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Microstructure and mechanical characteristics of AA6061-T6 joints produced by friction stir welding, friction stir vibration welding and tungsten inert gas welding: A comparative study

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  • Corresponding author:

    Behrouz Bagheri    E-mail: b.bagheri@aut.ac.ir

  • Received: 20 December 2019Revised: 26 April 2020Accepted: 30 April 2020Available online: 9 May 2020
  • This study compared the microstructure and mechanical characteristics of AA6061-T6 joints produced using friction stir welding (FSW), friction stir vibration welding (FSVW), and tungsten inert gas welding (TIG). FSVW is a modified version of FSW wherein the joining specimens are vibrated normal to the welding line during FSW. The results indicated that the weld region grains for FSVW and FSW were equiaxed and were smaller than the grains for TIG. In addition, the weld region grains for FSVW were finer compared with those for FSW. Results also showed that the strength, hardness, and toughness values of the joints produced by FSVW were higher than those of the other joints produced by FSW and TIG. The vibration during FSW enhanced dynamic recrystallization, which led to the development of finer grains. The weld efficiency of FSVW was approximately 81%, whereas those of FSW and TIG were approximately 74% and 67%, respectively.
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Microstructure and mechanical characteristics of AA6061-T6 joints produced by friction stir welding, friction stir vibration welding and tungsten inert gas welding: A comparative study

  • Corresponding author:

    Behrouz Bagheri    E-mail: b.bagheri@aut.ac.ir

  • 1. Department of Mining and Metallurgy, Amirkabir University of Technology, Tehran, Iran
  • 2. Faculty of Engineering, University of Kashan, Ravandi Blvd., Kashan, Iran
  • 3. Department of Materials Science and Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran

Abstract: This study compared the microstructure and mechanical characteristics of AA6061-T6 joints produced using friction stir welding (FSW), friction stir vibration welding (FSVW), and tungsten inert gas welding (TIG). FSVW is a modified version of FSW wherein the joining specimens are vibrated normal to the welding line during FSW. The results indicated that the weld region grains for FSVW and FSW were equiaxed and were smaller than the grains for TIG. In addition, the weld region grains for FSVW were finer compared with those for FSW. Results also showed that the strength, hardness, and toughness values of the joints produced by FSVW were higher than those of the other joints produced by FSW and TIG. The vibration during FSW enhanced dynamic recrystallization, which led to the development of finer grains. The weld efficiency of FSVW was approximately 81%, whereas those of FSW and TIG were approximately 74% and 67%, respectively.

    • AA6061-T6 is applied universally in the aerospace, automotive, and shipbuilding industries [1]. However, its welding is difficult because of high thermal conductivity, high hydrogen solubility, high oxygen reactivity, and so on. Tungsten inert gas welding (TIG) has been widely used to join aluminum alloys. The arc welding (such as TIG) of aluminum alloys produces some problems, such as porosity and cracking [2]. These problems develop in the liquid phase during welding.

      A solid-state welding process was invented by The Welding Institute in 1991 to overcome the difficulties [3]. A non-consumable tool is fed into a butt joint between two clamped workpieces and then traversed along the joint line after a short dwell time. Due to contact, heat is generated and leads to metal softening in the region near to the tool. The tool mechanically intermixes the two pieces of metal around the weld line and forces the plasticized material to move from the leading face of the tool to the rear face. This phenomenon causes severe plastic deformation of the material. As the tool is traversed along the joint line, a joint develops between the adjoining specimens [45].

      The liquid phase does not form during friction stir welding (FSW), thereby preventing porosity and cracking [6]. However, the formation of some defects in the stir zone of friction stir (FS)-welded specimens decreases the joint strength [78]. Different trials have been carried out to improve the microstructure and mechanical properties of FS-welded specimens and to reduce the forces on the FSW tool.

