*3.1. Hardness Evolution with Di*ff*erent Heat Treatment*

The 7A52 and 6061 Al alloys were T6-heat-treated, respectively, with the average hardness of 165 HV and 115 HV, and then joined by FSW. Figure 2 presents the hardness variation across the weld joint along the central line of the transverse section with different rotational speeds, where the different zones are indicated. The hardness curve of FSW joint is asymmetrical with respect to the weld center. The lowest hardness (62 HV) appears near the center of the WNZ on the 6061 side, the hardness gradually rises from WNZ to the TMAZ, to the HAZ, till the 6061 matrix, or to the center line on the opposite side. A sudden increase in hardness occurs at the center line from 98 HV on the 6061 side to 138 HV on the 7A52 side. Subsequently, the hardness drops slowly to approximately 120 HV in the TMAZ on the 7A52 side, and the hardness afterwards increases slowly from HAZ to the 7A52 matrix. As shown in Figure 2, the width of HAZ on the 7A52 side is affected by the rotational speed, the higher rotational speed selected, the wider HAZ formed.

**Figure 2.** Microhardness profile along the middle section of the weld joint of T6-treated 7A52 and 6061 Al alloys at different rotational speed.

After solid solution, the 7A52 and 6061 Al alloys were joined by FSW at the rotation speed of 600 rpm, and post-aging was then employed; Figure 3a presents the hardness variation across the weld joint along the central line of the transverse section. It is very interesting that the WNZ and HAZ is stronger than the matrix on the 7A52 side, although a narrow area with low hardness near 78 HV was found to be in TMAZ, as shown in Figure 3a. On the 6061 side, the hardness keeps constant from the WNZ to the matrix. After welding, aging treatment was employed with three different heating methods, as shown in Figure 3b–d; the hardness change occurs on the 7A52 side, not on the 6061 side. After post-aging, on the 7A52 side, the hardness in TMAZ significantly increased, the highest hardness was tested in WNZ, and then successively drops from WNZ to the matrix, via TMAZ and HAZ. Aging 7A52 and 6061 alloys separately obtained the highest hardness of 150 HV in WNZ on the 7A52 side (Figure 3d). Unfortunately, the hardness of the matrix is too low after welding, even cannot be increased after post-aging, because the matrix of 7A52 and 6061 were over aged during pre-heating at 150 ◦C for 2 h before welding.

**Figure 3.** Microhardness profile along the middle section of the weld joint of 7A52 and 6061 Al alloys with pre-solid-solution and different post-aging.

When considering the negative effect of pre-heating on the hardness of the matrix, solid solution treatment was employed after welding and before post-aging. The solid solution temperature of 6061 alloy is higher than that of the 7A52 alloy, the 7A52 side was solid-solution treated first at 475 ◦C for 1.5 h, and then the 6061 side was treated at 535 ◦C for 1 h. Affected by the thermal transmission, precipitation occurs in the 7A52 side during the subsequent solid solution of the 6061 side. Hence, after post-solid-solution-treatment, the hardness of 7A52 matrix (near 120 HV in Figure 4a) is higher than that of the solid solution treated 7A52 alloy (about 80 HV). As shown in Figure 4a, after the post-solid-solution treatment, the hardness on both side of the central line keeps constant from the WNZ to the matrix. However, the hardness on the 7A52 side is much higher than that on the 6061 side and an abrupt increase appears near the central line from 6061 side to 7A52 side. After the subsequent post-aging, as shown in Figure 4b–d, the hardness of the weld joint was significantly improved, and still kept steady on both sides without signs of softening. With different heating procedures for post-aging, different hardness is achieved on both sides. As is well known, the strength of a weld joint is dependent upon the weakest region. Hence, the study focused on how to improve the lowest hardness. From Figure 4, it can be seen that the weld joint undergoing two-stage artificial aging (SS + welding + SS + aging at 120 ◦C for 24 h + the second aging at 180 ◦C for 30 min.) has the lowest

hardness of about 100 HV on the 6061 side (Figure 4b), higher than that obtained by other post-aging treatments (Figure 4c,d), meanwhile the hardness of the 7A52 side is approximately 140 HV (Figure 4b). More importantly, the hardness evolution from the 7A52 side to 6061 side is gradual in Figure 4b–d, as opposed to the sudden shift seen in Figures 3 and 4a. This smooth change in hardness benefits the weld joint in stress concentration during service.

**Figure 4.** Microhardness profile along the middle section of the weld joint of 7A52 and 6061 Al alloys with pre-solid-solution and different post-heat-treatment (solid solution treatment and aging).

#### *3.2. Microstructures*
