This study investigates the effects of laser shock peening without coating (LSPwc) on the mechanical properties and surface integrity of 12Cr2Ni4A steel, with a focus on optimizing laser energy and impact counts to enhance its performance for aerospace applications. Experiments are conducted using an Nd:YAG laser system (wavelength: 532 nm, pulse width: 8 ns, spot diameter: 0.6 mm) under a coaxial water-jet configuration. The process parameters include laser energies of 70, 100, and 130 mJ, an overlap rate of 50%, a repetition frequency of 50 Hz, and impact counts of 1, 2, and 3 at 130 mJ. The microhardness, microstructure, residual stress, surface roughness, and surface morphology of both untreated and LSPwc-treated samples were systematically characterized and compared.
The results indicate that LSPwc induces plastic deformation in the near-surface region of 12Cr2Ni4A steel, forming a high-density dislocation structure and promoting the transformation of residual austenite to martensite, thereby enhancing microhardness and residual compressive stress. In terms of microhardness, with the increase of laser energy, the hardness of the treated samples reaches 574.78HV0.5, 605.52HV0.5, and 591.85HV0.5, representing increases of 4.1%, 9.7%, and 7.2%, respectively, compared with the untreated sample. Under the same laser energy (130 mJ), as the impact counts increase, the microhardness improvements are 6.7% and 3.7%. Residual stress analysis demonstrates that the maximum compressive residual stress occurs at a subsurface depth of about 50 µm, with values of 520.71, 611.66, and 666.2 MPa for energies of 70, 100, and 130 mJ, respectively. The affected layer depth increases with energy, reaching 400 µm at 130 mJ. Multiple impacts at 130 mJ further enhance the peak residual stress to 722.8 MPa (2 impacts) and 776.2 MPa (3 impacts), with corresponding depths extending to 500 and 550 µm. Kernel Average Misorientation (KAM) maps confirm intensified plastic deformation and increased dislocation density in the treated samples, consistent with the residual stress profiles. Surface roughness (Surface Arithmetic Average Height, Sa: The arithmetic mean of the absolute values of the height differences between all points in the sampling area and the reference plane.) exhibits a non-monotonic trend with increasing laser energy: it increases at 70 mJ due to localized plastic deformation and micro-cracking, decreases at 100 mJ as uniform plastic flow smoothed surface asperities, and rises again at 130 mJ owing to dominant thermal effects such as micro-melting and resolidification. Similarly, increasing the number of impacts leads to progressive roughness deterioration, attributing to cumulative plastic deformation and thermal damage. AFM topography further illustrates the evolution of surface morphology, with maximum peak-to-valley heights increasing significantly under high-energy and multi-impact conditions.
Overall, a single impact at 130 mJ achieves the optimal balance between the hardness gradient and residual stress distribution. Furthermore, the evolution of surface roughness and morphology in the treated samples is governed by the combined mechanical-thermal effects of laser shock peening. The trends of these changes with variations in laser energy and impact count are non-monotonic.
This investigation reveals that the influence of laser energy and impact counts on the mechanical properties and surface integrity of 12Cr2Ni4A steel is nonlinear. While increasing laser energy and impact count enhances the residual compressive stress, the improvement in microhardness does not follow a monotonic trend. This behavior is attributed to dislocation saturation and thermal damage effects induced by high energy levels and multiple impacts. To achieve high-performance laser shock peening for critical components such as aircraft gears and splines, it is essential to consider the combined effects of laser energy and impact counts and identify the optimal strengthening parameters.
Key words
laser shock peening without coating /
12Cr2Ni4A steel /
surface integrity /
process optimization
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Funding
National Key Research and Development Program of China (2022YFB4601703)
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