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Advanced Composites and Hybrid Materials

, Volume 2, Issue 3, pp 389–406 | Cite as

Green welding for various similar and dissimilar metals and alloys: present status and future possibilities

  • Rajesh Kumar BhushanEmail author
  • Deepak Sharma
Review
  • 331 Downloads

Abstract

Energy saving has become a priority for welding industry. This is due to the recent increase in energy demand and constraints in carbon emissions. Increasing environmental demands from governmental and customers strain the importance of reducing the environmental pollution while welding. Therefore, the minimum energy-oriented green welding process is must. Friction stir welding (FSW) is considered to be the most significant development in metal joining and is a “green” technology due to its energy efficiency, environment friendliness, and versatility. As compared to the conventional welding methods, FSW consumes considerably less energy. No shield gas or flux is used, thereby making the process environmentally friendly. No harmful gases are produced in FSW. Harmful gases adversely affect the surroundings. This creates health problems to persons carrying out welding and people living in nearby areas. Hence, there is a need to develop and use green welding techniques. An attempt has been made to study the present status and explore future possibilities in the field of FSW, so as to achieve the objectives of saving the electrical energy and protect the environment from pollution.

Graphical abstract

Friction stir welding process

Keywords

Green welding Friction stir welding Environment friendly Pollution free 

1 Introduction

The energy prices have seen a sharp increase and brought the cost for energy onto the priority of industries using welding. In addition, energy consumption during welding is responsible for a major part of the environmental burden. Therefore, the minimum energy-focused green welding process is necessary. Hence, there is an urgent need to develop welding processes which are pollution free and requires minimum electrical energy. Current techniques for welding in use are as follows: (i) shielded metal arc welding (SMAW). This is often used in the construction of steel structures and in industrial fabrication to weld iron and steel. (ii) Submerged arc welding (SAW). This is the most efficient fusion welding process in plate and structural work such as shipbuilding, bridge building, and pressure vessel fabrication. (iii) Flux-cored arc welding (FCAW). In ship and bridge building, metal FCW is habitually used for fillet welding of painted steel plate. (iv) Gas metal arc welding (GMAW/MIG). It uses a shielding gas along the wire electrode, which heats up the two metals to be joined. This method requires a constant voltage and direct-current power source, and is the most common industrial welding process. (v) Gas tungsten arc gas welding (GTAW/TIG). Welding together thick sections of stainless steel or non-ferrous metals is the most common use for this method. It is also an arc welding process that uses a tungsten electrode to produce the weld. This process is much more time consuming than the other three and much more complex too. (vi) Friction and resistance welding processes. The great forces needing to be inputted into the workpieces are disadvantageous and necessitate very sturdy clamping jigs. Friction stir welding is currently utilized in rail vehicle construction, and for the leak-tight welding of covers into hydraulic control parts. (vii) Beam welding technology. Electron beam welding advantages are an extremely high power density and thus a low heat input even in the case of the thickest welds of up to 250 mm and above. The availability of large vacuum chambers, up to 630 m3, permits the welding even of large-volume machine components. Laser beam welding has the great advantage that it can be used outside a vacuum, but in general it is only suitable for material thicknesses below 25 mm. The process is suitable for the manufacture of tailored blanks of different steel qualities and material thicknesses.

Friction stir welding (FSW) is a new process, which is less polluting and energy efficient as compared to other welding processes. The existing welding processes, i.e., gas welding, arc welding, which are normally used for welding of metals and alloys produce harmful gases like CO and CO2 and consume lots of electrical energy. If the use of these processes continues, this will increase pollution all over the world. Electrical energy is generated from fossil fuels. The increasing use of fossil fuels is badly affecting the environment. Literature for review has been selected from journals of ScienceDirect, Springer, and Taylor and Francis, based on future needs of welding of metals and alloys for aircraft and space applications. Research related to FSW published in the last 18 years has been mainly preferred for review. Literature related to similar and dissimilar metals/alloys tool parameters, welding parameters and mechanical properties, and microstructural characterization has been reviewed.

Currently, friction stir welding is in use for welding Al alloy like 1100, 2024, 2219, 5052, 5754, 5083, 6005, 6351, 6060, 6061, 6082, 7050, and 7075; Mg alloys like AM20, AZ80A, AZ31B-H24, AZ31B, NZ20K, AZ61A, and ZM21; type of steel like AISI 304 stainless steel, stainless steel AISI 304; and dissimilar alloys such as AA6061 and SS400 low carbon steel, AA6061 and copper, and AA5052 H32 and HSLA steel IRS M42. But defects have been noticed in FSW. An angular misalignment defect due to differential cooling or inadequate clamping during welding was seen to have a significant effect on the results of the tests conducted under axial (tensile) loading. Zigzag line is found as a common defect in FSW joint. Post weld heat treatment of FSW joints in needed for certain applications like automotive and aerospace. Tool wear and adhesion of the workpiece on the tool surface are serious problems. Further research is needed to find solution for these issues.

The aim of this paper is to look for the welding processes for joining metal and alloys which do not create any pollution and consume least energy.

2 FSW process

FSW is a solid-state process; this means that the parts are joined without reaching melting point. In FSW, a cylindrical shouldered tool with a profiled pin is rotated and plunged into the joint area between two parts of sheet or plate material. The parts have to be firmly clamped to prevent the joint faces from being moved apart. Frictional heat between the wear-resistant welding tool and the workpieces causes the workpieces to soften without reaching melting point, thus allowing the tool to traverse along the weld line. The plasticized material, which is transferred to the trailing edge of the tool pin, is forged through intimate contact with the tool shoulder and pin profile. On cooling, a solid phase bond is created between the parts.

Figure 1 shows the schematic diagram of the friction stir welding. FSW can be applied to different types of joints like butt joints, lap joints, T butt joints, fillet joints, and corner joints. Various weld zones produced in the FSW as (a) nugget zone, (b) thermo-mechanically affected zone (TMAZ), (c) heat-affected zone (HAZ), and (d) unaffected base metal. Frictional heating during FSW and intense plastic deformation result in generation of a recrystallized fine-grained microstructure within stirred zone. This region is normally referred to as nugget zone (or weld nugget) or dynamically recrystallized zone (DXZ). The TMAZ experiences both temperature and deformation during FSW. TMAZ undergoes plastic deformation but recrystallization does not occur in this zone due to insufficient deformation strain. However, dissolution of few precipitates is observed in the TMAZ. After the TMAZ, there is a heat-affected zone. This zone experiences a thermal cycle, but does not undergo any plastic deformation [1, 2]. Since there is no macroscopic melting, the controls needed in fusion welding to avoid phenomena such as solidification and liquation cracking, porosity, and loss of volatile solutes can be avoided. These recognized advantages of solid-state joining have led efforts to use FSW for a wide range of alloy [3]. The process of FSW has been widely used in the aerospace, shipbuilding, automobile industries, and other commercial applications. This is because of the number of its advantages over the conventional welding techniques. Some of advantages are very low distortion, no fumes, porosity, or spatter, no consumables (no filler wire), no special surface treatment, and no shielding gas requirements. FSW joints have improved mechanical properties and are free from porosity or blowholes as compared to conventionally welded materials. However, along with these advantages, there are a few disadvantages. These are also to be mentioned. At the end of the welding process, an exit hole is left behind when the tool is withdrawn. This hole is undesired in most of the applications. Hole can be removed by providing either an offset in the path for continuous trajectory, or by continuing into a dummy plate for non-continuous paths, or by cutting the undesired part with the hole. Rigid clamping of the plates to be welded is necessary for this process. Requirement of rigid clamping causes limitation in the applicability of this process to weld jobs with specific geometries [4].
Fig. 1

Schematic diagram of FSW [1]

3 Green technology

The requirement of a joining process has evolved over the years. Present need is fast, efficient process which is capable of producing defect-free joints, multipurpose, and environment friendly. FSW is the most significant process for metal joining and is a “green” technology due to its energy efficiency, environment friendliness, and versatility. As compared to the conventional welding methods, FSW consumes considerably less electrical energy. No cover gas or flux is used, which makes the process environmentally friendly [1]. Each year, lots of money is wasted because welding technology does not demonstrate the true cost of a weld and the enormous cost of “over welding.” True costs include the labor and materials, but what about the electrical energy consumed and the adverse effect on environmental. Green welding is making all other green technologies possible: solar, wind, waste management, and recycling. Machines and metal are needed for industrial activities. We cannot afford to have advances in one without advances in the other. Clara Musa et al. [5] compared the hot pressing (HP) and spark plasma sintering (SPS) techniques. Comparison was done in terms of end-product characteristics, process conditions, and energy efficiency. Comparison of the total energies consumed during each SPS or HP experiment showed that the use of the SPS technology allows for an energy saving in the order of 90–95%. Hence, a modern optimized technology, like SPS, is quite superior for energy saving. Xiong et al. [6] worked for decreasing material wastes and energy consumptions in GMAW-based additive manufacturing process. The experiments show that GMAW-based layered deposition process, which combines the intelligent detection and control system, is capable of saving materials and energies more than 10% compared with the open-loop control system.

