3.3.1. Features in the Single-Sided FSW

In the single-sided FSW, the residual stresses along the three lines have a similar distribution in both trend as well as magnitude. σLD is generally equal to σTD and larger than σND. The maximum tensile σLD and σTD are nearly the same, approximately 75 MPa. This stress value amounts to 37.5% of the yield strength of Al-6005A-T6 alloy, which is 200 MPa at room temperature.

The introduction of residual stresses is inevitable due to severe thermomechanical deformation by FSW. Therefore, it is important to control the amount of residual stress. Hassan et al. reported an optimum combination of rotational and travel speeds that gave the best mechanical performance [11]. This optimum condition shifted to a higher rotational speed when the travel speed was increased. High heat input associated with low traverse and high rotation speeds leads to more extensive softening in the weld region, resulting in an overall reduction in the magnitude of the longitudinal residual stress. It seems difficult to get both optimized mechanical properties and residual stresses. However, the present 6005A-T6 single-sided FSW achieved an acceptable condition for comprising the mechanical properties with a tensile strength of 74.4% of BM and residual stresses with peak magnitudes of approximately 37.5% yield strength of BM.

#### 3.3.2. Features in the Double-Sided FSW

In the double-sided FSW, σLD is generally larger than σND and σTD. There is large difference between σLD magnitudes on the three lines, as shown in Figure 11. The maximum tensile σLD is approximately 140 MPa on L1 line (*xt* = −10), 115 MPa on L2 line (*xt* = 0) and 110 MPa on L3 line (*xt* = 10) respectively. These stress values amount to 70%, 57.5% and 55% of the yield strength of BM. The residual stress profiles of the L1 line and L2 line show obvious asymmetric distributions. The highest stresses of those lines occur on the advancing side of the NZ compared to the retreating side. The maximum residual stress in the upper weld zone is 140 MPa, 27% larger than the bottom weld zone, where the stress is 110 MPa. As the parameters were the same for the first and second pass, an inference was made that the peak magnitude of residual stress produced by the first weld pass was approximately 140 MPa. The heat input by the second weld pass provided the post-treatment environment of stress relaxation for the first-pass weld zone.

**Figure 11.** Replot of longitudinal residual stresses in a single-sided FSW (**a**) and a double-sided FSW(**b**).

Although the tensile strength of the double-sided FSW was a little higher than the single-side FSW, the longitudinal residual stresses in the double-sided FSW are much larger than the single-sided FSW, as shown in Figure 11.

#### **4. Summary and Conclusions**

In this study, residual stresses, microstructures and mechanical properties of the 6005A-T6 single-sided and double-sided FSWs were studied. The 3D residual stresses were characterized using neutron diffraction. The microstructures were observed by TEM and EBSD.

In the 6005A-T6 single-sided FSW, there were acceptable mechanical properties with tensile strength 74.4% of BM, and low residual stresses with peak magnitudes of approximately 37.5% yield strength of BM were achieved. This indicated a good quality of weld with such low residual stresses. However, good mechanical properties with a tensile strength of 80.8% of BM, but high residual stresses with peak magnitudes of approximately 70% yield strength of BM were obtained in the 6005A-T6 double-sided FSW.

In the 6005A-T6 single-sided FSW, there was no precipitation in the NZ. The hardness of the NZ is related to the grain size. The less hardness at the upper NZ and high hardness at the bottom NZ coincided with the larger grain size at the upper NZ and smaller grain size at the bottom NZ, respectively.

In the 6005A-T6 double-sided FSW, there were lower residual stresses and larger hardness in the first-pass weld zone, but higher residual stresses and smaller hardness in the second-pass weld zone. This was due to the heat input by the second weld pass which provided an aging environment for the first-pass weld zone, where the dissolved phases were precipitated and residual stresses were relaxed.

**Author Contributions:** Conceptualization, X.L. and H.Z.; investigation, P.X., R.W., R.L. and M.L.; Writing—Original Draft preparation, X.L.; Writing—Review and Editing, W.L., Y.L. and D.C.; funding acquisition, Y.L. and D.C.

**Funding:** This research activity was financially supported by Ministry of Science and Technology of China (grant 2017YFA0403704), National Natural Science Foundation of China (grants 11605293 and 51327902).

**Conflicts of Interest:** The authors declare no conflict of interest.
