BACKGROUND

[0001] Three-dimensional (3D) printing may be an additive printing process used to make three-dimensional solid parts from a digital model. 3D printing is often used in rapid product prototyping, mold generation, mold master generation, and short run manufacturing. Some 3D printing techniques are considered additive processes because they involve the application of successive layers of material (which, in some examples, may include build material, binder and/or other printing liquid(s), or combinations thereof). This is unlike traditional machining processes, which often rely upon the removal of material to create the final part. Some 3D printing methods use chemical binders or adhesives to bind build materials together. Other 3D printing methods involve at least partial curing, thermal merging/fusing, melting, sintering, etc. of the build material, and the mechanism for material coalescence may depend upon the type of build material used. For some materials, at least partial melting may be accomplished using heat-assisted extrusion, and for some other materials (e.g., polymerizable materials), curing or fusing may be accomplished using, for example, ultra-violet light or infrared light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] Features of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear. [0003] Fig. 1 is a perspective view of a 3D printed object;

[0004] Fig. 2A is a perspective view of a first green part of a two-component design that is to be diffusion bonded, in accordance with the method disclosed herein, to a second green part (shown in Fig. 2B) to form the 3D printed object of Fig. 1 ;

[0005] Fig. 2B is a perspective view of the second green part of the two- component design that is to be diffusion bonded, in accordance with the method disclosed herein, to the first green part (shown in Fig. 2A) to form the 3D printed object of Fig. 1 ;

[0006] Fig. 3A through Fig. 3H schematically illustrate examples of the method disclosed herein where: a. Fig. 3A depicts spreading of a copper-based build material; b. Fig. 3B depicts the selective application of a binding agent; c. Fig. 3C depicts curing of the binding agent; d. Fig. 3D depicts a build material cake after a 3D printing process is performed; e. Fig. 3E depicts de-caking of the build material cake; f. Fig. 3F depicts the selective application of a diffusion bonding agent; and g. Fig. 3H depicts a final 3D object;

[0007] Fig. 4 is a schematic illustration of an example 3D printing system;

[0008] Fig. 5 is a schematic flow diagram illustrating an interface between two copper green parts during a multi-step heating process;

[0009] Fig. 6 is a schematic depiction of five green parts stacked for sintering in an example set forth herein;

[0010] Fig. 7 is a photograph, reproduced in black and white, of the five stacked green parts;

[0011] Fig. 8A is a low magnification stereomicrograph cross-section image of the sintered part formed from the five stacked green parts of Fig. 7;

[0012] Fig. 8B is a high magnification optical micrograph of the joint interface of the sintered part of Fig. 7; [0013] Fig. 9A is a scanning electron micrograph image of a 3D printed objected formed with a binding agent and Ag ink and sintered at 650°C; and

[0014] Fig. 9B is a scanning electron micrograph image of a 3D printed objected formed with a binding agent and Ag ink and sintered at 950°C;

[0015] Fig. 10 is a scanning electron micrograph image of a portion of the sintered part formed from the five stacked green parts, with markings shown for a spot Energy-dispersive X-ray spectroscopy (EDS) analysis;

[0016] Fig. 11A is a low magnification stereomicrograph cross-section image of a comparative sintered part formed from two green parts; and

[0017] Fig. 11 B is a high magnification optical micrograph of the joint interface of the comparative sintered part of Fig. 11 A.

DETAILED DESCRIPTION

[0018] In one three-dimensional (3D) printing technique, a binding agent is selectively applied to a layer of copper-based build material on a build platform, thereby patterning a selected region of the layer, and then another layer of the copperbased build material is applied thereon. The binding agent is then selectively applied to this other layer, and this patterning process is repeated to form a green (intermediate) part of a copper-based 3D part (i.e., 3D object) that is ultimately to be formed. With each application, the binding agent at least partially penetrates the layer of the copper-based build material onto which it is applied. The binding agent can spread around an exterior surface of the copper-based build material and at least partially fill void spaces between particles of the copper-based build material. The binding agent includes a binding material that temporarily holds the copper-based build material of the green part together. The green part may be cleaned to remove non-patterned copper-based build material, and then may be heated to remove the binding material and to sinter the metal-based build material particles of the green part. Sintering forms the copper-based 3D object.

[0019] In some instances, this 3D printing technique is used to form two or more individual 3D printed components, which are assembled post-sintering to form the final 3D object. Post-sintering assembling techniques may include mechanical assembly of the individual 3D printed components, such as bolting, riveting, caulking, shrink fitting, or adhesive bonding. Post-sintering assembling techniques may alternatively include metallurgical joining of individual 3D printed components, such as brazing/soldering, fusion welding, friction welding, or pressure welding.

[0020] The method disclosed herein eliminates having to perform post-sintering assembly, because two copper-based green parts are diffusion bonded during the sintering process. More specifically, an example of the diffusion bonding agent disclosed herein is selectively applied, either during the patterning process or after the green part(s) is/are generated, to designated bonding region(s) of one or both of the green parts that are to be bonded together. The diffusion bonding agent includes an element, or a reducible precursor of the element, that is capable of alloying with copper and that undergoes transient liquid phase sintering. Thus, the alloying element relatively seamlessly bonds the two copper-green parts at an interface adjoining the designated bonding region(s).

[0021] The method disclosed herein may be particularly suitable for forming a final 3D object that includes an internal feature, which may or may not have a complex geometry (e.g., sharp angles, curves, or the like). As used herein, an “internal feature” refers to a void in a gyroid structure (e.g., a fluid channel or line, a space for insertion of an electrical wire or other component, or the like) that has an inlet and an outlet, but is otherwise surrounded by the material of the 3D object.

[0022] A 3D object with several internal features having complex geometries is shown in Fig. 1. This 3D object 10 includes a fluid line 12, which fluidly communicates with several flow channels 14 within the interior of the 3D object. In this example, the flow channels 14 provide a high density lattice for efficiently cooling fluid directed through the 3D object 10. In order to form the flow channels 14 using the 3D printing technique described herein, the non-patterned, and thus unbound, copper-based build material has to be removed from the green part before sintering is performed. When 3D printed as a single green part, this removal process can be difficult due, in part, to the complex geometry of the interior flow channels 14.

[0023] With the method disclosed herein, the 3D object 10 (or any other 3D object with internal features) is converted into a two-green part design, where the internal features are accessible in the green part(s) but are enclosed within the interior of the final 3D object after diffusion bonding is performed during sintering.

[0024] An example of the two-component design for the 3D object 10 of Fig. 1 is depicted in Fig. 2A and Fig. 2B. Fig. 2A depicts the first green part 16A of the two- component design and Fig. 2B depicts the second green part 16B of the two- component design. In this example, the first green part 16A includes a portion 18A that will make up part of the exterior of the final 3D object 10, as well as the internal portion, which includes the flow channels 14 and the fluid line 12. As depicted, the internal portion of the first green part 16A is accessible so that non-patterned and unbound copper-based build material can be removed from the area(s) that will become flow channels 14. The first green part 16A also includes a first designated bonding region 20. As used herein, the phrase “designated bonding region” refers to an area specifically designed to be bonded to a designated area of another part (e.g., the second green part 16B). The second green part 16B includes another portion 18B that will make up another part of the exterior of the final 3D object 10. In this example, the second green part 16B is a lid for the internal portion of the first green part 16A. The second green part 16B also includes a second designated bonding region 22. In this particular example, the second designated bonding region 22 is an area specifically designed to be bonded to the first designated bonding region 20.

