Laser drilling in nickel super-alloy sheets with and without ultrasonic assistance characterized by transient in-process detection with indirect characterization after hole-drilling

•Real-time monitoring of water-based ultrasonic-assisted laser drilling is reported.•Increase of ultrasonic power expedites material removal for laser drilling.•A higher ultrasonic power is more helpful for drilling a hole through.•Higher laser pulse energy enhances material removal with more ejection and spatter.•Higher co-axial assist gas pressure reduces through-hole recast layer formation. The water-based ultrasonic-assisted laser drilling has been recently reported for improving hole-drilling quality and efficiency, but its material removal mechanism is currently still not well understood. In this work, the high-speed photography was used for studying the transient laser drilling processes with and without water-based ultrasonic vibrations. The effects of camera shooting angle, laser pulse energy, assist gas pressure and ultrasonic vibrations on laser drilling processes were investigated and compared systematically. The influential mechanism of water-based ultrasonic assistance on transient material removal for laser drilling was first reported using high-speed camera observation with indirect characterization after hole-drilling. It was found that the melt ejection with spattering and the vapor-plasma plume expulsion from a drilling hole were enhanced by using ultrasonic vibrations without assist gas blowing. When a hole was through, more ejection and spattering signals were detected above the hole entrance due to the combined effect of water-based ultrasonic vibrations and water-induced “closed state” for the hole exit. The increase of the ultrasonic power enhanced hole-drilling and material removal amount, which was useful for drilling through a hole. The through-hole entrance and exit diameters increased with the ultrasonic power.