Experimental and finite element analysis of ultrasonic vibration-assisted continuous-wave laser surface drilling

In pulsed laser drilling with co-axial assisted gas, material removal mechanism (surface vaporization and melt expulsion) determines the machining/drilling rate and quality of the drilled holes. Incomplete melt expulsion is one of the major causes of laser drilling defects. To improve the drilling efficiency and quality of holes, a novel ultrasonic vibration-assisted continuous-wave laser drilling (UVLD) approach is proposed. The application of ultrasonic vibrations (of frequency of 20kHz and vibration displacement of 23 mu m) during laser surface melting facilitates the melt expulsion in the form of sideways melt flow and droplet ejection from the drilling front. A systematic experimental study on the ultrasonic vibration-assisted laser drilling of AISI 316 stainless steel is performed to investigate the effect of working distance on the geometric features and surface quality of the holes. The experimental results based on high-speed photography indicate that the melt expulsion under the influence of ultrasonic vibrations initiates after the laser melted pool reaches a critical size/volume. Based on this underlying mechanism, a simplified finite element analysis is performed for the UVLD process to predict the hole volumes for the investigated working distances.