Heat (h) can be used, such as from a heat source 212 at the various layers (or group of layers, or after the three-dimensional green body object is formed) to remove solvent from the binding agent, and in accordance with the present disclosure, after the green body object is formed, to heat cure the green body object. In one example, heat can be applied from overhead (e.g., prior to application of the next layer of particulate build material, or after multiple layers are formed, etc.), and/or can be provided by the build platform from beneath the particulate build material and/or from the particulate build material source (preheating particulate build material prior to dispensing on the build platform or previously applied three-dimensional object layer). Again, these same heating sources or others can be used for heat curing the green body object 224 (shown in the process of being built) after formation. However, during the build, after individual layers are printed with binding agent, the build platform can be dropped a distance of (x), which can correspond to the thickness of a printed layer in one example, so that another layer of the particulate build material can be added thereon and printed with the binding agent, etc. The process can be repeated on a layer by layer basis until a green body object is formed that is stable enough to move to a fusing oven 214 suitable for fusing, e.g., sintering, annealing, melting, or the like. In this example, the fusing oven can alternatively be used for heat curing the green body object by soaking the green body object at heat curing temperatures prior to raising the temperature within the oven to the much higher fusing temperatures. The green body object in this example includes a three-dimensional object formed from solidified green body object layers, which include both particulate build material and binding agent that delivers the copper nitrate as a binder compound thereto. In other words, the materials and processes described herein can be used to form a three-dimensional green body object made up of particulate build material held together by the binder compound, e.g., the copper nitrate. In certain examples, the three-dimensional green body object can be formed by depositing thin layers of the particulate build material in a powder bed and then jetting a binding agent onto the layers to form individual slices of the three-dimensional green body. After forming one slice of the three-dimensional green body object in this way, a new layer of particulate build material can be deposited on the powder bed and the next slice can be formed by jetting the binding agent, and this can be repeated until a complete three-dimensional green body object is formed.

Regarding the application of heat to the green body object after formation, the green body object can be cured by bringing or retaining the temperature of the green body object to within the range of about 120° C. to about 200° C. for 30 minutes to 8 hours sufficient to convert from about 30 wt % to 100 wt % of the copper nitrate applied to the individual build material layers to copper hydroxynitrate. This can occur in the powder bed itself within the particulate build material 210 that was not contacted by the binding agent, in a separate heating device (such as a fusing oven 214), or a combination of both. In some cases, the powder bed can be used for heat curing the green body object for a first period of time within a first temperature range, e.g., from about 70° C. to about 120° C. for about 30 minutes to about 4 hours, and then a fusing oven or other heating device (outside of the powder bed) can be used for heat curing for a second period of time within a second temperature range, e.g., within the range of about 120° C. to about 200° C. for about 30 minutes to about 8 hours, or within the range of about 130° C. to about 180° C. for about 1 hour to about 4 hours. Thus, for clarity, the term “heat curing” and “fusing” represent multiple different temperature ranges and heating times and should not be confused. The present disclosure is drawn to heat curing the green body object in preparation for fusing to form the solidified metal object. However, in greater detail, there may also be one or two different heat curing events. For further clarity, heat curing of the green body object can occur in the powder bed material and/or a separate heat curing device; however, in some examples, heat curing in the powder bed may occur within lower temperature ranges to avoid green body object surface caking that can occur at higher temperatures, Heat curing in a heating oven may occur at a similar temperature range or can be at an even higher temperature. In accordance with this, heat curing in the powder bed or elsewhere within a lower temperature range, e.g., from 70° C. to about 120° C., can bring the green body object part flexural strength to within the range of about 6 MPa to about 10 MPa, in some examples. Heat curing at the higher temperatures can bring the part flexural strength to within the range of about 8 MPa to about 15 MPa, in some examples. “Flexural strength,” or the modulus of rupture (or bend strength) can be defined as the stress in a material just before it yields in a flexure test. An example test is shown in FIG. 5 hereinafter. This can sometimes be referred to here as “strength” or “green body object strength” or “part strength.”

