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Materials Science in Additive Manufacturing Thixotropic metal 3D printing

passivation layer . Cost-effective printing of these alloys whole process can be contained within one machine so
[1]
would lead to a revolutionary improvement on the that the need for melting and holding furnaces as well as
products of medical devices, engineering and industrial melt treatment are all avoided. Foundry cleanliness is easy
product and beyond. Therefore, a new technical approach to maintain, and energy requirements are less because
for breaking the physical limit set by an extremely low complete melting is not required, cycle times are reduced,
Ohnesorge number of molten alloy and enabling direct and scrap is minimized. (2) Shrinkage and cracking
printing is highly desired. within the mold are reduced because the alloy is already
partly solidified in cast. (3) Lower operating and pouring
1.3. Manufacturing process and system temperatures lead to an increase in the life of metal dies.
development (4) Composite materials can be readily produced by
The main objective of this research is to explore a novel adding microspheres, fibers, or other solid particulates
manufacturing system that is capable of thixotropic into the feedstock (often referred to as compo casting).
processing and 3D printing of low-melting-point alloys such Despite these advantages, all existing processes need a
as Al, Mg, and Zn alloys into various devices and products. mold for shaping. The thixotropy generated in existing
To enable direct extrusion of molten metal, one has to find processes is considered not adequate and suitable for 3D
a practically viable approach for substantially increasing printing. The solid inclusions in these processes are large in
the Ohnesorge number. From Equation (I), this requires size and irregular in shape, often containing large dendritic
increasing of viscosity or decreasing of surface tension. structures. In this research, a mixer was designed for
Therefore, one has to modify the material properties of the producing a printable micro-slurry from metallic alloys.
liquid metal by increasing the flow stress and decreasing the The desired micro-slurry should be a two-phase structure,
negative effects from the high surface tension. In this study, containing a fine β-phase grains (of a high melting point)
some low melting point alloys were made into a thixotropic uniformly dispersed in a molten α-phase. This micro-
fluid, like a toothpaste, having a desired non-Newtonian slurry is then directly fed into a filament extruder for 3D
fluid property for direct extrusion. A thixotropic fluid is printing, as illustrated in Figure 2. For demonstration
characterized by a high viscosity and a yield stress at a low purposes, several low-melting-point alloys were chosen in
strain rate, but the fluid thins at increasing strain rate. Inside the experimental work in this study.
the nozzle, the viscosity is low because of high shear rate, so
the flow is enabled. Outside the nozzle, the shear rate rapidly 2. Equipment design
vanishes, so a yield stress is produced to counterbalance 2.1. Thixotropic metal extruder design
the surface tension. This thixotropy not only permits the The core part of the thixotropic metal extruder was
printing fluid to form a stabilized paste-like filament during designed in conjunction with the following components
printing and furthermore allows the printed fluid not to sag (Figure 3): motor and gearbox, auger bit, hopper, material
in the absence of a mold, thus enabling direct 3D printing transfer channel, heating elements, and nozzle. The body
with high geometrical control.
of the extruder was modified by a Filabot EX2 polymer
Thixotropy is not completely new in alloy processing. filament extruder. The heating system comprise three
Actually, near net-shape manufacturing processes by sections: a high-temperature region for material melting,
semisolid processing are currently available, including a low-temperature region for semi-solid formation, and
thixocasting, rheocasting, and thixomolding [13-15] . a nozzle reheat region. The region of high-temperature
Semisolid processing has several advantages : (1) The heating is located near the material feeding hopper. When
[13]
capital investment and operating costs is significantly raw material pellets are inserted into the machine, the bit
lower compared with conventional casting methods. The transports them to the heating region. This section’s heating

Figure 2. Process design for thixotropic shearing-mixing and 3D printing of alloy with a dispersed grain structure.

Volume 1 Issue 1 (2022) 3 http://doi.org/10.18063/msam.v1i1.5