A duplex surface modification method for increasing bending fatigue life of aviation carburized gear steel considering its mechanical properties and microstructure gradients

As an effective surface modification technology, laser shock peening can improve the fatigue performance of aerospace components. However, for gear components with complex configurations, this technology exhibits significant technical limitations such as difficulties in adhering absorbed layers and distortion of the laser spot due to the curvature at tooth roots. This paper innovatively introduces a method combining laser shock peening without coatings and vibratory polishing, aiming to enhance the bending fatigue performance of gear steel. Underlying mechanisms of bending fatigue resistance have been elucidated through the examination of mechanical properties, surface integrity, and microstructural evolution during modification. After LSP without coatings combined with vibratory polishing, a gradient structure of nanocrystals-fine grains was formed along the depth direction, with significant increases in dislocation density and types of dislocations. The mechanical properties of samples were improved by introducing a work hardening layer with thickness of 570 μm and a residual compressive stress layer with thickness of 510 μm. Adverse surface factors such as ablation and remelting morphologies were mitigated. Compared to as-received samples, bending fatigue limit was significantly increased by approximately 86.1 %. Fatigue resistance mechanisms can be attributed to combined effects of the second-phase strengthening mechanism caused by amorphous dispersed carbides, residual compressive stress, high-density dislocations, and interlocking structure coupled with nanotwins and intergranular dislocations. The duplex surface modification process integrates the advantages of the shockwave force effect induced by laser with the exceptional surface integrity achieved through vibration polishing, which renders impetus for the researchers striving for aerospace gear performance improvement.