By heat curing a green body object within the powder bed, benefits can be achieved such as providing enough part flexural strength to allow for automation of movement and/or decaking of the green object. Subsequent heat curing in the fusing oven or other heating device can provide additional green body object flexural strength. It is noted that the example given above where heat curing occurs in the powder bed and in the fusing oven (or other heating device) is by example only, but provides one practical example of how the green body object can be prepared with good green body object part flexural strength, which can be beneficial when fusing the green body object to form the rigid and fused, e.g., sintered, metal object. Notably, “heat curing” should not be confused with “fusing” of the green body object to form the fused metal object.

In some examples, the three-dimensional green body object can have sufficient flexural strength to hold its own shape until the time that the three-dimensional green body object is sintered to form a final metal three-dimensional printed article. In further examples, the three-dimensional green body object can be sufficiently strong to withstand any processing and handling that occurs before sintering. For example, the three-dimensional green body object can be removed from the powder bed and cleaned to remove any loose build material particles that have become caked or weakly attached to the surface of the three-dimensional green body object. In some examples, this cleaning can be performed by brushing, sandblasting, or other methods. If the three-dimensional green body object does not have sufficient strength, then these processes can break off parts of the three-dimensional green body object, destroy surface details of the three-dimensional green body object, and so on. Thus, the green body objects formed using the multi-fluid kits, three-dimensional printing kits, and methods described herein can have increased strength, e.g., flexural strength, and can be removed from the powder bed and prepared for sintering with less risk of breakage. Sintering can include exposing the green body object to heat to sinter the metal particles of the particulate build material together and form a sintered three-dimensional printed object. The temperature at which the green body object is heated to fuse the metal particles and form the final three-dimensional printed object is referred to herein as the fusing temperature. In some examples, the polymeric binder can thermally decompose or be burned off during the sintering process.

Methods of Three-Dimensional Printing

In another example, as shown in FIG. 3, a method 300 of three-dimensional printing can include iteratively applying 310 individual layers of a particulate build material comprising from about 80 wt % to about 100 wt % metal particles, and based on a 3D object model, selectively and iteratively applying 320 a binding agent to the individual layers of the particulate build material to define a green body object formed from multiple individual layers within a powder bed of the particulate build material. In this example, the binding agent can include an aqueous liquid vehicle and a copper nitrate. The method can further include heat curing 330 the green body object by heat soaking the green body object at a temperature within the range of about 70° C. to about 200° C. for about 30 minutes to about 8 hours after the green body object is formed, wherein after heat curing, from about 30 wt % to 100 wt % of the copper nitrate applied to the green body object is in the form of copper hydroxynitrate.

Heat curing can occur at one or multiple locations, such as while the green body object remains in the powder bed (after the build) at from about 70° C. to about 120° C. for about 30 minutes to about 4 hours. At temperatures greater than 120° C. in the powder bed, excessive caking may occur at the surface of the green body object. Thus, in other examples, the green body object can be removed and heated outside of the powder bed. In some examples, the heat curing can likewise occur within the range of about 120° C. to about 200° C., or from about 130° C. to about 180° C., for a period of time for about 30 minutes to about 8 hours, or from about 1 hour to about 4 hours. Heat curing can likewise occur after formation of the green body object while in the powder bed, and then subsequently after removal from the powder bed using a two stage heat curing approach. In some examples, from about 75 wt % to 100 wt % of the copper nitrate applied to the individual build material layers can be converted to copper hydroxynitrate as a result of heat curing. In further detail, the method can also include fusing the green body object in a fusing oven at a fusing temperature from about 600° C. to about 3,500° C. for a period of time from about 1 hour to about 30 hours to form a heat-fused metal object. Thus, in one specific example, the heat curing can occur in the fusing oven prior to raising a temperature therein to the fusing temperature. The binding agent can include, for example, water, from about 0.01 wt % to about 5 wt % organic compounds, and from about 20 wt % to about 60 wt % of the copper nitrate. However, these compounds or classes of compounds can be included outside of these ranges as outlined previously with respect to the three-dimensional printing kits. In one example, at locations wherein the binding agent is selectively and iteratively applied, the binding agent can be applied to the individual layers or particulate build material such that from about 0.25 g to about 0.35 g of copper nitrate is applied per 1 cm³of particulate build material. In one specific example, this can be based on a 12 ng drop weight. The drop weight is given not to be limiting, but to establish an example suitable drop weight that can be used for three-dimensional printing at these copper nitrate (mass) to particulate build material (volume) ratios. Drop weights from about 6 ng to about 15 ng, from about 8 ng to about 14 ng, or from about 9 ng to about 12 ng, can be used alternatively in some examples.

