Interaction mechanisms for a laser-induced metallic boiling front

Abstract: This thesis is about fundamental interaction mechanisms of laser remote fusion cutting, RFC, which is based on the formation of a quasi-stationary laser-induced boiling front that causes drop ejection, preferably downwards. Laser cutting of metals, invented in 1967, has developed from a niche to a well established high quality cutting technique in the manufacturing industry. Usually a gas jet is employed concentric to the laser beam, to eject the molten metal. One technique option, interesting though hardly applied yet because of usually low quality and speed, is remote laser cutting. Two techniques are distinguished, remote ablation cutting, grooving down through a sheet, layer-by-layer, and the here addressed remote fusion cutting, by a single pass through the sheet. For the latter, the ablation pressure from laser-induced boiling at the cutting front continuously accelerates and ejects the melt downwards. Advantages of remote laser cutting, facilitated by high brilliance lasers during the last decade, are the possibility of a larger working distance along with the avoidance of cutting gas and of a gas jet nozzle. The review paper of the thesis surveys different laser remote cutting techniques, including their modelling, as well as the transition to keyhole welding, owing to similarities particularly from the boiling front and from root spatter ejection. The six Papers I-VI that compose the thesis address fundamental mechanisms of laser remote fusion cutting, theoretically and experimentally. In Paper I a simplified mathematical model of the RFC cutting front enables to estimate the geometrical and energetic conditions of the process. By evidence and post-modelling from high speed imaging, HSI, the simplified smooth cutting front model is developed further to a wavy topology in Paper III, for more sophisticated absorption analysis. As a systematic support, Paper II categorizes and analyses for the first time the different wavy topologies observed at the front, from HSI. The melt dynamics induced by a pulsed laser beam was studied in Paper IV, again from HSI. Apart from other interesting transient melt phenomena it was demonstrated that the ablation pressure can push the melt to a certain pending position during the laser pulse while the melt retreats by surface tension during the pulse break. To engage remote fusion cutting with additive manufacturing, Paper V introduces a novel technique where the drops ejected from RFC are transferred to a substrate, about a centimetre underneath, on which a continuous track forms. This technique can even be applied as an efficient recycling approach. In Paper VI a variant of the technique is presented, to develop a boiling front along the edge of a metal sheet from which the drop transfer takes place, in a different manner. This enables to systematically machine-off the entire sheet, which can be converted to a new shape and product. Summarizing, the thesis provides a variety of analysis of fundamental mechanisms of a laser-induced boiling front that bear a certain simplicity and in turn controllability, of interest for established as well as for new applications, in manufacturing and in other sectors, including remote fusion cutting.