      Santos et al. [9] presented a variant of FSW to minimize the root defects and improve the strength. Their concept was based on delivering a high-intensity current between the lower tip of the tool probe and the backplate during FSW. They found that electrical energy generated heat by Joule effect and increased material visco-plasticity, thereby minimizing the formation of root defects. In their concept, the electrical current flowed through the tool and the tool was one of the electrodes. Liu et al. [10] developed an electrically assisted FSW (EAFSW) in which a local electrical current field moved with the FSW tool and the tool was not one of the electrodes. They found that their welding method decreased the axial welding force and enhanced the joint quality. Terje et al. [11] used entitled induction-assisted FSW (IAFSW) in which an electromagnetic field was applied around the weld to induce an electric current inside the material and to heat the material. They applied IAFSW to join Al 6082 T5 specimens, and found that the forces on the tool decreased by about 50%.

      Gabriel [12] developed laser-assisted FSW (LAFSW) wherein a laser power was used to preheat the joining specimens at a localized area ahead of the tool. Subsequently, the specimens were joined in the same way as in the conventional FSW process. Campanelli et al. [13] applied LAFSW to join 5754H111 aluminum alloy plates with a thickness of 6 mm. They achieved satisfactory welds and concluded that tool wear and clamping force were reduced in LAFSW compared with FSW.

      Blaha and Langenecker [14] studied the effects of ultrasonic vibration (UV) on the plasticity of metals during FSW and called this process “UAFSW” [15]. UAFSW provides better superposition of UV with the plastic deformation of FSW, reduces the welding loads, and improves the formation, microstructure, and mechanical properties of Al-alloy welds. Liu and Lu [16] studied the effect of UAFSW on the welding force and weld quality. Ma et al. [17] analyzed the effect of ultrasonic power on the mechanical properties of welds between AA6061 alloy plates and found that the hardness and strength of welds increased with increasing ultrasonic power. Amini and Amiri [18] employed UAFSW to join AA6061-T6 plates. Ultrasonic vibrations were applied to the tool in pin direction and normal to the welding direction. They found that the temperature increased and the welding force decreased. During UAFSW, ultrasonic vibrations with a frequency of 20–50 kHz and amplitude of 10–50 μm were applied on the tool or workpieces [19]. The welding force and tool wear decreased and the weld quality improved with the application of UAFSW [18]. Ultrasonic softening is the primary function of the superimposed ultrasonic vibration in FSW. FSW with superimposed ultrasonic vibration can increase the material flow velocity, enlarge the flow region, decrease the viscosity, and enlarge the iso-viscosity region near the tool compared with the conventional FSW. Liu and Wu [20] indicated that the ultrasonic vibration could improve the plastic fluidity of the material around the welding tool pin because an additional deformation force and thermal action were superimposed. Superimposing ultrasonic vibration in FSW can minimize or eliminate the welding defects in the conventional FSW under certain welding conditions. Padhy et al. [21] investigated the effect of vibration on recrystallization fractions in the stirred zone of Al 6061-T6 friction stir welds and found that the superposition of the static load of FSW on residual ultrasonic softening induced subgrain formation.

      Despite these advantages, some utilities are required for the implementation of UAFSW, namely, the ultrasonic generator, piezoelectric converter, the amplifier, and the ultrasonic horn [22]. These utilities are expensive, and assembling these components and working with them require experience and expertise. In addition, ultrasonic vibration does not affect the temperature profile or mechanical properties during FSW. Shi et al. [23] illustrated that the temperature difference in UAFSW and FSW was less than 20 K and that the thermal effect of ultrasonic vibration was relatively small. Rahmi and Abbasi [24] presented a simple modified version of FSW called “friction stir vibration welding (FSVW)” to join AA5052 plates. Their method was inexpensive and simple. FSVW is based on the vibration of joining specimens through a camshaft mechanism. Vibration amplitude was 0.5 mm and its frequency was in the range of 20–50 Hz. They found that the grain size in the stir zone decreased and the weld quality increased in FSVW. Fouladi and Abbasi [25] applied FSW and FSVW to incorporate SiO2 particles within the microstructure of joints developed between AA5052 plates. They concluded that the particles were distributed homogenously in FSVW.

      The present study analyzed and compared the microstructure and mechanical properties of AA6061 joints produced using TIG, FSW, and FSVW.

