Melt expulsion during ultrasonic vibration-assisted laser surface processing of austenitic stainless steel

• Melt expulsion during ultrasonic vibration assisted continuous wave laser melting.
• Formation of craters and surface holes with the ultrasonic vibration assistance.
• Grain refinement in the resolidified layer of crater surfaces.
• Efficient material removal and improved microstructure of laser drilled craters.

Simultaneous application of ultrasonic vibrations during conventional materials processing (casting, welding) and material removal processes (machining) has recently been gaining widespread attention due to improvement in metallurgical quality and efficient material removal, respectively. In this paper, ultrasonic vibration-assisted laser surface melting of austenitic stainless steel (AISI 316) is reported. While the application of ultrasonic vibrations during laser processing delays the laser interaction with material due to enhancement of surface convection, it resulted in expulsion of melt from the irradiated region (forming craters) and transition from columnar to equiaxed dendritic grain structure in the resolidified melt films. Systematic investigations on the effect of ultrasonic vibrations (with vibrations frequency of 20 kHz and power output in the range of 20–40%) on the development of microstructure during laser surface melting (with laser power of 900 W and irradiation time in the range of 0.30–0.45 s) are reported. The results indicate that the proposed ultrasonic vibration-assisted laser processing can be designed for efficient material removal (laser machining) and improved equiaxed microstructure (laser surface modifications) during materials processing.