Above reviews clearly show that there is very high scope (up to 90%) for electrical energy saving and reduction in environmental pollution if FSW is used for welding instead of other techniques.

4 Similar and dissimilar metals/alloys

4.1 Perspectives and trends

The increasing applications of new competent materials in the field of engineering have created a need for development of new joining processes. These processes shall be effective, flexible, multipurpose, and capable of overcoming the problems associated with conventional joining processes. FSW fulfills all these requirements. It is capable of joining numerous similar and dissimilar combinations of metals and alloys, which is impossible to weld using conventional fusion welding processes. In literature the uses of FSW are from the joining of similar combinations of 1×××, 2×××, 5×××, 6×××, 7×××, and 8××× series aluminum alloys, copper, magnesium alloys, titanium alloys, and steels to numerous dissimilar combinations of aluminum alloys, aluminum alloys to copper, aluminum alloys to steels, aluminum alloys to magnesium alloys, aluminum alloys to titanium alloys, and so on. Some of these research works are as follows.

Jata et al. [7] in their research on FSW of 6.35-mm-thick plates of AA7050-T7451 suggested that the FS welding transformed the initial flat grains in the parent metal to fine equiaxed grains in the dynamically recrystallized zone. The width of the weld nugget zone was nearly equal to the diameter of the FS welding pin. Liu et al. [8] furthered the work by friction stir welding of 300 mm × 80 mm × 5 mm plates of AC4A cast Al alloy and observed that original casting voids were completely removed. Because of this, the mechanical properties of the weld nugget were superior to those of the base material. Woo et al. [9] successfully welded the plates of sizes 306 mm × 306 mm × 6.5 mm of AA6061 Al alloy with FSW process. They studied the microstructure and natural aging kinetics. The microstructural softening and the natural aging were detected in the DXZ and TMAZ. The softening in the heat-affected zone was because of the dissolution/growth of the precipitates and was not followed by the natural aging. In another study, Liu and Ma [10] had worked on FSW of 6061Al-T651 alloy plates of dimensions 400 mm × 75 mm × 6 mm. They concluded that the change in the shoulder and pin diameters and the rotation rate changed the position and inclination of the low hardness zones (LHZs), but that did not affect the hardness values along the LHZs.

FSW of Al 6061 alloy in “O” condition and “T6” condition with the dimensions of the plates as 300 mm × 50 mm × 5 mm was carried out in [11]. It was found that the strength increased as compared to parent metal, in the welding of alloy in “O” condition, at the cost of ductility. But the ductility is equal to or better than the parent metal in case of welding in “T6” condition. Chen and Lin [12] carried out the dissimilar FSW of AA6061 Al alloy plates (6 mm × 40 mm × 160 mm) with SS400 low carbon steel plates (6 mm × 40 mm × 180 mm). It was noticed that the rotation of the tool stirred and mixed the material. Translation of the tool moved the stirred material from front to the back of the tool pin. Elangovan et al. [13] studied the FSW of AA2219. They observed that the area of the weld zone decreased with decreasing the tool rotation speed. This affected the temperature distribution in the weld zone. Therefore lower heat input condition resulted in lack of stirring and yielded lower joint strength.

Year 2009 witnessed growing interest in FSW. Lakshminarayanan et al. [14] displayed comparison among GTAW, GMAW, and FSW of AA6061 plates of dimensions 300 mm × 150 mm × 6 mm. They concluded that the FSW joints showed highest strength and hardness among the three processes. The formation of fine, equiaxed grains and uniformly distributed, very fine strengthening precipitates in the weld region was the reason for superior tensile properties of FSW joints as compared to GTAW and GMAW joints. Moreira et al. [15] furthered the work by FSW butt joints of AA6061-T6 with 6082-T6 and found that the FSW AA6082-T6 material revealed lower yield and ultimate stresses, and the dissimilar joints exhibited intermediate properties. Furthermore, the fracture occurred near the weld edge line, corresponding to the transition between the TMAZ and the HAZ.

Kim et al. [16] carried out dissimilar FSW of AA5052 and A5J32 Al alloy plates of sizes 200 mm × 100 mm × 1.5 mm and 200 mm × 100 mm × 1.6 mm. They observed that where the AA5052 was fixed on the retreating side, few welding defects existed in the joint under certain welding conditions with a lower heat input. But, these defects did not affect the tensile properties. In continued research, Kwon et al. [17] in their study of FSW of AA5052 plates of size 160 mm × 30 mm × 2 mm revealed that the onion ring structure had appeared in the stir zone (SZ). The onion ring structure region became wider, as the tool rotation speed was increased. Zadpoor et al. [18] in their study of fracture mechanism for dissimilar FSW of AA2024 and AA7075 sheets of varied thicknesses found that in most cases the fracture surface showed the features of both ductile and cleavage fractures. The fracture location was noticed to be within the HAZ, while in some cases it was coincident with the WN or TMAZ. Muthukrishnan and Marimuthu [19], Babu et al. [20], and Adamowski and Szkodo [21] had worked on FSW of 6082-T6 and observed dark zones at the center of the weld. This indicated that the metal was affected by the welding process temperature. In weld nugget, the grains were refined and equiaxed as a result of dynamic recrystallization. Working on same lines, Patil and Soman [22] studied the effects of different welding speeds and tool pin profiles on the weld quality of AA6082-O plates of size 160 mm × 60 mm × 5 mm. It was discovered that pin profile has an important effect on the joint structure and the mechanical properties.

The focus of research was on microstructure of weld joint in year 2010. Kim et al. [23] had carried out a comparative study on GMAW and FSW of A6005-T5 extruded railway train panels of size 1000 mm × 180 mm × 4.8 mm. They noticed that the mechanical characteristics of FSW-welded regions were superior to those of GMAW-welded regions. FSW made it possible to reduce the thickness of Al materials, which reduced the weight of the railway trains. Hussain et al. [24] assessed the parameters of FSW for AA6351 plates of size 120 mm × 60 mm × 6 mm. Recommendation was that the alignment of workpiece welding line and tool should be considered as an important factor to obtain high tensile strength. They concluded that the FSW shall be used to avoid environment-related problems. Al-Badrawy [25] carried out a study on FSW of AA7075-T6. It was concluded that the microstructure of the FSW consisted of very fine and equiaxed grains. The fracture behavior of the broken specimens showed limited ductility. Further, the fracture surface was dominated with very fine dimples and breach crack was formed in the weld zone. This crack propagated in the direction normal to the stress direction. Shukla and Shah [26] studied the FSW of 150 mm × 60 mm × 2 mm sized sheets of AA6061 and copper. They observed that the imperfect joints were obtained when Al was in advancing side and Cu on retreating side. When plates were reversed so that Cu was in advancing side and Al on retreating side, sound joints were obtained.

