Measurement of melt ejection velocities during laser drilling of steel, using a novel droplet stream interception technique

Abstract Melt ejection is a mechanism of material removal that can occur in the laser drilling of metals. It arises from the high pressure gradients generated by vaporisation within the hole, which can expel surrounding molten material. A technique has been developed to determine the velocity of ejected droplets. This is based on the mechanical interception of droplets by a moving screen. The equipment involved is simple and robust. Results are presented from drilling of mild steel, using 0.5 ms single pulses from a Nd:YAG laser. The measured average particle velocity is determined to be ~1 m s −1 . Potential sources of error are considered, including the degree to which droplets decelerate in flight due to the drag effect of the air. It is inferred that fine droplets are emitted with relatively high velocity during the early stages of the process, but the measured value is strongly influenced by larger, slower-moving droplets generated towards the end of the drilling period.