Centrifugal sintering furnace

Abstract

A rotational sintering furnace, along with its production methods and simulation techniques, are disclosed. The design of the furnace and its associated systems are aimed at adjusting the grain compression that a part undergoes during the sintering process. The adjustment facilitates part homogenization during sintering. Additionally, the system and simulations are designed to prevent and model potential deformations in sintered parts.

Description

CENTRIFUGAL SINTERING FURNACESTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0001] This invention was made with government support under 2119832, 2119833, and 2138421 , awarded by the National Science Foundation, and DE-SC0022244, awarded by the Department of Energy. The government has certain rights in the invention.CROSS-REFERENCES TO RELATED APPLICATIONS

[0002] The current application claims the benefit of Provisional Application No. 63 / 616,154, filed December 29, 2023, the disclosure of which is incorporated herein by reference.FIELD OF THE INVENTION

[0003] This disclosure generally refers to systems and methods for sintering technology and systems. More specifically, this application relates to rotational sintering furnaces as well as methods, techniques, and systems for adjusting grain compression for sintering homogenization.BACKGROUND

[0004] Sintering is a manufacturing process that involves densifying a porous material, such as metals, ceramics, or plastics. This densification is achieved through thermal and / or pressure treatment. Sintering is crucial for the production of certain parts and is used in both traditional and advanced additive manufacturing technologies, such as binder jetting or stereolithography. The sintering process takes place in a high- temperature furnace and can often result in significant shrinkage of the parts. Shrinkage of up to 20% is common. Sintered parts and large sintered parts, in particular, can also be negatively affected by gravity. Gravitational force on the sintered part can cause geometric distortions or “slumping.” These distortions can often lead to cracks and result in rejected parts, lower yields, and increased production costs.SUMMARY OF THE INVENTION

[0005] Systems and methods in accordance with some embodiments of the invention are directed to manufacturing processes for sintering, producing sintered materials using a rotational furnace, stable environments for sintered materials production, and homogenization of sintered part production.

[0006] Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosure. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.

[0007] Many embodiments are directed towards devices comprising a cylindrical frame with a central point of rotation and at least one heater configured to heat the internal volume of the cylindrical frame, at least one fixture point on the interior surface of the cylindrical frame configured to affix a part to the interior surface of the cylindrical frame, and at least one rotations device configured to rotate the cylindrical frame about its central point of rotation.

[0008] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace including: a cylindrical frame wherein the cylindrical frame includes a central axis and an internal cavity, wherein the internal cavity is defined by an interior surface of the cylindrical frame; at least one heater configured to heat the internal cavity of the cylindrical frame; a fixture point configured to affix a part to the interior surface of the cylindrical frame; a rotator configured to rotate the cylindrical frame about the central axis of the cylindrical frame; and a controller configured to control a speed and a direction of rotation of the cylindrical frame wherein the rotation of the cylindrical frame exerts a pre-determined amount of centrifugal force on the part such that the part achieves a desired final geometry.

[0009] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the at least one heater is configured about the central axis of the cylindrical frame.

[0010] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the fixture point includes a clamp.

[0011] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the rotator includes at least two rollers in connection with an outer surface of the cylindrical frame.

[0012] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the rotator is configured to rotate the cylindrical frame about the central axis such that the affixed part is also rotated about the central axis of the cylindrical frame.

[0013] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the part is formed including additive manufacturing processes.

[0014] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the pre-determined centrifugal force imparts a pseudo- gravitational force.

[0015] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the pseudo-gravitational force is a negative force such that the part experiences reduced slumping due to gravity.

[0016] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the pseudo-gravitational force is a positive force such that the part experiences grain compression and densification.

[0017] In some embodiments, the techniques described herein relate to a centrifugal sintering furnace, wherein the pre-determined centrifugal force is determined by a sintering model embedded in an FEM software based on the continuum theory of sintering including: the desired geometry of a finished part; a sintering data for the part material and the desired geometry; an external load during sintering as expressed by a gravitational force and a centrifugal force; and a rotation speed expressed by a centrifugal acceleration and the gravitational force.

