U.S. patent application number 15/625161 was filed with the patent office on 2017-12-21 for structural direct-write additive manufacturing of molten metals.
The applicant listed for this patent is Eck Industries, Inc., UT-Battelle, LLC. Invention is credited to William G. CARTER, Michael S. KESLER, Orlando RIOS, Zachary C. SIMS, David WEISS.
Application Number | 20170362687 15/625161 |
Document ID | / |
Family ID | 60659315 |
Filed Date | 2017-12-21 |
United States Patent
Application |
20170362687 |
Kind Code |
A1 |
RIOS; Orlando ; et
al. |
December 21, 2017 |
STRUCTURAL DIRECT-WRITE ADDITIVE MANUFACTURING OF MOLTEN METALS
Abstract
An alloy for structural direct-writing additive manufacturing
comprising a base element selected from the group consisting of
aluminum (Al), nickel (Ni) and a combination thereof, and a rare
earth element selected from the group consisting of cerium (Ce),
lanthanide (La) and a combination thereof, and a eutectic
intermetallic present in said alloy in an amount ranging from about
0.5 wt. % to 7.5 wt. %. The invention is also directed to a method
of structural direct-write additive manufacturing using the
above-described alloy, as well as 3D objects produced by the
method. The invention is also directed to methods of producing the
above-described alloy.
Inventors: |
RIOS; Orlando; (Knoxville,
TN) ; WEISS; David; (Manitowoc, WI) ; SIMS;
Zachary C.; (Knoxville, TN) ; CARTER; William G.;
(Oak Ridge, TN) ; KESLER; Michael S.; (Knoxville,
TN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UT-Battelle, LLC
Eck Industries, Inc. |
Oak Ridge
Manitowoc |
TN
WI |
US
US |
|
|
Family ID: |
60659315 |
Appl. No.: |
15/625161 |
Filed: |
June 16, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62350749 |
Jun 16, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22D 23/003 20130101;
C22C 21/00 20130101; B33Y 10/00 20141201; B33Y 70/00 20141201 |
International
Class: |
C22C 21/00 20060101
C22C021/00; B33Y 10/00 20060101 B33Y010/00; B33Y 70/00 20060101
B33Y070/00; B22D 23/00 20060101 B22D023/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under Prime
Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. An alloy for structural direct-writing additive manufacturing
comprising a base element selected from the group consisting of
aluminum (Al), nickel (Ni) and a combination thereof, and a rare
earth element selected from the group consisting of cerium (Ce),
lanthanide (La) and a combination thereof, and a eutectic
intermetallic present in said alloy in an amount ranging from about
0.5 wt. % to 7.5 wt. %.
2. The alloy of claim 1, wherein said rare earth element is Ce,
wherein said Ce is present in said alloy in an amount up to 8 wt.
%.
3. The alloy of claim 1, wherein said rare earth element is La,
wherein said La is present in said alloy in an amount up to 10
weight percent %.
4. The alloy of claim 1, wherein said alloy further comprises at
least one additional alloying element selected from the group
consisting of iron (Fe), silicon (Si) and magnesium (Mg).
5. The alloy of claim 4, wherein Fe is present in said alloy in an
amount up to 2 wt. %, Si is present in said alloy in an amount up
to 2 wt. %, and Mg is present in said alloy in an amount up to 30
wt. %.
6. The alloy of claim 1, wherein said alloy further comprises an
additive selected from the group consisted of SiC, carbon nanotube
(CNT), alumina and boron nitride.
7. The alloy of claim 6, wherein said additive is present in said
alloy in an amount up to 30 vol %.
8. The alloy of claim 1, wherein said alloy is an Al--Ce alloy,
with said Ce present in an amount of about 0.5 to 7 wt. % by weight
of said alloy, and said eutectic intermetallic is
Al.sub.11Ce.sub.3.
9. The alloy of claim 1, wherein said alloy is an Al--La alloy, and
said eutectic intermetallic is Al.sub.11La.sub.3.
10. The alloy of claim 1, further comprising discrete units of said
alloy interconnected by an interface containing said eutectic
intermetallic, wherein said eutectic intermetallic is present at
said interface in an amount greater than that within each of said
discrete units.
11. A method of fabricating a three-dimensional (3D) metallic
object using direct-write additive manufacturing, the method
comprising the steps of: a. providing an alloy comprising a base
element selected from the group consisting of aluminum (Al), nickel
(Ni) and a combination thereof, and a rare earth element selected
from the group consisting of cerium (Ce), lanthanide (La) and a
combination thereof, and a eutectic intermetallic present in said
alloy in an amount ranging from about 0.5 wt. % to 7.5 wt. %; b.
heating said alloy to a temperature within 15% above or below a
melting point of said alloy in an inert atmosphere; c. extruding
said alloy through a nozzle in the presence of an oxygen-containing
atmosphere, including but not limited to ambient atmosphere, to
form beads of said alloy having a surface tension ranging from
about 0.3 N/m to 2.0 N/m, wherein before exiting said nozzle said
alloy remains in said inert atmosphere, and a stabilizing shell is
formed surrounding a liquid core of each of said beads when said
beads are exposed to said oxygen-containing atmosphere, wherein
said stabilizing shell comprises oxides of alloying elements of
said alloy and at least one metastable intermetallic of the
alloying elements; and d. depositing said beads on a substrate and
contacting said beads with each other, wherein said stabilizing
shells of adjacent beads fuse on contact as said beads are cooled
down in said oxygen-containing atmosphere, wherein during the
fusing of the beads, said eutectic intermetallic is formed at an
interface of said adjacent beads in an amount greater than that
within each of said adjacent beads.
