Q355GNHD high-weathering steel is a high-strength low-alloy structural steel with high yield strength, excellent low-temperature toughness, low carbon equivalent, and good weldability. With potential for lightweight design, strong impact and weather resistance, and high cost-effectiveness, this steel is a key material in rail transit, especially for high-speed train nose skins in cold regions, and it is also used in components such as heavy machinery, ship decks, and pressure vessels. During nose-skin forming, surface defects can easily induce stress concentration, thereby reducing the mechanical strength, fatigue life, and reliability of the component. Laser shock forming (LSF) is a flexible dieless forming technology based on ultra-high-speed cold plastic deformation. It uses nanosecond short-pulse lasers with a power density of up to 10⁹ W/cm² to impact the metal surface. After absorbing the energy, the protective layer undergoes explosive vaporization and generates GPa-level plasma shock waves. These shock waves can induce cumulative plastic strain within the material, enabling precision forming while improving fatigue resistance and corrosion resistance. Compared with traditional stamping, LSF offers non-contact processing and high flexibility. The depth of the induced residual compressive stress layer reaches 1.0 mm~2.0 mm, which is much greater than that achieved by mechanical shot peening, and it can process small-curvature components that are difficult to handle by shot peening. Existing studies have confirmed that laser process parameters such as spot size and overlap rate affect deformation and surface integrity, but studies on the “parameter-deformation-surface quality” relationship for Q355GNHD steel remain limited. This study aims to investigate the effects of spot size and overlap rate on the bending deformation and surface integrity (surface roughness, residual stress, and microhardness) of Q355GNHD steel, identify the optimal process parameters, and provide support for optimizing LSF in rail transit applications.

The test material was a 3 mm thick Q355GNHD high-weathering steel plate. Specimens measuring 150 mm × 30 mm were cut using a 6000 W laser cutter, followed by ultrasonic cleaning and drying for subsequent use. LSF experiments were conducted using a Nd:YAG laser. The specimens were fixed by unilateral clamping (clamping length: 20 mm), and the laser shock area was the central 110 mm × 30 mm region. After the specimen surfaces were cleaned with absolute ethanol, a 100 μm thick 3M-471 black tape was applied as the absorbing layer to prevent surface ablation, and a 1 mm~2 mm flowing deionized water layer was used as the confining layer to enhance the shock-wave pressure. The fixed parameters were a laser energy of 15 J, a pulse width of 15 ns, a repetition rate of 5 Hz, and a single impact. The variable parameters were spot size (3 mm, 4 mm, 5 mm) and overlap rate (0%, 10%, 30%). After the tests, the specimens were cleaned with absolute ethanol to remove residual tape. Bending deformation was measured using a Hexagon RigelScan3D scanner, with arc height as the evaluation index. Surface roughness was measured with a Bruker Contour GT-K white-light interferometer (5^× objective), and the R_a value was calculated. Residual stress was measured using a Proto-LXRD X-ray stress analyzer (sin(2ψ) method; Pearson Ⅶ fitting; Cr target K_α radiation; 30 kV and 20 mA). Microhardness was tested using an HVS-1000 Vickers hardness tester (load: 500 g, dwell time: 15 s). Six measurement points were selected along the x-axis at 75 mm from the specimen end along the length direction, and each point was tested four times to obtain the average value.

In terms of bending deformation, a smaller spot size led to a larger deformation (Fig.3). The arc heights for spot sizes of 3 mm, 4 mm, and 5 mm were 5.89 mm, 2.63 mm, and 1.86 mm, respectively, and the corresponding curvature radii were 476 mm, 1066 mm, and 1505 mm. Smaller spot sizes resulted in higher power density: at 15 J, the power densities for 3 mm, 4 mm, and 5 mm spots were 14.1 GW/cm², 7.96 GW/cm², and 5.09 GW/cm², respectively. However, the 3 mm spot caused edge ablation (Fig.2), whereas no such ablation was observed for the 4 mm and 5 mm spots. A higher overlap rate led to a larger deformation (Fig.5). The arc heights at overlap rates of 0%, 10%, and 30% were 2.29 mm, 2.63 mm, and 3.76 mm, respectively, and the corresponding radii of curvature were 1224 mm, 1066 mm, and 744 mm. No ablation occurred at any overlap rate (Fig.4), because a higher overlap rate means more laser impacts within the same area, resulting in a larger total bending moment. For surface roughness, it was negatively correlated with spot size (Fig.8): the roughness values for 3 mm, 4 mm, and 5 mm spots were 2.705μm, 1.960μm, and 1.330 μm, respectively. The 3 mm spot produced deeper indentations, the 4 mm spot showed a flat-top profile and the 5 mm spot produced shallower indentations but more burrs (Fig.6 and Fig.7). With increasing overlap rate, the surface roughness changed accordingly (Fig.11): the roughness values at 0%, 10%, and 30% overlap were 2.008μm, 1.960μm, and 2.205 μm, respectively. When the overlap rate exceeded 10%, the roughness deteriorated significantly, and “valleys” and “peaks” appeared on the surface at 30% overlap (Fig.9 and Fig.10). Residual stress (negative values indicate compressive stress) decreased as the spot size increased (Fig.12): −315.16 MPa for 3 mm, −272.69 MPa for 4 mm, and −245.82 MPa for 5 mm. It increased with overlap rate (Fig.13): −259.53 MPa at 0% overlap, −272.69 MPa at 10% overlap, −313.52 MPa at 30% overlap. Microhardness decreased as the spot size increased (Fig.14): 235.26 HV (↑10.2%) for 3 mm, 226.90 HV (↑6.3%) for 4 mm, and 222.54 HV (↑4.2%) for 5 mm. It increased with overlap rate (Fig.15): 223.5 HV (↑4.7%) at 0% overlap, 226.9 HV (↑6.3%) at 10% overlap, and 234.0 HV (↑9.6%) at 30% overlap, which was attributed to the enhanced work-hardening effect.

The optimal LSF parameters for Q355GNHD steel are a 4 mm spot size and 10% overlap rate. This parameter combination ensures a favorable deformation effect while maintaining low surface roughness and appropriate residual stress and microhardness. This study provides a theoretical basis for optimizing LSF of high-weathering steel in rail transit applications.