    2.   Experimental
    • AA6061-T6 sheet with a thickness of 3 mm was utilized as the base material. The chemical composition of this material was as follows (wt%): Al, 97.30; Zn, 0.20; Mn, 0.10; Si, 0.65, Fe, 0.35; Cu, 0.20; Mg, 0.85; Cr, 0.15; Ti, 0.10; other elements, 0.1. Rectangle specimens with the size of 200 mm × 100 mm were prepared, brushed with a steel wire brush to eliminate the oxide layer, and then washed with acetone to eliminate the oil, dirt, and so on. Each pair of specimens was clamped on the fixture in a butt position along the length.

      For TIG welding, the specimens were welded using V-butt weld geometry. They were fixed in the butt position and joined together. The Invertec V205-T AC/DC, automatic TIG welding machine was used to produce TIG-welded joints. Typical welding conditions are presented in Table 1. An AA4043 alloy rod with a diameter of 2.4 mm was employed as a filler. Argon shielding gas was utilized to prevent the weld pool from pollution by atmospheric gas. The sufficient conditions of welding parameters were established by trial and error. Welding current of 89 A, the voltage of 10 V, and welding speed of 30 mm/min were applied for TIG welding.

      ElectrodeTungsten + 2wt% thoria (ϕ3 mm)
      Tip angle / (°)45
      Arc length / mm3
      Shielding gasAr + 1vol% H2
      Flow rate / (L·min−1)10
      Welding directionHorizontal

      Table 1.  TIG welding conditions

      FSW and FSVW were implemented by applying a milling machine. For FSW, the fixture containing the joining specimens was fixed on the milling machine table. Optimum values of welding parameters were obtained using trial and error. The tansverse speed of the tool was 95 mm/min, the tool rotation speed was 1180 r/min, and the tilt angle was 2º. These values were also applied for FSVW. For FSVW, the fixture containing the joining specimens was installed on a machine prepared vibration during FSW. The machine contained a vibrating plate moving on two rails. The vibrating plate moved based on a camshaft mechanism. The rotating movement of a motor shaft was transformed into a linear and reciprocating movement of the vibrating plate through a camshaft. The vibrating plate moved in a direction normal to the welding line. For this machine, the vibration frequency was controlled by a driver, and the vibration amplitude was 0.5 mm. The schematic of the machine used for FSVW is presented in Fig. 1(a).

      Figure 1.  Views of the (a) machine and (b) tool utilized for friction stir vibration welding (Dimensions are in mm).

      A two-piece tool consisting of a shoulder from heat-treated M2 steel with a hardness of HRC 65 and a pin from carbide tungsten was used for FSW and FSVW. The schematic of the tool is presented in Fig. 1(b). As shown in the figure, the pin height was 2.8 mm. During FSW and FSVW, the pin penetrated into the space between the joining specimens and touched the surface of the fixture because of the downward force (about 3.9 kN) applied by a milling machine.

      Metallography techniques based on ASTM E3-11 [26] were applied to reveal the microstructures of the welded specimens. A solution consisting of HNO3 (5 mL), HCl (3 mL), HF (2 mL), and water (190 mL) was used as the etchant. Mean linear intercept [27] was performed to measure the grain size in the weld region. Microstructures were also analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanical characteristics of the welded specimens were investigated using a uniaxial tensile test based on ASTM E8 [28], Vickers hardness test based on ASTM E92-17 [29], and impact test based on ASTM E23-18 [30]. Specimens for these tests were prepared by wire cut (electro-discharge machining) in accordance with the method presented schematically in Fig. 2. All tests were carried out at room temperature.

      Figure 2.  Schematic of the specimens prepared from each welded specimen for different mechanical tests (All dimensions are in mm).

      Tensile tests were applied by an Instron universal testing machine with a strain rate of 10−5 s−1. A programmable hardness test machine was used to measure the hardness. During hardness tests, the load was 1 N and the dwell time was 10 s. Impact test specimens included a notch with a V configuration. The notch length was 2 mm, the angle was 45°, and the notch root radius was 0.25 mm. For each welding condition, three measurements were obtained for each test.