12. The method of claim 11, wherein said Alloy is an Al--Ce alloy,
and said eutectic intermetallic is Al.sub.11Ce.sub.3.
13. The method of claim 12, wherein at least one metastable
intermetallic is present in said stabilizing shell, and the at
least one metastable intermetallic comprises Al.sub.2Ce or
Al.sub.4Ce.
14. The method of claim 11, wherein said alloy is an Al--La alloy,
and said eutectic intermetallic is Al.sub.11La.sub.3.
15. The method of claim 11, wherein said alloy further comprises at
least one additional alloying element selected from the group
consisting of iron (Fe), silicon (Si), and magnesium (Mg).
16. The method of claim 11, wherein said stabilizing shell has a
thickness ranging from about 10 to 15 nm.
17. The method of claim 11, wherein said at least one metastable
intermetallic is present in said stabilizing shell, and the at
least one metastable intermetallic has a rare earth content
different than a rare earth content in said eutectic
intermetallic
18. The method of claim 11, wherein during said fusion, said oxide
of said rare earth element in said stabilizing shells of adjacent
beads dissolves and forms said eutectic intermetallic at said
interface.
19. The method of claim 11, wherein during said fusion, said at
least one metastable intermetallic is present in said stabilizing
shells of adjacent beads, and then decomposes and forms said
eutectic intermetallic at said interface.
20. The method of claim 11, wherein in step (d) said beads are
deposited in a layer-on-layer manner on said substrate to form said
3D metallic object, wherein said stabilizing shells of said beads
in adjacent layers fuse on contact as said beads are cooled down in
said oxygen-containing atmosphere, thereby welding said adjacent
layers, wherein, during said fusion, said eutectic intermetallic is
formed at an interface of said adjacent layer in an amount greater
than that within each of said layers.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application claims benefit of U.S. Provisional
Patent Application No. 62/350,749, filed on Jun. 16, 2016, all of
the contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention generally relates to the field of
direct-writing additive manufacturing. The invention relates, more
particularly, to direct-writing additive manufacturing of metallic
materials.
BACKGROUND OF THE INVENTION
[0004] Additive manufacturing or three-dimensional (3D) printing is
a process for making a 3D object of any shape by depositing and
joining materials layer by layer from 3D models. The capability of
freeform in fabrication of 3D complex objects without the need for
expensive tooling or machining has made additive manufacturing
promising for a wide variety of applications.
[0005] However, current additive manufacturing technologies lack
the capability to efficiently produce 3D objects composed of high
melting point materials, particularly metals and metal alloys. Most
metal additive manufacturing approaches are based on powder-bed
melting techniques, such as laser selective melting or electron
beam (e-beam) melting. These technologies are often restricted to a
select few materials (i.e., relatively expensive metal powders
having particles greater than 1 .mu.m in size) and involve the use
of expensive laser and e-beam systems. Therefore, metal additive
manufacturing has been heretofore economically unattractive for
most applications.
[0006] Welding-based additive manufacturing makes use of an
electrical arc generated by a non-consumable electrode or directly
from a feed material. A plasma plume is generated by the
interaction of the electric arc with the gas surrounding the build
zone. The high kinetic energy of the plasma coupled with the high
thermal gradients associated with the weld pool leads to poor
control of deposited materials resulting in low dimensional
tolerances and general build quality.
[0007] Structural direct-write additive-manufacturing is a method
wherein liquid material is deposited from a print head directly
onto a print bed, where the material solidifies, retaining an
intended shape and bonding with a layer of the same material upon
which it is deposited. Costs for structural direct-write additive
manufacturing systems are much lower than selective laser sintering
and e-beam commonly used in metal additive manufacturing. In
addition, structural direct-write additive manufacturing is less
time intensive than selective laser sintering and e-beam. However,
there remains a need to develop metallic materials that can be more
suitably integrated with structural direct-write additive
manufacturing.
SUMMARY OF THE INVENTION
[0008] In one aspect, the invention is directed to an alloy for
structural direct-write additive manufacturing. The alloy described
herein includes a base element selected from the group consisting
of aluminum and/or nickel, and a rare earth element selected from
the group consisting of cerium and/or lanthanide. A eutectic
intermetallic of the alloying elements is present in the alloy in
an amount ranging from about 0.5 wt. % to 7.5 wt. %.