[0025] Using the method disclosed herein, the non-patterned (and thus unbound) copper-based build material can be respectively removed from each of the first and the second green parts 16A, 16B before heating. Removal of the nonpatterned copper-based build material cleans the exterior of the green part 16A, 16B from unbound copper-based build material and also frees any internal features (e.g., fluid line 12 and flow channels 14) of the copper-based build material. Then, the green parts 16A, 16B can be diffusion bonded together at the designated bonding regions 20, 22 during heating, which forms the 3D object 10 (Fig. 1 ). As illustrated in this example two-component design, in addition to eliminating post-sintering assembly, the method disclosed herein also mitigates obstacles associated with removal of the nonpatterned copper-based build material from internal features, and expands the potential for even more complex internal geometries to be realized. [0026] Throughout this disclosure, a weight percentage that is referred to as “wt% active” refers to the loading of an active component of a dispersion or other formulation that is present in the diffusion bonding agent and/or the binding agent. For example, silver nanoparticles may be present in a water-based formulation (e.g., a stock solution or dispersion) before being incorporated into the diffusion bonding agent. In this example, the wt% actives of the silver nanoparticles accounts for the loading (as a weight percent) of the silver nanoparticles that are present in the diffusion bonding agent, and does not account for the weight of the other components (e.g., water, etc.) that are present in the stock solution or dispersion with the silver nanoparticles. The term “wt%,” without the term actives, refers to the loading of a 100% active component that does not include other non-active components therein. [0027] Some examples of the method disclosed herein utilize an inkjet compatible binding agent and diffusion bonding agent. By “inkjet compatible,” it is meant that the binding agent and diffusion bonding agent can be respectively ejected from a thermal inkjet printhead, a piezoelectric inkjet printhead, or both types of printheads. The printheads may be drop-on-demand inkjet printheads or continuous inkjet printheads.

[0028] Each example of the method disclosed herein also utilizes the copperbased build material. Each of the build material and the agents will now be described, followed by a description of examples of the method.

[0029] Copper-Based Build Material

[0030] The copper-based build material includes particles of copper or a copper alloy. In one example, the copper particles are pure copper. Some specific examples of copper particles include copper powders available from Goodfellow Corporation or Sandvik AB. Examples of suitable copper alloys include copper-zinc alloys (e.g., brass), copper-tin alloys (e.g., bronze), aluminum bronze, magnesium bronze, silicon bronze, phosphor bronze, copper-nickel alloys (e.g., cupronickel, monel, or the like), copper-chromium alloys, copper-gold alloys, copper-silver alloys, and others.

[0031] The copper or copper alloy particles can include similarly-sized particles or differently-sized particles. In some examples, the build material can have a D50 particle size from about 1 pm to about 150 pm, or from about 5 pm to about 50 pm, or from about 10 pm to about 30 pm. As used herein, “particle size” refers to a value of the diameter of spherical particles or in particles that are not spherical can refer to a longest dimension of that particle. The particle size can be presented as a Gaussian distribution or a Gaussian-like distribution (or normal or normal-like distribution). Gaussian-like distributions are distribution curves that can appear Gaussian in their distribution curve shape, but which can be slightly skewed in one direction or the other (toward the smaller end or toward the larger end of the particle size distribution range). That being stated, an example Gaussian-like distribution of the copper or copper alloy particles can be characterized using “D10,” “D50,” and “D90” particle size distribution values, where D10 refers to the particle size at the 10th percentile, D50 refers to the particle size at the 50th percentile, and D90 refers to the particle size at the 90th percentile. For example, a D50 value of 25 pm means that 50% of the particles (by weight or volume) have a particle size greater than 25 pm and 50% of the particles have a particle size less than 25 pm. Particle size distribution values may not be related to Gaussian distribution curves, but in one example of the present disclosure, the metal build particles can have a Gaussian distribution, or more typically a Gaussian-like distribution with offset peaks at about D50. In practice, true Gaussian distributions are not typically present, as some skewing can be present, but still, the Gaussian-like distribution can be considered to be “Gaussian” as used in practice. [0032] The shape of the copper or copper alloy particles can be spherical, non- spherical, random shapes, or a combination thereof.

[0033] In addition to the copper or copper alloy particles, the copper-based build material may also include a flow aid. Example flow aids include fumed silica, aluminum oxide, iron oxide, or other non-oxide flow aid materials suitable for copper. An example of a commercially available fumed silica flow aid is AEROSIL™ R812, available from Evonik. In an example, the flow aid is present in the copper-based build material in an amount ranging from greater than 0 wt% to less than 5 wt%, based upon the total weight of the copper-based build material, and the remainder of the copperbased build material is made up of the copper or copper alloy particles. [0034] Binding Agent

[0035] Any suitable binding agent for copper may be used in the examples set forth herein.

[0036] One example of the binding agent includes copper (II) nitrate or a hydrate thereof, a reaction inhibition additive, and a balance of water.

[0037] The copper (II) nitrate or a hydrate thereof functions as the binding material for the green parts formed with the binding agent. The copper (II) nitrate or hydrate thereof can be in the form of anhydrous copper (II) nitrate, copper (II) nitrate monohydrate, copper (II) nitrate sesquihydrate, copper (II) nitrate hemipentahydrate, copper (II) nitrate trihydrate, or copper (II) nitrate hexahydrate.

[0038] The copper (II) nitrate or hydrate thereof can be included in the binding agent in an amount ranging from about 20 wt% active to about 70 wt% active, or from about 30 wt% active to about 60 wt% active, or from about 35 wt% active to about 50 wt% active, based on a total weight of the binding agent.

[0039] In one particular example, the binding agent can include copper (II) nitrate trihydrate in an amount from about 20 wt% active to about 70 wt% active, based on the total weight of the binding agent.

[0040] The reaction inhibition additive may be added to the binding agent to inhibit reaction(s) between the metal particles and one or more components of the binding agent during the 3D printing process. For example, water and copper (II) nitrate can cause a reaction with copper or copper alloy particles. The reaction can produce gases, such as nitric oxide (NO) and nitrogen dioxide (NO2). The reaction may also oxidize the copper or copper alloy, forming copper (I) oxide (Cu ₂ O) and/or copper (II) oxide (CuO). If enough gas is released by this reaction, the gas can reduce the density of the three-dimensional printed green part 16A, 16B. For example, the gas can become trapped and form bubbles between particles in the green part 16A, 16B. The gas can also cause dimensional instability, such as bulging in the surface of the green part 16A, 16B. These defects can persist through the sintering process. Thus, the gas released by the reaction can affect the appearance and density of the final sintered metal object. Furthermore, voids created by the gas can negatively affect properties of the final sintered metal object, such as thermal conductivity, electrical conductivity, strength, and others. The reaction inhibition additive is included to inhibit these reaction(s), which can reduce gas evolution.

[0041 ] Examples of the reaction inhibition additive can be a copper oxide etchant, a water-soluble phosphate-containing compound, or a combination thereof. Examples of suitable copper oxide etchants include acids, such as acetic acid, phosphoric acid, formic acid, propionic acid, phosphonoacetic acid, oxalic acid, sulfuric acid, nitric acid, and others. Examples of water-soluble phosphate-containing compounds include ammonium dihydrogen phosphate, ammonium hydrogen phosphate, ammonium phosphate, phosphoric acid, sodium phosphate, sodium hydrogen phosphate, potassium phosphate, potassium hydrogen phosphate, potassium dihydrogen phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, cesium phosphate, and others. In certain examples, the binding agent can include phosphoric acid, ammonium dihydrogen phosphate, acetic acid, or a combination thereof. In further examples, the binding agent can include phosphoric acid, without any other reaction inhibition additives. In other examples, the binding agent can include acetic acid, without any additional reaction inhibition additives. In still further examples, the binding agent can include ammonium dihydrogen phosphate, without any additional reaction inhibition additives. In other examples, the binding agent can include a combination of acetic acid and ammonium dihydrogen phosphate or a combination of phosphoric acid and ammonium dihydrogen phosphate, without any other reaction inhibition additives.

