Drop on demand generation from a metal wire by means of an annular laser beam

Abstract

In the paper a novel system for drop-on-demand (DoD) generation from a metal wire is presented, whose main component is a newly developed laser droplet generation head, consisting of annular laser beam shaping optics and a wire feeding system. In the pendant droplet formation phase of the DoD generation, a laser pulse is used to melt the wire-end, which is fed into the focus of an annular laser beam. The formed pendant droplet is then detached by means of a detachment pulse, which induces Rayleigh-Plateau instability of the molten column of wire above the neck of the pendant droplet. The main process parameters, including the laser pulse and wire feeding parameters as well as the additional parameters which influence particular phases of the DoD generation process, have been identified. The empirical correlations between the influencing process parameters and the droplet characteristics, including droplet diameter and temperature, were determined, based on the analysis of high speed IR records of the process, images being acquired by an optical microscope and temperature data being acquired by pyrometers. As an example, DoD generation from a commercially pure 99.6 % Ni wire (Nickel 200) of 0.6 mm diameter is considered. It is shown that droplets with diameters ranging from 0.85 to 1.25 mm can be generated, with a resolution of 50 µm and a standard deviation of 15 µm. The temperature of the detached droplet remains above the melting point of the Ni wire, and increases with the droplet diameter within the range from 1650 °C to 1750 °C. Some examples of Ni droplets deposited on a Ti sheet surface are presented, with the aim of demonstrating the capability of the proposed system, and motivating further applications in which drops on demand having a high temperature and a precisely defined diameter need to be generated, while limiting the thermal loading of the surroundings.

Time series analysis based study of a mass-spring-like oscillation and detachment of a metal pendant droplet

“…In recent years, metal droplets have been used in many innovative technologies, such as the joining of electrical contacts [1][2][3], and 3D structuring [4]. Among various methods for metal droplet generation, which form a basis for the above technologies, laser droplet generation (LDG) from a wire [5,6] has been developed. In the LDG from a wire process, a laser beam is used to melt the end of the metal wire which is fed into the laser beam focus.…”

“…The sequence of droplets can be generated either as a discrete drop-on-demand sequence [6] or as a continuous droplet sequence [5]. In both cases, the pendant droplet detachment is the most crucial phase since it affects the dynamics of the droplet sequence generation as well as the properties of the detached droplets.…”

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

“…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…”

High-speed camera thermometry of laser droplet generation

“…Due to the presence of multiple flow phases and the very high temperatures of melt droplets (often in excess of 2000°C), only non-contact methods can be used to measure the temperature. Non-contact temperature measurements are most commonly performed by pyrometers or infrared cameras [5][6][7][8][9], but these devices have several operating limitations. Pyrometers are only capable of single-point temperature measurements, while the infrared cameras, though capable of measuring instantaneous temperature fields, are not well-suited for application in high-speed processes due to their limited spatial resolution and dynamic response as well as the high cost.…”