Rupture and growth of holes in the melt pool during laser keyhole welding of thin sheets

This study investigates the formation and growth of holes during laser keyhole welding of 100 μm thin 316 L stainless-steel sheets in overlap joints, represented by bead-on-plate welding of 200 μm thick foils. Using high-speed imaging, keyhole instabilities, promoted by increasing line energy, were identified as precursors to hole nucleation. Continuous weld seams were obtained up to approximately 7 kJ/m, while hole formation occurred beyond this regime and cutting became dominant above approximately 12 kJ/m. The hole growth mechanism is described by the competition between vertical and horizontal surface-tension components associated with the local melt pool curvature. Once nucleated, holes expanded at approximately 0.8–1.2 m/s over 1–3 ms and developed into elliptical shapes before forming permanent perforations or cuts. A geometric stability criterion based on the Young-Laplace force balance indicates that unstable growth occurs when the hole length becomes comparable to the sheet thickness. The thinner the foil, the stronger the vertical force component promoting hole growth, reaching 17 kN/m² here. A systematic flow chart linking the observed sequence of keyhole instability, hole nucleation, and hole growth provides a basis for identifying precursor signatures and suppressing hole formation during thin-sheet laser welding.