[0042] The reaction inhibition additive can be included in the binding agent in an amount ranging from about 0.01 wt% active to about 5.0 wt% active with respect to the total weight of the binding agent. In further examples, the amount of the reaction inhibition additive can range from about 0.05 wt% active to about 5 wt% active, or from about 0.075 wt% active to about 5 wt% active, or from about 0.1 wt% active to about 5 wt% active, or from about 0.25 wt% active to about 5 wt% active, or from about 0.5 wt% active to about 5 wt% active. As some specific examples, the binding agent can include acetic acid in an amount from about 0.5 wt% to about 2.5 wt%, and/or phosphoric acid in an amount from about 0.025 wt% to about 5 wt%, and/or ammonium dihydrogen phosphate in an amount from about 0.2 wt% to about 5 wt%. [0043] In some examples, the binding agent can include both a copper oxide etchant and a water-soluble phosphate-containing compound. In such examples, the concentration ranges given above can be the combined concentration of both the copper oxide etchant and the phosphate-containing compound. In some examples, the amount of the copper oxide etchant can be greater than the amount of water- soluble phosphate-containing compound included in the binding agent. In certain examples, a weight ratio of the copper oxide etchant to the water-soluble phosphate- containing compound can be from about 2: 1 to about 10:1 , or from about 2: 1 to about 5:1.

[0044] Another example of the binding agent includes a polymeric binder and a balance of water.

[0045] Examples of suitable polymeric binders include polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyamide, polyacrylic acid (PAA), or a polymer latex.

[0046] The term “polymer latex” refers to a stable dispersion of polymer particles in an aqueous medium. The polymer particles are heteropolymers or copolymers. The heteropolymers may include a more hydrophobic component and a more hydrophilic component. In these examples, the more hydrophilic component renders the particles dispersible in the binder agent, while the more hydrophobic component is capable of coalescing upon reaching its minimum film formation temperature (MFFT), thus binding the copper particles together.

[0047] Examples of monomers that may be used to form the more hydrophobic component include C4 to C8 alkyl acrylates or methacrylates, styrene, substituted methyl styrenes, polyol acrylates or methacrylates, vinyl monomers, vinyl esters, or the like. Some specific examples include methyl methacrylate, butyl acrylate, butyl methacrylate, hexyl acrylate, hexyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexy methacrylate, hydroxyethyl acrylate, lauryl acrylate, lauryl methacrylate, octadecyl acrylate, octadecyl methacrylate, isobornyl acrylate, isobornyl methacrylate, stearyl methacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetrahydrofurfuryl acrylate, alkoxylated tetrahydrofurfuryl acrylate, 2-phenoxyethyl methacrylate, benzyl acrylate, ethoxylated nonyl phenol methacrylate, cyclohexyl methacrylate, trimethyl cyclohexyl methacrylate, t-butyl methacrylate, n-octyl methacrylate, tridecyl methacrylate, isodecyl acrylate, dimethyl maleate, dioctyl maleate, acetoacetoxyethyl methacrylate, diacetone acrylamide, pentaerythritol tri-acrylate, pentaerythritol tetra-acrylate, pentaerythritol trimethacrylate, pentaerythritol tetra-methacrylate, divinylbenzene, styrene, methylstyrenes (e.g., a-methyl styrene, p-methyl styrene), vinyl chloride, vinylidene chloride, vinylbenzyl chloride, acrylonitrile, methacrylonitrile, N-vinyl imidazole, N- vinylcarbazole, N-vinyl-caprolactam, combinations thereof, derivatives thereof, or mixtures thereof.

[0048] The more hydrophilic monomer may be an acidic monomer. Examples of acidic monomers that can be polymerized in forming the latex polymer particles include acrylic acid, methacrylic acid, ethacrylic acid, dimethylacrylic acid, maleic anhydride, maleic acid, vinylsulfonate, cyanoacrylic acid, vinylacetic acid, allylacetic acid, ethylidineacetic acid, propylidineacetic acid, crotonoic acid, fumaric acid, itaconic acid, sorbic acid, angelic acid, cinnamic acid, styrylacrylic acid, citraconic acid, glutaconic acid, aconitic acid, phenylacrylic acid, acryloxypropionic acid, aconitic acid, phenylacrylic acid, acryloxypropionic acid, vinylbenzoic acid, N-vinylsuccinamidic acid, mesaconic acid, methacroylalanine, acryloylhydroxyglycine, sulfoethyl methacrylic acid, sulfopropyl acrylic acid, styrene sulfonic acid, sulfoethylacrylic acid, 2- methacryloyloxymethane-1 -sulfonic acid, 3-methacryoyloxypropane-1 -sulfonic acid, 3- (vinyloxy)propane-l -sulfonic acid, ethylenesulfonic acid, vinyl sulfuric acid, 4- vinylphenyl sulfuric acid, ethylene phosphonic acid, vinyl phosphoric acid, vinyl benzoic acid, 2 acrylamido-2-methyl-1 -propanesulfonic acid, combinations thereof, derivatives thereof, or mixtures thereof. Other examples of hydrophilic monomers include acrylamide, methacrylamide, monohydroxylated monomers, monoethoxylated monomers, polyhydroxylated monomers, or polyethoxylated monomers.

[0049] In some examples of the heteropolymers disclosed herein, the more hydrophobic component(s) make up from about 65% to about 100% of the copolymer, and the more hydrophilic component(s) make up from about 0.1 % to about 35% of the copolymer. [0050] Some specific examples of the polymer latex are formed with the following monomer combinations: i) butyl acrylate, styrene, methyl methacrylate, and methacrylic acid; or ii) butyl acrylate, methyl methacrylate, methacrylic acid, cyclohexyl methacrylate, cyclohexyl acrylate, and 2-phenoxyethyl methacrylate.

[0051] The polymeric binder can have a particle size ranging from 20 nm to 500 nm, from 50 nm to 350 nm, or from 150 nm to 270 nm.

[0052] In these examples of the binding agent, the polymeric binder is present in an amount ranging from about 2 wt% active to about 20 wt% active, based on a total weight of the binding agent. In other examples, the polymeric binder is present in the binding agent in an amount ranging from about 5 wt% active to about 15 wt% active, based on the total weight of the binding agent.

[0053] In any example, the balance of the binding agent is water. In some examples, water can be used as a solvent for the binding agent without any additional co-solvents. Accordingly, in some examples, the binding agent can consist of water, copper (II) nitrate or a hydrate thereof, and a reaction inhibition additive, or can consist of water and the polymeric binder. In further examples, the binding agent can consist of either of those ingredient sets (e.g., the copper (II) nitrate or hydrate thereof and the reaction inhibition additive or the polymeric binder) plus a co-solvent and/or one or more of the additives set forth herein. In any example, the amount of water in the binding agent can range from about 20 wt% to about 79 wt%.

[0054] When any example of the binding agent includes a co-solvent, any water soluble or water miscible organic co-solvent may be used. Suitable organic cosolvents include 2-pyrrolidone, 1-(2-hydroxyethyl)-2-pyrrolidone, tripropylene glycol mono methyl ether, dipropylene glycol mono methyl ether, dipropylene glycol mono propyl ether, tripropylene glycol mono n-butyl ether, propylene glycol phenyl ether, dipropylene glycol methyl ether acetate, diethylene glycol mono butyl ether, diethylene glycol mono hexyl ether, ethylene glycol phenyl ether, diethylene glycol mono n-butyl ether acetate, ethylene glycol mono n-butyl ether acetate, 2-methyl-1 ,3-propanediol, or combinations thereof. [0055] When included, the organic co-solvent is present in an amount ranging from amount about 0.1 wt% active to about 20 wt% active, or from about 0.1 wt% active to about 10 wt% active, or from about 0.1 wt% active to about 2 wt% active. [0056] In some examples, it may be desirable that the binding agent is free of an organic humectant or substantially free of an organic humectant (5 wt% or less). Organic humectants can include organic solvents having a boiling point of 120°C or higher.

