Vertical Compensation Friction Stir Welding of 6061-T6 Aluminum Alloy

Formation of VCFSW joint

The surface appearance of welding joint under the gap width of 1 mm and no compensation strip is shown in Figure 2. It is observed that the groove defect forms in the weld zone (WZ), which results from lower heat input and lack of plasticized materials. During FSW process, frictional heat is connected to effective contact area between rotational tool and welded workpieces. When the 1 mm gap appears, the reduction of effective contact area leads to the decrease of frictional heat, which in turn causes the insufficient plastic flow behavior. In addition, materials around rotational tool are softened by frictional heat and transferred from the front of the pin to the back of pin by combination of rotation and translation under bigger forging force [12]. However, since the FSW machine used in this study is a displacement-controlled machine, the introduction of gap causes insufficient forging effect of rotational tool, which makes that the WZ is difficult to be filled with plasticized materials. Based on the above mentioned, the groove defect easily appears owing to the existence of 1 mm gap. As a matter of fact, for pressure-controlled machine, the displacement of rotational tool can adjust with the change of forging force that is constantly monitored. Although the pressure-controlled machine may be propitious to the formation of joint, the thickness reduction of joint under the gap of 1 mm is much greater than that of joint without gap, which is unfavorable for load bearing of joint. Therefore, the existence of gap is adverse to FSW.

Figure 2:

Surface appearance of joints under the gap width of 1 mm and no compensation strip.

Figure 3 exhibits the appearance of joint when the gap is 1 mm and the compensation material (6061-T6 aluminum alloy) is same as BM. It is seen that the joint with smooth surface and small flashes is obtained, while no groove appears in the front surface of joint (Figure 3(a)), which also validates that VCFSW process can effectively solve the disadvantages caused by big gap. The back appearance of the joint is shown in Figure 3(b), where root flaws defect is hard to avoid. During VCFSW process, the compensation strip at the bottom of pin top is softened by frictional heat, which is generated by friction between pin top and materials. The frictional heat by pin top is not enough to soften and break up the compensation strip and then mix with BM, leading to the lack of root penetration (Figure 3(b)). Therefore, in order to successfully get high-quality joint and realize the joining process, the rational compensation material with lower melting temperature shall be chosen. The melting temperature of 6061-T6 aluminum alloy is about 580℃. The rational peak temperature during FSW process is supposed to be about 80–90% of the melting temperature of BM [8]. Therefore, the peak temperature of VCFSW of 6061-T6 alloy is supposed to be about 472–522℃, thereby, 2024-T4 aluminum alloy with the melting temperature of 503℃ is chosen as the compensation material in the following experiment. On one hand, under peak temperature of FSW of 6061 aluminum alloy, it is easy to make the flow stress much lower and then get excellent material flow behavior. On the other hand, it is also more susceptible to corrosion and etched out as dark color with Keller’s reagent owing to more Cu element.

Figure 3:

VCFSW joint of 6061-T6 aluminum alloy as compensation strip: (a) front and (b) back.

Figure 4 shows good surface formation of joints under the strips of 1 and 1.5 mm at the welding speed of 50 mm/min and rotational velocity of 1,800 rpm when the compensation strip is 2024-T4 aluminum alloy. However, the defect-free VCFSW joint was successful under the compensation strip of 1 mm width, but not to that in 1.5 mm width. During VCFSW process, the frictional heat is mainly divided into two parts: One is generated by friction between BM and tool, the other is produced by friction between compensation strip and tool. In this study, the melting temperature of 6061 alloy is far more than that of 2024 alloy and the frictional heat acting on 6061 alloy is much larger than that acting on 2024 alloy. As the width of strip increases from 1 to 1.5 mm, 6061 alloy in WZ decreases and 2024 alloy in WZ increases. Therefore, the frictional heat decreases with increasing width of compensation strip from 1 to 1.5 mm. The temperature curves under different strip widths are shown in Figure 5. Moreover, compared with 6061 alloy, 2024 alloy owns higher hardness and strength at elevated temperature, leading to higher flow stress. Therefore, when the width of strip is 1.5 mm, lower peak temperature and higher flow stress generate, which results in insufficient material flow behavior, leading to the formation of cavity defect.

Figure 4:

Surface morphology of joints under different widths of 2024-T4 alloy as compensation strip: (a) 1 mm and (b) 1.5 mm.

Figure 5:

Temperature profiles under the widths of 1 and 1.5 mm.

In terms of VCFSW process, the material around the tool is acted upon by shearing force of tool pin and pressure force of material in front of tool.