[0009] In another aspect, the invention is directed to a method of
fabricating a 3D metallic object using structural direct-write
additive manufacturing. The method generally involves heating the
above-described alloy to a temperature within 15% above or below a
melting point of the alloy in an inert atmosphere, extruding the
alloy through a nozzle in the presence of an oxygen-containing
atmosphere to form beads of the alloy having a surface tension
ranging from about 0.3 N/m to 2.0 N/m. A stabilizing shell is
formed surrounding a liquid core of each bead when the beads are
exposed to the oxygen-containing atmosphere. The stabilizing shell
is rare earth-rich, including oxides of alloying elements and at
least one metastable intermetallic of the alloying elements. The
method further includes depositing the beads on a substrate and
contacting the beads with each other. The rare earth-rich shells of
adjacent beads fuse on contact as the beads are cooled down in the
oxygen-containing atmosphere. During fusion, a eutectic
intermetallic of the alloying elements is formed at an interface of
the adjacent beads in an amount greater than that within each of
the adjacent beads.
BRIEF DESCRIPTION OF THE FIGURES
[0010] FIG. 1 is a schematic diagram showing a droplet of a molten
alloy of the present invention suspended from a nozzle.
[0011] FIG. 2 is a graph showing surface tension vs. drop-shape (ds
vs. de) for alloys with different densities including Al--Li,
Al--Mg, Al-rare earth alloys, Mg-rare earth alloys, and Fe-rare
earth alloys.
[0012] FIG. 3 is a graph showing bead size vs. nozzle diameter
defined by ds for various alloys with different densities including
Al-rare earth alloys, Mg-rare earth alloys, and Fe-rare earth
alloys.
[0013] FIG. 4 is a graph showing a ternary isotherm for an
Al--Ce--La alloy at 500.degree. C.
[0014] FIG. 5 is a low percentage binary phase diagram for an
Al--Ce alloy.
[0015] FIG. 6 is a low percentage binary phase diagram for an
Al--La alloy.
[0016] FIG. 7 is a depiction of a first continuous filament of
beads of an alloy of the present invention being deposited on a
substrate by a nozzle.
[0017] FIG. 8 is a depiction of a second continuous filament of
beads of the alloy of FIG. 7 being formed onto the first continuous
filament by the nozzle.
[0018] FIG. 9 is a cross-sectional view through A-A' of FIG. 8.
[0019] FIG. 10 is a cross-sectional diagram showing the outcome of
beads of the alloy of FIG. 7 being deposited too quickly and/or at
too high of a temperature.
[0020] FIG. 11 is a cross-sectional diagram showing the outcome of
beads of the alloy of FIG. 7 being deposited too slowly and/or at
too low of a temperature.
[0021] FIG. 12 is a flowchart illustrating a first mechanism that
may account for the formation of fused layers of beads of an alloy
of the present invention.
[0022] FIG. 13 is a flowchart illustrating a second mechanism that
may account for the formation of fused layers of beads of an alloy
of the present invention.
[0023] FIG. 14 is a flowchart illustrating a third mechanism that
may account for the formation of fused layers of beads of an alloy
of the present invention.
[0024] FIG. 15 is a flowchart illustrating a fourth mechanism that
may account for the formation of fused layers of beads of an alloy
of the present invention.
[0025] FIG. 16 is a flowchart illustrating a fourth mechanism that
may account for the formation of fused layers of beads of an alloy
of the present invention.
[0026] FIG. 17 is a graph showing photoelectron spectroscopy (XPS)
data for the surface of an as-cast sample of Al-6Ce. Intensity of
detected photoelectrons is measured in arbitrary units (a.u.)
[0027] FIG. 18 is a graph showing XPS data for an as-cast sample of
Al-6Ce after sputter etching to a depth of 12 nm.
[0028] FIG. 19 is a graph showing XPS data for the surface of a
heat treated sample of Al-6Ce.
[0029] FIG. 20 is a graph showing XPS data for a heat treated
sample of Al-6Ce after sputter etching to a depth of 12 nm.
[0030] FIG. 21 is a graph showing XPS data for an oxidized sample
of Al-6Ce.
[0031] FIG. 22 is a graph showing XPS data for an oxidized sample
of Al-6Ce after sputter etching to a depth of 12 nm.
[0032] FIG. 23 is a low-magnification scanning electron micrograph
(SEM) image of Al-6Ce.
[0033] FIG. 24 is a high magnification SEM image of Al-6Ce.
[0034] FIG. 25 is a graph showing differential scanning calorimetry
(DSC) data for Al-6Ce shown in FIGS. 25 and 26.
[0035] FIG. 26A is a SEM image of fused beads of Al-6Ce. FIG. 26B
is a higher magnification SEM image showing the wetting surface of
the fused beads of FIG. 26A.
[0036] FIG. 27 is a graph showing a ternary isotherm for Al--Ce--Si
alloy family at 639.degree. C.; a few of the many phases are
labeled.
[0037] FIG. 28 is a graph showing an enlarged portion of FIG. 27
below the 10 mass percent line. Details and phases of interest are
shown.
[0038] FIG. 29 is a graph showing the isopleth along the constant
93 wt. % Al composition line 42 shown in FIG. 27.
[0039] FIG. 30 is a graph showing the isopleth along the constant
97 wt. % Al composition line 44 shown in FIG. 27.
[0040] FIG. 31 is a graph showing the equilibrium phase fraction
diagram of Al--Ce--Si alloy family along the 93 wt. % Al line 42
shown in FIG. 27.
[0041] FIG. 32 is a graph showing the equilibrium phase fraction
diagram of Al--Ce--Si alloy family along the 97 wt. % Al line 44
shown in FIG. 27.