[0018] In some embodiments, the techniques described herein relate to a process of sintering including: attaching a cylindrical frame to a rotator wherein the cylindrical frame includes a central axis and an internal cavity further including an interior surface; attaching a part to the interior surface of the cylindrical frame's internal cavity; heating the internal cavity of the cylindrical frame such that the internal cavity has a consistent temperature; rotating the cylindrical frame about its central axis; and controlling the rotation of the cylindrical frame such that the rotation imparts a centrifugal force on the part.

[0019] In some embodiments, the techniques described herein relate to a process, further including modelling a desired final shape of the part to determine a rotation parameters including: determining a desired final geometry of a finished part; determining a sintering data for the finished part and the desired geometry; determining an external load during sintering as expressed by a gravitational force and a centrifugal force; and determining a rotation speed during sintering as expressed by a centrifugal acceleration and the gravitational force.

[0020] In some embodiments, the techniques described herein relate to a process, wherein the attaching a part to the interior surface of the cylindrical frame's internal cavity includes clamping the part at a fixture point.

[0021] In some embodiments, the techniques described herein relate to a process, wherein the rotation of the cylindrical frame includes rotating rollers in connection with an external surface of the cylindrical frame.

[0022] In some embodiments, the techniques described herein relate to a process, further including forming the part to be sintered including additive manufacturing processes.

[0023] In some embodiments, the techniques described herein relate to a process, wherein the heating the internal cavity of the cylindrical frame includes heating the internal cavity of the cylindrical frame to a uniform temperature.

[0024] In some embodiments, the techniques described herein relate to a process, wherein the rotation of the cylindrical frame about the central axis also rotates the part about the central axis of the cylindrical frame.

[0025] In some embodiments, the techniques described herein relate to a process, wherein the rotation imparts a pseudo-gravitational force.

[0026] In some embodiments, the techniques described herein relate to a process, wherein the pseudo-gravitational force includes a negative force such that the part does not experience slumping due to gravity.

[0027] In some embodiments, the techniques described herein relate to a process, wherein the pseudo-gravitational force includes a positive force such that the part experiences grain compression and densification.BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The description will be more fully understood with reference to the following figures, which are presented as embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:

[0029] Figures 1A to 1 C illustrate a centrifugal sintering furnace in accordance with some embodiments.

[0030] Figures 2A and 2B depict finite element sintering simulation of a hollow cylinder sintered in fixed configuration.

[0031] Figures 3A and 3B depict finite element sintering simulation of a hollow cylinder sintered in accordance with an exemplary embodiment.

[0032] Figures 4A to 4E depict distortions that can result from gravitational force on a sintered part and counteracting force applied to the sintered part in accordance with an exemplary embodiment.

[0033] Figures 5A to 5E depict distortion that can result from gravitational force on a sintered part and the centrifugal furnace reproducing the effect of gravitational force in accordance with an exemplary embodiment.

[0034] Figures 6A to 6F depict calculation of shape distortion of a part sintered in accordance with an exemplary embodiment.DETAILED DESCRIPTION OF THE INVENTION

[0035] It will be understood that the components of the embodiments, as generally described herein and illustrated in the appended figures, may be arranged and designed in a variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure but is merely representative of various embodiments. While various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

[0036] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.

[0037] Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages and similar language throughout this specification may, but do not necessarily, refer to the same embodiment.

[0038] Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.

[0039] Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may but do not necessarily, all refer to the same embodiment.

