The microstructure, hardness, lap shear strength and fracture energy of AA2139–TiAl6V4 spot joints produced by ultrasonic welding were investigated and related to the weld thermal cycle. No obvious intermetallic reaction layer was observed in the AA2139–TiAl6V4 welds, even using transmission electron microscopy. The hardness profile of AA2139 side after welding was studied, demonstrating that the heat introduced by the welding process leads to some softening with partial hardness recovery after natural aging. The effects of welding time on peak load and fracture energy were investigated. The peak load and fracture energy of welds increased with an increase in welding time and then reached a plateau, i.e., maximum peak load 5.3kN and maximum fracture energy 3.7kNmm. In all cases, failure occurred by fracture at the weld interface.