[0057] As mentioned, any example of the binding agent may include an additive. Any one or any combination of the following additives may be included: a surfactant, a pH adjusting agent, an anti-microbial agent, and an anti-kogation agent. [0058] Surfactants can be used to increase the wetting properties and the jettability of the binding agent. Examples of surfactants that can be used include: DOWFAX™ 2A1 from Dow Inc. (USA); SURFYNOL® SEF from Air Products and Chemicals, Inc. (USA); non-ionic fluorosurfactants, such as CAPSTONE® fluorosurfactants from DuPont (USA); ethoxylated low-foam wetting agents, such as SURFYNOL® 440 or SURFYNOL® CT-111 from Air Products and Chemicals Inc.

(USA); an ethoxylated wetting agent and molecular defoamer, such as SURFYNOL® 420 from Air Products and Chemicals, Inc. (USA); non-ionic wetting agents and molecular defoamers, such as SURFYNOL® 104E from Air Products and Chemicals Inc. (USA); or water-soluble, non-ionic surfactants, such as TERGITOL™ TMN-6 or TERGITOL™ 15-S-7 from Dow Inc. (USA).

[0059] A single surfactant or a combination of surfactants can be used. The total amount of surfactant in the binding agent can be from about 0.025 wt% active to about 2 wt% active, based on a total weight of the binding agent.

[0060] A pH-adjusting additive can also be included in the binding agent in some examples. The pH-adjusting additive can be included in a sufficient amount to render the pH of the binding agent from about 0 to about 3, or from about 1 to about 2. Additionally, the pH-adjusting additive can be free of elements that would remain in the copper-based 3D object after sintering. For example, some pH-adjusting additives, such as potassium hydroxide, can leave behind undesired elements, such as potassium, in the metal object after sintering. Accordingly, the pH-adjusting additives can include elements that can volatilize and/or combust during the sintering process so that no undesired elements are left behind in the sintered copper-based object. Some examples of pH-adjusting additives that can be used include ammonium acetate and ammonium hydroxide. The amount of pH-adjusting additive can range from about 0.1 wt% active to about 5 wt% active.

[0061] Example anti-microbial agents that may be included in the binding agent include NUOSEPT™ (Troy Corp., USA), UCARCIDE™ (Dow Chemical Co., USA), ACTICIDE® M20 (Thor, United Kingdom), an aqueous solution of 1 ,2-benzisothiazolin- 3-one such as PROXEL® GXL from Arch Chemicals, Inc. (USA), quaternary ammonium compounds such as BARDAC® 2250 and 2280, BARQUAT® 50-658, and CARBOQUAT® 250-T, all from Lonza Ltd. Corp. (Switzerland), an aqueous solution of methylisothiazolone such as KORDEK® MLX from Dow Chemical Co. (USA), or combinations thereof.

[0062] The anti-microbial agent may be added in any amount ranging from about 0.05 wt% active to about 0.5 wt% active with respect to the total weight of the binding agent.

[0063] As mentioned, an anti-kogation agent can also be included in the binding agent. Kogation refers to deposits formed on a heating element of a thermal inkjet printhead. Anti-kogation agents can be included to assist in preventing the buildup of kogation. Examples of suitable anti-kogation agents include oleth-3-phosphate such as CRODAFOS™ 03A or CRODAFOS™ N-3 acid from Croda (United Kingdom), or a combination of oleth-3-phosphate and a low molecular weight (e.g.,< 5,000) polyacrylic acid polymer such as CARBOSPERSE ^TM K-7028 Polyacrylate from Lubrizol (USA).

[0064] Whether a single anti-kogation agent is used or a combination of anti- kogation agents is used, the total amount of anti-kogation agent in the binding agent can range from about 0.2 wt% active to about 0.6 wt% active, based on the total weight of the binding agent.

[0065] Diffusion Bonding Agent

[0066] The diffusion bonding agent is an inkjettable fluid that includes a copper alloying element or a precursor to the copper alloying element in an aqueous vehicle. The precursor to the copper alloying element is also referred to herein as the copper alloying element precursor.

[0067] The copper alloying element may be any metal that is capable of alloying with copper. In one example, the copper alloying element is selected from the group consisting of silver, zinc, chromium, cadmium, zirconium, nickel, aluminum, antimony, magnesium, manganese, titanium, beryllium, bismuth, mercury, gold, indium, lead, arsenic, silicon, and tin. As such, in one example, the diffusion bonding agent is an inkjettable fluid that includes the copper alloying element; and the copper alloying element is selected from the group consisting of silver, zinc, chromium, cadmium, zirconium, nickel, aluminum, antimony, magnesium, manganese, titanium, beryllium, bismuth, mercury, gold, indium, lead, arsenic, silicon, and tin. Oxides of any of these materials may be used in the diffusion bonding agent as the precursor to the copper alloying element, as long as sintering is performed in a reducing environment that can reduce the oxide to generate the copper alloying element. As such, in another example, the diffusion bonding agent is an inkjettable fluid that includes the precursor to the copper alloying element; the precursor to the copper alloying element is an oxide of an element selected from the group consisting of silver, zinc, chromium, cadmium, zirconium, nickel, aluminum, antimony, magnesium, manganese, titanium, beryllium, bismuth, mercury, gold, indium, lead, arsenic, silicon, and tin; and the sintering is performed in a reducing environment.

[0068] The copper alloying element or precursor thereof is in the form of nanoparticles, having an average particle size ranging from about 0.5 nm to about 500 nm. In a specific example, the copper alloying element or precursor thereof is in the form of nanoparticles, having an average particle size of about 100 nm. When the precursor is used, the average particle size of the copper alloying element that is generated as a result of the reduction reaction will range from about 0.5 nm to about 200 nm. The large surface area of the alloying element nanoparticles enables the nanoparticles to melt at a lower temperature than its bulk material counterpart. As such, during the sintering process, the copper alloying element (initially present or the result of reduction) is able to melt, fill void spaces between the copper or copper alloy particles, and ultimately diffuse into the copper matrix to form an alloyed region containing copper and the copper alloying element.

[0069] In an example, the copper alloying element or precursor thereof is included in the diffusion bonding agent in an amount ranging from about 5 wt% active to about 60 wt% active, based on a total weight of the diffusion bonding agent.

[0070] In addition to the copper alloying element or precursor thereof, the diffusion bonding agent may further include a dispersing agent (i.e. , dispersant) to aid in dispersing the copper alloying element or precursor thereof throughout the diffusion bonding agent. The amount of the dispersant may range from about 0.1 wt% active to about 10 wt% active, based on the total weight of the diffusion bonding agent.

Examples of suitable dispersants include PLURONIC® 123 (symmetric triblock copolymer including poly(ethylene oxide) (PEO) and polypropylene oxide) (PPO), from BASF Corp.), BRIJ® series of surfactants (available from Croda, Inc.), IGEPAL® (surfactants derived from octylphenol, available from Rhodia), SURFONYL® series of surfactants (available from Air Products and Chemicals, Inc.), JONCRYL® series of dispersing agents (available from BASF Corp.), TWEEN® series of surfactants (available from Croda, Inc.), and DisperseBYK® series of wetting and dispersing additives (available from BYK).

[0071 ] In addition to the copper alloying element or precursor thereof (alone or in combination with the dispersant), the diffusion bonding agent may further include water, alone or in combination with any of a surfactant, an anti-kogation agent, an antimicrobial agent, a co-solvent, or combinations thereof. Any of the surfactants, anti- kogation agents, anti-microbial agents, and co-solvents set forth herein for the binding agent may be used in the diffusion bonding agent in the respective amounts set forth herein, based on the total weight of the diffusion bonding agent.

[0072] The diffusion bonding agent may be a commercially available metal nanoparticle ink, such as silver inks available under the tradename METALON® from NovaCentrix. [0073] Methods

[0074] In the methods disclosed herein, copper green parts are generated using the 3D printing technique described herein, and then the copper green parts are diffusion bonded together during sintering with the aid of the selectively applied diffusion bonding agent.

