镁/铝薄板搭接型式的搅拌摩擦焊接对于实现交通运载器具的结构轻量化有重要意义。本工作开展了镁合金与铝合金板超声振动强化搅拌摩擦搭接焊实验，并确定最优工艺参数为转速800 r/min、焊速90 mm/min，在焊接过程中对搭接接头匙孔区域采取急停+冷冻处理。对焊接后匙孔周围不同角度的垂直截面和匙孔水平截面的材料流动行为，以及镁合金侧匙孔周围水平截面的特征区域和过匙孔中心焊缝中心线上的各特征区域显微组织进行表征，并阐明了超声振动对搭接接头力学性能的影响。结果表明，搅拌摩擦搭接焊过程中通过引入超声振动，提高了镁/铝搭接接头的拉伸剪切强度以及有效板厚。此外，超声振动使得搅拌头驱动材料的体积增加，增强了材料的流动混合程度；在接头成形过程中，镁合金侧匙孔周围各区域晶粒尺寸分布更加均匀，且搅拌头后方晶粒的再结晶程度显著提高。

With growing emphasis on energy saving and emission reduction, lightweight structures have become a key development focus in vehicle manufacturing. Mg and Al alloys, as lightweight materials with excellent properties, have broad applications in aerospace, automobile manufacturing, 3C products, and other fields. Mg/Al composite components can fully leverage the advantages of both alloys. Therefore, achieving high-quality and high-efficiency joints of Mg/Al dissimilar alloys has become a critical challenge in the manufacturing industry. Among the important structural types of Mg/Al thin-plate dissimilar welded joints, lap joints have attracted considerable attention. Friction stir welding (FSW), a solid-state joining process, offers distinct advantages for producing high-quality, defect-free Mg/Al joints. Ultrasonic-assisted FSW can further broaden the process window and enhance joint strength. However, the mechanism by which ultrasound influences joint formation during welding remains unclear. In this study, ultrasonic vibration enhanced friction stir lap welding experiments were conducted on Mg alloy and Al alloy sheets. The optimal process parameters were determined to be a rotation speed of 800 r/min and a welding speed of 90 mm/min. During welding, the keyhole region of the lap joint was subjected to a sudden stop + freezing treatment. Material flow behavior was characterized on vertical cross-sections at various angles around the keyhole and on horizontal cross-sections. Microstructures of the characteristic regions on the horizontal cross-section around the keyhole and along the weld centerline passing through keyhole center near the Mg alloy side were characterized. The influence of ultrasonic vibration on the mechanical properties of the lap joints was also evaluated. The results show that introducing ultrasonic vibration during friction stir lap welding enhanced both the tensile shear strength and the effective sheet thickness of the Mg/Al lap joints. Furthermore, ultrasonic vibration increased the volume of material driven by the tool, promoting enhanced material flow and mixing. During joint formation, the grain size distribution around the keyhole on the Mg alloy side became more uniform, and the grains behind the tool underwent a significantly higher degree of recrystallization.