Some typical defects like delamination, fiber fracture, non-uniform resin distribution and springback are inevitable as titanium-based thermoplastic FMLs are formed by general stamping process at elevated temperature due to the significant differences of various constituent materials in mechanical and thermal properties as well as deformation mechanism. Thus, a novel ultrasonic impact sizing/shaping process method was proposed in the present work, in which a stepped horn and impact tools with a cylindrical tip end was designed by means of vibration modal and harmonic response analysis, and the corresponding experiment setup was established to verify the process mentioned above. Moreover, ultrasonic impact tests were carried out for titanium-based FMLs stacked by titanium sheet, thermoplastic resin film and carbon fiber reinforced fabric to reveal their sizing/shaping mechanism and the effect of key process parameters such as ultrasonic amplitude, scanning speed of impact tool and its tip end diameter on the surface and interface characteristics of titanium-based FMLs. The research results show that the ultrasonic impact scheme with the ultrasonic amplitude of 12 μm, the scanning speed of impact tool within 2 mm/s ∼ 3 mm/s, the tip end diameter of impact tool within Ø4mm∼Ø6mm are proper for the ultrasonic impact sizing/shaping process of titanium-based FMLs.

The distribution of surface integrity features directly affects the initiation and propagation of fatigue cracks. In this paper, the surface integrity characteristics changing law of turning and ultrasonic impacting specimens during high cycle fatigue loading has been studied, and the effect of surface modified layer on the fatigue properties of titanium alloy has been revealed. The results showed that the surface roughness increased with the increase of fatigue cycles. The compressive residual stress and its gradient distribution depth decreased continuously. The gradient distribution depth of residual stress in the ultrasonic-impacted surface rapidly decreased by about 50% near the fracture stage. Local cyclic hardening occurred at 20-50 μm from the surface of the specimen in the early stage of fatigue evolution, and then the microhardness continued to decrease. During this process, there were no significant changes in hardened layer depth. The fibrous microstructure of the ultrasonic-impacted surface undergoes a process from coarsening to gradual disintegration during the fatigue process. Its attenuation process needs a longer period of time. The fatigue source of the turned specimen was located at about 320 μm from the surface, and the fatigue source of ultrasonic impact was about 610 μm from the surface. The fatigue striation width of the ultrasonic impact specimen was about 20% narrower than that of the turned specimen. The fatigue life of the ultrasonic impact specimen was increased by 73.9% compared with the turned specimen. The research in this paper is of great significance for exploring the anti-fatigue mechanism and the ability of various surface integrity features.