Effect of Welding Speeds on Mechanical Properties of Level Compensation Friction Stir Welded 6061-T6 Aluminum Alloy

Macrostructure of the joints

The macrostructures of FSW and LCFSW joints are displayed in Figure 2. It can be seen that the material in shoulder affected zone (SAZ) of conventional FSW is concave under the action of axial force (Figure 2(a)). The thickness of weld zone due to the formation of flash is lower than that of BM. Figure 2(b) shows the macrostructure of LCFSW joint before milling. In weld zone, the flash and concave are also formed. The thickness of weld zone is lower than the total thickness of small convex platform and BM, but higher than that of BM. After welding, the convex platform can be removed using milling, so the flash and arc corrugation are also eliminated, increasing the beauty of joint. Compared with BM, therefore, the equal thickness joint without flash and corrugation is attained. From Figure 2(a) and 2(b), it is also found that the SAZ was markedly reduced. The thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were slightly reduced.

Figure 2:

Macrostructure of joint: (a) conventional FSW and (b) LCFSW.

Microhardness profile

The transverse microhardness profiles on the cross sections of the joints are shown in Figure 3. It is seen that the microhardness profiles under different welding speeds all exhibit W shape. The hardness profiles are corresponding to four distinct zones, i.e. weld nugget (WN), TMAZ, HAZ and BM. With respect to the LCFSW joints at different welding speed from 200 to 800 mm/min, although the WN experiences dynamic recrystallization (DRX) caused by plastic deformation and high temperature, the hardness of WN is lower than that of BM. The microhardness in the WN is higher than that of TMAZ or HAZ. The minimum hardness locates at the HAZ because of the softening phenomenon. During the welding process, the temperature on advancing side (AS) is higher than that on the retreating side (RS) because of the formation of flash and the difference of plastic deformation, which makes the microhardness of HAZ on AS lower than that on RS. Therefore, this hardness profiles in Figure 3 are asymmetry. With the increase of welding speed, the heat input in one unit decreases and the peak temperature of material in WN decreases, which results in the decrease of the width and softening degree of HAZ of welding joint. Moreover, under the constant rotational speed, the lower the peak temperature is, the finer the grains in WN are, which is beneficial for the increase of hardness. Therefore, the microhardness not only in HAZ but also in WN under the welding speed of 800 mm/min is higher than that under the lower welding speed varied from 200 to 600 mm/min.

Figure 3:

Microhardness profile of joints welded at different welding speeds.

Mechanical properties

The comparison of mechanical properties between LCFSW and conventional FSW is shown in Figure 4. It is observed that the mechanical properties of LCFSW and FSW show the same trends when the welding speed varies from 200 to 800 mm/min. During welding process, the frictional heat generated by shoulder and pin is related to the rotational speed and welding speed [14]. When the rotational speed of tool is constant, peak temperature in WN during the welding process is highest under the welding speed of 200 mm/min, resulting in the increase of the softening degree of and thus poor tensile strength and elongation of welding joints. With increasing the welding speed from 400 to 600 mm/min, peak temperature in WN decreases, which is beneficial for the decrease of softening degree and then results in the increase of mechanical properties of LCFSW joint. However, lower peak temperature means poor inadequate material flow, which may lead to cavity or lack of root penetration appear in WN and then deteriorate the quality of welding joint [15]. This may be the reason why the mechanical properties reduce when the welding speed further increases to 800 mm/min. The results of mechanical properties in this study are similar to that reported by Peel et al. [16]. In a word, under the constant rotational speed of 1800 rpm, the tensile strength of LCFSW joint increases firstly from 206 to 248 MPa and then decreases to 232 MPa. When the welding speed is 600 mm/min, the tensile strength and elongation of LCFSW joint both reach the maximum, whose values are, respectively, 248 MPa and 7.1%. The above-mentioned results show that there exists an optimum combination of the welding speed and the rotational velocity to produce a defect-free joint.

