Fatigue Performance Improvement of 7075-T651 Aluminum Alloy by Ultrasonic Nanocrystal Surface Modification

In this research, ultrasonic nanocrystal surface modification (UNSM) was used to process a 7075-T651 aluminum alloy. After UNSM treatment, a work-hardened layer with compressive residual stress up to 500 MPa was generated on the near-surface region of the sample. The surface microhardness of the alloy was increased from 195 ± 4 to 225 ± 7 HV, and the surface roughness was decreased from 1.32 ± 0.04 to 0.54 ± 0.27 µm. The stress concentration factor Kt of the as-received specimen was decreased from 1.080 to 1.069 after UNSM processing. Rotating–bending fatigue tests showed that UNSM significantly improved the fatigue performance. In addition, UNSM-treated 7075-T651 samples were able to endure pitting corrosion in a 3.5 wt.% NaCl solution for 72 h without any degradation in fatigue performance. These results demonstrated that UNSM is a robust surface modification method that can improve the rotating–bending fatigue resistance of the aluminum alloy.