[0075] One example of the method includes three-dimensionally printing: a first copper green part having a first designated bonding region, and a second copper green part having a second designated bonding region that is to be bonded to the first designated bonding region; selectively applying a diffusion bonding agent onto the first designated bonding region, the second designated bonding region, or both the first and second designated bonding regions, the diffusion bonding agent including a copper alloying element or a precursor to the copper alloying element; placing the first designated bonding region into contact with the second designated bonding region; and sintering the first and second copper green parts.

[0076] Another example of the method includes three-dimensionally printing a first copper green part having a patterned internal void and a first designated bonding region; three-dimensionally printing a second copper green part having a second designated bonding region that is to be bonded to the first designated bonding region; exposing the first copper green part to a powder removal process to remove unbound copper particles or copper alloy particles from the patterned internal void, thereby exposing a corresponding internal void; selectively applying a diffusion bonding agent onto the first designated bonding region, the second designated bonding region, or both the first and second designated bonding regions, the diffusion bonding agent including a copper alloying element or a precursor to the copper alloying element; placing the first designated bonding region into contact with the second designated bonding region, whereby the second copper green part at least partially covers the corresponding internal void; and sintering the first and second copper green parts.

[0077] Both examples of the method are schematically shown in Fig. 3A through Fig. 3H, which will now be described in detail. Fig. 3A through Fig. 3E schematically illustrate the 3D printing of one of the copper green parts 16A. While the description relates to formation of the first copper green part 16A, it is to be understood that the 3D printing technique may also be used to form the second copper green part 16B that is to be bonded to the first copper green part 16A.

[0078] The 3D printing technique used to generate the green part(s) 16A (or 16B) involves iteratively applying individual layers 24 of a copper-based build material 26 including particles of copper or a copper alloy (Fig. 3A and Fig. 3B); according to a 3D object model of a first 3D object that corresponds with the first copper green part 16A (or of a second 3D object that corresponds with the second copper green part 16B), selectively applying a binding agent 28 to each of the individual build material layers 24 (Fig. 3B); and heating the individual layers 24 and the selectively applied binding agent 28 (Fig. 3C). A printing system 30 may be used to generate the respective green parts 16A, 16B. The system 30 is shown in Fig. 4 and is described throughout the discussion of Fig. 3A through Fig. 3C. The printing system 30 may include a build area platform 32, a build material supply 34 containing the copperbased build material 26, and a build material distributor 36.

[0079] Fig. 3A illustrates the initial application of one individual layer 24 (see Fig. 3B) of the copper-based build material 26. The build area platform 32 receives the copper-based build material 26 from the build material supply 34. The build area platform 32 may be moved in a direction as denoted by the arrow 38, e.g., along the z- axis, so that the copper-based build material 26 may be delivered to the build area platform 32 or to a previously patterned layer. As used herein, a “patterned layer” refers to a layer 24 of the copper-based build material 26 having the binding agent 28 applied to at least a portion thereof.

[0080] In an example, when the copper-based build material 26 is to be delivered, the build area platform 32 may be programmed to advance (e.g., downward) enough so that the build material distributor 36 can push the copper-based build material 26 onto the build area platform 32 to form a substantially uniform layer 24 of the copper-based build material 26 thereon. The build area platform 32 may also be returned to its original position, for example, when a new green part 16A is to be built. [0081] The build material supply 34 may be a container, bed, or other surface that is to position the copper-based build material 26 between the build material distributor 36 and the build area platform 32. [0082] The build material distributor 36 may be moved in a direction as denoted by the arrow 40, e.g., along the y-axis, over the build material supply 34 and across the build area platform 32 to spread the copper-based build material 26 and form the layer 24 over the build area platform 32. The build material distributor 36 may also be returned to a position adjacent to the build material supply 34 following the spreading of the copper-based build material 26. The build material distributor 36 may be a blade (e.g., a doctor blade), a roller, a combination of a roller and a blade, and/or any other device capable of spreading the copper-based build material 26 over the build area platform 32. For instance, the build material distributor 36 may be a counterrotating roller.

[0083] As shown in Fig. 3A, the build material supply 34 may supply the copperbased build material 26 into a position so that the copper-based build material 26 is ready to be spread onto the build area platform 32. The build material distributor 36 may spread the supplied copper-based build material 26 onto the build area platform 32. A controller (identified by reference numeral 42 in Fig. 4) may process control supply data, and in response control the build material supply 34 to appropriately position the copper-based build material 26, and may process control spreader data, and in response control the build material distributor 36 to spread the supplied copperbased build material 26 over the build area platform 32 to form the layer 24. In other examples (not shown), the build distributor 36 may sprinkle the copper-based build material 26 over the build area platform 32 to form the layer 24 of copper-based build material 26 thereon. While several examples have been provided, it is to be understood that other techniques may be used to substantially uniformly apply the copper-based build material 26. As shown in Fig. 3B, one build material layer 24 has been formed.

[0084] The layer 24 of the copper-based build material 26 has a substantially uniform thickness across the build area platform 32. In an example, the thickness of the build material layer 24 is about 100 pm. In another example, the thickness of the build material layer 24 ranges from about 30 pm to about 300 pm, although thinner or thicker layers may also be used. For example, the thickness of the build material layer 24 may range from about 20 pm to about 500 pm, or from about 50 pm to about 80 pm. The layer thickness may be about 2x (i.e. , 2 times) the particle diameter at a minimum for finer part definition. In some examples, the layer thickness may be about 1 ,5x the particle diameter.

[0085] As shown in Fig. 3B, the binding agent 28 is then selectively applied to the layer 24. The binding agent 28 may be dispensed from an applicator 44. The applicator 44 may be a thermal inkjet printhead, a piezoelectric printhead, a continuous inkjet printhead, etc., and the selectively applying of the binding agent 28 may be accomplished by thermal inkjet printing, piezo electric inkjet printing, continuous inkjet printing, etc. The applicator 44 includes a reservoir of the binding agent 28 or is fluidly coupled to a separate supply of the binding agent 28, and the applicator 44 is directable to iteratively apply the binding agent 28 to layers 24 of the copper-based build material 26.

[0086] The controller 42 (see Fig. 4) directs the applicator 44. The controller 42 processes data for the corresponding layer 24 within the digital 3D object model. In response, the controller 42 controls the applicator 44 (e.g., in the directions indicated by the arrow 40’) to deposit the binding agent 28 onto predetermined portion(s) 46 of the build material layer 24 that are to become part of the green part 16A (and ultimately the final 3D object 10’, see Fig. 3H). In the example shown in Fig. 3B, the applicator 44 selectively applies the binding agent 28 on those portion(s) 46 of the build material layer 24 that is/are to become the first layer of the green part 16A.

[0087] The selective application of the binding agent 28 deposits the copper (II) nitrate or a hydrate thereof or the polymeric binder onto the build material layer 24. The liquid vehicle of the binding agent 28 is capable of wetting the copper-based build material 26 and the copper (II) nitrate or a hydrate thereof or the polymeric binder is capable of penetrating into the microscopic pores/voids of the build material layer 24 (i.e., the spaces between the particles of copper or copper alloy).

[0088] The amount of copper (II) nitrate or hydrate thereof or polymeric binder that is introduced into the portion(s) 46 of the build material layer 24 via the binding agent 28 can be sufficient to bind the copper or copper alloy particles together. In some examples, the concentration of copper (II) nitrate or hydrate thereof or polymeric binder in the portion(s) 46 can range from about 0.2 wt% to about 20 wt% based on the combined weight of copper or copper alloy particles and the copper (II) nitrate or hydrate thereof or polymeric binder. In other examples, the concentration of copper (II) nitrate or hydrate thereof or polymeric binder can range from about 0.2 wt% to about 15 wt% or from about 0.2 wt% to about 10 wt%, or from about 0.2 wt% to about 5 wt%, or from about 0.2 wt% to about 1 wt%, based on the combined weight of the copper or copper alloy particles and the copper (II) nitrate or hydrate thereof or polymeric binder.

