GH4169 is a nickel-based superalloy widely used in manufacturing aero-engine blades. However, it is affected by ultra-high cycle fatigue due to the low impact energy and small spot diameter of microscale laser shock peening. To address these issues, microscale laser shock peening reinforcement is employed to improve the fatigue properties of the material while controlling the depth of propagation of the shock wave to achieve macro-deformation of the thin blades and synergistic enhancement of fatigue properties. Currently, the effect of microscale laser shock peening on the ultra-high cycle fatigue properties of GH4169 alloy has not been investigated in domestic and international studies. Consequently, this effect is investigated in this study using an ultrasonic fatigue testing machine.

In this study, GH4169 alloy was used, and the fatigue specimen size was obtained through theoretical design and simulation. The specimens were subjected to microscale laser shock peening using three process parameters: 62 mJ impact energy and 1 time impact (62 mJ&1 time ), 62 mJ impact energy and 3 times impacts (62 mJ&3 times ), and 82 mJ impact energy and 1 time impact (82 mJ&1 time ). Subsequently, axially symmetric ultrasonic ultra-high cycle fatigue tests were conducted at room temperature, and fatigue fracture morphologies were analyzed using scanning electron microscope (SEM) with energy-dispersive spectroscope (EDS). Confocal laser scanning microscopy (CLSM) was used to analyze the surface morphologies strengthened by microscale laser shock peening, while X-ray diffraction (XRD) analysis was used to determine the distribution of residual stress in the surface layer. Additionally, the microstructure of the surface layer after microscale laser shock peening was observed using electron backscatter diffraction (EBSD). The analytical results were synthesized to reveal the strengthening mechanism of microscale laser shock peening in improving the ultra-high cycle fatigue properties of GH4169 alloy.

The S-N curves (Fig. 4) indicate that microscale laser shock peening improves the ultra-high cycle fatigue properties of GH4169. Fatigue fracture morphologies (Figs. 5?8) show that slip induces crack initiation, while microscale laser shock peening inhibits surface crack initiation. Additionally, surface morphology analysis (Figs. 9 and 10) reveals that microscale laser shock peening increases surface roughness. However, fatigue test results show that the increase in surface roughness due to laser shock peening is not the primary factor affecting the failure mechanism and properties of ultra-high cycle fatigue. Combining fatigue test results with surface residual stress distribution data (Fig. 11) indicate that the residual compressive stress introduced by microscale laser shock peening inhibits surface crack initiation. Furthermore, a combination of fatigue test results and microstructure EBSD analysis (Figs. 12 and 13, Table 4) reveals that refined surface grains and high-density dislocations contribute to fine grain and dislocation strengthening, respectively, thereby improving the ultra-high cycle fatigue properties of the material.

In this study, the influence of microscale laser shock peening on the ultra-high fatigue properties of GH4169 alloy is investigated using ultrasonic fatigue tests combined with the analysis of the fracture morphologies, surface morphologies, residual stress, and microstructure. This comprehensive approach enables a detailed understanding of the strengthening mechanism of microscale laser shock peening in improving the ultra-high cycle fatigue properties of GH4169 alloy. The main conclusions are as follows: microscale laser shock peening improves the ultra-high cycle fatigue properties of GH4169 alloy, allowing the material to withstand higher stress amplitudes at the same fatigue life. When the fatigue life reaches 1×10⁸ cycles, 62 mJ&1 time, 62 mJ&3 times, and 82 mJ&1 time specimens endure cyclic stress amplitudes of 450, 500, and 475 MPa, respectively, representing increases of 12.5%, 25%, and 18.75%. Microscale laser shock peening improves surface roughness, with R_a values of 0.926, 1.020, and 0.935 μm for 62 mJ&1 time, 62 mJ&3 times, and 82 mJ&1 time specimens, respectively. Increasing the impact energy and the numbers of impacts increases surface roughness but does not contribute to surface fatigue failure. Microscale laser shock peening introduces residual compressive stress within a depth range of 400?500 μm, with a gradient distribution. The 62 mJ&3 times specimen exhibits a maximum residual compressive stress of -761 MPa below the surface. Additionally, microscale laser shock peening refines grains of the surface layer and increases dislocation density, reducing the average diameters of the surface grains to 5.23, 4.09, and 4.68 μm, respectively. The average kernel average misoreintation (KAM) values of the surface layer reach 0.25°, 0.29°, and 0.27°, respectively, while the geometrically necessary dislocation (GND) densities increase to 0.78×10¹⁴, 0.9×10¹⁴, and 0.85×10¹⁴/m2, respectively. After microscale laser shock peening, the sources of fatigue cracks in GH4169 alloy shift from the surface to the interior because the residual compressive stress suppresses surface crack initiation and effectively balances the tensile stress. Additionally, surface layer grain refinement and the introduction of high-density dislocations contribute to fine grain and dislocation strengthening, respectively, further inhibiting surface crack initiation and improving the fatigue properties of the material.