Nonlinear Dynamic Force Balance Mass-spring-damper Modelof Laser Droplet Generation from a Metal Wire

Direct laser metal deposition is one of the established metal-based additive manufacturing processes [1]. It usually employs input material in shape of wire [2] or powder [3]. In contrast to the wire or powder deposition process, where the laser is used to generate melt pool on the workpiece surface, the process of metal droplet deposition significantly reduces the workpiece heat load, since the laser beam is used only for droplet generation. In consequence, metal droplets have been employed in various innovative technologies, including the droplet joining of electrical contacts, and of dissimilar and temperaturesensitive materials [4], as well as 3D printing [5]. A necessary basis for droplet-based technologies is a reliable method for the generation of metal droplets. To achieve this, a number of metal-droplet generation methods based on melt ejection from a nozzle have been developed, employing different techniques to generate the ejection pulse: piezoelectric [6], pneumatic [7], solenoid [8], and electromagnetic [9]. The above methods employ a heated crucible to maintain a reservoir of molten metal. Metals with relatively low melting temperatures are used, such as solder [9], indium, lead, zinc [7], and aluminum [10]. To avoid an energy-inefficient crucible and simplify the system, a method employing induction melting of the wire tip in the pressurized tube leading to the nozzle was proposed [11]. Common to all of the above methods is the nozzle, the diameter of which should be well below the diameter of the generated droplets and is subject to wear. To avoid melt oxidation, leading to nozzle clogging, a low-oxygen-content atmosphere should be provided [7]. Besides the methods based on melt ejection, several laser-based methods have been developed that take advantage of the high level of spatial and temporal control of the laser’s energy delivery. They differ in terms of the shape of the input metal material, which can be either a foil [12], a spherical preform [13], or a wire [14]. Although it achieves lower dropletgeneration frequencies compared to melt-ejection methods, the laser droplet generation from a wire using an annular laser beam [15] shows strong application potential, since it can employ metals with a high melting point, such as nickel [14], is not very sensitive to oxygen content, and the related system includes no wearable parts. In annular laser beam droplet generation (ALDG), a metal wire is fed axially into the center of the annular laser beam, which is focused on the circumference of the wire-end. ALDG can be performed either as drop-on-demand ALDG [14], where the pendant-droplet formation and detachment Nonlinear Dynamic Force Balance Mass-spring-damper Model of Laser Droplet Generation from a Metal Wire Jeromen, A. – Govekar, E. Andrej Jeromen* – Edvard Govekar University of Ljubljana, Faculty of Mechanical Engineering, Slovenia