In the later work, Bisadi et al. [27] examined the FSW of AA 5083 lap joints of plates of size 150 mm × 100 mm × 2.5 mm. They noticed that almost all of the fractures during the tensile tests took place at the heat-affected zone of the advancing side of the weld. Joshi et al. [28] researched on FSW of AA 5083 alloy to investigate the various methods of defect detection. They concluded that the ultrasonic technique to detect defect is better than radiography technique. Parida et al. [29] studied the mechanical properties and microstructure of the FS-welded commercial grade aluminum alloy of 6 mm thickness. They observed that the weld zone is having more grain refinement as compared to the HAZ zone. Cerri et al. [30] accomplished the FS welding of very thin sheets 0.8 mm in thickness for combinations of 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3. Complete weldability of these two alloys at the solid state was observed. No defects or porosity were detected neither in the 2024T3 nor in the 6082T6 joints, or in the dissimilar joints. The ductility of the thin joints was poor. Ductility was found to be independent of temperature and strain rate.

A research was carried out by Jean et al. [31] on joint properties of AA6061 plates of size 140 mm × 70 mm × 4 mm. It was revealed that the joints produced by the tool with tilting angle of spin were defect free. These joints were smoother than those produced by a tool having no tilt angle. In a recent work, Kumar and Balachandar [32] examined the metallurgical properties of FS-welded plates of 6063-O and 6063-O inserted with SiC powder of size 200 mm × 150 mm × 6 mm. Their results showed that the weld nugget zone consisted of relatively better distribution of SiC particles due to stirring process in FSW. Kumar et al. [33] furthered the work by studying the process forces and heat input with varying parameters in FSW of AA5083 alloy (250 mm × 50 mm × 12 mm). They stated that the axial thrust (Z-force) and heat input were most affected by tool diameter, rotational speed, and welding speed.

Emami and Saeid [34] focused on the effects of rotational and welding speeds on the microstructure and hardness of joints in FSW of brass. Welds were achieved under low heat input conditions at rotational and welding speeds of 400–800 r/min and 100–300 mm/min. Increasing the welding speed and/or decreasing the rotational speed caused the grain size of the stir zone to decrease. Due to this, average hardness of stir zone improved.

Farzadi and Matwiss [35] performed FSW on the AA7075-T6 sheets with the thicknesses of 5 mm using a 25-kW FSW machine. If no discontinuity exists in the stir zone, for the transverse tensile specimens, necking phenomenon occurs at the two sides of the stir zone, in accordance with, places with a minimum hardness. Due to this, fracture happens randomly on the advancing or retreating side independent of welding parameters.

Lin et al. [36] investigated the influences of FSW and post-weld heat treatment (PWHT) on the microstructures and tensile properties of Al–Cu–Li alloy. The strength loss of as-welded joint can be compensated by PWHT. The PRA meaningfully improved the yield strength (27%) and tensile strength (20%) of FSW joint without the ductility being excessively reduced. However, the RSRA led to slight increase of tensile strength and severe reduction of ductility.

4.2 Critical discussion—similar and dissimilar metals/alloys

The research work conducted so far indicates that the successful FSW of various aluminum alloys, titanium alloys, and steels has been carried out. Also the FSW of dissimilar metal pairs like AA 6061 with AA6082, AA6061 with copper, AA2024 with AA6082, AA6061 with SS400 low carbon steel, and AA5052 with A5J32 has been achieved. However from commercial point of view, there is a need to further carry out the research for FSW of similar and dissimilar pairs like various oxides dispersion strengthened (ODS) alloys, e.g., MA754, MA758, MA956, MA957, MA6000, etc. for advanced burner reactors (ABR), and 600, 700, and 800 series of Inconel alloys for pressurized water reactors.

5 Welding parameters and mechanical properties

5.1 Perspectives and trends

FSW is capable of operating at wide range of process parameters. The accomplishment of sound, defect-free and efficient FSW joints depends upon the selection of optimum welding parameters. The desired mechanical properties of joints are most influenced by the FSW process parameters like tool rotation speed, tool traverse speed, axial force, and tool tilt angle. Some of the typical studies about the effects of welding parameters on mechanical properties are compiled and reviewed here.

In an early work, Jata et al. [7] studied the FS welded AA7050-T7451 joints. Top side of the weld had consistently showed the lower hardness numbers. This was due to the fact that the top side has been in full contact with the FS welding tool shoulder and, thus, experienced direct heat from the rapidly rotating tool shoulder. The back side, on the other hand, has been in direct contact with a back plate that acted as a heat sink. The tensile test results indicated that the tensile strength of the weld joints was found to be less than that of base metal. In the continued work, Liu et al. [8] tested the tensile strength of AC4A cast Al alloy FSW joints at room temperature at a crosshead speed of 1 mm/min. CNC was used for FSW. The tensile strength, proof stress, and elongation were found to be (150 MPa, 85 MPa, and 1.6%) and (251 MPa, 96 MPa, and 14.4%) for base metal and the weld nugget of the joint, respectively, which indicated that the tensile properties of the weld nugget have been much higher than those of the base material. Elangovan et al. [13] in their investigation noticed that the tensile strength of AA2219 Al alloy joints increased with an increase in welding speed and axial force.

Chen and Lin [12] studied the C-notch Charpy impact values for dissimilar metals joints of AA6061 and SS400 low carbon steel. They found that higher impact strength was obtained at the lower transverse speed and rotation speed. Specimen at rotation speed 800 rpm had given the worst (almost zero) impact strength, whereas at transverse speed of 0.9 mm/s, it gave the best impact strength. In a further work, on FSW of AA6082-T6, Babu et al. [20] and Adamowski and Szkodo [21] found that the tensile strength of FS welds was directly proportional to the travel/welding speed. Tensile strength of FS welds was found to be lower than that of the parent metal. The softening was mostly apparent in the HAZ on the advancing side of the welds. The hardness of both the HAZ and the weld nugget was found to be lower than that of base metal (BM), respectively, by 15–20 and 7–10%. Liu and Ma [10] investigated the mechanical properties of friction-stir-welded 6061-T651 Al alloy and found that the hardness along the LHZs was similar (65 to 70 HV) for all three welds. Hardness increased from 57 to 62, to 69, to 75 HV with increasing the welding speed. The tensile strength increased with increasing the welding speed from 200 to 600 mm/min for the same tool geometry and tool rotation rate. Increasing the tool rotation rate from 900 to 1400 rpm did not change the strength of the welds for the same tool dimension and a constant welding speed of 200 mm/min.

D’Urso et al. [37] carried out FSW of AA6060 alloy. They used three rotational speeds of 1000, 1500, and 2000 rpm and feed rates of 150, 300, 600, 900, and 1200 mm/min. For the basic material (ultimate tensile strength (UTS) = 215 MPa, strain = 0.061 in correspondence of the UTS), the best condition in terms of UTS (186 MPa) was obtained using the threaded tool at a rotational speed of 1500 rpm and a feed rate of 150 mm/min. Kwon et al. [17]carried out FSW of AA5052 plates. They found that at 500, 1000, and 2000 rpm, the tensile strength of the FS welded plates was reported to be similar to that of the base metal (about 204 MPa) and the elongation was found to be lower than that of the base metal (about 22%). But, maximum elongation of about 21% was seen at 1000 rpm. The Vickers microhardness tests showed that at a rotational speed of 500 rpm, the average hardness increased remarkably. It reached a level of about 33% greater than that of the base metal.

Kim et al. [16] compared the GMAW and FSW of AA6005-T5. They found that the tensile strength of the FSW-welded regions was 7.5% higher as compared with the GMAW-welded regions. The hardness of the FSW-welded regions was higher than that of the GMAW-welded regions, but lower than that of the base metal. The fatigue strength was found to decrease with increase in both rotational speed and welding speed. In a further work, Muthukrishnan and Marimuthu [19] while welding AA6082-T6 noticed that at constant rotational speed, mechanical resistance of joints increased with increasing the travel speed. The average hardness of the nugget zone was found to be remarkably lower than the hardness of the base alloy. The smallest hardness was observable in the heat-affected zone. Patil and Soman [22] had conducted a study on welding of AA6082-O aluminum. They concluded that the joints fabricated by taper screw threaded pin tool at the welding speed of 70 mm/min had demonstrated more ultimate and yield strength in comparison with other welding speed of 60 and 75 mm/min. The tensile strength for joints fabricated by tri-flute pin was more at the welding speed of 60 mm/min in comparison with other welding speeds of 70 and 85 mm/min. The UTS of the joints fabricated by taper screw threaded pin and tri-flute pin was 92.30 and 58.97% of the base metal UTS, respectively. In a continued work, Ahmed et al. [24] indicated that increase in rotational speed had resulted in increase of tensile strength. The maximum tensile strength of 172 MPa was found at 1350 rpm (for 115 mm/min feed). A higher range of rotational speed with lower weld speed is best suited to achieve maximum tensile strength for IS 64430 AA6351. The Vickers hardness of the weldment was 85 HV as of 93.5 HV of parent metal.

