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Materials Science in Additive Manufacturing Thixotropic metal 3D printing

(according to ISO/ASTM 52900 Standard) . In recent and both are simple compared with beam-based melting/
[2]
years, the material extrusion technique has already evolved sintering machines. 3D printing machines are now widely
into a mature technique for several other materials, such available from leading manufacturers, including 3D
as polymer, composites, and hydrogel [3-5] . However, liquid- System, Stratasys, XJET, HP, Voxeljet, ExOne, Desktop,
state deposition of molten metal with accuracy suitable for and Prodways. They can be divided into two groups:
AM remains to be an immense challenge [6,7] . The difficulty direct deposition and binder deposition. Nevertheless,
arises from the unfavorable rheology of molten metals, for metallic materials, all commercially available jetting/
that is, extremely low viscosity but exceptionally high extrusion machines rely on a binder material (mostly
surface tension. This leads to uncontrollable coalescence organic materials) for formulating a printable compound;
and breakup of the fluid under normal conditions, as such direct printing of molten metal for controllable freeform
neither controllable droplet nor stable liquid threads can 3D fabrication has not yet been achieved.
be formed.
This difficulty in liquid-state deposition of molten metal
1.1. Thixotropic property and semi-solid metal is understandable by examining the Ohnesorge number,
(SSM) processing Oh, defined as a ratio of viscous force over the combined
inertia and surface tension forces,
Thixotropy refers to a material’s yielding and shear-
thinning properties. When the material is sheared, it Oh = µ ρσ L (1)
thins, but when the material is allowed to stand, it thickens
again . For a suspension or slurry, this behavior is related where µ is viscosity, ρ is density, σ is surface tension,
[8]
to the disintegration of solid particle agglomerates in and L is characteristic size. For molten metal, the surface
the presence of shear and their reformation after the tension is typically about several 100s of mN/m, while
shear is removed. For a metallic alloy under cooling, the viscosity is very low, about a few centipoises (mPa⋅s).
this behavior is observed when the microstructure is For a 0.1 mm sized droplet, the Oh number is calculated
composed of solid spheroids suspended in a liquid to be on the order of 0.005 (assuming ρ ≈ 5000 Kg/m ).
3
matrix (Figure 1) . Thixotropy forms the theoretical For jetting of droplets, the needed Oh number is typically
[8]
foundation for SSM processing, and as a result, several in the range of 0.1 – 1 [10,11] . On the other hand, for stable
SSM processing methods have already been developed formation of a liquid filament, a larger Oh number much
and commercialized for molding and casting applications. >1 is needed. It then becomes apparent that neither
In particular, continuous stirring and extrusion is droplets nor filaments as desired in 3D printing can be
one popular SSM method for the alloy industry, and formed in a stable, controllable manner for molten metal.
thixomolded magnesium alloy has become one of the Actually, molten metal is extremely hard to harness under
major products for light-weight structural applications normal extrusion conditions [6,7] ; the ultrahigh surface
including components for automotive. tension dictates instability, such as coalescence and
breakup, especially the sensitivity to process variations
1.2. Problem statement and local geometrical changes. The large surface tension
Liquid-state deposition by jetting or by extrusion represents also makes adhesion to the substrate difficult. As such,
the most cost-effective and successful techniques in AM . liquid-state deposition/printing of molten metal is still
[9]
In jetting, liquid droplets are created while in extrusion, a immensely difficult.
liquid filament is generated. Jetting-based and extrusion- From the perspectives of fluid physics, the surface
based machines share many common features in design,
tension in general is not easy to modify, but the viscosity is.
A B Therefore, one may increase the viscosity (or flow resistance
forces) so that the Oh number becomes sufficiently large
for filament formation. Actually, some limited work [7,12] has
been conducted to improve filament formation of liquid
metal by increasing flow resistance, for example, through
alloy formulation. However, the existing ideas could only
yield very limited initial results and still remain to be
explored.
For low-melting and chemically reactive alloys such as
Al-, Zn-, and Mg-based alloys, enabling direct extrusion
Figure 1. Microstructure of solidified alloy. (A) Dendritic microstructure
in an as-cast sample and (B) a globular microstructure in a semisolid alloy is of particular significance, since powder-based fusion
sample. processes do not work well for these alloys due to the

Volume 1 Issue 1 (2022) 2 http://doi.org/10.18063/msam.v1i1.5