Laser shock micro-hydraulic bulging (LSMHB) is a novel high-strain-rate forming technology that combines the advantages of laser shock forming and hydraulic shock forming. It is characterized by high strain rate, excellent surface quality, and stable forming, effectively improving the forming performance of metallic materials, which has attracted increasing attention. Previous studies have mainly focused on the use of cylindrical liquid chambers, however the influence of liquid chamber geometry on forming performance has not been extensively explored. In conventional LSMHB, excessive thinning and localized stress concentrations are often observed at the fillet regions, leading to surface defects and even failure. Therefore, this study aims to improve the forming process by using a conical frustum liquid chamber, replacing the conventional cylindrical chamber. This research systematically investigates the effects of laser energy on material forming depth, thickness distribution, surface quality, and microhardness, using TC4 titanium alloy foil as the experimental material. The goal is to improve forming uniformity and address the excessive thinning at the fillet regions.

To evaluate the performance of the conical frustum liquid chamber in the LSMHB process, a series of experiments were conducted at varying laser energy levels (30%?90%). The TC4 titanium alloy foil used in this study had a thickness of 50 μm. The liquid chamber design was optimized to improve pressure distribution based on numerical simulations and the response surface methodology. During the experiments, the laser system generated a high-intensity nanosecond pulse, and the laser energy was precisely adjusted to ensure repeatability and consistency. Forming depth and thickness distribution were measured using the digital microscope, and surface roughness and 3D morphology were detected using the microscope. Microhardness analysis was performed using cross-sectional samples obtained through cold mounting and measured with the micro Vickers hardness tester to assess the impact of laser energy on material strengthening. Finally, the experimental results were analyzed to assess the advantages of the conical frustum liquid chamber in improving forming performance compared to the cylindrical liquid chamber.

The experimental results show that as laser energy increases, the forming depth increases uniformly. At 90% laser energy, the maximum forming depth reaches 287.30 μm, surpassing the forming depth of the cylindrical liquid chamber under the same conditions. This improvement is attributed to the optimized conical frustum liquid chamber, which allows the laser shock wave to be transmitted more evenly to the workpiece surface. Additionally, the thinning ratio at the fillet region remains stable across all laser energy levels, avoiding the excessive thinning and microhardness spikes typically observed in conventional LSMHB processes. Surface roughness analysis indicates that as laser energy increases, the surface roughness changes moderately. Microhardness tests show a consistent increase in material hardness with increasing laser energy and a minimum hardness increase of 17.28% at 90% laser energy. This enhancement is attributed to the high-strain-rate deformation mechanism, which promotes dislocation accumulation and strain hardening, thus strengthening the material. The study on the forming performance of TC4 shows that the optimized liquid chamber design effectively improves the forming capability of LSMHB process.

This study demonstrates that the introduction of the optimized liquid chamber in the LSMHB process results in a smoother thickness transition at the fillet region, improving the microstructural hardness of TC4 titanium alloy foil and significantly enhancing the forming performance. The conical frustum liquid chamber allows for a more uniform transmission of the laser shock wave, effectively solving the excessive thinning and microhardness spikes observed in the fillet region of traditional cylindrical liquid chambers. The results provide valuable theoretical and experimental support for optimizing the LSMHB process.