Following application of the binding agent, the particulate build material and fluid(s) applied thereto can be heated to an elevated temperature to assist with enhancing the object flexural strength via heat curing. As noted, heat curing can occur while the three-dimensional object is still in the powder bed of particulate build material and/or heat curing can occur at a separate location using a heating device when the green body object is removed from the powder bed material. When heat curing in the powder bed, heat soaking for a period of time can be at a temperature ranging from about 70° C. to about 120° C., from about 80° C. to about 120° C., from about 80° C. to about 110° C., or from about 90° C. to about 120° C. Heat curing in the powder bed can occur at from about 30 minutes to about 4 hours, from about 1 hour to about 4 hours, from about 30 minutes to about 2 hours, or from about 1 hour to about 3 hours. The times and temperatures for heat curing in a device outside of the powder bed can be as follows: heat curing the green body object outside of the powder bed at a temperature within the range of about 70° C. to about 200° C., from about 120° C. to about 200° C., from about 130° C. to about 180° C., from about 140° C. to about 200° C., or from about 90° C. to about 180° C.; and heat soaking for a period of time during heat curing for a time increment from about 30 minutes to about 8 hours, from about 30 minutes to about 4 hours, from about 1 hour to about 8 hours, or from about 1 hour to about 4 hours. As a note, the heating device that can be used when the green body object is removed from the powder bed may be the same device that is ultimately used for fusing the green body object to form the heat fused metal object. In this scenario, the heating device would be ramped up to a temperature within one or more of the ranges recited above and allowed to heat soak for a period of time as set forth in the time ranges above. After heat curing, the temperature could then be ramped up for heat fusing as outlined in further detail herein. It is noted that these temperature ranges and time increments can also be used in the context of the systems of three-dimensional printing or used with the three-dimensional printing kits described herein.

Upon heat curing, the three-dimensional green body object with enhanced flexural strength can be moved to a heating device (if not already moved there), such as a fusing oven. In some examples, the heat curing in the powder bed material may be used to promote a partial cure and to provide some enhanced object strength prior to heat curing in the heating device outside of the powder bed, or a more fragile part can be moved to the heating device for heat curing prior to fusing. In some examples, with respect to fusing, the heating device can also be the fusing device, e.g., a sintering oven. In one example, after heat curing, the three-dimensional green body object can be heated to a sintering or fusing temperature ranging from about 500° C. to about 3,500° C., including at a temperature within the range where the metal particles are fused together. In another example, the temperature can range from about 600° C. to about 1,500° C., or from about 800° C. to about 1200° C., or from about 1,000° C. to about 2,000° C. In further detail, the fusing temperature range can vary, depending on the material, but in one example, the fusing temperature can range from about 10° C. below the melting temperature of the metal particles of the particulate build material to about 100° C. below the melting temperature of the metal particles of the particulate build material. In another example, the fusing temperature can range from about 50° C. below the melting temperature of the metal particles of the particulate build material to about 100° C. below the melting temperature of the metal particles of the particulate build material. The fusing temperature can depend upon the particle size and period of time that heating occurs, e.g., at a high temperature for a sufficient time to cause particle surfaces to become physically merged or composited together. For example, a fusing temperature for stainless steel can be about 1400° C. and an example of a fusing temperature for aluminum or aluminum alloys can range from about 550° C. to about 620° C. Temperatures outside of these ranges can be used as determined on a case by case basis.

In certain examples, the three-dimensional green body object can be gradually heated from the heat curing temperature to the fusing or sintering temperature. As the binder compound used in these examples is copper nitrate, the copper content of the copper nitrate can become incorporated into the fused three-dimensional object, unlike polymer binder which would normally burn off or become decomposed at about 300° C. to about 500° C.

In some examples, during heating in the oven, the heating device can include an inert atmosphere to avoid oxidation of the metal particles. In one example, the inert atmosphere can be oxygen-free and can include a noble gas, an inert gas, or combination thereof. For example, the inert atmosphere can include a noble gas or an inert gas selected from argon, nitrogen, helium, neon, krypton, xenon, radon, hydrogen, or a combination thereof. Upon removal of the fused three-dimensional object from the oven and cooling (or annealing by controlling the cool down rate in the oven), the fused three-dimensional object can be treated or polished, such as by sand blasting, bead blasting, air jetting, tumble finishing such as barrel finishing, vibratory finishing, or a combination thereof. Tumble or vibratory finishing techniques can be performed wet (involving liquid lubricants, cleaners, or abrasives) or dry.