    3.   Results and discussion
    • The top surface morphologies of the FS-, friction stir vibration (FSV)-, and TIG-welded specimens are shown in Fig. 3. The surfaces of all specimens were free of voids, cracks, and flashes. The FS- and FSV-welded specimens had a smooth surface, whereas the surface of the TIG-welded specimen was rough and contained a weld bead on the face. Onion-ring patterns were observed for the FS- and FSV-welded specimens; however, these patterns were more obvious in the FS-welded specimen than in the FSV-welded specimen. This result can be attributed to the higher temperature during FSVW than FSW when joining the specimens. Barooni et al. [31] found that the vibration in FSVW increased the friction between the tool and the joining specimens and that the temperature was higher in FSVW than in FSW. As a result, the material in the stir zone was softer and the resistance to material flow was lower in FSVW than in FSW.

      Figure 3.  Surfaces of the (a) FS-, (b) FSV-, and (c) TIG-welded specimens.

      The weld pool shape, weld metal composition, cooling rate, and grain growth rate are all interrelated to the heat input in the base metal during welding. Weld thinning has different negative effects, including formation of flashes and reduction of serviceable load and stress concentration. With respect to some fusion welding processes, the height of the welded zone is higher than that of the base metal zone, indicating that the cross-sectional feature of the welded zone is a convex arc shape in geometry. Fig. 4 shows the cross-sectional comparison of the welded zones between FSW joints and fusion-welded joints. In general, the cross-section of the butted welded joint was convex on both sides due to the welding process [3233].

      Figure 4.  Comparison between the fusion-welded and FSW samples (BMZ—Base metal zone; HAZ—Heat affected zone; SZ—Stir zone; TMAZ—Thermo-mechanical affected zone; h1—Height of recession; h2—Height of protrusion).

      The cross-section macrostructures of the welded specimens are presented in Fig. 5. No voids or pores were observed in these sections. Fig. 5 shows that the stir zone of the FSV-welded specimen was larger than that of the FS-welded specimen. This result can be ascribed to the fact that workpiece vibration enhances the stir zone size.

      Figure 5.  Cross-sectional views of the (a) TIG-welded, (b) FSW-, and (c) FSVW-welded specimens (BM—Base metal; WZ—Weld zone).

      The microstructures of the FS- and FSV-welded specimens in the stir zone and the weld zone microstructure of the TIG-welded specimen are shown in Fig. 6. The grains of the TIG-welded specimen were columnar and large, whereas those of the FS- and FSV-welded specimens were equiaxed and fine. In addition, the grains of the FSV-welded specimen were finer than those of the FS-welded specimen.

      Figure 6.  Microstructures of the welded zones for (a) TIG, (b) FSW, and (c) FSVW.

      TIG is a fusion welding process wherein the molten solidification starts with nucleation and proceeds by the growth of nuclei [34]. These nuclei finally take directional growth and grow in the direction opposite to the direction of heat flow; as a result, columnar grains form [34]. However, grain development during FSW and FSVW is due to severe plastic deformation and dynamic recrystallization (DR) [35]. The density of dislocations increases during FSW by severe plastic deformation [35]. High temperature due to severe contact between the tool and joining workpieces allows dislocations to rearrange and form low-angle grain boundaries (LAGBs) within the major grains. As deformation proceeds, the misorientation between sub-grains increases and LAGBs transform to high-angle grain boundaries (HAGBs). Consequently, fine grains develop in the microstructure [3536]. The material deformation in the stir zone was higher in FSVW than in FSW because of the higher vibration in the former. More deformation corresponds to more density of dislocations. Thus, DR was enhanced and grains were finer in FSVW than in FSW. The higher density of dislocations in the stir zone of the FSV-welded specimen compared with the FS-welded specimen is shown in Fig. 7. Fig. 7 represents the TEM images of the stir zones of the FS- and FSV-welded specimens. TEM images also revealed the presence of some particles. SEM–energy-dispersive X-ray spectroscopy (EDS) analyses indicated that they were Mg2Si precipitates.

      Figure 7.  TEM microstructure images of (a) FS- and (b) FSV-welded specimen and (c) SEM–EDS result for the characterization of precipitates.