Shivaraj et al. [38] furthered the work by examining the FS welding of 2024-T4 and 7075-T6. It was observed that the joint strength for 2024-T4 and 7075-T6 reached 60 and 75% of the UTS of the parent material, respectively. It was concluded that a local material softening had occurred in the weld because of the thermal action of the welding process: in particular, a localized softening in the HAZ. Al-Badrawy and El-Nasr [25] inspected the AA7075-T6 joints. They discovered that the maximum elongation with UTS of 445 MPa was obtained at weld speed of 840 rpm and feed rate of 122 mm/min; however, the same was gradually decreased with increasing feed rate for all the working conditions. The hardness profile indicated a decrease in the hardness compared with that of the base alloy. Shukla and Shah [26] carried out FSW of AA6061 alloy to copper. They found that the increasing rotational speed resulted in lower tensile strength. This was mainly due to increased amount of intermetallic compounds formed at the Al/Cu interface. The hardness in the stir zone has been slightly higher than the base metals. The average hardness of AA6061 (on retreating side) and copper (on advancing side) has been 110 and 105 HV, respectively. The tensile strength of parent material of AA6061 and copper was 313.2 and 272 MPa, respectively. Tensile strength of Cu/Al joint at 1000 rpm, welding speed 20 mm/min, and at 1250 rpm, welding speed 20 mm/min has been observed as 62.2 and 56.3 MPa, respectively.

Parida et al. [29] carried out FSW of AA6082-T6 at tool rotation speed of 1810 rpm and at a feed rate of 460 mm/min. The tensile property of the weld improved considerably as compared to base metal. They further disclosed that the displacement before failure for welded plate was found to be 13 mm as compared to 7.5 mm in case of base material. This indicated the increase in ductility. The results obtained from the microhardness test carried out at a load of 0.98 N showed that the hardness values of the weld zone or the nugget is lower than the base materials. In a further work, Cerri et al. [30] investigated 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin FSW joints. The 2024T3-2024T3 weld has the highest strength and ductility among the three joints, whereas the 6082T6-6082T6 welds showed the least strength. The dissimilar joint has showed a medium strength but has the lower ductility. 6082T6-6082T6 joint at 473 K showed an increase in strength with the strain rate. But the elongation remained almost constant.

Shah and Tosunoglu [39] in FSW observed that down force, welding speed, the rotation speed of the welding tool, and tilting angle must be controlled. It was also proposed that the down force (Z-axis) is the most important control feature which enables the generation of frictional heat to soften the material and provides robust control during higher welding speeds. Jean et al. [31] continued the work to achieve an optimization of tensile properties of the FS-welded AA6061. They found out the butt joint strength at room temperature. Distribution of tensile strength of butt joints was found to be varied from average 120 MPa to average 140 MPa. The tensile strength of substrate matrix was 125 MPa. In case of 0° spin tilted angle, the tensile strength was found 160 MPa, whereas at and above 2.5° of spin and 500 rpm, the highest strength was obtained. Joint strength was 105 MPa at 1800 rpm indicating the same below the substrate matrix. The maximum joint strength was 201 MPa. This is 1.68 times than that of substrate matrix. Further, it was observed that in case of tilting angle of 5°, the tensile strength of 200 MPa was recorded when welding speed was kept below 100 mm/s. Kumar and Balachandar [32] discovered that the weld nugget in the 6063-O alloy FSW joints possesses higher hardness values as compared to other zones of weld, i.e., unaffected material, HAZ, and TMAZ. The addition of silicon carbide powder in the plates resulted in higher microhardness values at weld nugget zone as compared to that of without SiC powder. In a recent work, Kumar et al. [33] investigated the AA5083 FSW joints and observed that the increment in rotational speed resulted in decreased Z-force (Fz). The Fz was found to be least affected by welding speed and reported to increase from 16.88 to 17.94 kN when welding speed was increased from 80 to 120 mm/min. Awang et al. [40] butt-welded AA6061-T6 sheets in three different passes: single pass, double pass on the same side, and double pass on different sides. Double passes experienced smaller wormhole defect as compared to single pass. Microstructure investigation shows slight difference in the grain size among all passes. All regions show lower hardness values as compared to the parent material in all combinations of passes. Rakesh et al. [41] studied the fatigue behavior of friction stir-welded single-sided AA6061-T651 butt joints under bending and tension load. This was done through small-scale specimen testing and fracture mechanics analysis. The effect of angular misalignment on the fatigue life of welded joints was also studied. Results showed that the fatigue performance of FSW joints is better than typical arc-welded joints and top side of the weld has better fatigue performance as compared to the root or bottom side of the weld under bending load. Even a slight misalignment of the order of 0.5° can generate significant secondary stress. Small fatigue crack initiation and growth behavior for AA7075 FSW joints were investigated with the replica technique in [42]. Results showed that with the decrease of stress ratio to − 0.3, the small cracks grow until the cycle numbers are about 30 to 45% of the fatigue life. The phase of small crack initiation and propagation accounts for about 50 to 80% of the fatigue life. Summary of mechanical properties of FSW joints for various metals/alloys are given in Table 1.
Table 1

Summary of mechanical properties of FSW joints for various metals/alloys

Workpiece material

Tensile strength of base metal

Tensile strength after FSW

Hardness of base metal

Hardness after FSW

Fatigue strength of base metal

Fatigue strength after FSW

Impact value of FSW joint

Reference

AA6061

334 MPa

295 MPa

105 VHN

85 VHN

   

Chen (2008)

AA6061

125 MPa

Average 120 MPa to average 140 MPa

     

Jean et al. (2012)

AW6082-T6

≥ 310 MPa

150–240 MPa

94 HV

84 HV

   

Parida et al. (2011)

AA6082-O

130 MPa

56–120 MPa

     

Patil and Soman (2010)

AA6351

250 MPa

160–172 MPa

93.5 VHN

85–90.2 VHN

   

Ahmed et al. (2010)

AA6060

215 MPa

186 MPa

     

D’Urso et al. (2009)

A6005-T5

269.73 MPa

201.74 MPa

100–104 HV

62–66 HV

110 N/mm2

90 N/mm2

 

Kim et al. (2010)

A5052-O

204 MPa

200 MPa

55 HV

60–75 HV

   

Kwon et al. (2009)

AA2219-T87

402 MPa

150–240 MPa

     

Elangovan et al.(2008)

AA2014-T4

440 MPa

264 MPa

145 HV

140 HV

   

Cerri et al. (2011)

AA7075-T6

510–538 MPa

445 MPa

170 HV

145–155 HV

   

Al-Badrawy and Nasr (2010)

AA7050-T7541

555 MPa

429 MPa

54 HRA

43–50 HRA

   

Jata et al. (2000)

AC4A cast aluminum alloy

150 MPa

150–251 MPa

     

Liu et al. (2004)

AISI 304 stainless steel

515 MPa

246–484 MPa

     

Meran and Canyurt (2010)

6061-T6 and 6082-T6 dissimilar alloys

306.3 MPa-6061 and 276.2 MPa-6082

140.5 MPa

112 HV-6061 and 108 HV-6082

75–78 HV

   

Moreira et al. (2009)

AA6061 aluminum alloy and SS400 low carbon steel

      

50.997 J

Chen and Lin (2008)

AA6061 and copper

313.2 MPa-AA6061 and 272 MPa-copper

62.2 MPa

110 HV-AA6061 and 105 HV-copper

120 HV

   

Shukla and Shah (2010)

