The processing of a large number of high-quality gas film holes in novel materials with higher thermal resistance, lower mass density, and certain formability has been identified as a critical issue that should be addressed for the development and application of advanced high thrust-to-weight ratio aerospace engines. However, the preparation of small holes with large depth-to-diameter ratios has been significantly constrained when using a single conventional or non-conventional machining technique. In recent years, hybrid machining technologies, particularly laser and electrochemical hybrid machining, have been widely favored due to their complementary advantages. However, the transmission of the laser is constrained within the salt solution, leading to a laser utilization of less than 45% (for tubular electrode lengths exceeding 45 mm). Furthermore, the laser utilization is further diminished with the increase in the depth-to-diameter ratio. Meanwhile, in existing laser-electrochemical hybrid machining technology, Gaussian laser beams are employed, causing the energy to be concentrated in the central region, while the energy distribution in the peripheral areas is significantly reduced. Additionally, the mechanisms of interaction among the laser, electrolyte fluid, and electrochemical reactions as well as the processes of material ablation are inadequately defined. Therefore, improvements in laser utilization and energy distribution during hybrid machining, along with the elucidation of multiphysics coupling mechanisms involved, are deemed essential for the advancement of deep small hole machining using laser-electrochemical hybrid technology.

An annular laser and electrochemical hybrid machining technology that combines the uniform distribution of energy from the annular laser with the characteristics of tube electrode electrolysis, which includes defect-free, lossless processing, and the capability to penetrate into the workpiece interior, is proposed in this article. The technology employs a ring fiber to transmit the laser and a metal tube (tube 1) nested within the ring fiber to deliver the electrolyte, and another metal tube (tube 2) nested around the exterior of the ring fiber is used to conduct the current. This configuration effectively mitigates laser attenuation during transmission, significantly enhancing the laser utilization efficiency on the workpiece surface. Furthermore, the finite element analysis is also used to examine the multiphysics field coupling mechanisms and material removal mechanism during the hybrid processing. Subsequently, deep small holes machining experiments are conducted on a TC4 substrate (size of 20 mm×20 mm×15 mm) using a custom-designed hybrid processing setup (Fig. 13). The microstructural morphologies of the deep small holes, prepared by electrochemical machining and hybrid processing, are comparatively analyzed using scanning electron microscope, optical microscope, and energy-dispersive X-ray diffraction.

The flow field transmission characteristics during the hybrid processing significantly influence the stability of the multi-energy field interactions. Therefore, the distributions of the flow fields under different inter-electrode gaps, inlet pressures, and depth-to-diameter ratios are presented (Fig. 4). The results indicate that electrolyte rapidly spreads across the surface after it impacts the workpiece surface, resulting in the maximum electrolyte flow velocity on the workpiece surface. Moreover, increasing the processing gap and depth-to-diameter ratio significantly reduce the electrolyte flow velocity, while increasing the inlet pressure enhances the flow velocity. According to Equations (10) and (11), the distribution of current density on the workpiece surface during the hybrid processing is closely related to the electrolyte conductivity, gas volume fraction, and temperature. Therefore, the electrolyte conductivity, gas volume fraction, and electrolyte temperature as a function of laser power, voltage, and machining time are presented (Figs. 5?8). The results indicate that the conductivity within the annular fiber region exhibits a Gaussian distribution, reaching a maximum value at approximately 188 μm from the central origin. As the voltage and laser power increase, the electrolyte temperature and gas volume fraction also increase. Therefore, increasing voltage and laser power enhance the current density and improve the localized removal capability. When the laser power is below 25 W, the material removal mechanism is characterized by laser heating-assisted electrochemical machining. Conversely, when the laser power exceeds this threshold, the process shifts to electrochemical-assisted laser machining (Fig.9). Deep small hole machining experiments demonstrate that employing this hybrid technology results in significantly improving machining quality of the deep small holes.

This study addresses the challenge of achieving high efficiency and high-quality deep small hole machining with existing technologies by the proposed annular laser and tube-electrode electrochemical hybrid processing based on laser-current-solution separated transmission. Furthermore, in this study, the multi-physics field coupling mechanisms involved in the hybrid processing are investigated. The main conclusions are as follows: 1) Increasing the processing gap and depth-to-diameter ratio significantly reduce the electrolyte flow velocity. Increasing the inlet pressure facilitates the rapid renewal of the electrolyte within the processing gap. 2) Increasing the voltage and laser power increase the workpiece surface temperature and electrolyte temperature in the processing gap, as well as increase the electrolyte conductivity, gas volume fraction, and current density. 3) As the processing voltage and laser power increase, the anode workpiece removal rate accelerates. When the laser power is low, laser heating activates the workpiece surface and increases the electrolyte temperature. This leads to accelerated electrochemical dissolution rate, making electrochemical dissolution the predominant removal mechanism. Conversely, when the laser power increases to the point where the workpiece is melted, the material removal rate via laser processing is significantly higher than that of electrochemical machining, making laser processing the predominant removal mechanism.