Influence of number of laser shock peening impacts on surface integrity of GH625 superalloy

To improve the surface integrity of GH625 superalloy used in the lugs of a first-stage turbine guide in a turboshaft aero-engine, enhance its fatigue and wear resistance, and improve aircraft flight safety and extend service life, experimental research and theoretical analysis on surface modification of GH625 superalloy under different numbers of laser shock peening (LSP) impacts were conducted using the LSP process. Microstructure observation, phase analysis, surface roughness measurement, residual stress detection, microhardness measurement, and wear testing were performed using an Axio Imager Mzm optical microscope, D8 Advance X-ray diffractometer, PS50 non-contact profilometer, XL-640 stress measurement instrument, HXD-1000TMSC/LCD hardness tester, and T50 tribometer, respectively. The results showed that multiple LSP impacts significantly refined the surface grains of the GH625 superalloy (Figure 1). The average grain size of the unimpacted GH625 superalloy was 23.2 μm, while the average grain sizes of the surface layer of GH625 superalloy after 1, 3, and 5 impacts were 12.8 μm, 10.0 μm, and 8.8 μm, respectively. Compared with the unimpacted GH625 superalloy, the surface average grain size decreased by 44.8%, 56.9%, and 62.1% after 1, 3, and 5 impacts, respectively, indicating that the grain refinement degree in the surface layer of GH625 decreased with increasing impact numbers. When the number of impacts was 0, 1, 3, and 5, the Vickers hardness of the GH625 impacted surfaces was 265.5 HV, 333.0 HV, 348.0 HV, and 355.0 HV (Figure 2). The surface roughness Ra was 0.46 µm, 0.91 µm, 1.31 µm, and 1.72 µm, respectively. The residual stress values were −56.3 MPa, −352.5 MPa, −448.9 MPa, and −490.5 MPa (Figure 3), the full width at half maximum (FWHM) of the (111) diffraction peak was 0.33°, 0.43°, 0.48° and 0.51° (Figure 4 and Table 3), and the wear rates were 19.74×10−6 mm³·s⁻¹·N⁻¹, 18.38×10−6 mm³·s⁻¹·N⁻¹, 13.82×10−6 mm³·s⁻¹·N⁻¹, and 4.6×10−6 mm³·s⁻¹·N⁻¹, respectively (Table 4, Figures 5 and 6). Compared with the unimpacted GH625 superalloy, the surface roughness Ra increased by 97.8%, 184.8%, and 273.9%, the surface microhardness increased by 25.4%, 31.1%, and 33.7%, and the wear rate decreased by 6.9%, 29.9% and 68.4% after 1, 3, and 5 impacts, respectively. This indicated that as the number of impacts increased, the growth amplitude of roughness Ra continuously increased, the increase amplitude of surface hardness progressively decreased, and the reduction amplitude of wear rate gradually increased. With increasing impact numbers, the maximum residual compressive stress on the impacted surface increased continuously, and the affected layer depth increased from 0.6 mm to 1.4 mm. Multiple impacts did not change the phase composition of GH625 superalloy, but the full width at half maximum (FWHM) of the diffraction peaks increased with increasing impact numbers. Multiple impacts have significantly improved the surface integrity of GH625 superalloy, providing experimental and theoretical foundations for optimizing multi-impact processing techniques and industrial applications.