[0042] FIG. 33 is a graph showing minimum ratios needed to achieve
0.549 weight % non-Al phase content.
[0043] FIG. 34 is a graph showing minimum ratios needed to achieve
0.549 weight % non-Al phase content.
[0044] FIG. 35 is a diagram showing Al--Si--Ce ternary alloy
isotherm at 100.degree. C.
[0045] FIG. 36 is a diagram showing Al--Si--Ce ternary alloy
isotherm at 500.degree. C.
[0046] FIG. 37 is a diagram showing Al--Si--Ce ternary alloy
isotherm at 650.degree. C.
[0047] FIG. 38 is a diagram showing Al--Si--Ce ternary alloy
liquidus.
DETAILED DESCRIPTION OF THE INVENTION
[0048] In one aspect, the invention is directed to an alloy that is
suitable for structural direct-write additive manufacturing. The
alloy described herein includes a base element selected from the
group consisting of aluminum (Al) and/or nickel (Ni), and a rare
earth element selected from the group consisting of cerium (Ce)
and/or lanthanide (La). In some embodiments, the alloy is a binary
alloy, such as an Al--Ce alloy, Ni--Ce alloy, Al--La alloy, or
Ni--Al alloy. In other embodiments, the alloy is a ternary alloy,
such as an Al--Ni--Ce alloy, Al--Ni--La alloy, Al--Ce--La alloy,
Ni--Ce--Al alloy, Al--Ce--Si alloy, Ni--Ce--Si alloy, Al--La--Si
alloy, or Ni--Al--Si alloy. In yet other embodiments, the alloy is
a quaternary alloy, such as an Al--Ni--Ce--La alloy, Al--Ni--Ce--Si
alloy, Al--Ni--La--Si alloy, Al--Ce--La--Si alloy, or
Ni--Ce--Al--Si alloy. Any of the above alloy compositions may or
may not also include iron (Fe) and/or magnesium (Mg) and/or carbon
(C) to form a higher alloy.
[0049] The alloy of the present invention has the following
characteristics which make it feasible to be used in structural
direct-write additive manufacturing. First, when molten, a surface
tension of the molten alloy falls within a specific range in
ambient atmosphere such that the molten alloy forms free-standing
beads as the molten alloy is extruded from a nozzle of a 3D
printing system and deposited either upon a substrate or upon a
previously deposited layer of beads. In one embodiment, the molten
alloy has a surface tension ranging from about 0.3 N/m to 2.0 N/m.
This range encompasses unprintable metals, such as mercury and
molten iron. It has herein been found that a molten alloy having a
surface tension below 0.3 N/m is largely incapable of forming
beads, and instead flows freely, which is deleterious to structural
direct-write additive manufacturing. It has also herein been found
that a molten alloy having a surface tension above 2.0 N/M is
largely incapable of forming beads, and instead forms clumps, which
is also deleterious to structural direct-write additive
manufacturing.
[0050] FIG. 1 depicts a droplet (i.e., bead) of a molten alloy 10
suspended from a nozzle 12. The surface tension of the droplet can
be calculated by the following formula:
.gamma.=(.DELTA.pgde2).times.(1/H)
where .gamma. is surface tension, .DELTA.p is density difference
between the two fluids of interest (e.g., for Al extrusion in a
gaseous atmosphere the density of aluminum is an appropriate
approximation), g is gravity acceleration, and 1/H is a correction
factor determined from ds/de via second order partial differential
Laplacian approximations of droplet shape. Additional information
on the calculation of the surface tension of droplet can be found
in J. M. Andreas, et al. "Boundary Tension by Pendant Drops",
presented at the Fifteenth Colloid Symposium, held at Cambridge,
Mass., Jun. 9-11, 1938, the contents of which are herein
incorporated by reference in their entirety. FIG. 2 shows surface
tension vs. droplet aspect ratio (ds/de) for alloys with different
densities, e.g., alloys with low density, medium-to-low density,
medium-to-high density, and high density. These alloys include
Al--Li, Al--Mg, Al-rare earth alloys having densities in the range
of 1-2.7 g/cm.sup.3, Mg-rare earth alloys having densities in the
range of 1.5-2.2 g/cm.sup.3, and Fe-rare earth alloys having
densities in the range of 7-8 g/cm.sup.3. It can be seen that
operable surface tension for alloys of different densities
generally falls in the range of 0.3 N/m to 2.0 N/m.
[0051] Beads of the alloy may have any shape. Typically, the beads
are substantially spherical or ovoid. The sizes of the beads are
primarily determined by the size of the nozzle 12 and physical
properties of the alloy. FIG. 3 shows projected bead size for
various alloys with different densities in the operable surface
tension range vs. nozzle diameter. These alloys include Al-rare
earth alloys having densities in the range of 1-2.7 g/cm.sup.3,
Mg-rare earth alloys having having densities in the range of
1.5-2.2 g/cm.sup.3, and Fe-rare earth alloys having densities in
the range of 7-8 g/cm.sup.3. It can be seen that, for a given
nozzle size, the bead size increases with higher alloy density.
