**1. Introduction**

As high strength aluminum alloys, Al-Zn-Mg alloys of the 7000 series family and Al-Mg-Si alloys of the 6000 series family are widely used in the defense, aerospace, automotive, and structural applications. In these heat-treatable alloys, Mg combines with Zn or Si (Cu) to form a large number of strengthening precipitates during aging, such as the metastable precursors of η-MgZn2 [1,2] and β"-Mg5Si6, L, and/or Q-phase [3,4], contributing to the high strength of these alloys. Presently, there is great interest in these alloys to advance strengthening to further reduce the weight of structures in high-speed train, aircraft, and other applications. However, to join components that were made by these alloys is a large challenge in assembly, because of the sensitivity of the strengthening metastable precipitates in these alloys to thermal cycles introduced by traditional fusion welding. The strength of the weld would be significantly reduced due to precipitates coarsening in the heat affected zone (HAZ) [5]. As with the fusion welding joint of Al-Mg-Si alloys, the strength of the HAZ is even lower than that of the weld zone, due to over-aged HAZ resulting from heat input. Moreover, the conventional fusion welding processes are not suitable for dissimilar welding applications, due to the different thermodynamic behavior of dissimilar materials.

Friction stir welding (FSW) is an effective method for joining many combinations of dissimilar materials at the solid state, being pioneered and practiced by Wayne Thomas in 1991 [6]. As a new family of solid-state joining techniques, benefitting from the decreasing of heat input and the introduction of deformation during FSW, the joint might be free from the dendritic structure typical of a fusion-weld joint, and it has finer microstructures and better mechanical properties than the respective fusion welding joint. These welding processes have been successfully applied to join dissimilar aluminum alloys [7–13]. In the literatures, investigators have paid more attention to study the effect of FSW parameters on microstructures, mechanical properties, and defects formation [7,9,11–24]. Researchers found that FSW parameters, such as tool design, tool rotation and traverse speed, depth of tool plunge, angle of tool tilt, tool pin offset, and welding gap, etc., may lead to defect formation, if they are not selected properly [8,25,26].

Beyond a weld joint without defects, engineers strive to pursue higher mechanical properties. Thus, researchers need to understand the microstructural evolution and the underlying physics of FSW. For precipitation-hardened Al alloys, such as 6000 and 7000 series, the thermos-mechanical process of FSW results in the change of precipitate structure, size, distribution, and density in different zones [27]. Generally, this change is negative for the strength of the FSW joint. In the nugget zone of the weld (WNZ), the pre-existed hardening precipitates will dissolve due to the sufficient high temperature that is generated during FSW. The region outside of the WNZ is termed the thermo-mechanical affected zone (TMAZ), where precipitates will be sheared or surrounded by dislocations introduced by severe plastic deformation. Outside of the TMAZ is the heat affected zone (HAZ), in which precipitates coarsen and transform into the stable partner, which makes HAZ over-aged and soften. Obviously, an important reason for the drop of strength in FSW joint is the loss of strengthening precipitates. Researchers naturally adopted post-heat-treatment to reform the strengthening precipitates. The effect of post-weld heat treatment on FSW joints of dissimilar Al alloys were investigated [28–31], they found that, after post-weld heat treatment, the strength of the weld can be improved if there are new precipitates forming during post heat treatment, the location and mode of fracture under tension are completely different from the as-weld joints [32–35]. However, not all post-weld heat treatment is effective. Mahoney et al. found that the post-weld T6 aging treatment would not help in the improvement of the mechanical properties of FSW joints of AA7075-T651 [36]. Sullivan and Robson even found that post weld heat treatment (T7 temper) coarsened nano-sized precipitates in matrix, WNZ, and TMAZ of FSW AA7449 joints, which results in the decrease of hardness in these area [37]. The above conflicting results revealed that the aging conditions of the as-received materials and the precipitates in the as-weld sample should be considered before the application of post-weld heat treatment.

In this work, the effect of pre- and post- heat treatment on the hardness of the weld joints between two technologically important age hardenable Al alloys (7A52 and 6061) was investigated from the point view of microstructural evolution at different scales. Moreover, the underlying physics of precipitation kinetics was revealed by high-resolution transmission electron microscopy (HRTEM) (Thermo Fisher Scientific, Waltham mass, Massachusetts, USA). A new strategy of two-stage aging was proposed to restore precipitation strengthening in the weld based on the above.