[0040] Many embodiments are directed towards a centrifugal sintering furnace. The centrifugal sintering furnace of many embodiments may comprise a cylindrical frame having an internal volume and a central axis. The internal volume in various embodiments is defined by the interior surface of the cylindrical frame. The centrifugal sintering furnace of several embodiments further comprises one or more heaters, configured to heat the internal volume of the cylindrical frame to a uniform temperature. In many embodiments the one or more heaters are proximally located about the central axis of the cylindrical frame. In many embodiments the cylindrical frame is rotated during at least a portion of the sintering process. The centrifugal sintering furnace of several embodiments further comprises a rotator. In some embodiments, the rotator may be a set of rollers in connection with the external surface of the cylindrical frame. The rotation of the rollers is configured to rotate the cylindrical frame about its central axis in many embodiments. In some embodiments, the interior surface of the cylindrical frame further comprises a fixture point. The fixture point of many embodiments is configured to affix a part to be sintered to the interior surface of the cylindrical frame. In further embodiments, the attachment of the part to the interior surface of the cylindrical frame is configured to rotate the part about the cylindrical frame’s central axis as the cylindrical frame is rotated. Several embodiments are directed towards sintering a part within the centrifugal sintering furnace that has been produced by additive manufacturing. In many embodiments, the rotation of the cylindrical frame is configured to impart a centrifugal force on the part being sintered.

[0041] The centrifugal sintering furnace in accordance with many embodiments imparts a centrifugal force on a part during the sintering process. In several embodiments, the centrifugal force created by the centrifugal sintering furnace imparts a pseudo- gravitational force on the part during the sintering process. In many embodiments a negative pseudo-gravitational force is applied. In several embodiments, a positive pseudo-gravitational force is applied.

[0042] Many embodiments of the centrifugal sintering furnace are configured to impart a pseudo-gravitational force determined by a finite element model of the external load the part experiences during the sintering process. In several embodiments, the external loadexperienced by the part is expressed by the gravitational force and centrifugal force of the sintering environment.

[0043] Figure 1A provides an exemplary embodiment of a centrifugal sintering furnace. In many embodiments, the centrifugal sintering furnace comprises a cylindrical frame 102 that further comprises a central axis 104. Many embodiments comprise one or more heaters 106 configured to heat the internal volume of the cylindrical frame 102. In the embodiment of Figure 1A, the one or more heaters 106 is centrally located within the cylindrical frame. In some embodiments the one or more heaters may be affixed to one interior surface of the cylindrical frame. In many embodiments the one or more heaters may be affixed to several internal surfaces of the cylindrical frame. In several embodiments, the one or more heaters may be affixed to the outer surface of the cylindrical frame in one or more places. In some embodiments the one or more heaters is not affixed to the cylindrical frame. Though the embodiment of Figure 1A provides the heater as centrally located, it should be understood that the one or more heaters can be configured in any position such that it is capable of heating the internal volume of the cylindrical frame to a uniform temperature.

[0044] In the embodiment of Figure 1A, a part 108 to be sintered is configured to be affixed to the internal surface of the cylindrical frame 102 such that the part 108 rotates about the central axis 104 as the cylindrical frame 102 rotates. Figure 1A and Figure 1 B provide the different locations of the part 108 during the centrifugal sintering process. In various embodiments, the fixture point may comprise, but is not limited to, a clamp, a clip, an adhesive, a strap, and / or any combination thereof. Regardless of the manner in which the part is affixed, it may be affixed such that it remains securely fastened to the interior surface of the cylindrical frame during the rotational sintering. In some embodiments, the part is formed and then sintered to form the final part. In many embodiments, the part may be produced through additive manufacturing processes.

[0045] In the embodiment of Figure 1A, the rotator 110 is configured to rotate the cylindrical frame 102 about the central axis 104 of the cylindrical frame 102. In some embodiments the rotator may be a set of rollers. In many embodiments the rotational device may be a set of gears in connection with the outer surface of the cylindrical frame.In several embodiments the rotational device may be an axel in connection with the cylindrical frame. Regardless of the manner in which the cylindrical frame is rotated, it may be understood that the rotator is configured to rotate the cylindrical frame about its central axis such that the part is similarly rotated about the central axis of the cylindrical frame.

