STRUCTURAL DIRECT-WRITE ADDITIVE MANUFACTURING OF MOLTEN METALS
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.
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.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCHThis 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.
FIELD OF THE INVENTIONThis 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 INVENTIONAdditive 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.
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 μm 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.
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.
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 INVENTIONIn 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. %.
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.
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.
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.
γ=(Δpgde2)×(1/H)
where γ is surface tension, Δp 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.
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.
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.
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.
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.
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, Al11Ce3.
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, Al11La3.
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.
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.
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
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° 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.
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 Al2Ce and/or Al4Ce. As the bead cools, the liquid core begins solidifying.
Next, referring to
Referring to
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
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.
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.
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.
The data in
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.
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.
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 Al11Ce3.
9. The alloy of claim 1, wherein said alloy is an Al—La alloy, and said eutectic intermetallic is Al11La3.
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 Al11Ce3.
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 Al2Ce or Al4Ce.
14. The method of claim 11, wherein said alloy is an Al—La alloy, and said eutectic intermetallic is Al11La3.
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.
Type: Application
Filed: Jun 16, 2017
Publication Date: Dec 21, 2017
Inventors: Orlando RIOS (Knoxville, TN), David WEISS (Manitowoc, WI), Zachary C. SIMS (Knoxville, TN), William G. CARTER (Oak Ridge, TN), Michael S. KESLER (Knoxville, TN)
Application Number: 15/625,161