      Zener–Hollomon parameter (Z) describes the simultaneous effect of temperature (T) and strain rate ($ \dot{\varepsilon } $), which is defined as follows [37]:

      Q and R are the activation energy and the gas constant, respectively. The relation between the Z parameter and the grain size (D) is presented as follows [38]:

      where a and b are constants. On the basis of Eq. (2), the grain size decreases as the Z parameter increases. In fact, the vibration of the joining workpieces and the rotation and transverse motion of the tool increase the material deformation in the stir zone. In plasticity, material deformation is presented by strain. Thus, the strain and strain rate of the material in the stir zone for FSVW were higher than those related to FSW. In this regard, the Z parameter for FSVW was higher than that for FSW. According to Eq. (2), the grain size (D) for FSVW was lower than that for FSW.

      Electron backscattered diffraction (EBSD) is an SEM-based system to distinguish the substructure and crystallographic orientation of a material. EBSD images of the FS- and FSV-welded specimens are presented in Fig. 8 to reveal their microstructures. As shown in Fig. 8, the stir zone grains of the FSV-welded specimen were smaller than those of the FS-welded specimen. This result is in agreement with the observations in Fig. 6.

      Figure 8.  EBSD maps of the welded areas for FS- and FSV-welded specimens (ND—Normal direction; WD—Width direction; TD—Transverse direction).

      Microstructures of the thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) regions are also presented in Fig. 8. The HAZ is a region around the stir zone that is affected by the heat produced during welding, and TMAZ is a transition zone between the stir zone and the HAZ [39]. As shown in Fig. 8, the TMAZ grains of the FSV-welded specimen were smaller than those of the FS-welded specimen, whereas the HAZ grains of the FSV-welded specimen were slightly larger than those of the FS-welded specimen. Grains in TMAZ are partially affected by the heat and mechanical deformation by the tool, and the occurrence of DR is inevitable [40]. Given that the material deformation in the stir zone for FSVW was higher than that for FSW, the TMAZ grains of the FSV-welded specimen were smaller than those of the FS-welded specimen. The larger grains in the HAZ of the FSV-welded specimen can be attributed to the higher temperature of the weld zone during FSVW than during FSW [31].

      However, the lower grain size in the stir zone of the FSV-welded specimen compared with that of the FS-welded specimen indicated that the effect of mechanical deformation and DR on grain refinement is greater than that of temperature.

      Histograms showing the stir zone grain size distribution of the FS- and FSV-welded specimens are presented in Fig. 9. Histograms showing the misorientation angle distribution for grain boundaries in the stir zone of the welded specimens are also presented in Fig. 9. As shown in Figs. 9(a) and 9(c), the grains of the FSV-welded specimen were smaller than those of the FS-welded specimen. As shown in Figs. 9(b) and 9(d), the fraction of grain boundaries with a misorientation angle higher than 15º in the FSV-welded specimen was greater than that in the FS-welded specimen. Grain boundaries with a misorientation angle higher than 15º are enumerated as HAGBs [40]. As mentioned before, the higher deformation of the material in FSVW intensifies the DR and produces higher HAGBs because of the vibration of the workpieces.

      Figure 9.  Histograms showing the grain size distributions and misorientation angle distributions for (a, b) FS- and (c, d) FSV-welded specimens.

    • Table 2 summarizes the influence of welding conditions on the mechanical characteristics, including strength, elongation, formability index, and impact toughness, of the welded specimens. The mechanical characteristics of the FSV-welded specimen were higher than those of the other specimens, and TIG-welded specimen had the weakest ones. These results can be related to the effect of grain size and shape within the microstructure of the welded specimens. The grains in the weld region of TIG-welded specimen were large and columnar, whereas the stir zone grains of the FS- and FSV-welded specimens were equiaxed and fine. In addition, the stir zone grains of the FSV-welded specimen were finer than those of the FS-welded specimen.