Cu–(33.8 wt%) Zn alloy plate

  

105 HV

123 HV

  

Microstructure in the SZ consisted of fine equiaxed grains, and their sizes decreased with increasing welding speed and/or decreasing rotational speed

Emami and Saeid 2015

AA 6061-T6

  

61 HV

Double passes 63.1 HV

   

Awang et al. 2015

AA7075-T6

522 MPa

374.5 MPa

117 HV

129 HV

   

Farzadi, 2017

Al–Cu–Li alloy

362 MPa

321 MPa

92 HV

116 HV

  

Improved the yield strength (27%)

Lin 2018

AA6061-T651

     

Secondary stresses of ± 38 MPa

 

Rakesh 2019

AA7075

     

Small cracks begin to grow when the cycle number are about 30 to 45% of the fatigue life

 

Guoqin 2019

5.2 Critical discussion—mechanical properties

The research conducted so far includes the assessment of various mechanical properties of FSW joints of various combinations of metal/alloys with respect to a wide range of welding parameters. The results of various tests performed to evaluate the mechanical properties show the success of the FSW process. The mechanical properties possessed by FSW joints have proved to be much better than that of other fusion welding processes like GTAW and GMAW. The fatigue behavior of butt joints was studied under bending and tension load through small-scale specimen testing and fracture mechanics analysis. Fatigue performance of the FSW joints is better than typical arc-welded joints. The review shows that tensile strength and hardness are the most studied mechanical properties for all combinations of materials. Initiation and propagation of small cracks during fatigue loading are mainly affected by strengthening particles, grain boundaries, rough pits, and hardness distribution. Small cracks begin to grow when the cycle numbers are about less than 20% of the fatigue life at the stress ratio of 0.1. From mechanical characterization point of view, there is a need to further carry out the research to study the fatigue strength and bending strength of FSW joints. Further, the studies are needed to evaluate the wear characteristics of the FSW joints.

6 Microstructural characterization

6.1 Perspectives and trends

FSW joints are characterized by their unique microstructure. The frictional heat results in plasticization of the material in the vicinity of the FSW tool. The rotation and traverse movement of FSW tool deform and dynamically recrystalize the grains of the work material and finally result in fine equiaxed grained microstructure in the nugget zone or weld nugget at the center of the weld. Progressing in the direction of base metal from the center of the weld, the next zone is thermo-mechanically affected zone which is characterized by partially deformed and recrystallized grains, due to the action of the tool shoulder. Next to the TMAZ, another zone called heat-affected zone has appeared. The grains here are affected only by the heat and no deformation or recrystallization is visible in this region.

In an early work, Jata et al. [7] in their study on 7050-T7451 welds observed that the width of the weld nugget zone was nearly equal to the diameter of the FS welding pin. Optical microscopy and TEM examination of the weld nugget region indicated that the FS welding process transformed the initial millimeter-sized pancake-shaped grains in the parent material to fine 1 to 5 μm dynamically recrystallized grains. The TEM investigation of the weld nugget region showed some grains to contain a high dislocation density. Some of the dislocations were also pinned by Al7Cu2Fe inclusions and second-phase particles. Higher-magnification TEM micrographs displayed that the strengthening phases were still present in the HAZ region. Reynolds et al. [43] in the study of 304L stainless steel joints used optical microscope. They found the noteworthy difference in the grain size in the nuggets of two welds completed at two different rotational speeds. The grain size in the 500-rpm weld was approximately 13 μm while the grain size in the 300 rpm weld was 7.6 μm. In addition to the grain refinement in the weld nuggets, narrow bands were observed in both welds. The 300-rpm weld demonstrated higher strength than the 500-rpm weld. Continuing on same lines, Mishra and Ma [1] have acknowledged three different microstructural zones in friction stir weld. These are (i) nugget region, experiencing intense plastic deformation and high-temperature exposure and characterized by fine and equiaxed recrystallized grains; (ii) thermo-mechanically affected region, experiencing medium temperature and deformation and characterized by deformed; and (iii) un-recrystallized grains, and heat-affected region, experiencing only temperature and characterized by precipitate coarsening. Khaled [2] suggested a generalized grouping of the weld zone consisting of the nugget, TMAZ, and the region under the shoulder, and it was proposed to classify all of them as different regions of the SZ.

Woo et al. [9] studied the TEM bright-field images of FS welds of AA6061-T6 at 200 μm magnification. They reported that the microstructure in the BM showed a high density of the needle-shaped precipitates and the coarse spherical precipitates. The dissolution of the needle-shaped precipitates and the advent of new fine pin-dot-like precipitates was discovered in the dynamically recrystallized zone of Case-I. The dissolution of the needle-shaped precipitates and the development of thick plates were noted at the interface of the coarse spherical particles in the HAZ (root) of Case-II. Further, the TMAZ (face) of Case-II also showed dissolution of the needle-shape precipitates and re-precipitation of particles. Kumbhar and Bhanumurthy [11] used digital low magnification and BSE images at a magnification of 100 μm. This was done to study the microstructure of FS welds of AA6061. Three regions were identified as parent material TMAZ and NZ. It was suggested that the parent material consisted of elongated grains of size 100 μm. The nugget consisted of fine equiaxed grains in order less in magnitude to that of the parent material ranging between 15 and 20 μm. The TMAZ consisted of grains having similar size as that of the parent material. Liu and Ma [10] furthered the work by examining the FS joints of AA6061-T651 using optical micrographs and TEM images. This was done at a magnification of 100 μm in the three subzones of the HAZ. They found that the HAZ I was described by a reasonably high density of needle-shaped precipitates and few rod-shaped precipitates. HAZ II contained only a low density of rod-shaped precipitates. In HAZ III, both types of the precipitates vanished fundamentally. With increasing the welding speed from 200 to 600 mm/min, the rod-shaped precipitates have a tendency to reduce.

Kim et al. [16] probed the microstructure of dissimilar FS welds of AA5052 and A5J32 alloys at a magnification of 50 μm. They examined that in the case where the AA5052 was fixed on the retreating side, excessive agglomeration occurred in a narrow region of the welded zone. This was attributable to restricted flow. In continued work, Lakshminarayanan et al. [14] inspected FSW of AA6061 using optical micrographs with 50 μm magnification. It discovered that the base metal contained coarse and elongated grains. Uniformly distributed very fine precipitates were also found in base metal. The fusion zone of GMAW and GTAW joints contained dendritic structure. But, the weld region of FSW joint contained very fine equiaxed grains. The fractured surface of tensile specimens of welded joints was examined using SEM at 50 μm magnification. It was found that the fractographs invariably consisted of dimples, which were an indication that most of the tensile specimens displayed a ductile fracture under the action of tensile loading. An intergranular fracture feature was observed in GMAW joints. Coarse dimples were seen in GTAW joints and fine dimples were seen in FSW joints. Continuing with same work, Moreira et al. [15] in their experiment on FSW butt joints of AA6061-T6 with AA6082-T6 investigated the micrographs at 20 and 200 μm magnification. They revealed that the retreating side presented more flash. It was found to be the location of the lower values of hardness. The mixture of the two different alloys can be identified in the nugget zone. AA6061-T6 was characterized by larger grains and the AA6082-T6 by precipitates. Kwon et al. [17] studied the AA5052 joints using optical micrographs. They discovered that the area of the stir zone has increased with the tool rotation speed and onion ring structure was formed in the tool rotation speeds ranging from 500 to 2000 rpm. In addition, the increase of the tool rotation speed led to the increase in the region of the onion ring structure. The base metal contained non-uniform, non-equiaxed large grains with size over 100 μm. In contrast to BM, the SZ consisted of smaller and equiaxed grains. Further, the grain size increased in spite of the increase in the tool rotation speed. This led to the increase in the strain and strain rate.

