This study investigates the microstructural evolution and mechanical performance of Al–Mg–Si alloy plates repaired by preheating-assisted additive friction stir deposition (AFSD) at different rotational speeds. The results show that preheating-assisted AFSD significantly improves joint bonding and strength compared to non-preheated conditions. Increasing the tool rotation speed enhances metallurgical bonding and reduces defects. Under preheating conditions, ultimate tensile strength (UTS) and elongation are positively correlated with rotation speed. At a tool rotation speed of 1000 r/min and preheated substrate, defect-free specimens were obtained with tensile fracture occurring in the substrate rather than the repaired zone. The UTS and elongation reached maximum values of 164.2 MPa and 13.4%, equivalent to 99.4% and 140% of the heated substrate, respectively. AFSD is a solid-state additive manufacturing technology that can effectively overcome solidification defects. It produces high-quality interfaces and refines the microstructure of cast aluminum alloys. Compared to other additive manufacturing technologies, AFSD is more efficient. It is also used for structural repair, with some studies showing its effectiveness in repairing through-holes and grooves in Al alloys. However, the effect of preheating on repair quality by AFSD has not been studied. This study introduces a simplified AFSD method for blind hole repair, with preheating of the substrate to improve material flow and bonding. The microstructure evolution and mechanical properties of the repaired Al–Mg–Si alloy plates at different rotational speeds and preheating conditions were investigated to understand the relationship between repair quality and heat input. The results indicate that preheating and increasing rotation speed improve repair quality.This study investigates the microstructural evolution and mechanical performance of Al–Mg–Si alloy plates repaired by preheating-assisted additive friction stir deposition (AFSD) at different rotational speeds. The results show that preheating-assisted AFSD significantly improves joint bonding and strength compared to non-preheated conditions. Increasing the tool rotation speed enhances metallurgical bonding and reduces defects. Under preheating conditions, ultimate tensile strength (UTS) and elongation are positively correlated with rotation speed. At a tool rotation speed of 1000 r/min and preheated substrate, defect-free specimens were obtained with tensile fracture occurring in the substrate rather than the repaired zone. The UTS and elongation reached maximum values of 164.2 MPa and 13.4%, equivalent to 99.4% and 140% of the heated substrate, respectively. AFSD is a solid-state additive manufacturing technology that can effectively overcome solidification defects. It produces high-quality interfaces and refines the microstructure of cast aluminum alloys. Compared to other additive manufacturing technologies, AFSD is more efficient. It is also used for structural repair, with some studies showing its effectiveness in repairing through-holes and grooves in Al alloys. However, the effect of preheating on repair quality by AFSD has not been studied. This study introduces a simplified AFSD method for blind hole repair, with preheating of the substrate to improve material flow and bonding. The microstructure evolution and mechanical properties of the repaired Al–Mg–Si alloy plates at different rotational speeds and preheating conditions were investigated to understand the relationship between repair quality and heat input. The results indicate that preheating and increasing rotation speed improve repair quality.