a NFSP, b SFSP [40]

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Heirani et al. [41] carried out the friction stir welding of Al5083 alloy under various processing parameters conditions and the effect of welding environments like water and air on the microstructure and the mechanical properties were examined. The results illustrated water environment leading to a high cooling rate so that the grains did not have adequate time to grow. Sabariet al. [42] investigated underwater and normal FSW on armour grade AA2519-T87 aluminium alloy. They have identified the width of thermo mechanically affected zone of normal FSW is higher when compared to the underwater FSW. Macrostructure of normal and underwater FSW is shown in Fig. 16.

Fig. 16
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Macrostructure a FSW joint, b UWFSW joint [42]

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Defects are formed in UFSW due to insufficient material flow, inadequate heat generation and improper selection of welding parameters. urrow defects, Tunnel defect, Groove defect and voids are generally observed in UFSW (see Fig. 17).

Fig. 17
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a–d Welding defects in UFSW

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Lin et al. [43] have studied the microstructure of friction stir welded 7055 aluminium alloy under air and water cooling. They have reported that the water cooled samples produce finer grains than the air cooled samples. Figure 18 shows the optical images for the air and water cooled FSW samples.

Fig. 18
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OM images of the cross sections a air-cooled and b water-cooled [43]

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Tan et al. [44] carried out underwater friction stir welding process for AA 3003 aluminium alloy with different initial microstructures. Microstructural evolution and mechanical properties of the underwater friction stir joints were investigated. Wang et al. [6] did friction stir welding of the 5083Al-H19 aluminium alloy at tool rotational speeds of 800 and 1200 rpm, with/without additional water cooling. They have obtained the equiaxed grains of 800 nm in the nugget zone at the lowest heat input condition due to the additional water cooling. Wang et al. [45] performed underwater friction stir welding at a rotational speed of 950 rpm and a travel speed 60 mm/min. X-ray diffractometer, scanning electron microscope and transmission electron microscope were used for examining the microstructural of the FSW joint. They obtained an ultrafine-grained microstructure with the grain size of 0.7 μm in the weld nugget zone using water cooling. Wang et al. [46] did successful welding of Spray forming Al–Zn–Mg–Cu alloy using underwater Friction stir welding. EDS, XRD, DSC, and TEM were used for studying the evolution of the strengthening phases. The results of Energy Dispersive Spectroscopy (EDS) analysis for normal and underwater joints are shown in Table 3.The authors have found element content of Mg, Zn and Cu are higher in underwater FSW joint when compared to normal FSW joint.

Table 3 EDS analysis for normal and underwater joints [46]
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3 Modeling and Optimization of SFSW Process

3.1 Modeling

Process modelling and optimization are two important issues in any manufacturing process. Earlier researchers have been used various modelling techniques in FSW process like numerical, analytical and mathematical model to find out the theoretical value of torque, power, heat transfer, material flow, residual thermal stresses, etc. [47,48,49]. Research articles of the limited number have been found for modelling and optimization of submerged friction stir welding techniques. The various approaches of Lagrangian–Eulerian, Finite Element Analysis (FEA), Response Surface Methodology (RSM) approaches are discussed in detail below.

Ghetiya and Patel [50] carried out immersed FSW an AA2014 alloy based on the Box–Behnken design by varying the tool shoulder diameter, welding speed, and rotational speed. Response Surface Methodology (RSM) was used for developing a mathematical model for the tensile strength. ANOVA was used for finding the most influential process parameter. Among the three parameters, welding speed was found having a predominant effect on the tensile strength of the welded sample. Hajinezhad and Azizi [51] investigated the finite element modelling of friction stir welding in air and underwater for controlling the thermal cycles on Al6061-T6 alloys. Three-dimensional modelling with ANSYS was used for this purpose. The model results were then checked by experimental data, and a reasonable agreement was observed as shown in Fig. 19.