Muthukrishnan and Marimuthu [19], Babu et al. [20], and Adamowski and Szkodo [21] studied the microstructure of various weld zones in the Al-6082-T6 FS welds at 100 and 200 μm magnifications. They concluded that in weld nugget the grains were refined, equiaxed. This was the result of dynamic recrystallization. The microstructure in heat-affected zone was similar to that of BM. The grains were slightly overgrown as a result of the exposure to welding heat. In the continued work, Shivaraj et al. [38] in their experiment on joints of AA2024-T4 and AA7075-T6 found that the welded zone has fine recrystallized grains and FSW modified the microstructure. This removed the casting defects. Al-Badrawy and Nasr [25] executed the FS welding on 7075-T6 alloy. They used the optical and scanning electron microscopy. They found that a nearly V-shaped weld zone was clearly apparent in the optical microstructure at 400 and 600 μm magnifications. In the SEM micrograph at 200 μm magnification, grain boundaries could be identified in the weld nugget. The average grain size was measured to be 4 μm. Large MgZn2 precipitates were uniformly distributed in the weld nugget. Further, Meran and Canyurt [44] probed the microstructure of the welding zone of AISI 304 stainless steel joints and macrographs of the HAZ by SEM. The investigation discovered that the average grain size in the SZ was between 3 and 7 μm, which was smaller than that in the BM. The dark and narrow bands detected in the microstructure of the transition zone were supposed to be the Cr2O3 oxide layers. These over the surface of stainless steels may have ruptured during FSW.

Cerri et al. [30] examined the 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin FSW joints. They discovered that high density of large rod-shaped particles containing Al, Mn, and Cu (probably Al2O Mn3Cu3) was visible in the 2024T3 similar joints. These were distributed homogeneously in the matrix and aligned along rolling direction in different grains. During TEM analysis of 6082T6-6082T6 similar joints at magnifications of 0.5, 1, and 200 μm, homogeneously distributed large spherical insoluble particles of Al (Fe, Mn, Cu, Cr) Si type were detected in the matrix. Further, Davies et al. [4] in the study of FS welding of Ti-6Al-4V alloy discovered that the high peak weld temperatures resulted in a supertransus zone (STZ). STZ contained predominantly fully lamellar microstructure due to the phase transformation on cooling. During SEM analysis, the fine-grained microstructure did not occur at the surface of supertransus welds made using conventional tools. Also Bisadi et al. [27] inspected the optical microstructures of the weld zone in FS welds of AA5083 at magnifications of 20 and 50 μm. They pointed out that at tool rotational speed of 825 rpm and welding speed of 32 mm/min the heat-affected zone had the highest grain size, whereas in the stir zone the finest grain size was seen. Kumar and Balachandar [32] accomplished FS welding on 6063-O and 6063-O inserted with SiC powder. Optical microscopy and scanning electron microscopy were used for microstructural analysis. They observed that the voids were generated in the weld nugget due to the difference in material transport. The micrographs discovered the grain refinement in the HAZ of advancing side and flow of material from retreating side to advancing side. This led to deposition of SiC powder at the advancing side.

Ramachandran et al. [45] successfully joined AA5052 H32 and HSLA steel IRS M42 using FSW process in a butt joint. The effect of tool rotational speed and tool tilt angle on the mechanical and metallographic characteristics of the joint was investigated. In EDS, it was seen that intermetallic compound layer is formed at the joint interface. The highest joint strength of about 94% of the UTS the base Al alloy was obtained at a tool rotational speed of 450 rev/min, a welding speed of 45 mm/min, an axial load of 7 kN, and a tool tilt angle of 1.5°. The distributed steel flakes and intermetallic compounds strengthened the SZ at the Al alloy side. The highest UTS (196 MPa, about 94% of the UTS of base Al alloy) was obtained at a tool rotational speed of 450 rev/min.

Kalemba and Dymek [46] investigated the microstructure and properties of FSW Al alloys. FSW joints show a change in the microstructure of their cross sections. The weld has a relatively homogeneous microstructure characterized by small uniaxial grains. This microstructure was the result of substantial plastic deformation combined with the physical flow of material around the mandrel (stirring) and a significant increase in temperature. Andreas et al. [47] used positron annihilation spectroscopy (PAS) with a scanning positron beam to detect crystallographic defects and changes of the microstructure of FSW. The observations made through PAS were employed to explain the trends in the mechanical properties. The varying UTS could be clearly attributed to dissolution and re-precipitation phenomena, which occur during and after welding. Summary of microstructural characteristics of FSW joints of various metals/alloys are given in Table 2.
Table 2

Summary of microstructural characteristics of FSW joints of various metals/alloys

Workpiece material

Base metal/alloy

Heat-affected zone

Thermo-mechanically affected zone

Weld nugget/nugget zone/DXZ/SZ

References

Al 6061

Elongated grains with size 100 μm

 

Same grain size as that in base metal with a bent morphology

Fine equiaxed grains with size from 15 to 20 μm

Kumbhar and Bhanumurthy (2008)

AA6061

Coarse and elongated grains with uniformly distributed very fine precipitates

  

Very fine equiaxed grains

Lakshminarayanan et al. (2009)

6061-T6

High density of the needle-shaped precipitates and the coarse spherical precipitates

Dissolution of the needle-shaped precipitates and a growth of thick plates

Dissolution of the needle-shape precipitates and re-precipitation to the pin-dot-like precipitates similar to DXZ

Fine pin-dot-like precipitates

Moreira et al. (2009)

6061Al-T651

Elongated grains several hundred micron long and 80 μm wide

High density of needle-shaped precipitates and few rod-shaped precipitates

 

Fine and equiaxed grains with average grain size of 12 μm

Liu and Ma (2008)

AW6082-T6

Elongated grains

Grains similar to that of base metal but slightly overgrown

Deformed, elongated and rotated grains due to applied strain

Refined and equiaxed grains with average diameter 1–5 μm

Muthukrishnan and Marimuthu (2010); Babu et al. (2008); Adamowski and Szkodo (2007)

Al7075-T6

Grains with large size

  

Fine equiaxed grains with size 4 μm, also uniformly distributed large MgZn2 particles are present

Al-Badrawy and Nasr (2010)

Al 7050-T 7451

1 mm pancake-shaped grains with 1–10 μm sub-grains or strengthening precipitates within these grains

Grain size similar to base alloy but strengthening precipitates coarsened by a factor of 5

 

1–5 μm sized fine equiaxed grains pinned with Al3Zr, Al7Cu2Fe inclusions and second-phase particles

Jata et al. (2000)

Stainless steel AISI 304

Coarse grains

Grain size about 20 μm, i.e., half of that in base metal

 

Fine equiaxed grains with size 3–7 μm

Meran and Canyurt (2010)

Stainless steel 304 L

Equiaxed grains of size 15 μm with some annealing twins

  

Equiaxed grains of sizes 13 μm in 500 rpm welds and 7.6 μm in 300 rpm welds

Reynolds et al. (2003)

Al Alloy AA5052 H32 and HSLA steel IRS M42

AA5052 60 ± 4 HV HSLA steel 234 ± 5 HV highest UTS (196 MPa, about 94% of the UTS of base Al alloy)

Intermetallic compound layer is formed at the joint interface

Non-uniform variation in hardness

Non-uniform variation in hardness

Ramachandran et al. (2015)

Aluminum alloys

Homogeneous microstructure 200 HV

120–140 HV

 

Onion rings structure

Kalemba and Dymek (2016)

AW-2219

Materials characterization by Doppler-broadening spectroscopy

   

Andreas (2019)

6.2 Critical discussion—microstructural characterization

The microstructural characteristics largely affect the mechanical properties of FSW joints. PAS with a scanning positron beam to detect crystallographic defects and changes of the microstructure of FSW was used. Numerous studies mentioned in the literature reveal that the FSW results in grain refinement due to the deformation and dynamic recrystallization. Three extensive microstructural zones in the FSW joints have been mentioned, i.e., NZ, TMAZ, and HZ. Nugget zone has been called differently by different authors, e.g., stir zone, STZ, and dynamically recrystallized zone. The nugget zone of the FSW joint has been characterized by the finest grains in contrast to the grain in TMAZ, HAZ, and BM. The nugget zone of 6061 Al alloy joints noticed a grain refinement from several hundred micron to 12 μm, whereas in Al7050 joints, 1 mm pancake-shaped grains have been refined to a size of 1–5 μm. Presence of Al3Zr and Al7Cu2Fe inclusions was clearly visible. Coarse grains of austenitic stainless steel have also been refined to a size as small as 3–7 μm.

