Media 1 Realtime 3D printing of an aluminum ring structure using the MagnetoJet process
Dr. Furlani's interdisciplinary group of electrical and chemical engineers are helping advance Vader Systems' revolutionary new additive manufacturing process via multiphysics and multiscale computational modelling.
The Furlani research group is currently in an ongoing collaboration with local startup business Vader Systems (www.vadersystems.com) to perform advanced computational modelling of a novel method for drop-on-demand (DOD) additive manufacturing (AM) process. The process, referred to under the tradename "MagnetoJet", involves printing of 3D solid metal structures using liquid metal droplets using a magnetic coil. Vader Systems are currently in the process of commecrializing this magnetohydrodynamic liquid metal 3D printer. Furlani's group focuses on computational modelling of the complex AM technology, which involves the study and modelling of magnetohydrodynamics (MHD), electromagnetics, power electronic circuit design, two-phase fluidics, thermofluidics, solid heat transfer and others. Dr. Furlani's group uses Comsol Multiphysics and FLOW3D to model the MagnetoJet process via droplet generation and droplet deposition simulations.
Figure 1B COMSOL simulation model showing the magnetic field generated by a pulsed magnetic coil as well as the volume fraction of ejected liquid aluminum
Figure 1A A cross-sectional view of printhead and process overview
Drop-on-demand inkjet printing is a well-established method for commercial and consumer image reproduction. The same principles that drive this technology can also be applied in the fields of functional printing and additive manufacturing. Conventional inkjet technology has been used to print a variety of functional media, tissues and devices by depositing and patterning materials that range from polymers to living cells [1, 2]. The focus of this work is on the extension of inkjet-based technology to the printing of 3D solid metal structures [3, 4]. Currently, most 3D metal printing applications involve deposited metal powder sintering or melting under the influence of an external directed energy source such as a laser (e.g. Selective Laser Sintering[5] and Direct Laser Metal Sintering[6]) or an electron beam (e.g. Electron Beam Melting[7]) to form solid objects. However, such methods have disadvantages in terms of cost and complexity, e.g. the travel to a substrate where they coalesce and solidify to form extended solid structures. Three-dimensional structures of arbitrary shape can be printed layer-by-layer using a moving substrate that enables precise patterned deposition of the incident droplets. This technology has been pioneered and commercialized by Vader Systems (www.vadersystems.com) under the tradename MagnetoJet. The advantages of a MagnetoJet printing process includes the printing of 3D metallic structures of arbitrary shape at relatively high deposition rates and with low material costs. In this work, we discuss the MagnetoJet prototype printing process and demonstrate sample 3D printed structures. We also introduce computational models that enable rational design and prediction of device performance.
Media 2 Flow 3D simulation of molten aluminum droplet ejection through the orifice
To model the droplet generation process, Comsol Multiphysics' level set and phase field two-phase fluidic methods were initially used. In addition a custom FORTRAN 90 code was developed to calculate the Lorentz force density around an axisymmetric solenoid as a function of time, spatial conductivity, number of coil turns and the applied current amplitude and wave shape. This code was used in customizing the commercially available FLOW-3D program by including the Lorentz force density as a force acting on individual fluid elements. This CFD analysis was used to study MHD-based droplet pinch-off and ejection behavior, as well as effective pressure generation. The most significant component of the MHD model is the magnetohydrodynamic force density, which causes the motion of the metal in the reservoir and through the ejection orifice. A time dependent eddy current computation, based on the finite element method in Comsol and on analytical equations in Flow3D was performed to calculate this force density.
Figure 2 Plot of current excitation pulse applied to the ejection coil, along with droplet generation and ejection as a function of time
Figure 2 shows an example of a current excitation pulse applied to the ejection coil, along with 2D temporal slices of aluminum droplet ejection from a 300 μm diameter orifice. The velocity of the droplet was predicted to be 2 m/s, which agrees with the average experimental velocity of 2.5 m/s. It is important to note that the slope of this excitation current corresponds to the generated pressure, i.e. a positive current slope will generate a downward pressure and a negative slope will cause a retraction pressure. Both of these pressures, corresponding to the direction of the MHD force density, are necessary to push liquid out of the orifice and then immediately pull it back to ensure a droplet pinch-off. Since the effective time for the droplet generation and droplet pinch-off rate are in the order of 400 μs, the current pulse is half of this time, allowing for a DOD ejection of liquid metal. Since the droplet ejection is in an on-demand mode, it is reasonable to use the data for droplet size and velocity from the first model as input into the droplet deposition model, which is discussed next.
Media 3 Thermofluidic simulation of the solid fraction of solidifying aluminum droplets.
Figure 3 Flow 3D simulation showing the effect of overlap fraction on the drop-on-demand additive manufacturing process : 10 droplets at 100 Hz with (a) 0.5 overlap fraction; (b) 0.8 overlap fraction; (c) 1.0 overlap fraction (d) Three-layered structure at 400 Hz.
In the droplet deposition model, we designed CFD models to investigate the droplet deposition, coalescence and solidification on a heated substrate. A Finite Volume thermo-fluidic analysis was performed using the solidification model in FLOW-3D. The temperature-dependent physical properties of molten aluminum, such as viscosity, density, heat capacity and thermal conductivity were obtained from [11-14]. Morphology of inclined pillars were studied as a function of droplet overlap fraction. In this analysis, spherical droplets of molten aluminum at 1023 K impact a stainless steel substrate, kept at 473 K, from a height of 3 mm. The droplets have a diameter of 450 μm and travel with an initial velocity of 2.5 m/s. Various droplet ejection frequencies were studied and it was observed that at frequencies above 100 Hz, the temperature of the solidified droplet is too high such that when the next droplet impacts it, excessive re-melting takes place and the structure is not stable. This effect was also observed during experiments, with the frequency limited to 20Hz to ensure full coalescence and solidification of each droplet. When building pillar structures using the DOD process, it is critical to control the droplet overlap fraction, which is defined as the ratio of the maximum overlap length between any two droplets to the outside diameter either droplet. This variable is controlled by the relative velocities of the droplet and the moving substrate during experiments, and via x-coordinate placement of the droplet in the model. Structures of 10 droplets at an ejection frequency of 100 Hz are presented in Fig. 3, with overlap fractions of 0.50, 0.80 and 1.00 (vertical pillar). In Figure 3a, the overlap fraction is assumed to be 0.5 and the result is a flat coalesced layer. In Fig. 3b, the overlap fraction is 0.8 which leads to formation of an inclined structure at 45° whereas in Fig. 3c the overlap fraction is 1.0 and as expected, the result is a vertical pillar. Pillars with a range of inclination angles can be fabricated by varying the overlap fraction during deposition. This is shown in Figure 3d, which is a snapshot of the experimentally printed structures.
Media 4 Thermofluidic simulation of the temperature profile of solidifying aluminum droplets.
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