Magnetohydrodynamic Drop-on-Demand Droplet Formation and Coalescence

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In recent years there has been a meteoric rise in interest and research in the field of metal additive manufacturing in both academia and industry. Such processes represent an attractive alternative to more traditional metal casting techniques [1]. This has led to the development of several new metal additive manufacturing techniques including, but not limited to: Material Jetting [2], Powder Bed Fusion [3], Material Extrusion [4], Directed Energy Deposition [5] and Sheet Lamination [6]. The MagnetoJet process [2], which is being commercialized by Vader Systems (vadersystems.com), belongs to the material jetting group of processes. It is a novel approach where a pulsed magnetic field induces a MHD-based pressure pulse within the liquid metal inside a nozzle that causes a droplet to be ejected. This method is appealing because of its lower cost due to the use of commodity wire as input, thus eliminating the requirement of powderizing the metal in advance of the 3D printing process. In the MagnetoJet process, droplets are ejected with a velocity that typically ranges from 1 to 10 m/s and cool slightly during flight before impacting the substrate. The ability to control patterning and solidification of droplets on the substrate is critical to the formation of precise 3D solid structures. Precise patterning is achieved using a high resolution 3D translation stage, which allows metal droplet placement accuracy up to 1 μm. There are several factors that influence the solidification of droplets on the substrate surface, and hence the patterning, such as droplet size, spacing, velocity, frequency, printhead temperature, substrate temperature and so on. In this study, a parametric CFD analysis is presented using the FLOW-3D CFD software package (www.flow3d.com). The FLOW-3D program is customized to include the effects of magnetohydrodynamic forces. An analysis is performed of the droplet ejection process for multiple ejections under various current pulses. Moreover, multilayer droplet coalescence and solidification on a heated substrate are investigated. In this analysis, spherical droplets of liquid aluminum with a radius of 229 μm and a velocity of 2.5 m/s impact a heated high strength steel (Dual-Ten 590 steel) substrate from a height of 5 mm. The droplets have an initial temperature of 1223 K and the substrate is heated at 573 K. The droplet spacing is 500 μm at a frequency of 400 Hz. The results of this computational analysis appear to correlate well with experimental results obtained on a prototype 3D printer. Overall, this study will help promote understanding of the underlying physical phenomena regarding droplet-substrate interactions and droplet fusion. The results should prove useful to experimentalists in the field of drop-on-demand additive manufacturing.

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Journal: TechConnect Briefs
Volume: 4, Informatics, Electronics and Microsystems: TechConnect Briefs 2018
Published: May 13, 2018
Pages: 114 - 117
Industry sector: Advanced Materials & Manufacturing
Topics: 3D Printing
ISBN: 978-0-9988782-1-8