[0089] It is to be understood that portions 47 of the build material layer 24 that do not have the binding agent 28 applied thereto also do not have the copper (II) nitrate or hydrate thereof or polymeric binder introduced thereto. As such, these portions 47 do not become part of the green part 16A or of the final 3D object 10’ that is ultimately formed.

[0090] Fig. 3C illustrates the curing of the selectively applied binding agent 28 to form a compound 50 that can bind the particles of the copper-based build material 26 together. As used herein, “curing” refers to a process of heating the copper-based build material 26 and the binding agent 28 i) so that water and co-solvent(s) in the binding agent 28 evaporate and the copper (II) nitrate or hydrate thereof is dehydrated or partially dehydrated, or ii) so that water and co-solvent(s) in the binding agent 28 evaporate and the polymeric binder is able to flow. In a specific example, the binding agent 28 can include copper (II) nitrate trihydrate, and curing can include heating the selectively applied binding agent 28 until the copper (II) nitrate trihydrate decomposes to form copper hydroxy nitrate. In another specific example, the binding agent 28 can include the polymeric binder, and curing can include heating the selectively applied binding agent 28 to a temperature sufficient to reflow the polymeric binder such that polymer penetrates into spaces between copper particles of the build material 26 to adhesively bond the copper particles together.

[0091 ] The heat for curing can be provided by a curing heater 48 that is positioned to heat the copper-based build material 26 on the build area platform 32. In some examples, as shown in Fig. 4, the curing heater 48 can be a stationary heater that is positioned above the build area platform 32, and thus above the patterned layers. In other examples, as shown in Fig. 3C and in Fig. 4, the curing heater(s) 48 can be affixed to a carriage 49 that moves the applicator 44, and thus the heater(s) 48 can be translated back and forth (e.g., in the directions indicated by the arrow 40”) with the applicator 44 above the build area platform 32, and thus above the patterned layers. In other examples, the build area platform 32 beneath the patterned layers can be heated. Heaters can also be positioned on sides of the build area platform 32. A combination of these heaters can also be used.

[0092] The temperature at which the copper-based build material 26 and the selectively deposited binding agent 28 are cured can range from about 70°C to about 250°C. As other examples, the curing temperature can be from about 70°C to about 160°C, or from about 70°C to about 120°C, or from about 100°C to about 160°C, or from about 140°C to about 160°C, or from about 140°C to about 250°C.

[0093] The curing time can range from about 1 second to about 4 hours. In some examples, curing can be performed on individual patterned layers, and the curing time can be from about 1 second to about 1 minute per layer. In other examples, the curing can be performed on the entire green part 16A, and the curing time can be from about 15 minutes to about 4 hours, or from about 20 minutes to about 3 hours, or from about 30 minutes to about 2 hours, or from about 1 hour to about 2 hours.

[0094] In some examples, the green part 16A can be fully cured in the 3D printing system 30. In other examples, a first curing stage can be performed in the 3D printing system 30, and then a second curing stage can be performed in another location, such as in a curing oven. In certain examples, the second curing stage can expose the green part 16A to a higher curing temperature than the first curing stage. In one example, the green part 16A can be cured in a first curing stage within the 3D printing system 30 at a first curing temperature from about 70°C to about 160°C, and then additionally cured in a second curing stage at a second curing temperature from about 140°C to about 250°C.

[0095] As mentioned, curing can dehydrate or partially dehydrate the copper (II) nitrate in the binding agent, forming a compound 50 that can bind the copper particles or copper alloy particles of the copper-based build material 26 together. The bound particles in the single layer 24 are shown in Fig. 3C. [0096] The processes shown in Fig. 3A through Fig. 3C may be repeated to iteratively build up several green part layers to form the green part 16A (shown in Fig. 3D). As such, some examples of the method include repeating the applying of the copper-based build material 26, the selectively applying of the binding agent 28, and heating of the patterned layer to cure the patterned portion 46. In other examples, the application of the copper-based build material 26 and the selective application of the binding agent 28 are repeated, and all of the patterned layers are exposed to heating at the same time to cure. In either example, a build material cake 52, as shown in Fig. 3D, is formed, which includes the green part 16A residing within the non-patterned portions 47 of each of the build material layers 24. The green part 16A is a volume of the build material cake 52 that is filled with copper-based build material 26 bound by the compound 50 (represented by the speckling in Fig. 3D). The remainder of the build material cake 52 is made up of non-patterned and thus unbound particles of the copper-based build material 26.

[0097] In the example shown in Fig. 3D, the green part 16A includes a patterned internal void 54 and the first designated bonding region 20.

[0098] The patterned internal void 54 is an area of non-patterned copper-based build material 26 that is at least partially surrounded by patterned copper-based build material 26 (held together by the compound 50) and that is to form a void 56 (see Fig. 3D) upon removal of non-patterned copper-based build material 26 from the green part 16A. In the examples disclosed herein, at least a portion of the patterned internal void(s) 54 is positioned so that the non-patterned copper-based build material 26 contained therein can be easily removed.

[0099] The methods disclosed herein also involve extracting the green part 16A (and 16B) from the build material cake 52. The green part 16A or 16B may be extracted by any suitable means. In an example, the green part 16A or 16B may be extracted by lifting the green part 16A or 16B from the non-patterned copper-based build material 26 using an extraction tool. In another example, the green part 16A or 16B may be extracted using a wet or a dry extraction process. In the example shown in Fig. 3E, the wet extraction process is used to remove the non-patterned copperbased build material 26, and thus extract the green part 16A. In an example, the wet extraction process may include spraying the build material cake 52 with water using wet extraction tool(s) 58, such as a hose and a sprayer, a spray gun, etc. In other examples, the wet extraction process may include sonicating the build material cake 52 in a water bath or soaking the build material cake 52 in water. In some examples, dry extraction of non-patterned copper-based build material 26 from the build material cake 52 may be used in place of wet extraction. As an example, non-patterned copper-based build material 26 may be removed from the build material cake 52 by suction from a vacuum hose. In this example, the removed copper-based build material 26 can be collected in a reservoir for future use.

[00100] It is to be understood that copper-based build material 26 from nonpatterned regions (e.g., 47, 54 in Fig. 3D) that remain bound to the green part 16A after the extraction process may be removed by cleaning with a brush and/or an air jet. [00101] As part of the extraction process shown in Fig. 3E, the first copper green part 16A is exposed to a powder removal process to remove unbound copper particles or copper alloy particles (build material 26) from the patterned internal void 54, thereby exposing a corresponding internal void 56. The interval void 56 is shown at a surface of the green part 16A for ease of removal of the non-patterned copper-based build material 26. However, it is to be understood that the void 56 will become enclosed when the second green part 16B is diffusion bonded to the first green part 16A using the method described herein (see Fig. 3H).

[00102] Fig. 3F depicts the green part 16A, and a second green part 16B that has been 3D printed using the method described in reference to Fig. 3A through Fig. 3B. Each of the first and second green parts 16A, 16B has a respective designated bonding region 20, 22 which are to be bonded together using the method disclosed herein.

[00103] For bonding the two green parts 16A, 16B, the method includes selectively applying any example of the diffusion bonding agent 60 disclosed herein onto the first designated bonding region 20, the second designated bonding region 22, or both the first and second designated bonding regions 20, 22 (Fig. 3F); placing the first designated bonding region 20 into contact with the second designated bonding region 22 (Fig. 3G); and sintering the first and second copper green parts 16A, 16B (Fig. 3H).

[00104] The selective application of the diffusion bonding agent 60 may be performed i) digitally via an inkjet printhead (e.g., applicator 44’ shown in Fig. 3F) or ii) manually. To apply the diffusion bonding agent 60 digitally, the green part 16A, 16B may be placed back into the printing system 30, which may be equipped with the applicator 44’, or into another system that contains the applicator 44’. The applicator 44’ may be any example of the applicator 44, and may include a reservoir containing the diffusion bonding agent 60 or may be in fluid contact with a supply of the diffusion bonding agent 60. The controller 42 (shown in Fig. 4) of the printing system 30 or of the other system directs the applicator 44’. To apply the diffusion bonding agent 60 manually, a brush, spray coater, a syringe, or other manually operated coating apparatus may be used. The diffusion bonding agent 60 may be applied to all or a portion of the first designated bonding region 20 and/or the second designated bonding region 22.