The directions of the shearing force and pressure force are opposite for the materials on the advancing side (AS), while the force directions are the same for the materials on the retreating side (RS). Therefore, more materials flow toward the RS of weld behind the tool pin, which makes the hole or cavity defect usually appear on the AS of weld. This phenomenon also appears in the joint of traditional FSW [13]. In this study, different tool offsets on the AS were further investigated in order to eliminate cavity defect. Therein, tool offset represents the distance between the rotational axis of tool and the centerline of weld, as revealed in Figure 6. It is seen from Figure 7 that the dimensions of cavity decrease with increasing tool offsets varying from 0 to 0.8 mm. In this study, because of 3 mm in diameter of pin top, when the offset reaches 1 mm, the WZ near the pin tip is difficult to cover the interface between compensation strip and welded workpieces and join workpieces (Figure 6), which is not further studied.

Figure 6:

Schematic diagram of tool offset.

For VCFSW process, the cavity defect of joint by the designed tool in this study is difficult to avoid when the width of compensation strip is 1.5 mm. Perhaps, higher rotational velocity, lower welding speed and rational tool geometry may avoid the emergence of cavity defect in VCFSW joint under the 1.5 mm wide strip, which isn’t further investigated.

Figure 7:

Macrostructure of cross section of joints under different tool offsets: (a) 0 mm, (b) 0.2 mm, (c) 0.4 mm, (d) 0.6 mm and (e) 0.8 mm.

Microstructures of VCFSW joint

The morphologies of cross section of welding joint under different welding speeds and the constant strip width of 1 mm are investigated (Figure 8(a) and (b)). The WZ is filled with vortex-like structure owing to different etching response to the Keller’s reagent. The vortex-like structure is composed of white and black bands, which is similar to onion ring referred to in the literatures [14–16]. The formation mechanism of the vortex is thought to be attributed to the geometry of tool pin and plastic flow behavior of two materials [17, 18].

Figure 8:

Typical morphologies of cross section of VCFSW joints at different welding speeds: (a) 200 mm/min and (b) 50 mm/min.

Positions marked in Figure 8(a) and (b) are selected to perform elemental analysis, which is used to distinguish the black and white bands. 2024 alloy is Al–Cu alloy and 6061 is Al–Mg–Si alloy. Therefore, only Al, Cu and Mg were mainly measured by EDX in order to discuss the mixture of 2024 and 6061 alloys. The results of elemental analysis are shown in Table 1. The black and white band structures both involve Al, Mg and Cu. The minimum value of wt.% of Cu in the black band reaches 0.68%. Ordinarily speaking, wt.% of Cu in 6061 alloy varies from 0.15% to 0.4%, which is lower than 0.68%, so the black band is considered as 2024-T4 alloy. Accordingly the white band is regarded as 6061-T6 alloy. It is well known that the chemical reaction of Cu element and HNO₃ easily happens during etching metallographic specimen, which leads to the loss of Cu element. Therefore, wt.% of Cu in 6061-T6 alloy only reduces but not increases. This is also the convincing evidence that the white band is 6061-T6 alloy. During VCFSW process, frictional heat is produced by the shoulder and pin, while the amount of frictional heat generated by shoulder is larger than that by pin. The temperature of material near the bottom of WZ is lower than that near the top of WZ. The flow velocity of material is relatively high near the shoulder and decreases with the increase of distance away from the top surface of workpiece [17]. Therefore, the compensation strip near the top can be more easily smashed using the rotational tool and mix with BM (Figure 8(a) and (b)). The compensation strip near the bottom is stretched and crooked, owing to lower thermal cycle and material flow velocity (Figure 8(a)). The peak temperature increases with the decrease of welding speed. The higher temperature in the WZ during the welding process is beneficial to reduce the flow stress of material. The flow behavior of material at the welding speed of 50 mm/min is better than that at the welding speed of 200 mm/min. da Silva stated that increasing rotational velocity was propitious to formation of onion ring structure [19]. Therefore, the mixing pattern characterized by vortex-like structure appears in the upper region of the joint at the welding speed of 50 mm/min rather than 200 mm/min. Moreover, the area of region containing 2024 alloy in the WZ in Figure 8(b) is bigger than that in Figure 8(a), which is related to the difference of welding temperature.

The microstructures of joints under different welding speeds are shown in Figure 9. In this study, since 2024 alloy is added into the gap as compensation material and WZ is filled with 2024 alloy, the microstructure of 2024 alloy is investigated and studied. During VCFSW process, the compensation material is acted by rotational tool and undergoes higher temperature and larger plastic deformation [20]. Therefore, the microstructures in the top and bottom of WZ are characterized by fine and equiaxed grains because of dynamic recrystallization, as exhibited in Figure 9. During VCFSW process, frictional heat generated by shoulder is much more than that by pin, which leads to higher temperature in the top of joint compared with that in the bottom of joint. Therefore, it can be found that the grain size of microstructure in Figure 9(a) is slightly bigger than that in Figure 9(b) due to the difference of temperature between top and bottom when the welding speed is 50 mm/min. The same phenomenon also happens in the joint obtained using the welding speed of 200 mm/min. Compared with Figure 9(a) and (b), the grain size of microstructures in Figure 9(c) and (d) is finer. This is because that increasing welding speed results in the decrease of heat input, which causes shorter time for recrystallized grain to coarsen.