      SampleYield strength / MPaUTS / MPaElongation / %Formability index / (MPa·%)Impact
      toughness / J
      Welding coefficient / %
      AA6061-T6255–260305–31510–133050–396514
      TIG-welded sample1282025.41090.8 866.22
      FS-welded sample1332255.5212421573.77
      FSV-welded sample1482477.21778.42180.98

      Table 2.  Influence of welding methods on the mechanical characteristics of welded zones

      Grain boundaries impede the movement of dislocations and increase strength [4144]. According to Hall–Petch relation $ \sigma = {\sigma _0} + k{D^{ - \frac{1}{2}}}$ (σ is flow stress; $ {\sigma _0}$ is the resistance to dislocation motion; k is constant) [41], the strength σ increases as the grain size D decreases. Thus, the highest strength values for the FSV-welded specimen are predictable. The higher strength of the FSV-welded specimen compared with that of the FS-welded specimen may also be related to the effect of Mg2Si precipitates. Mg2Si particles precipitate within the microstructure of AA6061 alloys [45]. As shown in Fig. 7, the precipitate content was higher in the FSV-welded specimen than in the FS-welded specimen; however, these precipitates were smaller and the interparticle distances between them were smaller. According to Orowan–Ashby relation (Eq. 3) [41],

      where G and b are the shear modulus and the Burgers vector, respectively. Precipitation strengthening (Δσ) depends on the size of precipitates (r) and interparticle distance (λ). For a constant volume fraction of precipitates, the interparticle distance decreases as the size of precipitates decreases [41]. In this condition, the movement of dislocations decreases further and the strengthening is improved.

      Ductility or elongation, defined as the strain at fracture, is obtained from the uniaxial tensile test and depends largely on grain size [46]. Different ideas have been presented with regard to the effect of grain size on ductility. Some believe that the volume fraction of grain boundaries increases as the grain size decreases [46]. Grain boundaries decrease the growth of cracks, and fracture occurs at high strain values. Some researchers [47] believe that fracture mechanism changes from intergranular fracture to transgranular fracture as the grain size decreases; as a result, ductility increases as grain size decreases. A previous study [48] reported the presence of geometrically necessary dislocations (GNDs) in grain boundaries and their presence in accommodating the plastic strain between adjacent grains to distribute the strain homogenously. On the basis of this idea, the higher volume fraction of grain boundaries results in the more density of GNDs and ductility increases as grain size decreases.

      The formability index, which is defined as the ability of a substance to deform without being fractured, is presented by ultimate tensile strength (UTS)× elongation [49]. The formability index can be regarded as a measure of toughness. For the FSV-welded specimen with the highest UTS and elongation values, the formability index was the highest. For the TIG-welded specimen, the formability index was the lowest. Impact test results for the welded specimens indicated that the impact toughness value of the TIG-welded specimen was lower than the base metal, whereas that of the FSV-welded specimen was higher than that of the base metal. Toughness increases as grain size decreases [41]. Another parameter that can decrease the toughness of TIG-welded specimens is the presence of voids and pores in the weld region. Voids and pores might form in the weld region because of molten solidification [50]. Porosities lead to stress concentration and decrease the strength and toughness of welded specimens [41]. Weld efficiency is defined as the ratio of the tensile strength of the welded specimen to the tensile strength of the base metal [51]. The weld efficiency of the FSV-welded specimen was around 81%, whereas those of the FS- and TIG-welded specimens were approximately 74% and 67%, respectively.

      Fig. 10 presents the hardness distribution along the cross-section of the studied specimens. The hardness distribution diagrams of the FS- and FSV-welded specimens were W-like. This result indicated that the stir zone hardness value was higher than the hardness values of adjacent regions (HAZ and TMAZ) and that the hardness value of the base material was higher than those of the noted regions.

      Figure 10.  Hardness profiles of the samples produced with different welding processes.

      Hardness is a measure of the resistance of the material to localized plastic deformation, and it increases as the movement of dislocations decreases [41]. Grain boundaries decrease the movement of dislocations and hardness increases as grain size decreases. As shown in Fig. 8, the grains in the stir zone were smaller than those in the HAZ and TMAZ. Thus, higher hardness values were predicted for the stir zone compared with the HAZ and TMAZ. Higher hardness of base metal compared with the stir zone may be related to the development of tensile residual stress in the stir zone. Kumar et al. [52] noted that tensile residual stresses formed in the stir zone after FSW and FSVW. Tensile residual stresses facilitate the movement of dislocations and decrease hardness. The fusion zone of the TIG joint mainly consists of relatively coarsened dendritic grains, and the matrix is closest to a supersaturated solid solution [53]. The main strengthening mechanism of this zone is solution strengthening. Therefore, the hardness was lower. During TIG, the non-recrystallized structure remains in the HAZ and the aging strengthening precipitates in the HAZ coarsen remarkably. Therefore, the strengthening mechanism of the HAZ is fundamentally the same as the base metal. However, the aging strengthening of the HAZ is lower than that of the base metal. This finding explains why the hardness of the HAZ was lower than that of the base metal but higher than that of the fusion zone.

