Melt ejection during laser drilling of metals

Abstract In laser drilling of metals, melt ejection can be a significant mechanism of material removal. Vaporisation within the hole creates high pressure gradients, which expel molten material from the hole. Results are presented for a range of metals drilled with single pulses with durations of 0.1 and 0.5 ms, using a Nd:YAG laser. Power intensities across the focussed beam were of the order of 0.2 MW mm −2 . Ejected droplets were collected and characterised, using several experimental techniques. The particle size distribution, angle of trajectory, molten layer thickness and temporal variation of melt ejection were determined. Two complementary methods, high speed photography and a particle stream interruption technique, were used to determine the ejection velocity. The experimental results obtained have been used to gain insight into the overall process of melt ejection. Melt ejection commences with the ejection of small (∼10 μm) droplets, moving at velocities of up to 30 m s −1 . This is followed towards the end of the process by the ejection of larger (∼100 μm), slower-moving droplets, with velocities of ∼1 m s −1 . Increasing the pulse intensity increases the ejection velocity and decreases the average particle size. This is attributed to the molten layers around the cavity being thinner, as a consequence of the higher thermal gradients. To a first approximation, typical particle diameters appear to be of the order of the molten layer thickness during drilling.