Fig. 19
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Modeling and actual results of temperature histories

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Fratini et al. [9] focused on the effects of water cooling treatment during friction stir welding of AA7075-T6 alloy butt joints in experimental and theoretical work. The lagrangian–eulerian formulation was selected for the numerical simulations for their work. The experimental results were also highlighted and simulated through finite element model. Yazdipour et al. [52] investigated the effect of cooling rate on the grain size in the stirred zone of the friction stir processed Al5083. They have developed a new microstructural evolution model for predicting the grain size in the stir zone. The simulation result showed the achievement of a superior mechanical property by rapid cooling rate through refining the microstructure. Rathinasuriyan and Senthil Kumar [53] have conducted the submerged friction stir welding experiments based on three factors, three levels, and the Box–Benhan design with the full replication technique. The effect of SFSW process parameters was analysed, using response surface methodology (RSM), and a mathematical model was also developed on the weld of AA6061-T6. Zhang et al. [54] have developed a mathematical model to optimize the welding parameters for the maximum tensile strength of the 2219-T6 aluminium alloy. The effect of process parameters was also studied. Box–Behnken experimental design was selected for finding out the relationship between the response and the variables.

3.2 Optimization

A new way of thinking is essential for meeting global competition in the market to change and improve the existing technology and to develop products at an economical price. It means not only investment in procuring new machinery but also effective control over the process parameters involved in any manufacturing process. These process parameters must be measured, controlled and optimized to obtain the desired and valuable outputs. The typical process parameters for a submerged friction stir welding process which affect the desired output for a welding process are rotational speed welding speed, axial load tool geometry etc.

Santhanam et al. [55] used L9 orthogonal array (Taguchi method) for conducting trials and SFSW experiment on the AA6063-O alloy. The optimum process parameters identified using the TOPSIS approach, were the tapered pin profile, the rotational speed of 1200 rpm and a welding speed of 120 mm/min. The percentage of contribution of SFSW process parameters was calculated using ANOVA. It was found that the tool pin profile, welding speed, and rotational speed contribute to 44%, 33% and 20% respectively. Lokesh et al. [30] took up the process parameters such as rotational speed, welding speed and tool pin profiles (cylindrical, threaded and tapered) for friction stir welding the AA 6063 alloy under the submerged condition. They concluded that the optimum parameter combinations such as tapered pin profile, the rotational speed of 1200 rpm and a welding speed of 120 mm/min provide higher hardness based on the signal to noise ratio analysis. Santhanam et al. [56] have investigated the submerged FSW of AA 6063-O alloy, and optimized the process parameter using grey relational analysis. The optimum parameters are rotational speed of 1200 rpm, welding speed of 120 mm/min and the tool pin profile of taper respectively. Gao et al. [57] did the joining of a polyethylene sheet using submerged friction stir welding by varying rotational and welding speeds. They have investigated the effects of process parameters on microstructure and mechanical property. They found the optimum welding parameters by experimentally such as rotational speed of 1800 rpm and the tool welding speed of 45 mm/min respectively. Zhang et al. [54] have obtained the maximum tensile strength of 360 MPa through underwater FSW. This value was 6% higher than the maximum tensile strength of normal FSW. The optimum welding parameters for maximize the tensile strength are presented Table 4.

Table 4 Optimum welding parameters [54]
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Dumpala et al. [58] conducted friction stir welding process on Al 6061 & Al 6063 alloy at both room temperature and in under water temperature at different rotational speeds of 1200 & 1400 rpm and feed rates of 22 & 44 mm/min respectively. Mechanical properties such as tensile, hardness and Charpy impact were evaluated. Variations in parameters were made for obtaining the optimum values, these results of optimum speed and feed rate for welding was 1200 rpm and 22 mm/min.

4 Current Status and Development of SFSW

Submerged friction stir welding process is initially applied in aluminum alloys and its successful execution into other non-ferrous materials such as magnesium, copper, titanium, and steels. Now a day, SFSW was carried out on similar and dissimilar materials with varying plate thickness, and different environments of air, water, gas respectively.