[0052] The proportions of the alloying elements in the alloy are
chosen such that a eutectic intermetallic of the alloying elements
is present in the alloy as a segregated phase in an amount ranging
from about 0.5 to 7.5 wt. %, preferably about 0.5 to 7 wt. %. In
different embodiments, the eutectic intermetallic is present in an
amount of, for example, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5,
5.5, 6, 6.5, 7, or 7.5 wt. %, or within a range bounded by any two
of the foregoing values. The eutectic intermetallic can affect the
viscosity of the molten alloy, thereby affecting its
printability.
[0053] The alloy of the present invention also exhibits a unique
coupling of surface tension and surface chemistry to stabilize the
material mechanical properties once printed. Once the molten alloy
is extruded through the nozzle 12 to form freestanding beads, each
bead is stabilized mechanically by a stabilizing shell that is
formed spontaneously on its surface when the beads are exposed to
an oxygen-containing atmosphere, such as, for example, ambient
atmosphere. The stabilizing shell stabilizes the surface tension on
material flow and protects the bulk composition of the alloy from
degradation. The stabilizing shell is composed of oxides of the
alloying elements (e.g., native oxides of the alloy elements and
secondary oxide) and at least one metastable intermetallic of the
alloying elements. The stabilizing shell has a rare earth content
higher than that in the eutectic intermetallic (i.e., it is
"rare-earth rich"). The stabilizing shell may be formed to have a
thickness ranging from, for example, about 10 to 15 nm.
[0054] Beads of the alloy of the present invention also have a
self-welding ability. Upon contact with each other, the reaction
between the stabilizing shells of adjacent beads produces a higher
concentration eutectic intermetallic at the interface, thereby
fusing the beads together.
[0055] In some embodiments, the alloy may contain Ce in an amount
up to about 8 wt. %, such as an alloy containing Al and Ce with a
Ce content in an amount up to about 8 wt. %. In one embodiment, the
alloy includes Ce in an amount of about 0.5 to about 7 wt. %, or
more specifically, about 1 to about 6 wt. %. The eutectic
intermetallic present in the Al--Ce alloy may be, for example,
Al.sub.11Ce.sub.3.
[0056] In some embodiments, at least a portion of the Ce in the
alloy is substituted by La. In some embodiments, each part by
weight of Ce is substituted by 1.2 parts by weight of La. In other
embodiments, all of the Ce in the alloy is substituted by La, and
the alloy includes Al and La, with a La content in an amount up to
about 10 wt. %. The eutectic intermetallic present in the Al--La
alloy may be, for example, Al.sub.11La.sub.3.
[0057] FIG. 4 is an isotherm for an Al--Ce--La alloy at 500.degree.
C. Mirroring in the phases is observed as compositional amounts of
Ce and La are varied, indicating the high similarity between Ce and
La. FIGS. 5 and 6 are low percentage binary phase diagrams for an
Al--Ce alloy and an Al--La alloy. Notably, the foregoing phase
diagrams appear nearly identical except for a slight suppression in
the melting temperature of the hyper and hypo-eutectic regions.
[0058] In some embodiments, the alloy may also include one or more
additional alloying elements such as, for example, iron (Fe),
silicon (Si), and magnesium (Mg). The alloy may contain up to about
2 wt. % of Fe, up to about 2 wt. % of Si, or up to about 30 wt. %
of Mg. Some particular examples of alloys include Al--Ce, Al--La,
Al--Ce--La, Al--Ce--La--Si, Al--Ce--Si, Al--La--Si, Al--Ce--Ni, and
Ce--Ni alloys.
[0059] In some embodiments, the alloy may further include a minor
additive, such as SiC, carbon nanotube (CNT), alumina, or boron
nitride. The minor additive can typically constitute up to about 30
vol % of the alloy. The minor additive may be employed, for
example, to increase the loss modulus (i.e., viscosity) of the
molten alloy. Increasing the loss modulus can stabilize the molten
and semi-solid beads as the beads cool down.
[0060] In another aspect, the invention is directed to a method for
fabricating a 3-D metallic object from the alloy described above
using structural direct-write additive manufacturing. The method
entails first providing an alloy, as described above, and feeding
the alloy into a feed chamber of a 3D printing system. The feed
chamber is connected to the nozzle 12, shown in FIG. 1. In some
embodiments, the nozzle 12 has a diameter up to about 10 mm, or
more typically from about 0.1 mm to 2 mm. Inert gas, such as
nitrogen or argon, is supplied to the feed chamber and the nozzle
12 to create an inert atmosphere; thus, before exiting the nozzle
12, the alloy is protected by the inert atmosphere. The alloy can
be provided in any suitable form, such as an alloy wire, alloy
ingot, or as a mixture of constituent alloying element powders.
