Abstract Ultrasonic spot welding (USW) is a solid-state joining process highly applicable to lightweight alloy materials with high diffusivity rates in the melt state. To improve the understanding of the USW process and to enhance the weld strength in similar Al alloy welding, this work focused on a parametric study using some process parameters such as vibration amplitude and welding time. Experiments were conducted using lateral-drive USW machine operating in time control mode. Weld quality of USWed joint was analyzed based on the weld strength obtained by lap-shear tests and the fracture pattern subsequently characterized. Mixed fracture pattern of shear and pull-out exhibited a higher lap-shear failure load, while solely shear and pull-out was a distinct fracture pattern in lower lap-shear failure load. Shorter welding time was needed for higher vibration amplitude to generate enough temperature and to prevent over welding. Joint microstructures revealed that longer welding time produced over welding which gave short but intensified weld interface waviness, resulting to a shorter fracture path. The horn and anvil tip surface patterns influenced on the variation in temperature rise measured at the weld part and the extent of part marking/sticking. The temperature rise measured on the weld surface was useful to determine over welding condition. Weldability map of USWed Al alloy sheets could be constructed from the results of weld strength and fracture type, and an effective welding energy input was determined from welding process parameters combination.