Research Progress of Liquid-assisted Laser Micro-hole Processing

With the rapid progress of laser processing technology, micro-hole processing with laser is becoming more and more popular. However, it is still away from qualifying real applications, especially when high aspect ratio micro-hole is required. Facing the situation, hybrid micro-hole drilling methods, i.e., laser beam-based and multi-energy field-assisted hole drilling techniques gradually emerge. With the purpose of elucidating the inspiring development in this important field, the work aims to summarize the methods of underwater laser drilling, water-based ultrasonic vibration-assisted laser drilling, water-based ultrasonic & magnetic field assisted laser drilling and electrolyte/water jet assisted laser drilling. The advantages and shortages of each method are provided and discussed as follows. Compared to laser beam drilling, underwater laser drilling means that the workpiece is immersed into water and the laser beam passes through water to drill holes. By tuning water layer thickness and laser parameters, the quality of micro-hole can be improved. On the one hand, the existence of water takes away accumulated heat thus achieving more accurate edge quality; on the other hand, the cooling effect makes it easy for the expelled slags to re-accumulate inside the drilled voids, which seriously affects the incoming laser beam absorption. The latter one limits its applicability in industry. Water-based ultrasonic vibration assisted micro-hole processing is based on underwater laser drilling. In addition, ultrasonic vibration source is employed. The ultrasonic vibration can break the bubbles generated during the drill course. It can also decrease the interference of bubbles on the laser beam and reduce debris repositioning in the processing area. All the above effects favor a cleaner surface profile and a better aspect ratio. As for water-based ultrasonic-magnetic field assisted laser micro-hole drilling method, an ultrasonic vibration field and a magnetic field are coupled simultaneously into the underwater micro-hole drilling procedures. The addition of magnetic field can weaken the shielding effect of the plasma on the incident laser beam, increase the effective energy injection of the incident laser beam and enhance the mixing effect caused by the electromagnetic force inside the molten metal. It can improve the laser energy absorption inside the drilled hole and promote material removal efficiency. Meantime, it can also suppress the formation of recast layers and reduce residual stress. Water jet assisted micro-hole drilling method is another novel way for micro-hole drilling. In this procedure, water jet is used not only for putting workpiece inside water tank. Water jet provides powerful water flow. The water flow can cool down the generated slags and take them away, which is unavailable for underwater laser drilling. Water jet can also wash the recast layer to improve the quality of the drilled hole. Furthermore, electrolyte can be introduced into the water flow to become into hybrid liquid flow. By virtue of its corrosive effect and the laser beam ablation, material removal rate speeds up. As the results, hole drilling efficiency is obviously increased. As for the criterions to evaluate the quality of laser drilled micro-holes, several parameters are usually considered, e.g., taper angle of micro-hole, aspect ratio, roundness of hole, thickness of recast layer, heat-affected zone, micro-crack and inner side surface roughness. In the study, these indicators of each processing methods are classified and analyzed. Finally, the laser micro-hole drilling procedures are summarized. Based on the above analysis, short prospects of potential research on micro-hole drilling are also given. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.