7 Correlation between microstructure and mechanical properties

Feng et al. [48] evaluated microstructural evolution and strain hardening behavior of a friction stir welded (FSWed) high-strength AA7075Al-T651. The nugget zone consists of fine and equiaxed recrystallized grains with a low dislocation density and free of original precipitates. But containing uniformly distributed dispersoids was there in nugget zone. The strength, joint efficiency, and ductility of the FSWed joints increased with increasing welding speed. A joint efficiency of 91% was achieved at a welding speed of 400 mm/min and rotational rate of 800 r/min. But the ductility remained basically the same as that of the base metal. While the ductility decreased with increasing strain rate, the yield strength and UST were nearly independent of the strain rate. This indicated the absence of strain rate sensitivity in both base metal and welded joints of the AA7075Al-T651. Izabela Kalemba and Stanisław [46] carried out a detailed study into the microstructure and mechanical properties of FSW welds of Al–Zn–Mg–Cu alloys. Tests of joints created at various tool rotation speeds had shown that joints of suitable quality, in terms of microstructure and properties, can be obtained for a relatively wide range of process parameters. The tool rotation speeds applied during the welding process did not have a significant influence on the quality of the welds. Mohammad et al. [49] investigated the effect of welding parameters on microstructure and tensile strength of FSW joints. The effects of pin profile, tool rotational speed, and welding speed on the mechanical and microstructural properties of 5-mm 7075-T6 sheet FSW joints were investigated. The maximum UTS belongs to the threaded conical pin. This is attributed to a finer grain size generated in the weld nugget zone. Also it can be said that when the tool rotational speed varies from 800 to 1600 rpm, the tensile strength increases, while with the tool rotational speed further increasing to 2000 rpm, the UTS decreases. Increasing the tool rotational speed and the welding speed leads to the increase in hardness in WNZ. It is related to the formation of very fine recrystallized microstructure.

Velichko et al. [50] developed a technology for single-pass FSW of 11 and 35-mm-thick plates of Al–Mg–Sc alloys. In welding the 35-mm-thick 1570C alloy, 80 kN axial force of the tool is sufficient to produce defect-free welded joints. The welded joints obtained under the optimum welding conditions were free from macro defects. The strength of the welded joint equals 98% of the strength of the parent metal, which is higher than the strength of fusion-welded joints. It is concluded that the FSW of thick plates of Al–Mg–Sc alloy can be used efficiently in practice. Farzadi et al. [35] examined microstructural evolution and mechanical properties of FSW AA7075-T6. A significant grain refinement and dissolution of η′ precipitates in the stir zone were found. But chromium-bearing dispersoids remained nearly unchanged. Hardness profiles of the welds were explained by precipitate distributions. The formation of Guinier-Preston zones in the stir zone and some parts of the heat-affected zone during post-weld natural aging resulted in increase of the hardness. In transverse tensile specimens, fracture occurred in a location with the minimum hardness at either advancing or retreating side randomly. Zhou et al. [51] designed tool system for dual-rotation friction stir welding. AA6061-T6 was welded using the self-designed tool system. Fine equiaxed recrystallized grains were found in both weld nugget zone and shoulder affected zone. The microhardness showed a noticeable decline in the weld zone, and the lowest value was located in heat-affected zone. Tensile test results demonstrated that all welded joints fractured at the interface of thermo-mechanically affected zone and heat-affected zone with ductile fracture mode. Table 3 shows the correlation between microstructure and mechanical properties.
Table 3

Correlation between microstructure and mechanical properties

Workpiece material

Microstructure before FSW

Microstructure after FSW

Tensile strength after FSW

Hardness after FSW

Joint efficiency

References

High-strength 7075Al-T651 alloy

Fine and equiaxed recrystallized grains

Nugget zone after friction stir welding was observed to consist of fine and equiaxed recrystallized grains with a relatively low dislocation density and free of original precipitates

Increased with increasing welding speed

Fine equiaxed grains with size from 15 to 20 μm

91%

Feng et al. (2014)

Al–Zn–Mg–Cu alloys

Homogeneous microstructure 200 HV

Onion rings structure

 

120–140 HV

 

Kalemba and Dymek (2016)

7075-T6

No defects such as cracks and voids UTS 488 MPa

Finer grain size generated in the weld nugget zone

370 MPa

180 HV

 

Mohammad et al. (2016)

Al–Mg–Sc alloys

Uniform microstructure

Free from macrodefects. Non-uniform microstructure and mechanical properties

 

86 HV at z = 3 mm, 98 HV at z = 17 mm, and 106 HV at z = 33 mm

98%

Velichko et al. (2016)

AA7075-T6

Elongated grains stir zone consists of fine equiaxed grains

Significant grain refinement but chromium-bearing dispersoids remained nearly unchanged

374.5 MPa

129 HV

 

Farzadi et al. (2017)

AA6061-T6

Fine equiaxed recrystallized grains in both weld nugget zone and shoulder-affected zone

Micro hardness showed a noticeable decline in the weld zone, and the lowest value was located in heat-affected zone

219 MPa at shoulder rotation speed of 400 rpm

  

Zhou et al. (2019)

7.1 Critical discussion—correlation between microstructure and mechanical properties

Use of tool system for dual-rotation FSW showed that the microhardness was greatly affected by shoulder rotation speed. The highest value was observed at BM. The lowest value was observed at HAZ. Hardness increased with the increase of shoulder rotation speed. The maximum tensile strength of 215 MPa was obtained at shoulder rotation speed of 400 rpm. The main particles in the stir zone and heat-affected zone were η precipitates as well as Guinier-Preston zones formed during post-weld natural aging. Hardness profiles of the welds were explained by precipitate distributions. FSW resulted in the reversion and coarsening of η′ precipitates. The formation of Guinier-Preston zones in the stir zone and some parts of the heat-affected zone during post-weld natural aging increased the hardness. Tensile properties of the weld nugget have been much higher than those of the base material. When a tensile load is applied to the joint, the stress and strain concentration takes place in the lower-strength part or region of the joint, and consequently, the joint is fractured in this region. The relationship between evolved microstructure and mechanical properties like tensile strength and hardness is vastly studied. Still there is a need to carry out the research to assess the influence of microstructure on other mechanical properties of FSW joints like fatigue strength, fracture behavior, and impact strength. Further studies are needed to be extended towards the evolution of the microstructure during the FSW of various similar and dissimilar combinations of metal matrix composites and alloys.

