Constraints and optimization of the laser microwelding process of thin metal foils

Abstract

Laser microwelding of thin metal foils of various materials is established in several fields of application. Within all these application fields, a sufficient and reliable welding process is required to join thin metal foils successfully. One crucial function of the weld is often a gas-tight sealing. For instance, titanium foils provide hermetic and sterile packaging of medical implants, and vacuum insulation panels and pressure sensors are assembled from stainless steel foils. Even if gas-tightness is not the aim, weld defects lower the functionality of the product. In the case of a roll imprint process in order to structure optical surfaces on films and panels, where nano-structured nickel foils serve as masters, weld imperfections lead to failures in the optical structure. Furthermore, insufficiently welded cathode foils, which are made of aluminum, cause an increasing electric resistivity. Weld defects during laser microwelding are provoked by thermally induced distortion due to small foil thickness. The distortion causes gaps between the joining partners and can lead to a partial process abortion. Hence, metal foils have to be joined in the deep welding mode, which causes less heat conduction losses than conduction welding. In addition to a defect-free weld seam, a broad process window is required for a reliable and robust process. The process window in laser microwelding is determined by the transitions from a full penetration weld to a fusion cut regime and to an incomplete penetration weld. Furthermore, the onset of humping at higher feed rates decreases the cross-sectional area of the join and, as a consequence, the tensile strength, and has to be avoided in order to achieve a high weld quality. For the purpose of considering all mentioned constraints (deep welding regime, process window broadness, and onset of humping) while designing the process with due regard to the applied material, focal diameter, laser power, and feed rate, a deeper understanding of the process is necessary. The present work gives a qualitative and quantitative overview of the occurrence of these constraints in laser microwelding and determines the ideal working range with respect to the combination of process parameters.