Enhancing joint strength of bobbin tool friction stir welded Al–Mg–Si alloy via post-weld aging process

For the precipitation-strengthened aluminum alloys such as Al–Mg–Si alloys, the friction stir welded (FSW) joints produced with T6-treated plates usually exhibit obvious softening in the heat-affected zones (HAZ), significantly reducing the joint strength. When bobbin tool FSW (BT-FSW) is used, severer softening of the HAZ occurred compared to conventional FSW. It is therefore crucial to achieve precipitate morphology control in the HAZ for BT-FSW through original plate state selecting, welding parameter optimizing, and post-weld artificial aging (PWAA). Compared to the conventional welding process (plates welded directly under T6 condition, T6-BT), in the present work, a post-weld aging process was proposed: the plates under T4 condition were BT-FSWed using optimizing parameters (T4-BT), and then subjected to PWAA (T4-BT-PWAA). The intrinsic microstructural differences between the T6-BT and T4-BT-PWAA joints were revealed. More attentions were paid on the microstructural evolution in the lowest hardness zone (LHZ) during PWAA for the T4-BT joint. The LHZ of the T6-BT joint exhibited an over-aged state characterized by coarsened β′ phase and the hardness in this zone had very limited enhancing space when subjected to post-weld peak aging. For the T4-BT joint, the LHZ was characterized by a low density of precipitates and presented excellent re-precipitating ability of β'' phase during PWAA, with the ultimate tensile strength increased by ∼11.4% compared to its T6 counterpart, which is very beneficial for engineering applications.