Figure 9:

Microstructures of joints under different welding speeds: (a) top and (b) bottom at the welding speed of 50 mm/min; (c) top and (b) bottom at the welding speed of 200 mm/min.

Mechanical properties of VCFSW joint

The tensile specimens and bending specimens of joints under the welding speeds of 50 mm/min and 200 mm/min are revealed in Figure 10. During bending test of VCFSW joint, the bending property of specimen is closely linked to the mixture of two materials. If lack of root penetration, hole or tunnel defect emerges in the weld, the bending angle of surface bending and root bending is difficult to reach 180° [7]. In this study, when the welding speed is 200 mm/min, because the section of compensation strip near the bottom cannot be broken into pieces and fully mixed with BM, kissing bond possibly appears near the bottom and degrades the mechanical properties (Figure 11a). The view of line scanning shows that no Al is detected in the middle zone (Figure 11(b)), which validates the formation of kissing bond. The dimensions of compensation strip not broken up in Figure 8(a) is bigger than that in Figure 8(b), which may cause the failure of root bending of welding joint at the welding speed of 200 mm/min instead of 50 mm/min. Table 2 shows the mechanical properties of VCFSW joint. During VCFSW process, the higher the welding speed, the lower the tensile strength and elongation. Although mechanical property is related to grain size, the degree of mixture of two materials and percentage of compensation strip broken into pieces along the thickness direction during VCFSW process are more significant [21–22]. It can be seen from Figure 8 that the degree of mixture of joint at 200 mm/min is not patch on that of joint at 50 mm/min, which is the main reason why mechanical property of joint under the welding speed of 200 mm/min is lower. Therefore, the tensile strength and elongation of joint at the welding speed of 50 mm/min and constant rotational velocity of 1,800 rpm reaches the maximum values of 203 MPa and 5.2%, equivalent to 71.5% and 54.7% of BM, respectively.

Figure 10:

Specimens of tensile tests and bending tests of joints under different welding speeds and the 1 mm wide strip: (a) test specimens and (b) bending specimens under the welding speed of 50 mm/min; (c) test specimens and (d) bending specimens under the welding speed of 200 mm/min.

Figure 11:

(a) SEM micrograph from the area marked by the white arrow in Figure 8(a) and (b) view of the associated line scanning showing the aluminum.

Microhardness of VCFSW joint

Microhardness value of joints under different welding speeds was measured along the dashed line marked in Figure 8, which are 1 and 3 mm to the top surface, while the result is shown in Figure 12. It is observed from Figure 12 that microhardness of WZ presents the typical W-shape distribution and the minimum hardness appears in heat affected zone (HAZ). Since 2024-T4 and 6061-T6 aluminum alloys belong to precipitation strengthening alloys, the softening phenomenon is a typical characteristic of joint because of evolution of the precipitation of strengthening phase [23–25]. Therefore, the hardness in WZ, HAZ or thermal-mechanically affected zone (TMAZ) is lower than that of BM. WZ is made up of the large amount of fine equiaxed grains, while the grain growth of material in HAZ happens. Therefore, the hardness of material in WZ is higher than that in HAZ. The average hardness values of BMs of 2024-T4 alloy and 6061-T6 alloy are 145 and 98 Hv, respectively. After undergoing the softening phenomenon, the hardness of 2024 alloy in WZ is higher than that of 6061 alloy [22]. This is why the uneven distribution of hardness appears in WZ, as shown in Figure 12(a) and (b). In this study, the hardness of black band region reaches about 100 Hv, equivalent to BM of 6061-T6 alloy approximately. The hardness of white band region is only 75 Hv or so. Therefore, the addition of 2024-T4 alloy as compensation material plays a strengthening effect on WZ and is beneficial to enhance property of the weld. Compared with Figure 12(a), the hardness value at the welding speed of 200 mm/min (Figure 12(b)) is slightly higher, which results from the lower heat input and finer grains.

Figure 12:

Microhardness of joints under different welding speeds: (a) 50 mm/min and (b) 200 mm/min.

Fracture surface of VCFSW joint

The fracture morphologies of VCFSW joint of 6061-T6 aluminum alloy are shown in Figure 13, by which the mechanical properties are further discussed. It is seen that the fracture surface morphologies are featured by large amounts of dimples and present the typical ductile fracture. According to Figure 13, it is concluded that welding speed exerts significant influence on fracture surface morphology of VCFSW joint, which in turn influences mechanical properties. There are larger and deeper dimples in Figure 13(a), while shallower and smaller amounts of dimples appear in Figure 13(c). Generally speaking, the larger and deeper the dimples are, the better the ductile of joint is, which in turn results in higher elongation. Therefore, the ductile and elongation of VCFSW joint are the best at the welding speed of 50 mm/min and the worst at the welding speed of 200 mm/min.

Figure 13:

Fracture morphologies of welding joints at different welding speeds: (a) 50 mm/min, (b) 100 mm/min and (c) 200 mm/min.