For FSW joint, WN can be divided into SAZ and pin affected zone (PAZ), as shown in Figure 2. During the welding process, the friction heat generated by shoulder is much more than that by pin. Therefore, temperature of material in SAZ is higher than that of pin, which results in the bigger DRX grains in SAZ. So, for defect-free joint, SAZ is weak area compared with PAZ. After the milling process for LCFSW, SAZ, HAZ and TMAZ are partly removed, which is beneficial for the mechanical properties. As mentioned above, material in WN is concave for the conventional FSW joint while the thickness of LCFSW joint is equal to that of BM. Therefore, compared with conventional FSW, the bearing area of LCFSW joint is bigger. Moreover, when the welding joint bears the tensile force, the existence of concave and arc corrugation easily causes the geometrical stress concentration, which goes against the quality of welding joint. The above-mentioned three reasons makes that the mechanical properties of LCFSW joint after removing the convex platform are higher than those of conventional FSW joint under the same welding parameters, as shown in Figure 4. In fact, some previous studies also investigated the tensile strength of conventional FSW joints of 6061 alloy [6–9, 17]. Heidarzadeh et al. [9] and Rajakumar et al. [18] attain the welding joint of conventional FSW with the maximum tensile strength of 159 and 224 MPa. The result reported by Liu et al. [6–8] is also lower than that in this study. Therefore, LCFSW is advantageous to improve the mechanical properties of welding joint.

Figure 4:

Tensile strength and elongation of LCFSW and FSW joints at different speeds.

Fracture of welding joint

The fracture locations of LCFSW joints are mainly discussed, as shown in Figure 5. It is seen from the figure that all the defect-free joints exhibit the obvious necking. It is well known that the tensile failure is likely to occur in the region with minimum hardness [18–20]. For LCFSW joint, the minimum microhardness locates at HAZ (Figure 3), so the fracture easily occurs in the HAZ adjacent to the TMAZ on AS, as shown in Figure 5. The material in SAZ experiences high temperature, which results in coarser grains and then possesses lower hardness. Therefore, the direction of fracture presents 45° from bottom surface to top surface and the final fracture position is located in SAZ. Because SAZ and HAZ of LCFSW joint are only partly removed by milling, the rest also owns lower or minimum microhardness compared with the other region of welding joint. Therefore, the fracture feature of conventional FSW joint in this study is similar to that of LCFSW joint, which is also reported by other researchers [7, 21]. Liu et al. [7] concluded that the fracture of the defect-free friction stir welded 6061-T6 joints occurs in the HAZ adjacent to the TMAZ. Wang et al. showed that the fracture of conventional FSW joints of 6061 aluminum alloy occurs in the HAZ and then the crack expands to the weld [21].

Figure 5:

Fracture location of LCFSW welds at the rotational velocity of 1800 rpm with different welding speed: (a) 200 mm/min, (b) 400 mm/min, (c) 600 mm/min and (d) 800 mm/min.

SEM micrographs of fracture surfaces of joints are shown in Figure 6. It is observed that all fracture surfaces of welding joints show dimples of varying size and depth, which represents the typical ductile fracture. The difference of fracture surfaces shows that the welding speed has significant effect on fracture morphology. Generally speaking, the higher the elongation of joint is, the better the ductility of joint is. Under the welding speed of 200 mm/min, the elongation of welding joint is lowest (Figure 4), so the fracture surface in Figure 6(a) is covered with shallower and smaller dimples, indicating the worst ductility. Under the welding speed of 400 and 800 mm/min, the elongations are both higher than that under the welding speed of 200 mm/min, which causes the increase of larger and deeper dimples (Figure 6(b) and 6(d)). From the viewpoint of dimple characteristics, it can be concluded that the ductility of joint under the welding speed of 600 mm/min is the best. In summary, the experimental results of fracture morphologies of LCFSW joints are in agreement with those of elongations.

Figure 6:

Fracture morphology of the joints welded at different welding speeds: (a) 200 mm/min, (b) 400 mm/min, (c) 600 mm/min and (d) 800 mm/min.