The TC4 titanium alloy is the earliest and most widely used titanium alloy in aviation and aerospace structures. However, the TC4 titanium alloy exhibits high sensitivity to notches, which decreases its fatigue resistance and leads to fatigue fracture during use. The detail fatigue rating (DFR) is a parameter that is commonly used in civil aviation to assess the fatigue resistance of the TC4 titanium alloy. Laser shock peening (LSP) is a novel strengthening method that has been proven to effectively improve the fatigue life of the TC4 titanium alloy. Previous research on LSP primarily assesses the fatigue resistance of the TC4 titanium alloy by using methods such as analyzing the S-N (residual stress versus fatigue life) curve of the residual stress, examining fracture surfaces, or conducting finite element simulations. However, there is a limited number of studies that directly investigate the enhancement of TC4 fatigue resistance via LSP based on the DFR_cutoff value. In addition, during the LSP of TC4 samples, plasma easily penetrates the water flow layer in the notch area, and the laser directly impacts the surface of the TC4 notch area, which leads to the ablation and damage of the titanium alloy surface. This, in turn, reduces the DFR_cutoff value and service life of the titanium alloy. Therefore, this article presents a novel LSP strategy that utilizes a discontinuous composite path, consisting of a C-shaped path and a matrix, with a zero overlapping rate. The objective is to investigate the potential of LSP to enhance the fatigue resistance of TC4 materials by considering the DFR_cutoff value. This study serves as a valuable reference for future research on the improvement of fatigue resistance in aviation TC4 alloys via LSP.

First, the TC4 alloy was fabricated into specimens with a gage section geometry of 290 mm×40 mm×7 mm. Subsequently, the specimens underwent a laser shock peening treatment. The surface roughness of the TC4 alloy treated by LSP was measured using a PROTO Proflometer LP200 laser profilometer. The microstructure and morphologies were characterized using an FEI Talos F200X G2 (200 kV) for focused ion beam scanning electron microscopy (FIB-SEM). The residual stress was tested by using a PROTO LXRD X-ray diffractometer with the sin2ψ method. A fatigue life test was performed using an SDZ-0100 fatigue tester at a frequency of approximately 20 Hz, following the guidelines of HB 7110—1994.

The results indicate that there is no significant difference in surface roughness between the areas treated with LSP and the untreated areas for the TC4 titanium alloy when the LSP is conducted at a power density of 10 GW/cm² with varying overlap rates of 0, 25%, and 50%. The average residual compressive stress on the surface of the sample after impact strengthening reaches its peak and varies as the laser power density increases at the same overlapping rate. The residual compressive stress increases with an increase in the overlapping rate under fixed laser power density conditions. The maximum value of the residual compressive stress is reached when the spot overlapping rate increases from 0 to 25%. The DFR_cutoff values of the samples increase with the increase in laser power density owing to LSP treatment at 8 GW/cm², 10 GW/cm², and 12 GW/cm², compared with the DFR_cutoff values of untreated TC4 samples. The DFR_cutoff value increases by 20.76%, compared with the untreated TC4 sample. This is mainly because the laser energy absorbed on the surface of the TC4 sample increases with the increase in laser energy. As a result, the shock wave generated by the laser induces greater plastic deformation and grain refinement on the surface of TC4, which leads to the formation of more dislocations. These dislocations effectively counteract the normal stress and inhibit the generation and propagation of cracks during cyclic loading. Consequently, this leads to a reduction in the local effective load of the sample and thereby enhances the fatigue resistance of the TC4 sample after the LSP treatment and results in a higher DFR_cutoff value.

In this study, a novel approach is employed to systematically investigate the effect of LSP on the cutoff value of the detail fatigue rated strength of the aerospace titanium alloy TC4. This approach involves a unique combination of a rectangular path and a C-shaped path, which together form a discontinuous composite path. The investigation focuses on the impact of a non-continuous composite path with a zero overlapping rate on the surface treatment of LSP. The results indicate that a laser power density of 12 GW/cm² results in a maximum residual compressive stress of -669.67 MPa, with a corresponding compressive stress depth of 0.8 mm. Additionally, fatigue testing reveals that setting the laser power density to 12 GW/cm² leads to a 20.76% increase in the DFR_cutoff value of the TC4 titanium alloy, compared with its original value.