      As shown in Fig. 10, the stir zone hardness of the FSV-welded specimen was higher than that of the FS-welded specimen, and both were largely higher than the hardness value in the weld region of the TIG-welded specimen. These results can be related to the role of grain size on hardness as previously discussed.

      Tensile test specimens after fracture are shown in Fig. 11. Observations indicated that apart from the TIG-welded specimen that fractured from HAZ, other specimens mostly fractured from the stir zone. This result implies that deformation finally concentrates in the stir zone and fracture initiates from the stir zone but not from the HAZ during the tensile test. This result may be related to the role of residual stresses developing in the stir zone and HAZ. During FSW and FSVW, the materials in the stir zone is forged and extruded extensively, and they are under compressive stresses, whereas the material in the HAZ is under tensile stresses due to thermal expansion [39]. After welding, tensile and compressive residual stresses develop in the stir zone and HAZ, respectively [39]. During the tensile test, the tensile residual stresses in the stir zone superpose on the applied tensile stresses. As a result, deformation concentrates in the stir zone, and fracture finally occurs in the stir zone.

      Figure 11.  Fracture locations in joints made by different welding conditions.

      Fig. 12 shows the SEM fracture surface images of the studied welded specimens as well as the base material. A dimpled fracture surface, which is characteristic of ductile metals, was observed for all specimens. Ductile fracture initiates by nucleation of voids and proceeds by the growth of voids and their coalescence to form a crack [39,54]. As shown Fig. 12, the voids in the FSV-welded specimen were smaller than those in the other specimens, and large voids were observed in the TIG-welded specimen. Voids normally form in dislocation locks, grain boundary junctions, and around the second phase particles as such inclusions [55]. However, the voids formed around the second phase particles are larger compared with those formed in the dislocation locks and dislocation junctions [55]. Second phase particles are normally found in the microstructure. The presence of large voids in the fracture surface of the TIG-welded specimen indicates that second phase particles as such inclusions might be present in the microstructure of this specimen. As grain size decreases, the volume fraction of grain boundary junctions increases. For the FSV-welded specimen with the finest grains in the stir zone, the fraction of voids formed around the grain boundary junctions was high and the voids were small.

      Figure 12.  SEM analysis of surface fracture for (a) base metal, (b) TIG sample, (c) FSW, and (d) FSVW.

    4.   Conclusions
    • This study investigated the relationships between metallurgical and mechanical characteristics of the AA6061-T6 welded joints produced using FSW, FSVW, and TIG. Microstructure observation revealed that fine and equiaxed structures were produced in the FS- and FSV-welded specimens, whereas coarse and columnar grains were formed in the TIG-welded specimen. The following results were obtained:

      (1) The UTS of the FSV-welded specimen was about 22% and 10% higher than those of the TIG- and FS-welded specimens, respectively, and the elongation values of the FSV-welded specimen were about 33% and 30% higher than those of the TIG- and FS-welded specimens, respectively.

      (2) All specimens showed a ductile fracture surface, although the fracture surfaces of the FS- and FSV-welded specimens were full of small dimples and voids.

      (3) The highest impact toughness was related to the FSV-welded specimen (21 J), and the lowest one was related to the TIG-welded one (8 J).

      (4) The results indicated that the hardness values in the weld regions for all welded specimens were lower than the base metal hardness value. The hardness distribution for the FS- and FSV-welded specimens in the weld region was a W-like distribution with a high hardness value in the stir zone and a low hardness value in the HAZ, whereas it showed a U-like distribution in the weld region of the TIG-welded specimen.

    Acknowledgement
    • The authors would like to thank the Amirkabir University of Technology for their support during this research.

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