[0061] The method further includes heating the alloy to provide a
molten alloy. The alloy can be heated to a temperature at which the
alloy can consistently be extruded through the nozzle 12. The alloy
is typically heated to a temperature within 15%, within 10%, within
5%, or within 2% above or below a melting point of the alloy. For
example, when the alloy is an Al--Ce alloy, the Al--Ce alloy can
generally be heated above 650.degree. C. In some embodiments,
heating the alloy includes heating with one or more heating
elements surrounding the feed chamber and the nozzle 12. Energy can
be coupled to the alloy inside the feed chamber and the nozzle 12
either via conduction heating or electromagnetic heating in order
to precisely control the extrusion temperature of the alloy. In
some embodiments, one or more heating elements are resistance
heating elements. In some embodiments, the one or more heating
elements are disposed inside the nozzle 12. In some embodiments,
the one or more heating elements are configured to provide a nozzle
temperature that increases toward an extrusion opening of the
nozzle 12. In some embodiments, the heating elements are arranged
in increasing density toward the extrusion opening of the nozzle
12, operating at increasing power toward the extrusion opening of
the nozzle 12, or both.
[0062] Subsequently, the molten alloy is extruded through the
nozzle 12 via pneumatic or mechanical pressure to form beads. These
beads have a surface tension ranging from about 0.3 N/m to 2.0 N/m.
Upon exiting the nozzle 12, a stabilizing shell composed of oxides
of the alloying elements and at least one metastable intermetallic
of the alloying elements is formed on a surface of each alloy bead,
surrounding a liquid core of the bead as the bead is exposed to an
oxygen-containing atmosphere (e.g., ambient atmosphere). The at
least one metastable intermetallic can be a rare earth-rich
intermetallic having a rare earth content greater than that in the
eutectic intermetallic or a rare earth-deficient intermetallic
having a rare earth content lower than that in the eutectic
intermetallic. In one embodiment, when the alloy is an Al--Ce
alloy, the at least one metastable intermetallic formed in the
stabilizing shell may include Al.sub.2Ce and/or Al.sub.4Ce. As the
bead cools, the liquid core begins solidifying.
[0063] Next, referring to FIG. 7, the beads of the alloy are
deposited one by one by contacting previously deposited beads on a
substrate 16 from the nozzle 12. The stabilizing shells of adjacent
beads fuse on contact as beads are cooled down in the
oxygen-containing atmosphere at ambient temperature, forming a
first continuous filament 14 on the substrate 16 in a predetermined
pattern. After fusion, the eutectic intermetallic is formed at an
interface of adjacent beads in a higher concentration than that
within each bead.
[0064] Referring to FIGS. 8 and 9, once the first continuous
filament 14 is formed, deposition of the beads begins one by one on
top of the previous layer of fused beads (e.g., the first continues
filament 14) to form a second continuous filament 18. The
stabilizing shells of beads in the second continuous filament 18
fuse with the rare earth-rich shells of beads in the first
continuous filament 14 on contact, welding the first and second
continuous filaments 14, 18 together. At the time of fusion, the
lower bead 14' has a stabilizing shell 15 and an at least partially
solidified core 20, while the upper bead 18' has a stabilizing
shell 19 and a mostly liquid core 22. Deposition of beads of the
alloy continues in this manner, layer by layer, until the entire 3D
metallic object is completed. During deposition, the nozzle 12 may
be moved with respect to the substrate 16. In one embodiment,
either the nozzle 12 may be moved or the substrate 16 may be moved.
In another embodiment, both of the nozzle 12 and the substrate 12
may be moved to cause relative motion between the nozzle 12 and the
substrate 16). The dotted line spanning across A and A' in FIG. 8
refers to the cross-section. The cross-sectional view through A-A'
is provided in FIG. 9.
[0065] The printing conditions employed, such as extruding rate and
melting temperature, are carefully controlled to ensure proper
welding of the consecutive layers. For example, if beads of the
alloy are deposited too quickly and/or at too high of a
temperature, the beads will be deformed, as shown in FIG. 10, and
may even collapse. Conversely, if beads of the alloy are deposited
too slowly and/or at too low of a temperature, the beads will
solidify too quickly, resulting in unsatisfactory self-welding, as
shown in FIG. 11.
[0066] Strong interlayer welding is important to prevent the
material from delaminating during printing and end user
application. Depending on the composition of the stabilizing shell
formed, the self-welding of layers of beads of the alloy of the
present invention can be attributed to the following reactions
occurred at layer-to-layer interface or bead-to-bead interface in
the same layer.
[0067] FIG. 12 is a flowchart illustrating a first mechanism that
may account for the formation of the fused layers of beads of the
alloy. As shown, a molten alloy of the present invention is
extruded onto a substrate, thereby forming a first layer of beads.
A stabilizing shell composed of native oxides of alloying elements
is formed on a surface of each bead in the first layer once the
bead is contacted with the ambient atmosphere. As beads in the
first layer cool, cores of beads partially solidify under ambient
temperature. A second layer of beads is then deposited on top of
the first layer. A stabilizing shell composed of native oxides of
alloying elements is formed on a surface of each bead in the second
layer once the bead is contacted with ambient atmosphere. As beads
in the second layer cool, an exothermic reaction occurs between the
base element and the rare earth oxide present at interfaces of the
beads to result in the eutectic intermetallic at these interfaces;
the rare earth oxide thus dissolves at the interfaces of the beads.
The exothermic reaction causes localized welding of layers. The
remaining oxide precipitates of the base element help to strengthen
the bonds between layers.
[0068] FIG. 13 is a flowchart illustrating a second mechanism that
may account for the formation of fused layers of beads of the
alloy. As shown, a molten alloy of the present invention is
extruded onto a substrate, thereby forming a first layer of beads.