[00105] In one example, the diffusion bonding agent 60 is applied after the green part 16A, 16B is formed. This is depicted in Fig. 3F, where the diffusion bonding agent 60 is applied to the exterior surface of the green part 16A at the first designated bonding region 20 and/or to the exterior surface of the green part 16B at the second designated bonding region 22. In this example, the diffusion bonding agent 60, and in particular the copper alloying element 68 (see Fig. 5) or the copper alloying element precursor (not shown), may sit on the exterior surface and may penetrate somewhat into the green part 16A and/or 16B at areas that are devoid of the compound 50. [00106] In another example, the diffusion bonding agent 60 may be applied during the 3D printing method. In this example, the diffusion bonding agent 60 is selectively applied to an outermost of the individual build material layers 24 during the three-dimensional printing of the first copper green part 16A or the second copper green part 16B. In this example, the binding agent 28 and the diffusion bonding agent 60 may be applied to the outermost layer(s) (e.g., the 3-6 outermost layers) at the first designated bonding region 20 and/or at the second designated bonding region 22. In this example, the diffusion bonding agent 60, and in particular the copper alloying element 68 or the copper alloying element precursor, will be introduced right into the build material layer(s) 24. In this example, the copper alloying element 68 or the copper alloying element precursor will be exposed to the curing process described herein.

[00107] The printed copper alloying element 68 or the copper alloying element precursor can coat the copper-based build material 26 and occupy the void spaces between the copper-based build material 26.

[00108] Whether the diffusion bonding agent 60 is applied to the green part(s) 16A, 16B after curing or during layer 24 patterning (and thus before curing), once the designated bonding regions 20 and/or 22 include the diffusion bonding agent 60, the regions 20, 22 may be placed into contact with one another. The positioning of the regions 20, 22 is in accordance with the two-component design for the final 3D object 10’. The placement of the two green parts 16A, 16B into contact with one another may be performed manually or via an automated process. An interface 66 is formed where the regions 20, 22 are placed in contact with one another.

[00109] In Fig. 3G, the two green parts 16A, 16B are placed into contact with one another in a sintering oven 62, where the sintering process may be performed.

[00110] The terms “sinter,” “sintered,” “sintering,” or the like refers to the consolidation and physical bonding of the copper-based build material 26 together (after temporary binding using the binding agent 28) by solid state diffusion bonding, partial melting of the copper-based build particles, or a combination of solid state diffusion bonding and partial melting.

[00111 ] The sintering process takes place at the end of a multi-step heating process that is schematically depicted in Fig. 5. In the example shown in Fig. 5, the diffusion bonding agent 60 includes the copper alloying element 68.

[00112] The multi-step heating process involves first heating to a first temperature that is below the melting temperature of the copper-based build material 26 and that can melt the copper alloying element 68 in the diffusion bonding agent 60. The first temperature depends upon the copper alloying element 68 that is used and the size of the copper alloying element 68. It is to be understood that, generally, the melting point of the copper alloying element 68 increases with increasing particle size. As such, the nanoparticle size of the copper alloying element 68 enables it to melt at a lower temperature than its corresponding bulk material. For example, silver nanoparticles can melt at about 650°C, which is well below the ~962°C melting point of bulk silver. As another example, tin nanoparticles can melt at about 177°C, which is well below the ~232°C melting point of bulk tin. As yet another example, zinc nanoparticles can melt at about 325°C to about 377°C, which is well below the ~420°C melting point of bulk zinc.

[00113] The first temperature may range from about 100°C to about 850°C. In an example, the first temperature ranges from about 550°C to about 850°C. In another example, the first temperature ranges from about 100°C to about 250°C. In yet another example, the first temperature ranges from about 250°C to about 450°C. [00114] As shown in Fig. 5, the melted copper alloying element 68’ can fill spaces between the copper-based build material 26 at the interface 66. The melted copper alloying element 68’ can bond the copper-based build material 26.

[00115] Also at the first temperature, hydrogen, oxygen, and nitrogen of the compound 50 (i.e. , the decomposed copper (II) nitrate or a hydrate thereof) can be driven off as gases, leaving copper, which will remain as a part of the sintered 3D object 10’.

[00116] The first temperature can then be raised to a second temperature. The second temperature is at or above the melting temperature of the copper-based build material 26. The second temperature will depend upon whether the copper-based build material includes copper particles or copper alloy particles. In some examples, the second temperature can be from about 850°C to about 1300°C. In further examples, the second temperature can be from about 800°C to about 1300°C, or from about 900°C to about 1300°C, or from about 1000°C to about 1300°C, or from about 1100°C to about 1300°C, or from about 1200°C to about 1300°C, or from about 800°C to about 1200°C, or from about 900°C to about 1200°C, or from about 1000°C to about 1200°C, or from about 1100°C to about 1200°C, or from about 800°C to about 1100°C, or from about 900°C to about 1100°C, or from about 1000°C to about 1100°C. Once the second temperature is reached, it can be held for a sintering time of from about 10 minutes to about 20 hours, or from about 30 minutes to about 10 hours, or from about 1 hour to about 5 hours, in some examples.

[00117] The multi-step heating process can be performed in an atmosphere or vacuum. In some examples, the sintering atmosphere can be an inert gas, a low- reactivity gas, a reducing gas, or a combination thereof. Some gases that can be used in the sintering atmosphere include hydrogen, helium, argon, neon, xenon, krypton, nitrogen, carbon monoxide, and combinations thereof.

[00118] As the temperature of the sintering oven 62 is raised to the second temperature, the melted copper alloying element 68’ can diffuse from the interface 66 and into the matrix of the bound copper-based build material 26. The diffusion enables an alloy of copper and the copper alloying element 68 to be formed at and near the interface 66. More specifically, after sufficient inter-diffusion, the composition at the local interface 66 reaches the vicinity of the eutectic alloy (e.g., Ag-Cu) composition, which has a lower melting point than either copper or the bulk alloying element 68. Thus, the alloyed regions 64 (see Fig. 3H and Fig. 5) will experience surface melting of the copper-based build material 26 at the lower temperature, which leads to enhanced sintering and pore closure at the joining interface(s) 66. As sintering progresses, the copper alloying element 68 diffuses further into the matrix of the bound copper-based build material 26, and the composition is normalize based on the equilibrium solubility of the copper alloying element 68 in copper. After sintering is complete, the interface(s) 66 at which the green parts 16A, 16B are joined results in sound metallurgical bonding.

[00119] The rate at which the green parts 16A, 16B are heated may be dependent, for example, on one or more of: characteristics of the sintering oven 62, characteristics of the copper-based build material 26 (e.g., type, particle size, etc.), and/or the characteristics of the green parts 16A, 16B (e.g., wall thickness). In an example, the green parts 16A, 16B may be heated to the first temperature and then to the second temperature at a rate ranging from about 1 °C/minute to about 50°C/minute. [00120] The multi-step heating process (and in particular sintering) forms the final 3D object 10’, which includes the internal void 56. The final 3D object 10’ is made up of the sintered green parts 16A, 16B and may be densified relative to the green parts 16A, 16B. For example, as a result of sintering, the density may go from 50% density to over 90%, and in some cases very close to 100% of the theoretical density.

[00121] When the diffusion bonding agent 60 includes the copper alloying element precursor, it is to be understood that the multi-step heating process that is schematically depicted in Fig. 5 is initiated in the presence of a reducing environment, such as hydrogen gas. In an example, the hydrogen gas may be introduced into the sintering oven 62 at the outset of the multi-step heating process. In this example, the copper alloying element precursor is first reduced to the copper alloying element 68, and then the processes shown in Fig. 5 take place. It is to be understood that the reducing gas may be used throughout the remainder of the heating process (e.g., sintering) to prevent oxidation. Alternatively, the reducing gas may be replaced with an oxygen free inert gas, such as Ar or N ₂ , after the reduction stage is complete and for the remaining of the heating process.

