This study investigates the microstructural evolution and mechanical performance of Al–Mg–Si alloy plates repaired using preheating-assisted additive friction stir deposition (AFSD). The AFSD technique was employed to repair 5 mm deep blind holes on 6061 aluminum alloy substrates, with the aim of evaluating the effects of tool rotation speed and substrate preheating on repair quality. The results indicate that preheat-assisted AFSD significantly improved joint bonding and strength compared to non-preheat conditions. Increasing the rotation speed also enhanced metallurgical bonding and prevented volume defects. Under preheating conditions, the ultimate tensile strength (UTS) and elongation were positively correlated with rotation speed. At a tool rotation speed of 1000 r/min and preheating, defect-free specimens were obtained, with the UTS and elongation reaching 164.2 MPa and 13.4%, respectively, which are 99.4% and 140% of the heated substrate's values. The study highlights the potential benefits and limitations of AFSD as a solid-state additive repair method for Al–Mg–Si alloys.This study investigates the microstructural evolution and mechanical performance of Al–Mg–Si alloy plates repaired using preheating-assisted additive friction stir deposition (AFSD). The AFSD technique was employed to repair 5 mm deep blind holes on 6061 aluminum alloy substrates, with the aim of evaluating the effects of tool rotation speed and substrate preheating on repair quality. The results indicate that preheat-assisted AFSD significantly improved joint bonding and strength compared to non-preheat conditions. Increasing the rotation speed also enhanced metallurgical bonding and prevented volume defects. Under preheating conditions, the ultimate tensile strength (UTS) and elongation were positively correlated with rotation speed. At a tool rotation speed of 1000 r/min and preheating, defect-free specimens were obtained, with the UTS and elongation reaching 164.2 MPa and 13.4%, respectively, which are 99.4% and 140% of the heated substrate's values. The study highlights the potential benefits and limitations of AFSD as a solid-state additive repair method for Al–Mg–Si alloys.