A stabilizing shell composed of metastable intermetallics and mixed
oxides of alloying elements is formed on a surface of each bead in
the first layer once the bead is contacted with the ambient
atmosphere. As beads in the first layer cool, cores of beads
partially solidify under ambient temperature. A second layer of
beads is deposited on top of the first layer. A stabilizing shell
composed of metastable intermetallics and mixed oxides of alloying
elements is formed on a surface of each bead in the second layer
once the bead is contacted with the ambient atmosphere. Oxides at
the layer-to-layer interface dissolve due to the presence of
energetically favored metastable intermetallics. In addition, as
the beads in the second layer cool, the metastable intermetallics
in the stabilizing shells at the layer-to-layer interface decompose
and transform into a eutectic intermetallic at the bead-to-bead
interfaces. These reactions cause localized welding of layers.
[0069] FIG. 14 is a flowchart illustrating a third mechanism that
may account for the formation of fused layers of beads of the
alloy. As shown, a molten alloy of the present invention is
extruded onto a substrate, thereby forming a first layer of beads.
A stabilizing shell composed of native oxides of alloying elements
and metastable intermetallics is formed on a surface of each bead
in the first layer once the bead is contacted with the ambient
atmosphere. As beads in the first layer cool, cores of beads
partially solidify under ambient temperature. A second layer of
beads is deposited on top of the first layer. A stabilizing shell
composed of native oxides of alloying elements and metastable
intermetallics is formed on a surface of each bead in the second
layer once the bead is contacted with the ambient atmosphere. As
beads in the second layer cools, rare earth oxide and metastable
intermetallics at the layer-to-layer interface decompose, thereby
forming a eutectic intermetallic at the interface. The exothermic
reaction causes localized welding of layers.
[0070] FIG. 15 is a flowchart illustrating a fourth mechanism that
may account for the formation of fused layers of alloy beads. As
shown, a molten alloy of the present invention is extruded onto a
substrate, thereby forming a first layer of beads. A stabilizing
shell composed of native oxides of alloying elements and metastable
intermetallics is formed on a surface of each bead in the first
layer once the bead is contacted with the ambient atmosphere. As
beads in the first layer cool, cores of beads partially solidify
under ambient temperature. A second layer of beads is deposited on
top of the first layer. A stabilizing shell composed of native
oxides of alloying elements and metastable intermetallics is formed
on a surface of each bead in the second layer once the bead is
contacted with ambient atmosphere. As beads in the second layer
cools, oxides break up and metastable intermetallics decompose at
the layer-to-layer interface, thereby forming a eutectic
intermetallic at the interface. The exothermic reaction causes
localized welding of layers.
[0071] FIG. 16 is a flowchart illustrating a fifth mechanism that
may account for the formation of fused layers of alloy beads. As
shown, a molten alloy of the present invention is extruded onto a
substrate, thereby forming a first layer of beads. A stabilizing
shell composed of metastable intermetallics is formed on a surface
of each bead in the first layer once the bead is contacted with the
ambient atmosphere. As beads in the first layer cool, cores of
beads partially solidify under the ambient temperature. A second
layer of beads is deposited on top of the first layer. A
stabilizing shell composed of metastable intermetallics is formed
on a surface of each bead in the second layer once the bead is
contacted with the ambient atmosphere. As beads in the second layer
cools, the metastable intermetallics decompose at the
layer-to-layer interface, thereby forming a eutectic intermetallic
at the interface. The exothermic reaction causes localized welding
of layers.
Examples
Characteristics of Al-6Ce Alloy
[0072] An Al-6Ce alloy containing 6 wt. % of Ce was tested and
found to have suitable beading characteristics attributable to
formation of a rare earth-rich shell at the exposed surface of the
bead.
[0073] FIG. 17 is a plot of photoelectron spectroscopy (XPS) data
for an as-cast sample of an Al-6Ce alloy, while FIG. 18 shows XPS
data for the Al-6Ce alloy following sputter etching to remove about
12 nm of surface material. The peak resulting from the presence of
oxide has near zero intensity, which indicates that the stabilizing
shell is on the order of 10-15 nm in thickness.
[0074] FIGS. 19 and 20 show respective XPS data for heat-treated
and heat-treated sputter-etched samples of the Al-6Ce alloy. FIGS.
21 and 22 show respective XPS data for oxidized and oxidized
sputter-etched samples of the Al-6Ce alloy. The data show that,
after 12 nm of etching, the oxide present at the surface of the
material has dissipated and the internal structure is bulk metallic
material.
[0075] FIGS. 23 and 24 show respective low and high magnification
SEM images of the Al-6Ce alloy respectively. The SEM images show a
low content (.about.6.7 weight %) of a eutectic Al.sub.11Ce.sub.3
intermetallic phase. Al.sub.11Ce.sub.3 eutectic intermetallic and
Al microconstituent surrounds the primary Al FCC grains. The
microstructure shows a divorced morphology.
[0076] FIG. 25 shows DSC data for the Al-6Ce alloy. It can be seen
that the Al-6Ce alloy starts to melt at approximately 640.degree.