[00122] In some examples of the method, weight may be added to the uppermost surface of the stack of green parts 16A, 16B to be diffusion bonded during high temperature sintering. As an example, a heat resistant weight may be applied to the surface of the green part 16B in Fig. 3G. The additional weight can help to ensure intimate contact between bonding regions 20, 22 of the green parts 16A, 16B.

[00123] 3D Printed Article

[00124] In an example, the 3D printed article (e.g., 3D object 10’) includes a first copper-based portion 70A (e.g., the sintered green part 16A) including an internal void 56; a second copper-based portion 70B (e.g., the sintered green part 16B) bonded to the first copper-based portion 70A at a designated bonding region (e.g., interface 66 formed by the regions 20, 22 in contact with each other), the second copper-based portion 70B at least partially covering the internal void 56 of the first copper-based portion 70A; and a copper alloy 64 within the first copper-based portion 70A, the second copper-based portion 70B, or both the first and second copper-based portions 70A, 70B adjacent to the designated bonding region 66, 20, 22, the copper alloy being selected from the group consisting of a copper-silver alloy, a copper-zinc alloy, and a copper-tin alloy. In an example, the copper alloy 64 extends a predetermined depth into the first copper-based portion 70A, the second copper-based portion 70B, or both the first and second copper-based portions 70A, 70B adjacent to the designated bonding region 66, 20, 22. This predetermined depth may be controlled by controlling the time for which the copper alloying element 68 is allowed to diffuse.

[00125] Sets and Kits

[00126] The binding agent 28 and the diffusion bonding agent 60 may be part of a multi-fluid kit. The multi-fluid kit may also be part of a 3D printing kit with the copperbased build material 26.

[00127] It is to be understood that the agents 28, 60 of the multi-fluid kit and the agents 28, 60 and the copper-based build material 26 of the 3D printing kits may be maintained separately until used together in examples of the method disclosed herein. The agents 28, 60 and copper-based build material 26 may each be contained in one or more containers prior to and during the method, but may be combined together during the method. The containers can be any type of a vessel (e.g., a reservoir), box, or receptacle made of any material.

[00128] To further illustrate the present disclosure, an example is given herein. It is to be understood that this example is provided for illustrative purposes and is not to be construed as limiting the scope of the present disclosure.

EXAMPLE

[00129] A binding agent with the formulation set forth in Table 1 was prepared.

Table 1: Composition of Binding Agent [00130] Pure copper powder from Sandvik Inc. was used. The powder particles had spherical morphology, and the particle size distribution (PSD) analysis confirmed a unimodal distribution with an average particle size of 12-13 pm.

[00131 ] A commercially available silver (Ag) nanoparticle ink (METALON™ N261 ink from Nova Centrix) with a loading of 40 wt% and a density of about 1 .6 g/ml was used.

[00132] Green parts were printed and assembled for sintering as shown in Fig. 6. For this example, four green parts were 3D printed with the binding agent (GP1 , GP2, GP3, GP4) and one interface green part was 3D printed with both the binding agent and the Ag nanoparticle ink (IGP). To print GP1 , GP2, GP3, GP4, the copper powder was spread into layers, and the binding agent was jetted (at 0.15 g per 1 cm ³ to 0.27 g per 1 cm ³ ) onto the copper powder. To print IGP, the copper powder was spread into layers, and both the binding agent (at 0.15 g per 1 cm ³ to 0.27 g per 1 cm ³ ) and the Ag ink were jetted (0.08 g per cm ³ to 0.16 g per cm ³ ) on the copper power. This Ag ink loading will deliver up to 3 wt% of the silver to the binding regions.

[00133] The patterned layers were exposed to thermal cycles to evaporate solvent from the binding agent and to partially dehydrate/partially decompose the copper (II) nitrate in every layer. After printing, the entire powder bed of parts was subjected to a post-cure treatment at 90°C for 1 hour. After curing and the post-cure treatment, the green parts were de-caked and cleaned of background powder.

[00134] The green parts GP1 , GP2, GP3, GP4, IGP were assembled as shown in Fig. 6. A photograph of the stacked green parts was taken and is shown in Fig. 7. [00135] The stacked green parts were sintered in a high temperature furnace with the following sintering cycle: 300°C/1 hour (hold) — 550°C/2 hours (hold) — 650°C/1 hour (hold) — 1050°C/4 hours (hold). Sintering was performed in an ArH ₂ . [00136] A low magnification stereomicrograph cross-section image of the sintered part is shown in Fig. 8A. Fig. 8B shows a high magnification optical micrograph of a portion of the joint interface (identified by the added dashed line) where the Ag ink was printed with the binding agent. The results demonstrate sound metallurgical bonding without any interfacial defects at the joint interface. At areas where copper green parts were directly bonded (without Ag ink, e.g., GP1-GP3), cracks were observed. These results demonstrate that Ag ink regions can create excellent metallurgical bonding with the potential to produce leak proof parts.

[00137] Two additional parts were 3D printed in the same manner as IGP. The first (IGP2) was sintered at 650°C and the second (IGP3) was sintered at 950°C.

Scanning electron microscope images (SEMS) of each of these 3D objects is shown, respectively, in Fig. 9A and Fig. 9B. As shown in Fig. 9A, the 3D object sintered at 650°C included Ag regions encapsulating the copper particles. As shown in Fig. 9B, the further heat treatment resulted in dissolution of Ag into the copper matrix. Interdiffusion of Ag into Cu across the interface can reduce the melting point of copper, due to alloying, and contribute to higher sintering kinetics. Higher sintering leads to better bonding and higher density at the joint interfaces. At the sinter temperature of 1050°C, Ag diffused further into the copper matrix and homogenized (Fig. 8A and Fig. 8B).

[00138] The chemical composition across the joint interface was evaluated using SEM-EDS analysis. Spot EDS analysis was carried out according to the marking indicated on Fig. 10, and Table 2 provides the corresponding Ag composition. The results show that Ag had diffused through the interface and into Cu part of 0.8 mm thickness.

Table 2: Chemical Composition across Joint Interface

[00139] Comparative 3D objects were generated for comparison. Green parts were prepared in the same manner as GP1-GP4, using the copper build material and the binding agent. The green parts were stacked without any Ag ink at the interface, and the stacked parts were exposed to sintering as described herein to form the comparative 3D objects. [00140] A low magnification stereomicrograph cross-section image of the sintered part is shown in Fig. 11 A. Fig. 11 B shows a high magnification optical micrograph of the joint interface between the two copper green parts. The results demonstrate interfacial defects at the joint interface when two copper green parts are directly sintered together.

[00141] It is to be understood that the ranges provided herein include the stated range and any value or sub-range within the stated range, as if such values or subranges were explicitly recited. For example, from about 20 wt% active to about 70 wt% active should be interpreted to include not only the explicitly recited limits of from about 20 wt% active to about 70 wt% active, but also to include individual values, such as about 25 wt% active, 63 wt% active, etc., and sub-ranges, such as from about 30 wt% active to about 60 wt% active, from about 22 wt% active to about 68 wt% active, etc. Furthermore, the term “about” as used herein in reference to a numerical value or range, allows for a degree of variability in the value or range, for example, within 10%, or, in one aspect within 5%, of a stated value or of a stated limit of a range.

[00142] Reference throughout the specification to “one example”, “another example”, “an example”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the example is included in at least one example described herein, and may or may not be present in other examples. In addition, it is to be understood that the described elements for any example may be combined in any suitable manner in the various examples unless the context clearly dictates otherwise.

[00143] It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise.

[00144] While several examples have been described in detail, it is to be understood that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting.