Water-jet guided laser (WJGL) machining utilizes a hair-thin water jet as an optical fiber to guide the laser beam for precision cutting. It offers advantages such as minimal thermal effect, no need for real-time focus adjustment, parallel kerf walls, high aspect ratio, and pollution-free. However, during the WJGL machining, the intense interaction among the laser, the high-pressure water jet, and the material forms a highly complex multi-energy-field coupling process involving optics, hydrodynamics, and thermodynamics. Investigating the influence of temperature evolution and fluid flow during WJGL machining is extremely intricate, as the physical mechanisms of material removal significantly impact the machining quality. Nevertheless, experimental investigation of these mechanisms is challenging.

To investigate the material removal mechanisms under multi-field coupling in WJGL machining, a multi-pulse transient numerical model incorporating solid?liquid?gas three-phase multi-field coupling was established for WJGL processing of monocrystalline silicon. Experimental validation was conducted to confirm the model's effectiveness and accuracy. The melting and vaporization processes of monocrystalline silicon during WJGL machining involve solid?liquid and gas?liquid interface tracking problems. In this study, the enthalpy?porosity method was employed for solid?liquid interface tracking, while the volume of fluid (VOF) method was applied for gas?liquid interface tracking. From the perspectives of heat transfer, mass transfer, and dynamics, the material removal mechanisms were systematically explored to provide technical support for the development of high-quality and high-efficiency WJGL machining methods.

The comparison between simulation and experimental results demonstrates the high reliability of the developed model (Fig. 4). Simulation results reveal that the thermal effect of laser power can approximately reach a dynamic equilibrium with the cooling effect of the water jet (Fig. 6), where laser power is proportional to hole depth and the hole surface temperature distribution under four laser powers exhibits an overall Gaussian-like profile (Fig. 5). The relationship between water pressure and hole depth is influenced by multiple factors (Fig. 7), leading to the hole depth at 10 MPa exceeding that at 12 MPa after 200 pulses. The thermal accumulation effect in high-frequency pulses enhances the material removal rate, causing the maximum temperature at 301 kHz to gradually surpass that at 251 kHz and 276 kHz as the number of pulses increases (Fig. 9). However, this also induces increased thermal damage, while reducing the pulse frequency promotes better cooling during the inter-pulse interval (Fig. 10). The molten pool generated at 119 W laser power completely dissipates under the impact and cooling of the water jet, whereas the molten pool at higher powers is not fully removed before the next pulse cycle arrives. The velocity vector distribution of the molten pool shows high consistency with the streamline pattern of water flow in the hole, with the flow velocity minimal at the bottom of the molten pool and relatively higher at the periphery (Fig. 12).

We established a multi-pulse transient material removal model for WJGL machining, incorporating the solid?liquid?gas three-phase evolution of monocrystalline silicon, and validated the model experimentally. Based on this model, the influence mechanisms of different process parameters on the temperature field and morphological evolution during WJGL material removal were analyzed from the perspective of heat and mass transfer. The dynamics of material removal were analyzed from the perspective of fluid flow, revealing the interaction mechanism between the water jet and the molten material. Simulation results indicate that laser power is proportional to the energy absorbed by the material, with higher laser power yielding higher temperatures and deeper holes. The thermal effect of laser power at 119 W and 139 W essentially reaches a dynamic equilibrium with the cooling effect of the water jet. Under different water pressure conditions, the material removal process is influenced by the combined effects of water jet cooling, impact force, and hole depth. The velocity vector distribution of the molten pool shows high consistency with the streamline pattern of water flow within the hole. The deflection angle of the water flow after impacting the hole exceeds 90°, and the difficulty of melt expulsion increases with hole depth. This research provides a reference for understanding the mechanisms of process control in WJGL machining; future work will explore the effects of coupling different process parameters on the machining process.