8 FSW process optimization

Sundaravel  et al. [52] carried out the optimization of process parameters in FSW of AA 5083. The L9 orthogonal array of Taguchi experimental design was used for optimizing the FSW process parameters. The process parameters considered for optimization were the rotational speed of the tool in revolution per minute, transverse speed in millimeter/minute, and the axial force in kilonewton. The objective was to find the optimum levels of the process parameters at which FSW yields maximum tensile strength and consumes minimum power. Based on the gray relational grade, optimum levels of parameters have been identified, and significant contribution of parameters is determined by ANOVA. The optimum levels of the process parameters were determined and validated by the confirmation run. Chi-Hui  et al. [53] considered four major controllable four-level factors, i.e., the tool rotation speed, transverse speed (feed rate), tool tilt angle with respect to the workpiece surface, and pin tool length. The uncontrollable factors were the UTS and elongation rate. These can be converted to signal-to-noise ratios, the larger the better. A gray relational grade obtained from gray relational analysis was used as the multiple performance characteristic. The resulting optimum process parameters were rotation speed at 1800 rpm, transverse speed at 180 mm/min, tool tilt angle at 1°, and pin tool length at 2.9 mm for the best multiple performance characteristics with minimum cost. Rajakumar and Balasubramanian [54] optimized the FSW parameters (rotational speed, welding speed, axial force, shoulder diameter, pin diameter, and tool hardness) using multi-objective optimization in the RSM. Objective was to obtain the maximum strength and minimum corrosion rate. For AA1100, maximum tensile strength of 105 MPa, hardness value of 67 HV, and minimum corrosion rate of 0.69 10−4 in the stir zone region were observed in FSW joints. Joints were fabricated at optimized parameters of 893 rpm rotational speed, 100 mm/min welding speed, 6.5 kN axial force, shoulder diameter of 14.8 mm, pin diameter of 4.9 mm, and tool material hardness of 45.4 HRc. Vidal and Infante [55] used Taguchi method, to optimize the FSW parameters for improving mechanical behavior of AA2024-T351 joints. The parameters considered were vertically downward forging force, tool travel speed, and probe length. An orthogonal array of L9 (34) was used. ANOVA analyses were carried out to identify the significant factors affecting tensile strength, bending strength, and hardness field. The percentage contribution of each parameter was also determined. As a result of the Taguchi analysis, the probe length is the most significant parameter on tensile strength, and the tool travel speed is the most important parameter affecting both the bending strength and the hardness field. An algebraic model for predicting the best mechanical performance, namely fatigue resistance, was developed. Optimal FSW combination was determined using this model. The results obtained were validated by conducting confirmation tests. Khethier et al. [56] performed friction stir spot welding (FSSW) for dissimilar AA2024-T3 and AA5754-H114 sheets of 2 mm thick at different tool rotational speeds, plunging times, and tool pin profile or geometry. Process parameters were optimized by using Taguchi technique. Data analysis based on the Taguchi method was performed by utilizing the Minitab 17 to estimate the significant factors of the FSSW. Maximum shear force (2860 N) was obtained at best welding process parameters: 800 rpm of rotation speed, 60 s of plunging time, and taper cylindrical pin. Pareto chart of the standardized effects of tensile shear results showed that the pin profile was the most effective parameter, than other welding parameters (rotation speed and plunging time). It was also found that the contribution percentage was 61.5% for pin profile followed by tool rotation speed 20.1% and plunging time 18.4%. Trueba et al. [57] performed process optimization of self-reacting friction stir welding (SR-FSW). SR-FSW, also called bobbin-tool friction stir welding (BT-FSW), is a solid-state welding process similar to FSW except that the tool has two opposing shoulders instead of the shoulder and a backing plate found in FSW. The tool configuration results in greater heat input and a symmetrical weld macrostructure. An optimization experiment was performed using a factorial design to evaluate the effect of process parameters on the weld temperature, surface and internal quality, and mechanical properties of self-reacting friction stir welded AA 6061-T6 butt joints. The parameters evaluated were tool rotational speed, traverse speed, and tool plunge force. A correlation between weld temperature, defect formation, and mechanical properties was found. Optimum parameters were determined for the welding of 8-mm-thick 6061-T6 plate. Table 4 shows FSW process optimization.
Table 4

FSW process optimization

Workpiece material

Optimization technique

Experimental design

Process parameters considered for optimization

Responses

Optimum levels of the process parameters/significant parameter

References

Aluminum alloy AA 5083

Multiple responses based on orthogonal array with gray relational analysis

Taguchi experimental design

Rotational speed of the tool in rpm, transverse speed in mm/min, and the axial force in kN

Maximum tensile strength (270 MPa) and consumes minimum power

Rotational speed, transverse speed, and axial force were set at 650 rpm, 115 mm/min, and 9 kN

Vijayan et al. (2010)

AA5083

Multiple performance characteristics with minimum cost

Gray-based Taguchi method

Tool rotation speed, transverse speed (feed rate), tool tilt angle

Workpiece surface and pin tool length

Rotation speed at 1800 rpm, transverse speed at 180 mm/min, tool tilt angle at 1°, and pin tool length at 2.9 mm

Chien et al. (2011)

AA1100

Maximum strength and minimum corrosion rate

Multi-objective optimization in the RSM

Rotational speed, welding speed, axial force, shoulder diameter, pin diameter, and tool hardness

Maximum tensile strength of 105 MPa, hardness value of 67 HV, and minimum corrosion rate of 0.69910_4 in the stir zone region

893 rpm rotational speed, 100 mm/min welding speed, 6.5 kN axial force, shoulder diameter of 14.8 mm, pin diameter of 4.9 mm, and tool material hardness of 45.4 HRc

Rajakumar and Balasubramanian, (2012)

AA2024-T351

Orthogonal array of L9 (34)

Taguchi method

Vertical downward forging force, tool travel speed, and probe length

Tensile strength (global efficiency to tensile strength—GETS), bending strength (global efficiency to bending—GEB), and hardness field

Probe length is the most significant parameter on GETS, and the tool travel speed is the most important parameter affecting both the GEB and the hardness field

Vidal and Infante (2013)

AA2024-T3 and AA5754-H114

(L9 orthogonal array)

Taguchi technique and depending on design of experiment Minitab 17

Tool rotational speeds plunging times and tool pin profile

Maximum shear force was (2860 N)

Best welding process parameters: 800 rpm of rotation speed, 60 s of plunging time and taper cylindrical pin

Pin profile was the most effective parameter than other welding parameters

Contribution percentage was 61.5% for pin profile followed by tool rotation speed 20.1% and plunging time 18.4%

Abbass et al. (2016)

6061-T6

Factorial experiment

Design incorporated three levels of rotational and traverse speeds and four levels of plunge force

Tool rotational speed, traverse speed, and tool plunge force

Ductility and tensile strength

A rotational speed of 450 rpm, traverse speed of 508 mm/min, and a plunge force of 6.01 kN

Luis et al. (2018)

8.1 Critical discussion—FSW process optimization

Gray-based Taguchi method, design of experiment (DOE), and Taguchi method have been used to optimize the FSW parameters. Based on the gray relational grade, optimum levels of parameters have been identified, and significant contribution of parameters is determined by ANOVA. Major controllable factors are the tool rotation speed, transverse speed (feed rate), tool tilt angle, and tool feed force. ANOVA analyses were carried out to identify the significant factors affecting tensile strength, bending strength, and hardness. The objective was to find the optimum levels of the process parameters at which FSW yields maximum strength and consumes minimum power. The optimum levels of the process parameters are determined and validated by the confirmation run.

9 Assessment

The increasing applications of light alloy in automobiles, aerospace industry, and nuclear power plants provide motivation and further scope for research to be carried out in the field of FSW of similar and dissimilar light alloy. Although FSW of light alloys has been attempted, still further investigations are needed towards the attainment of defect-free joints during the FSW of various similar and dissimilar combinations of alloys. The joints are to have sufficient fracture resistance, creep, fatigue, and impact strength.

10 Summary and challenges

From commercial point of view, there is a need to further carry out the research for FSW of similar and dissimilar pairs like various ODS alloys, e.g., MA754, MA758, MA956, MA957, MA6000, etc. for ABR, and 600, 700, and 800 series of Inconel alloys for pressurized water reactors. It was revealed that the use of the SPS technology allows for an energy saving in the order of 90–95%. Although the relationship between evolved microstructure and mechanical properties like tensile strength and hardness is vastly studied, yet there is a need to carry out the research to assess the influence of microstructure on other mechanical properties of FSW joints like fatigue strength, fracture behavior, and impact strength.

Optimum levels of the various input process parameters to achieve the desired values of responses have been summarized. FSW experiments can be conducted at these optimum values. The share of FSW in welding market is limited. The reasons are the high cost of setup, difficulty in using, big size, making it difficult to transport near the point of use, and requirement of trained personnel. The present environmentally polluting techniques are to be discarded to save the environment. Hence, further research is needed to make this green welding technique (FSW) commercially viable and user friendly.

11 Urgent research directions for further investigation

Based on the above review, research is recommended in the following areas:
  1. i.

    To investigate the kinds of misalignment in various configurations of joints that can result from the FSW process. Further, to relate amount of misalignment defect to fatigue performance.

     
  2. ii.

    To develop techniques to detect FSW weld defects.

     
  3. iii.

    To identify the suitable material/process to reduce tool pin wear and adhesion of weld material on tool pin.

     

Notes

Compliance with ethical standards

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Guru Ghasidas Vishwavidyalaya (A Central University)BilaspurIndia
  2. 2.Shri Mata Vaishno Devi UniversityKatraIndia

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