Definitions

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.

The term “about” as used herein, when referring 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. The term “about” when modifying a numerical range is also understood to include as one numerical subrange a range defined by the exact numerical value indicated, e.g., the range of about 1 wt % to about 5 wt % includes 1 wt % to 5 wt % as an explicitly supported sub-range.

As used herein, “devoid” refers to a numerical quantity that can be zero or can be substantially zero, e.g., a quantity may be permissible in trace amounts, such as up to 0.1 wt % of a formulation or composition.

As used herein, the term “green” when referring to a green part, green body object, green body object layer, etc. refers to any intermediate structure that has been solidified and/or cured (prior to heat fusing), furthermore, a green body object can include particulate build material and the copper nitrate as a binder compound to bind the particulate build material together. The term “green body” is used to refer to a multi-layered object that is (weakly) bound together, but upon some water removal, can exhibit sufficient flexural strength to be moved to a fusing oven, for example. However, in accordance with the present disclosure, the green body object may still be considered to be a green body object after it is heat cured in accordance with the present disclosure. A cured green body object can be further solidified to have enhanced strength but is still not in a state of having been fused to form a metal object. It is to be understood that any build material that is not patterned with binding agent is not considered to be part of the green body, even if it is adjacent to or surrounds the green body. For example, unprinted particulate build material acts to support the green body object while contained therein, but the particulate build material is not part of the green body object unless it is printed with binding agent to generate a solidified part prior to fusing.

As used herein, the terms “three-dimensional part,” “three-dimensional object,” or the like, refers to the target three-dimensional object that is being built, and can be a green body three-dimensional object or a fused three-dimensional object, depending on the context. However, in some instances, for clarity, the three-dimensional object can be referred to as a “fused” three-dimensional object, indicating it has been fused, e.g., sintered, annealed, melted, etc., or a “green body,” “three-dimensional green body object,” or “green” three-dimensional object, indicating it has been solidified or is in the process of solidification sufficient for movement, but not yet heat fused.

“Binding agent” refers to a fluid that includes water and copper nitrate as a binding compound, and which is effective for binding layers of particulate build material when forming a green body. The binding agent is typically applied to form a three-dimensional green body object. Upon heat curing, the copper nitrate remains, even if some of the water and/or organic compound(s) have been removed.

The term “fluid” does not infer that the composition is free of particulate solids, but rather, can include solids dispersed therein.

As used herein, “kit” can be synonymous with and understood to include a plurality of compositions where the different compositions can be separately contained in separate containers prior to and/or during use, e.g., building a green three-dimensional object for subsequent fusing. These compositions of the “kit” can be combined together during a three-dimensional build process. The containers can be any type of a vessel, box, or receptacle made of any material.

The term “fuse,” “fusing,” “fusion,” or the like refers to the joining of the material of adjacent particles of a particulate build material, such as by sintering, annealing, melting, or the like, and can include a complete fusing of adjacent particles into a common structure, e.g., melting together, or can include surface fusing where particles are not fully melted to a point of liquefaction, but which allow for individual particles of the particulate build material to become bound to one another, e.g., forming material bridges between particles at or near a point of contact.

As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though the various members of the list are individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list based on their presentation in a common group without indications to the contrary.

Concentrations, dimensions, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include the numerical values explicitly recited as the limits of the range, as well as to include all the individual numerical values or sub-ranges encompassed within that range as if the various numerical values and sub-ranges are explicitly recited. For example, a weight ratio range of about 1 wt % to about 20 wt % should be interpreted to include the explicitly recited limits of 1 wt % and 20 wt % and to include individual weights such as about 2 wt %, about 11 wt %, about 14 wt %, and sub-ranges such as about 10 wt % to about 20 wt %, about 5 wt % to about 15 wt %, etc.

EXAMPLES

The following illustrates examples of the present disclosure. However, it is to be understood that the following are illustrative of the application of the principles of the present disclosure. Numerous modifications and alternative compositions, methods, and systems may be devised without departing from the spirit and scope of the present disclosure. The appended claims are intended to cover such modifications and arrangements.