IN718 alloy is commonly adopted in the manufacturing of components such as turbine engine hot-end parts, blades, discs, and critical fasteners due to its high strength, oxidation resistance, and weldability at high temperatures. With the development of new coating technologies, high-temperature-resistant coatings have reduced the actual service temperature of IN718 components. However, the enhancement of equipment performance and deterioration of operating environments have gradually made the surface structure and properties of nickel-based alloys cannot meet the high-end equipment demands. Currently, laser shock peening (LSP) technology has been employed to improve the surface properties of IN718 components. Nevertheless, the short interaction time of LSP with the material surface has caused limitations in enhancing the material surface properties. Meanwhile, LSP induces fish-scale-like plastic deformation pits on the material surface, thus increasing the surface roughness of processed workpieces. To mitigate surface roughness, existing studies often reduce laser energy density to decrease plastic deformation on the sample surface, which significantly reduces LSP enhancement performance. We utilize the ultrasonic-assisted LSP (ULSP) technique to treat the surface of the IN718 nickel-based alloy. The combination of laser shock waves and ultrasonic shock waves is employed to regulate the material surface structure and properties. The microstructural characteristics induced by ULSP and their evolution are analyzed, with their effects on microhardness and residual stress investigated. The surface strengthening mechanism of ULSP on IN718 nickel-based alloy is elucidated to provide support for enhancing the service life of IN718 nickel-based alloy in extreme environments such as fatigue and wear.

The material selected for this experiment is IN718 nickel-based alloy, which is machined into specimens with the dimension of 10 mm×10 mm×2 mm using a wire cutting machine. The LSP experiments are conducted using a Nimma2000 device with a wavelength of 1064 nm and a pulse width of 9 ns. The laser energy density is set at 9 GW/cm², with a laser spot diameter of 1.5 mm, a frequency of 1 Hz, a scanning speed of 0.5 m/s, and an overlap rate of 50%. Subsequently, ultrasonic peening (UP) experiments are performed using custom ultrasonic impact equipment, with a frequency of 20 kHz, a preload of 50 N, an ultrasonic amplitude of 15 μm, a scanning speed of 0.5 m/s, and an overlap rate of 50%. Both LSP and UP utilize an S-type scanning strategy. After the specimen processing, surface morphology measurements are conducted using laser scanning confocal microscopy, while hardness and stress are measured using a microhardness tester and an XRD (X-ray diffraction) stress analyzer respectively. Finally, microstructural testing is carried out using transmission electron microscope (TEM) and XRD. In summary, the characterization includes assessments of surface roughness, surface hardness, residual stress, phase structure, and microstructural evolution of the material.

The surface protrusions of the prepared ULSP specimens become relatively flat, and the surface roughness reduces to 2.19 μm, which decreases by 26.0% compared to UP specimens and by 44.0% compared to LSP specimens (Fig. 2). From the perspective of microstructural evolution, the combined action of laser shock waves and ultrasonic shock waves promotes the evolution of dislocations towards low-energy state structures, thus forming numerous subgrain boundaries, twin boundaries, and a large number of smaller-scale subgrains. Meanwhile, the duration of ultrasonic shock waves is significantly increased compared to laser shock, leading to more precipitation of γ' phases within the IN718 nickel-based alloy. Additionally, high-density dislocation entanglements exist within the grains of ULSP specimens, with dislocation lines distributed perpendicular to them for forming numerous dislocation walls within the grains (Fig. 5). Regarding the phase structure, no new phases are generated after LSP and ULSP, but the diffraction peaks significantly broaden, which indicates a higher dislocation density. Furthermore, the main diffraction peak shifts to the right, further proving that ULSP has a better grain refinement effect than LSP (Fig. 6). Under different laser energies, the microhardness of ULSP specimen surfaces ranges from 352.7 HV to 377.5 HV with a maximum increase of 10.4% compared to the LSP-1.6 J specimen, and the microhardness of ULSP specimens increases with the rising laser energy (Fig. 7). Analysis suggests that the induction of higher dislocation density, smaller grain size, and more γ' phases are the main reasons for the increase in microhardness. Under the same laser energy, the residual stress of ULSP specimens is significantly higher than that of LSP specimens, with the surface residual stress of ULSP-1.6 J specimens reaching up to 344.6 MPa, an increase of approximately 31.4% compared to LSP-1.6 J (Fig. 8). Analysis suggests that ULSP can significantly increase the amplitude of residual stress on the specimen surface by inducing greater plastic deformation, and the residual stress increases with the rising laser energy.

We adopt ULSP technology to perform surface enhancement on IN718 nickel-based alloy. Meanwhile, we conduct a comparative study on the surface roughness, phase structure, dislocation proliferation, microhardness, and residual stress of untreated specimens, LSP specimens, UP specimens, and ULSP specimens to reveal the organizational evolution and strengthening mechanism of ULSP on IN718 nickel-based alloy. The study finds that due to the plastic deformation induced by shock waves, the surface roughness of ULSP and LSP specimens is significantly higher than that of untreated specimens. However, ULSP utilizes the rolling effect of ultrasonic shock waves on laser-induced craters, significantly reducing the surface roughness R_a compared to LSP specimens. Additionally, under the combined action of laser shock waves and ultrasonic shock waves, ULSP can form a high-density dislocation structure and γ' strengthening phase on the specimen surface, while promoting dislocation evolution and generating a large number of twins and subgrains. Finally, a significant refinement of the specimen surface grains compared to LSP specimens is realized. Furthermore, based on the effects of dislocation proliferation, grain refinement strengthening, and precipitation strengthening, ULSP increases the microhardness and residual compressive stress of the material surface compared to LSP at the same laser energy level.