Hard and brittle dielectric materials represented by quartz are widely used in aerospace, consumer electronics,weapons and other fields. In view of the defects such as re-condensation and micro-cracks in the process of femtosecond laser machining such materials, the ultrasonic gas jet assisted femtosecond laser machining technology is proposed. The steady-state jet is transformed into pulsating jet through the coupling of ultrasonic and gas jet. With the help of the purging effect of gas jet and the high-frequency scouring and crushing effect of pulsating jet, the re-condensate produced in the machining process is taken away,which reduces the adhesion of re-condensate on the machined wall,improves the machining efficiency and improves the machining quality. Firstly,this paper proposes an ultrasonic gas jet device and analyzes the coupling mechanism of ultrasonic and gas jet. The coupling effect of ultrasonic and gas jet is mainly manifested in two aspects. One is that the local shock wave structure is formed by the direct coupling of ultrasonic and inlet jet. The formation of shock wave structure originates from the combination of sound pressure and jet pressure. Due to the alternating propagation of ultrasonic sound pressure with time,the combination of sound pressure and jet pressure makes the gas jet change from a steady jet to pulsating jet with alternating high and low pressure. Second,the ultrasonic transducer is mechanically fixed with the jet nozzle, and the high-frequency micro-displacement generated by the transducer is transmitted to the nozzle. The high-frequency displacement of the nozzle drives the whole flow field to generate high-frequency vibration,thus high- frequency scouring the machined surface. The coupling effect of ultrasonic sound pressure and jet pressure is deduced theoretically,and simulated with ANSYS Fluent software. The theoretical analysis and simulation results show that the steady-state jet is transformed into a pulsating jet, alternating high and low pressure under the action of ultrasound. Secondly, the AT tangential quartz wafers were etched by femtosecond laser with a wavelength of 1 030 nm,pulse width of 290 fs and repetition rate of 20 kHz under the condition of 45 degrees between the central axis of the jet nozzle and the laser axis. The effects of ultrasonic frequency,ultrasonic power and gas inlet pressure on the depth and width ratio of femtosecond laser etching quartz microgrooves were investigated, and the morphology of femtosecond laser etching quartz microgrooves with or without ultrasonic gas jet was compared and analyzed. The experimental results show that:1)Under the assistance of gas jet and ultrasonic gas jet, the depth and aspect ratio of femtosecond laser etching quartz microgrooves have been greatly improved. Under the same conditions,the gas jet and ultrasonic gas jet have little change in the depth and aspect ratio of etching. 2) There is little difference between the etching depth and aspect ratio of quartz microgrooves under different ultrasonic frequencies,but the etching depth increases slightly with the increase of ultrasonic frequency. There is no significant difference in ultrasonic power(ultrasonic amplitude) etching. Ultrasonic power has little effect on the etching depth and aspect ratio of femtosecond laser etching quartz microgrooves. Under the same laser processing parameters,the depth and depth-width ratio of microgrooves increase with the increase of inlet pressure. When the inlet pressure reaches a certain value(about 0. 4 MPa),the depth and depth-width ratio of microgrooves reach the maximum value. When the pressure is above the value,the depth and depth-width ratio of quartz microgrooves change little. 3) Under the single laser etching, the particles on the surface of quartz microgrooves are disorderly and irregular,and the size is large. The particle size of quartz microgrooves tends to be consistent under the assistance of jet. Compared with the single laser etching,the particle size of quartz microgrooves is reduced. Under the assistance of ultrasonic gas jet,the particle size of quartz microgrooves further decreases and tends to be consistent,and the wall quality is significantly improved and improved. The reason is that the high- speed pulsating jet formed by the coupling of ultrasonic and gas jet breaks and forms an impact in the laser action zone. The impact takes away the laser removal near the molten pool and accelerates the melt injection, thereby reducing the adhesion of laser removal on the machined surface. In addition,under the high- frequency impact of pulsating jet,the large particle slag generated in the laser action zone is decomposed into small particles. Some of these small particle slags are brought out with the gas, and even some are attached to the machining surface. Compared with the attachment of large particle slag,the quality of the machining surface is improved.
