Ultrasound-assisted water-confined laser micromachining (UWLM) of metals: Experimental study and time-resolved observation

This paper reports the experimental studies and time-resolved observations of the ultrasound-assisted water-confined laser micromachining (UWLM) process on metal workpieces using 532-nm nanosecond laser pulses. UWLM is a new machining process proposed by the corresponding author (Wu, 2014). During UWLM, a laser beam interacts with a workpiece's front surface location immersed in water to ablate the workpiece, and in-situ ultrasonic waves in water are also applied (for example, from above the front surface using an ultrasonic horn) to the laser ablation site (which energize the water and generate cavitations) to improve the machining process. This paper reports the experimental study of the UWLM process on multiple types of metal workpieces and time-resolved in-situ observations of the process. This kind of study on UWLM has been rarely reported in the literature. Under the studied conditions, it has been found that the ablation depths produced by UWLM at 200 laser pulses are about 2.3–3.8 times the depths by laser ablation in water without ultrasound. Through in-situ time-resolved shadowgraph imaging observations, it has been found that one important likely underlying mechanism for the enhanced material removal rate in the studied UWLM process is that the in-situ ultrasound in UWLM can reduce the material cloud resulted from the previous laser pulse ablation that lingers around laser ablation site, which can enhance the coupling of the subsequent laser pulse(s)’ energy to the workpiece.