Page 63 - MSAM-1-1

Materials Science in Additive Manufacturing Thixotropic metal 3D printing

Figure 3. Illustration of the components in the thixotropic metal extruder.

temperature is slightly higher than the material’s melting of the material. The temperature proportional, integral,
point. The purpose of establishing high temperature in and derivative (PID) control system consists of two
this section is to melt or soften the raw material. Dual heating regions. Both heating regions are controlled by
element heating tape (from Omega Engineering) is chosen a bench top PID controller (from Omega Engineering).
as a heating element in high-temperature heating regions. According to the measurements, the actual temperature
This heating tape has a maximum heating temperature of received inside the channel is on average 5°C< the PID
1400°F (760°C), which is suitable for the majority of low setup temperature. As a result, the input temperature for
melting metals. Alternatively, the heating element can be heat tape 1 is set to 78°C, while the input temperature
replaced with an induction heater so that the metal melts for heat tape 2 is set to 69°C. Due to the fact that the
while simultaneously generating globular microstructure. printing material is produced as pellets, the melting rate
The region of low-temperature heating is equipped with a is significantly faster than with a conventional ingot. The
fiberglass heating rope (from Omega Engineering) capable auger bit rotates at a speed of 60 rpm, depending on the
of producing heat up to 900°F (482°C). The material in melting condition of the material. An alloy processed by
this section is cooled and sheared as the auger bit rotates. the extruder is extruded out as a semi-solid slurry using
When the temperature decreases, the molten metal 1 – 1.5 mm outlet nozzle. The material is printed on the
begins to transition from the liquid to the solid phase. substrate as a result of gravity and extrusion force.
The shearing action within the channel aids in grain size
reduction and the formation of near-globular grains. 3. Simulation and prediction
After passing through the low-temperature section, the 3.1. Modeling and simulation of thixotropic metal
processed material is reheated and squeezed out through 3D printing
the nozzle to form small beads. The nozzle is connected Computer simulations were conducted to study the free
to a band heater in this section to maintain a constant surface flow during extrusion and 3D printing. A funnel-
heating temperature. With reheating, a semi-solid slurry’s like nozzle is used in the geometrical model, as shown
solid fraction is reduced, and the material’s viscosity is in Figure 5A. The funnel has entry and exit diameters of
decreased for easy flow. 40 mm and 2 mm. The free-surface flow is modeled in
2.2. Thixotropic printing system design an axisymmetric 2D domain (Figure 5B). A thixotropic
metal slurry enters the funnel from the inlet and then
The thixotropic printing system is composed of an flows out of the nozzle tip to evolve into a free-surface flow
extrusion printing nozzle mounted on a support frame surrounded by air. Surface tension is applied at the slurry
that also houses the reservoir and extrusion mechanism. to air interface. The rheological properties (including yield
Figure 4 illustrates the frame design and the major stress and viscosity) of a thixotropic fluid are supposed
controlling system mounted on it. The frame is supported to depend on the solid fraction and structure. In this
by steel struts and features an adjustable central holding preliminary study, only the effect of variable viscosity of
area for the reservoir. A motion controller controls an the thixotropic fluid was considered, leading to a variable
electric linear actuator (IP66 series from Progressive Ohnesorge number (Equation II). The density is set to a
Automations) that moves at a speed of 3.8–38.1 mm/s. Three representative value of 4000 Kg/m . For the nozzle size of
3
control systems are incorporated into the design. Through 2 mm, the Oh number used in the simulation ranges from
the extrusion motion controlling system, the actuator and 0.001 to 1000. In moving boundary flow simulation, the
motion controller are connected. The controller can adjust free surface is often tracked using the level set method or
the extrusion speed based on the extrusion requirements the phase field method. In this preliminary study, the level

Volume 1 Issue 1 (2022) 4 http://doi.org/10.18063/msam.v1i1.5