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Materials Science in Additive Manufacturing Thixotropic metal 3D printing

point were chosen for initial tests. A preliminary study A paste-like slurry was formed around 75°C at which
was conducted to test the feasibility of thixotropic alloy the material fluidity was significantly decreased. This
as an AM material. This section describes the results from thixotropic material was manually printed by movement
an indium-gallium alloy. Indium-gallium alloy is well- of a graphite extruder (with 1 mm hole). Line patterns with
known for its electrochemical and rheological properties. sharp tips (Figure 9E) were successfully printed.
Therefore, some researchers have focused on developing
electronic circuits and microchannels using In-Ga based 4.2. Printability study
alloys [19-21] . Following the In-Ga phase diagram [22-24] 4.2.1. Effects of nozzle diameter
(shown in Figure 9A), we decided to choose a 70/30 In-Ga It should be noticed that all the experiments in this
composition for testing. From the phase diagram, it is printability study were conducted 4 times for statistically
anticipated that when the molten alloy is cooled to about average values. With the steady-state thermal control
75°C, a two-phase structure containing a liquid phase and of the extrusion and printing system, a stable material
a solid phase would be formed. Specifically, indium was solid fraction inside the reservoir can be developed. The
melted at 170°C, and then gallium was added and mixed printing quality resulting from this steady-state system was
well at this temperature. The alloy was then gradually then evaluated using nozzles with three different outlet
cooled to about 50°C while under vigorous mixing. diameters: 1.5 mm, 1.0 mm, and 0.8 mm. All these printing
The resulting mixture at 50°C was a paste-like material, nozzles were machined as a single piece to mount to the
which was able to form sharp shapes when deformed and bottom of the reservoir. Throughout the extrusion process,
sustain the deformed shape, indicating formation of a the key process parameters including PID set value,
thixotropic fluid. This material was used for 3D printing extrusion speed, X-Y platform moving speed, and gap
using a syringe-type printer, as shown in Figure 9B. The distance between nozzle tip and substrate were maintained
printing results are compared in Figure 9C and D. For (Table 1). The printed lines were measured, and their mean
molten gallium, the motorized syringe was not able to values were used to determine the printing resolution (line
control the extrusion of the fluid when depositing on the width) (Table 2). The comparison of printed lines from
glass substrate, leaving droplets of uncontrollable sizes nozzles with different sizes is shown in Figure 10.
(Figure 9C). In contrast, the In-Ga alloy was able to be As illustrated in Table 2, a 1.5 mm diameter nozzle
printed into lines (Figure 9D). These results indicate that can extrude a continuous line with a width of 1.85 mm.
by forming a thixotropic paste, a two-phase alloy may be
printed using an extrusion-based 3D printer. Under the same conditions, a 1.0 mm nozzle can print
lines as thick as 1.32 mm, while a 0.8 mm nozzle can print
In addition, a 56/44 bismuth-lead alloy was also tested. lines as thin as 0.8 mm. As observed, lines printed by a
Typically, the bismuth-lead alloy was used as a solder or an 1.5 mm nozzle are 23% larger in diameter than their outlet
easy-handle material for casting. The alloy was heated to diameter. Lines extruded from 1 mm and 0.8 mm nozzles,
150°C to form a liquid alloy, and then the temperature was on the other hand, are approximately 31% larger than the
reduced. The alloy was vigorously mixed during cooling. outlet diameter. From these results, it can be seen that there

A B

C D E

Figure 9. Thixotropic processing and 3D printing of two testing alloys. (A) Phase diagram of In-Ga alloy. (B) Syringe-based 3D printer. (C) Printed molten
gallium metal. (D) Manually printed lines from thixotropic 70In-30Ga alloy on glass substrate. (E) Manually printed lines from thixo-tropic 56Bi-44Pb
alloy.

Volume 1 Issue 1 (2022) 7 http://doi.org/10.18063/msam.v1i1.5