Baillie et al. [59] welded S275 steel plates using friction stir welding in the air and underwater environments. They have assessed the heat input, distortion tensile strength, microhardness, fatigue and Charpy impact toughness performance in each welding process. They observed the heat input of 3.01, 4.29 kJ/mm for air and underwater welded plate respectively. Gao et al. [60] demonstrated the submerged friction stir lap welding of acrylonitrile butadiene styrene sheets and high-density polyethylene. They have studied the effects of different rotational speeds, plunge depths, and welding speeds on the tensile strength of joints. They have identified the maximal tensile strength of 14.7 MPa at a rotational speed of 2500 rpm, welding speed of 30 mm/min, and plunge depth of 0.2 mm. Jalili et al. [61] have investigated the temperature distribution and final distortion of Al-5052 plates in normal FSW and cooling with CO2 gas during friction stir welding. They have identified the maximum peak temperature with and without cooling, as 200 and 303 K. They concluded that the cooling method resulted in 34% reduction of the peak temperature. Zhang et al. [62] employed underwater friction stir welding for fabricating lap weld in aluminium/copper (Al/Cu) metallic couple and via comparison with the weld obtained under same process parameters classical friction stir welding. The K-type thermocouple was utilized for measuring weld temperature. They have found decreased peak temperature and shorter thermal cycle time in underwater friction stir welding. Stewart [63] carried out underwater FSW in 3.5% NaCl water on HY-80 steel. The main purpose of this study was to determine the feasibility of underwater FSW of high-strength low carbon steels. The authors have concluded that the underwater FSW can be welded in seawater condition of steel due to obtaining the defect-free sample. Zhao et al. [23] carried out underwater friction stir welding of 6013 Al alloy and AZ31 Mg alloy for reducing heat input and to control the formation of brittle intermetallic compounds. The intermetallic compounds of the water welded specimen were less than the air welded specimen. It means that the formation of brittle intermetallic compounds is lesser in UFSW when compared to conventional FSW. Mehta et al. [64] carried out the heating and cooling assisted-friction stir welding for dissimilar Cu and Al materials. Tensile strength and hardness of the welded specimen were evaluated after the welding process. Significant improvement in tensile strength and hardness were found for weld of cooling assisted FSW compared to the heating assisted FSW. Bloodworth [16] adopted submerged FSW to improve the strength of the normal FSW joint of aluminium alloy. This literature survey shows the development of gas bubbles developed during the submerged friction stir welding process. These would create voids in the nugget and thermo-mechanically affected zone as a result of the formation of porosity. These defects depend very much on the water depth, i.e. water level from the surface of the welded sample. Rathinasuriyan and Senthil Kumar [34] were taken the water head as one of the submerged friction stir welding process parameters in his work. They have found that the tensile strength is increased by 3.22% on SFSW at the water head of 20 mm, when compared to SFSW at the water head of 30 mm. Raju et al. [65] have conducted the friction stir welding of thin sheets of aluminium alloy and steel in air and water medium. They have reported that underwater FSW better weld strength when compared to the normal FSW. Submerged friction stir welding is truly an innovative and novel technique of friction stir welding. Fleming et al. [66] have conducted underwater friction stir welding for demonstrating the release of hydrogen gas. Triangular funnel was used to collect the hydrogen gas generated during the welding process. The funnels move with the tool and remain at a constant distance from the tool during the experiment. The funnels were coupled with hydrogen fuel cell through the tubing. The chemical formula for the reaction is stated to be most likely

$$ 2 {\text{Al }} + {\text{ 3H}}_{ 2} {\text{O}} \to {\text{Al}}_{ 2} {\text{O}}_{ 3} + {\text{ 3H}}_{ 2} $$

In this present era, few researchers have done the investigations in the field of submerged friction stir welding. Submerged friction stir welding is required for improve the weld properties, for further research must attempting in this field [67, 68].

5 Conclusion

  • For the past two decades, researchers have overcome many difficulties and made considerable advancement in submerged friction stir welding of heat treatable aluminium alloys. The results hold high hopes of useful applications. SFSW have the capability of joining of heat treatable aluminium alloy with excellent weld strength through control of the welding thermal cycles.

  • The advantages of submerged friction stir welding process that have transformed ferrous alloy may not witness quick progress, but many applications will get the benefit over the coming years.

  • The increasing awareness of environmental issues has also placed pressure on weight reduction in many applications. So that the FSW process offers many potential benefits for joining of materials. In future, FSW can be replaced by SFSW with more advantages.

6 Scope of the Future Work

This review points out to the potential of submerged friction stir welding which is one of the effective techniques, for enhancing the mechanical and microstructural properties. For further research can be focused on the followings:

  • Welding of dissimilar materials in submerged friction stir welding process is limited. Investigate of the detailed study of dissimilar materials in SFSW, has to the take-up.

  • In general, there is a lack of study relating to establish an analytical model for heat generation by submerged friction stir welding process.

  • Investigation of the corrosion resistance, impact strength and fatigue of submerged friction welding is necessary.