3.1.2. Features in the Double-Sided FSW

Figure 7 shows the grain map and the corresponding grain size distribution, TEM and its diffraction pattern in the double-sided FSW. The average grain size in the NZ was 7.48, 6.25 and 9.69 μm on L1, L2 and L3 lines respectively. The grain size in the first pass is much larger than that in the second pass. No precipitation was observed in the NZ of the second-pass region and the middle region. As compared to the BM, fewer precipitations were observed in the NZ of the first-pass region.

**Figure 5.** The grain map (on the left), grain size distribution (in the middle), TEM and diffraction pattern (on the right) of the NZ on L1 (**a**), NZ on L2 (**b**) and NZ on L3 (**c**) in the single-sided FSW.

**Figure 6.** The microstructure of the base metal (BM) observed by SEM (**a**), TEM (**b**) and the diffraction pattern (**c**).

**Figure 7.** The grain map (on the left), grain size distribution (in the middle), TEM and diffraction pattern (on the right) of the NZ on L1 (**a**), NZ on L2 (**b**) and NZ on L3 (**c**) in the double-sided FSW.

The precipitations were dissolved in the second-pass NZ, while the precipitations were observed in the first-pass NZ. As the parameters were the same for the first and second pass, the authors inferred that the precipitations in the NZ were dissolved during the process of dynamic recrystallization in the first weld pass. The heat input by the second weld pass provided the aging temperature that the strengthening phases were precipitated in the first-pass NZ with grain growth occurring at the same time.

3.1.3. Comparison of Grain Size between Single-Sided and Double-Sided FSWs

The grain size of the NZ in the single-sided FSW is larger than the second pass of the double-sided FSW. The temperature of the NZ in the single-sided FSW is 7.9 ◦C higher than the double-sided FSW. The higher temperature is useful to activate the dynamic recrystallization and grain growth. This comparison verified the effect of the heat input on the recrystallized grain size.

#### *3.2. Mechanical Properties*

#### 3.2.1. Hardness Distribution

Figure 8 shows the hardness profiles in the weld zone of single-sided and double-sided FSWs. The obtained profiles follow the general features with a central plateau and two valleys, which is a typical hardness behavior for all FSW heat treatable aluminium alloys [6].

**Figure 8.** The hardness distribution in the single-sided FSW (**a**) and the double-sided FSW (**b**).

In the single-sided FSW, the minimum hardness on three lines is nearly the same with a value of approximately 54 HV and all appear in the HAZ. The valleys on L1 line, L2 line and L3 line are located approximately 20, 15 and 10 mm away from the weld center, respectively. The distance between two valleys decreases towards the bottom of the weld, which corresponds to the shape of the rotating pin. The average hardness of the NZ (central plateau) is 59.2, 62.8 and 66.4 HV on L1, L2, L3 lines, respectively. The hardness in the NZ increases in the order of the upper, middle and bottom regions. With the absence of precipitation in the NZ, fine grains usually benefit the strength [6]. Therefore, the hardness of NZ in the bottom region (grain size 7.38 μm) is 12.2% larger than that in the upper region (grain size 9.16 μm).

In the double-sided FSW, the hardness is generally larger than the single-sided FSW. The minimum hardness on three lines is nearly the same with approximately 58 HV and all appear in the HAZ. The valleys on L1 line, L2 line and L3 line are located approximately 18, 13 and 17 mm away from the weld center, respectively. The average hardness of the NZ (central plateau) is 66.0, 71.1 and 76.9 HV on L1, L2 and L3 lines respectively. The hardness of the middle NZ is 7.7% larger than that of the second-pass NZ. This is caused by the grain size in the middle NZ with 6.25 μm and in the second-pass NZ with 7.48 μm. The hardness of the first-pass NZ is approximately 16.5% larger than the second-pass NZ. The second-pass NZ has no precipitation and smaller grains, while the first-pass NZ has a small quantity of precipitations and larger grains. It is concluded that the strengthening precipitations overwhelm larger grain size in controlling the hardness.

#### 3.2.2. Tensile Strength

The tensile strength of a single-sided FSW and a double-sided FSW was 186 ± 1 MPa and 202 ± 2 MPa, equivalent to 74.4% and 80.8% of BM (250 MPa), respectively. The larger tensile strength coincides with the larger hardness of the double-sided FSW. Figure 9 shows the fracture location of the single-sided FSW and the double-sided FSW. In the single-sided FSW, the fracture occurred in the HAZ of the advancing side. In the double-sided FSW, the crack initiated in the HAZ of the advancing side of the second pass and propagated to the HAZ of the retreating side of the first pass.

**Figure 9.** The fracture location of the single-sided FSW (**a**) and the double-sided FSW (**b**).
