Effect of bubble dynamic behaviors on machining performance of laser-induced microjet-assisted ablation

Laser-induced microjet-assisted ablation (LIMJAA) has been proven to be effective in fabricating high-quality surface structures. This achievement is attributed to the spontaneous generation of a continuous and stable microjet in liquid film, which prevents the recasting of ablation debris and eliminates the light-scattering effect of laser-induced cavitation bubbles. However, our experiments show that recasting occurs within the fabricated structures when laser repetition frequency exceeds a critical value in the LIMJAA process. Theoretical calculations and analyses confirm that the formation of a stable microjet requires a minimum pulse interval time exceeding the sum of the laser-induced cavitation bubble's lifetime and the microjet outflow time. This constraint of pulse interval time limits the maximum repetition frequency that LIMJAA can utilize, thereby affecting the method's overall processing efficiency. We found that reducing the liquid thickness and increasing liquid flowing velocity results in a shorter cavitation bubble lifetime and a quicker microjet outflow time, respectively, enabling shorter applicable pulse intervals. This significantly raises the usable maximum pulse repetition frequencies from approximately 40 kHz to 100 kHz. This study enhances our understanding of material removal mechanisms in LIMJAA technology and provides valuable theoretical insights for optimizing liquid-assisted laser micromachining performance. It paves the way for advancements in high-precision and efficient laser manufacturing.