The ejection threshold of molten aluminum induced by millisecond pulsed laser

The laser pulse energy density threshold of melt ejection induced by millisecond pulsed laser can vary widely due to the variation of focused laser spot area. An axisymmetric model of the flow of liquid Aluminum under the effect of recoil force from evaporation is built up to analyze the threshold of ejection through the temporal evolution of the temperature of melted pool. Temperature of the central point of focused laser spot, drag force induced by surface tension in melted pool, and recoil force of evaporation on molten liquid surface could be obtained through resolving heat conduction equation analytically and numerically with assumed Gaussian temperature distribution function. The relationship of laser pulse energy density threshold of the ejection and area of the focused laser spot is obtained through the theoretical calculation. The shape of the melted pool will be different when the relative magnitude of recoil force and drag force is varied. The validity of the obtained threshold is verified through comparison of melted pool shape with the experimental result reported in literature. The criterion proposed in this paper provides a reliable fast-diagnostic method for laser processing.