C., and there are no measurable phase shifts before the Al-6Ce
alloy melts.
[0077] FIG. 26A shows a SEM image of fused beads of the Al-6Ce
alloy. FIG. 26B is a higher magnification SEM image showing the
wetting surface of the fused beads of FIG. 26A. The light gray
areas represent eutectic Al.sub.11Ce.sub.3 phases, and the dark
gray area represents an Al phase. It can be seen the amount of the
eutectic Al.sub.11Ce.sub.3 phases in the interface of the beads is
greater than that of the eutectic Al.sub.11Ce.sub.3 phases within
the beads. The formation of the Al.sub.11Ce.sub.3 eutectic
intermetallic at the interface is believed to be primarily involved
in the welding of the beads.
Characteristics of Al--Ce--Si Alloys
[0078] Al--Ce--Si alloys exhibit similar rheological properties to
that of the binary Al--Ce alloy. At high intermetallic contents
(>7.5 wt. %), the Al--Ce--Si alloys flow too well and, as a
result, will not bead sufficiently for printing. Thus, it is
necessary to maintain an overall low content of intermetallic (0.50
to up to 7.5 wt %) when utilizing ternary or quaternary additions
to the binary alloy.
[0079] FIGS. 27-32 provide detailed characteristics of the
Al--Ce--Si alloy family and intermetallic properties thereof. FIG.
27 shows a full ternary isotherm for the Al--Ce--Si ternary alloy
family at 639.degree. C., and FIG. 28 shows an enlarged portion of
the Al--Ce--Si ternary alloy family shown in FIG. 27 below the 10
wt. % line 40. Heavy dashed lines 42 and 44 represent respective
position of isopleths and property diagrams at 93 wt. % Al and 97
wt. % Al. FIGS. 29 and 31 show the isopleth and property diagrams
along the 93 wt. % Al composition line A shown in FIG. 27. FIGS. 30
and 32 show the isopleth and property diagrams along 97 wt. % Al
composition line B shown in FIG. 27.
[0080] The data in FIGS. 27-32 show that in instances where Al
content is held constant but the Si and Ce are varied to balance,
the different phases which precipitate form out of the melt and the
base Al-6Ce contains roughly 7 weight % intermetallic, which is
near the maximum intermetallic content that is suitable for
structural direct-write additive manufacturing. Thus, in order to
precipitate the same weight percent of Al--Ce--Si intermetallic, it
is required that the overall composition is greater than 7 wt. %
alloying additions. In the case of the Al--Ce--Si alloy, the
highest weight % of alloying elements which produce the necessary
low amount of ternary intermetallic is about 9 wt. % of alloying
additions, including 2 weight % of Si and 7 wt. % of Ce.
[0081] A low percentage of intermetallic (also called non-aluminum)
phase fraction is preferable for the alloy. For example, for
Al--Ce--Si alloys, an upper limit for 7 weight % of Ce results in
an intermetallic content of about 6.66 weight %. Taking 6.66 weight
% of intermetallic phase to be the upper limit for the Al--Ce--Si
alloys, the amount of Ce and Si that should be present in the alloy
can be calculated. Taking into account the lower limit of 0.5 wt. %
Ce, which results in an intermetallic content of 0.549 wt. %
intermetallic, the same relationship can be drawn.
[0082] FIG. 33 shows a plot of the lower limits of Ce and Si that
result in 0.549 wt. % of intermetallic phase, where (Si)=-0.4004
(Ce)+0.5254. An asymptotic behavior is observed at 0.5 wt. %
cerium, wherein a small addition of Si greatly increases the amount
of alloying addition necessary to reach the 0.549 wt. %
intermetallic phase due at least in part to the good solubility of
Si in Al. However, following the quick asymptotic behavior, the
relation is linear. The intercept value reflects the weight
percentage of Si necessary to obtain 0.549 wt. % of non-aluminum
phase.
[0083] FIG. 34 shows a similar relationship for the upper limits of
Ce and Si that result in 6.66 wt. % of intermetallic phase, where
(Si)=-0.454 (Ce)+6.896. At the upper limit of 7 wt. % Ce, there is
a distinct asymptotic behavior, but as Si is added, the
relationship quickly becomes linear.
[0084] FIG. 35 is a ternary phase diagram that shows a more
graphical representation of the data presented in FIGS. 33 and 34.
FIG. 36 shows an Al--Ce--Si ternary isotherm at 100.degree. C. in
the Al-rich region of the ternary phase diagram. FIG. 37 shows an
Al--Ce--Si ternary isotherm at 500.degree. C. FIG. 38 shows the
presence of the liquid phase in the Al-rich region of the
Al--Ce--Si ternary liquidus phase diagram. Heavy lines 52, 54 in
FIGS. 35-38 represent phase compositional boundaries within which
intermetallic content limits are satisfied. The area within lines
52, 54 represents Ce and Si contents of suitable compositions for
structural direct-write additive manufacturing. Reference is also
made to the respective isothermal lines in FIGS. 27 and 28.
[0085] While there have been shown and described what are at
present considered the preferred embodiments of the invention,
those skilled in the art may make various changes and modifications
which remain within the scope of the invention defined by the
appended claims.
* * * * *