**2. Experimental Details**

## *2.1. Materials*

The base metal used in this investigation was 37 mm thick 6005A-T6 aluminium alloy with the chemical composition and mechanical properties shown in Table 1. The dimensions of the 6005A-T6 test plates are shown in Figure 1.


**Table 1.** The chemical composition and mechanical properties of 6005A aluminium alloy.

Further, FSW was carried out on a LM-FSW-5025 machine (China FSW Center, Beijing, China). For the single-sided FSW, the tool consisted of a 35 mm diameter shoulder with a pin length of 36.5 mm and a root diameter of 18 mm. The single-sided FSW process was performed with a rotation speed of 350 rpm and a traverse speed of 40 mm/min. The down force was approximately 8–9 kN. For the double-sided FSW, the tool consisted of a 32 mm diameter shoulder with a pin length of 18 mm and a root diameter of 14.4 mm. To apply the second pass of the FSW, the plates were rotated around their welding axes after completing the first weld. All welding passes were conducted with a rotation speed of 650 rpm and a traverse speed of 200 mm/min. The down force was approximately 13–14 kN.

**Figure 1.** The layout of single-sided and double-sided friction stir welding (FSWs).

The temperature of the NZ was estimated using the following relation [38]:

$$\frac{T}{T\_m} = \mathcal{K} \left( \frac{w^2}{v \times 10^4} \right)^\alpha \tag{1}$$

where *Tm* is the melting temperature of 6005A aluminium alloy 654 ◦C, exponent α is a dimensionless constant selected as 0.05, K is a dimensionless constant selected as 0.7, ν is the traverse speed and ω is the rotation speed [39]. According to Equation (1), the temperature of the nugget zone in the single-sided FSW is 431.4 ◦C, while in the double-sided, FSW is 423.5 ◦C.

## *2.2. Neutron Di*ff*raction*

Due to the high penetration ability of thermal neutrons, neutron diffraction is an excellent engineering tool for providing 3D residual stresses nondestructively in bulk components. Therefore, neutron diffraction was applied to characterize the residual stresses in thick 6005A-T6 aluminium alloy FSWs, and was performed on the dedicated residual stress neutron diffractometer E3 at HZB, Germany.

As shown in Figure 1, the middle section with 200 mm in length was cut using electrical discharge machining (EDM, LA350, SSG, Suzhou, China) from the specimen for a neutron diffraction measurement. The longitudinal direction (LD), transverse direction (TD) and normal direction (ND) were assumed to be the principal directions in the bulk of the components and were measured as three orthogonal directions. The points along three lines, L1, L2 and L3, in the cross section were chosen to characterize the residual stresses, as shown in Figure 1. The diffraction peaks of the marked points were measured with the scattering vector parallel to three orthogonal directions. The peak positions, 2θ, were analyzed using the least square Gaussian fitting method [40].

The measurements were made using the Al(311) Bragg reflection, which is the strongest diffraction reflection and also is weakly affected by intergranular strains [36]. The E3 is equipped with a perfectly bent Si(400) crystal monochromator providing a wavelength of 1.47Å. Therefore, the Al(311) reflection was at a scattering angle of 2θ~74◦. Figure 2 shows the experimental setup on E3 for the measurement of the transverse component. The gauge volume was defined by an incident primary slit with 3 mm and a secondary radial collimator with 2 mm. The height of the primary slit was set to 10 mm for transverse and normal measurements, and 3 mm for longitudinal measurements.

**Figure 2.** The experimental setup.

The elastic lattice strains ε*<sup>i</sup>* in the *i*-direction (*i* = LD, TD, ND) were calculated using the following equation [36]:

$$
\varepsilon\_i = \frac{d\_i - d\_0}{d\_0} \tag{2}
$$

The elastic strains were converted to residual stresses (σLD, σTD, σND) using the generalized Hooke's law [36]:

$$\sigma\_{i} = \frac{E\_{hkl}}{1 + \upsilon\_{hkl}} \left[ \varepsilon\_{i} + \frac{\upsilon\_{hkl}}{1 - 2\upsilon\_{hkl}} (\varepsilon\_{\rm LD} + \varepsilon\_{\rm TD} + \varepsilon\_{\rm ND}) \right] \tag{3}$$

where *i* is the LD, TD or ND component corresponding to the three orthogonal directions. The diffraction elastic constants (*E*311) of 69 GPa and Poisson's ratio (ν311) of 0.35 were computed using the Kröner model via the software, IsoDEC [41].

To obtain a precise stress-free reference lattice parameter, *d0*, is an important part of the diffraction-based, residual strain/stress experiment. To address a possible issue of *d0* variation due to microstructural changes, a full stress analysis was performed on 5-mm slices made by EDM from the specimen. The measurements were repeated in the same positions as for the specimen. Then, in an approximation of a biaxial stress state and the condition of the through thickness component (longitudinal in this case) to be zero (σLD = 0), the calculation of the stress-free parameters was made according to the following equation:

$$d\_0 = \frac{1 - \upsilon\_{\rm bkl}}{1 + \upsilon\_{\rm hkl}} d\_{\rm LD} + \frac{\upsilon\_{\rm hkl}}{1 + \upsilon\_{\rm hkl}} (d\_{\rm TD} + d\_{\rm ND}) \tag{4}$$

As well as the TD and ND stress components in the slice as a by-product of the analysis,

$$
\sigma\_{\rm TD} = \frac{E\_{\rm bkl}}{1 + \upsilon\_{\rm bkl}} [\varepsilon\_{\rm TD} - \varepsilon\_{\rm LD}], \ \sigma\_{\rm ND} = \frac{E\_{\rm bkl}}{1 + \upsilon\_{\rm bkl}} [\varepsilon\_{\rm ND} - \varepsilon\_{\rm LD}].\tag{5}
$$

## *2.3. Microstructure Characterization*

The metallographic samples were cut perpendicular to the welding direction using EDM to avoid thermal degradation. The optical microstructure examination was performed on an Olympus microscope.

In order to compare the difference in precipitation, the microstructure examination in the NZ was conducted by a transmission electron microscope (TEM, Tecnai G2 F20, FEI, Hillsboro, OR, USA). The metallographic samples were polished down to a thickness of 80~100 μm. The final thickness reduction was obtained by electro-polishing with a HNO3 solution (HNO3 30% in volume in methanol at ~30 ◦C under 9V).

The electron backscatter diffraction (EBSD) samples were examined in a high resolution Philips XL30 field emission gun (FEG) SEM (Philips, Amsterdam, The Netherlands) interfaced to an HKL Channel EBSD orientation mapping system. The resulting EBSD orientation maps, with a step size of 0.1–0.25 μm and an area of 145 × 127 μm, were used to characterize the grain structures present in the NZ at three positions on cross sections along the weld center line, namely, the top (10 mm above the mid plane), the center (at the mid plane), and the bottom of the nugget (10 mm below the mid plane). The maps were processed using in house software (VMap) to determine the grain size.
