Influence of Tool Relief Angle on Surface Integrity and Fatigue Behavior During Ultrasonic Peening Drilling of Ti-6al-4v

Imposing the compressive residual stress field around a fastening hole serves as a universal method to enhance the hole fatigue strength in the aircraft assembly filed. Ultrasonic peening drilling (UPD) is a recently proposed hybrid hole making process, which can achieve an integration of strengthening and precision-machining with a one-shot drilling operation. Due to the ironing effect of the tool relief surface, UPD can introduce large compressive residual stress filed in hole subsurface. In order to reveal the strengthening mechanism of UPD, this paper analyzed the characteristics of dynamic relief angle and its corresponding effect on surface integrity in UPD. The experiments were conducted to verify the effect of tool relief angle on surface integrity and fatigue behavior of Ti-6Al-4V hole in UPD. The results indicate that the specimen features smaller surface roughness, higher micro-hardness, plastic deformation degree and circumferential compress residual stress under smaller tool nominal relief angle and higher vibration amplitude. The maximum thickness multiple of plastic deformation layer achieved by UPD is 6.25 times when vibration amplitude increases from 0 μm to 5 μm at 7°, in which the surface circumferential compressive residual stress reaches -982.6 MPa. The fatigue life increases along with the decrease of tool nominal relief angle or the increase of vibration amplitude, and the fatigue source site in UPD shifts from the surface to the subsurface comparing with that without vibration assistance. The results demonstrates that a better strengthening effect can be obtained by reasonably controlling the tool relief angle in the process of UPD.