Abstract: Trailing ultrasonic impact welding (TUIW), impacted the welds by controlling the impact-tool up and down, with periodic ultrasonic energy conducted to the molten pool at the same time. Metal flowing patterns changed periodically under the influence of periodic ultrasonic energy, obvious scales formed in the welds surface similar to the fish-scale. Compared with conventional TIG, it turns out that TUIW periodic ultrasonic energy leads to periodic crystallization during Al-Cu alloy molten pool solidification process. With ultrasonic in, the weld grains were refined and the elongated eutectics were broken down, the width of small grain zone depend linearly on welding velocity and the ultrasonic action time in a single cycle. When ultrasonic began to import, banded structure formed in the weld, θ phase was small and distributed evenly within the organization. Meanwhile, eutectic organization precipitated linearly and tended to be gathered.