With the development of laser technology, femtosecond laser has been developed. Because the peak power of femtosecond laser is very high, and the pulse width is very short, its processing has the characteristics of wide material applicability, high precision, small heat affected zone, thin recast layer and so on, which has become a research hotspot in the field of laser drilling [1], [2], [3], [4], [5], [6].

In the 1990s, Chichkov et al. in Germany confirmed the advantages of femtosecond laser in the field of small hole manufacturing. However, it was found that the prerequisite for obtaining holes without recast layer was that the laser fluence was slightly higher than the material removal threshold, which led to low drilling efficiency, especially for drilling holes with larger aspect ratio [7]. Improving the laser fluence is an effective method to improve the drilling efficiency [8], but heat accumulation will occur under high laser fluence, especially under the high pulse repetition rate, thermal damage will be occurred, particle residues will appear on the hole sidewall, and microcracks and recast layers may also appear on the hole sidewall [9], [10], [11]. Therefore, it is an urgent problem to improve the efficiency of femtosecond laser drilling, improve the quality of hole sidewall, and reduce the thermal damage under high laser fluence and high pulse repetition rate. In order to improve the quality of femtosecond laser drilling, in recent years, more and more assisted technologies have been applied to femtosecond laser drilling, such as ultrasonic vibration assistance [12], magnetic assistance [13], water assistance [11], [14], etc.

At present, the femtosecond laser drilling has achieved corresponding research results, but the mechanism of assistance methods to improve the quality of femtosecond laser drilling is still unclear. Water assistance is an effective method to improve the quality of femtosecond laser drilling. Behera et al. [14] studied the water assisted laser drilling process with different pulse width (millisecond, nanosecond and femtosecond), and compared the performance with the traditional laser drilling method, proving the superiority of water assisted laser drilling, and the shorter the pulse width, the better the drilling quality. The research indicated that the mechanism of water assisted laser drilling mainly included the transmission (absorption) of light in liquid, the heating and vaporization of liquid, the formation, growth and collapse of cavitation, and the heating, melting and vaporization of materials. However, the mechanism of water assisted femtosecond laser drilling had not been discussed in detail. Wang et al. [11] used the water assisted femtosecond laser drilling method to drill holes on 500μm thickness 4H-SiC material, and obtained high quality through holes without cracks, surface material shedding and heat affected zone (HAZ), which proved the advantage of water assisted method in laser drilling high aspect ratio holes on hard and brittle materials. Tan et al. [15] carried out the experiment of water assisted femtosecond laser drilling high aspect ratio holes on thick glass, and the maximum hole aspect ratio reached 30:1. It was found that femtosecond laser induced cavitation of water produced bubbles, which promoted the removal of debris during laser drilling. Wu et al. [16] observed that bubbles appeared at the incident point of the laser beam and diffused around in the experiment of liquid assisted processing of silicon carbide, and the laser-induced periodic plasma shock propagated outward with the bubbles. With water assistance, the surface quality of silicon carbide was improved and the roughness was reduced. As for the interaction mechanism between femtosecond laser and liquid, other researchers had also carried out corresponding observation experiments [17], [18]. In addition, our previous research indicated that transverse magnetic assisted method could improve the quality and efficiency of femtosecond laser blind hole drilling. The transverse magnetic could also reduce the hole taper angle and improve the hole sidewall quality during femtosecond laser through hole drilling [13], [19].

Combining the advantages of water assistance and magnetic assistance in femtosecond laser drilling, this paper used water-based magnetic assistance femtosecond laser drilling method, and used laser layered ring trepanning method in combination with high-speed scanning galvanometer technology. Firstly, the effects of different environments (in air, magnetic assistance, water assistance and water-based magnetic assistance) on the hole the entrance and exit, cross-section and sidewall morphology of femtosecond laser drilling were analyzed, and the hole diameter, taper angle and sidewall roughness under different environments were compared and analyzed. Then the influence of magnetic flux density on the hole diameter, taper angle and sidewall quality with water-based magnetic assistance were discussed. Finally, based on the experimental results, the mechanism of water-based magnetic assisted femtosecond laser layered ring trepanning was revealed preliminary, which provided theoretical basis and technical support for femtosecond laser drilling. Our research had a good prospect of industrial application.