[0046] In conventional sintering or firing, a part is exposed to gravity and can deform under its own weight when subjected to high temperatures. If the geometric deviation is significant, the parts are often rejected. As a result, for certain applications, the industry would look for alternative production technologies with better yields.

[0047] Increasing and / or balancing the load on the sintered component through rotation, along with effective heat treatment, can ensure that complex shapes are maintained during the sintering process, improving yields, and making sintering a viable production process for more industry applications.

[0048] At high temperatures, such as 1400°C for stainless steel, materials become less viscous, and gravity can significantly affect them, leading to shape distortion under free-sintering conditions. This distortion often results in cracks and leads to costly rejections of parts in industrial production settings.

[0049] Figures 2A and 2B show the simulated shape deformation of a part sintered in a fixed position. The intended shape 201 of the part is a hollow cylinder. However, when sintered under a fixed position and subject to gravity, the simulated finished part 202 is deformed such that it is no longer the intended shape 201 .

[0050] Under other conditions, the forces present (e.g., from gravity or the lack thereof) are too low to provide sufficient grain compression and removal of pore buoyancy. Such conditions result in lower sintered densities and can also lead to increased distortions and lower yields.

[0051] Conventionally manufactured parts, as well as parts produced via additive manufacturing (AM) technologies, often require sufficient grain compression for effective sintering. Maintaining the shape without distortion is a challenge and one that is especially prevalent in the production of AM sintered parts. AM enables the production of complex and hollow geometries that require precise tolerances. Such AM parts are particularlysusceptible to failures that result from distortion, and AM production necessitates careful attention to preserving geometry during sintering.

[0052] The centrifugal force applied in accordance with various embodiments works as an additional driving force for the sintering process. In accordance with various embodiments, slowly rotating the part at high temperatures (where the material’s viscosity is at its lowest but the density is high) results in the gravitational load being offset or augmented and distributed more evenly across the part and allows for better shape retention. The shape of the part can be preserved without the need for intermediary supports or alterations to the sintering cycle.

[0053] Amongst factors that lead to distortion of a part during its free sintering, gravity is one of the most important especially for heavy metal powders. The simulation of sintering for binder-jetted additively manufactured green compacts, in accordance with various embodiments, is important based of the unique microstructure of the additively manufactured component leading to anisotropic shrinkage. Sintering shrinkage and structural analysis of parts, of various embodiments, derived from these green bodies reveal that an initial anisotropic layered structure typically results in shape distortion and final material property anisotropy. Advancements in computer tomography enable detailed analysis of powder bed structure evolution during binder-jetted processing and highlight the periodic density fluctuation mainly in the build-up direction.

[0054] Using a finite element method (FEM) model of sintering based on the continuum theory of sintering (Equation 1 ), it is possible to model a part's densification and behavior during the heating process according to various embodiments. The simulation aids in quantifying the shape distortion that occurs during sintering.

[0055] the stress tensor (Pa), represents the externally applied stress, J?0is the shear viscosity of the fully dense material (Pa ■ s) , <p and i are respectively normalized shear and bulk viscosity moduli, E^ is the strain rate tensor (s-1), e is the first strain rate tensor invariant (s-1), corresponding to the volumetric shrinkage, <5iyis the Kronecker delta and PLis the effective sintering stress (Pa).

[0056] (p, ip and PLcan be expressed as a function of porosity, q, with modifiedSkorokhod-Olevsky model and constitutive parameters as expressed in Equations 2 thru 4 respectively, in accordance with an embodiment for SS136L.

[0057] In accordance with an embodiment of a sintered part material of SS136L, with the surface tension a = 1.2 J / m2, the initial particle size is Go = 8- 10'6m. On its side, the initial density is considered uniform with a mean value of 4.38 g / cm3(54.75% relative density). Equation 5 provides the generalized expression of the gravity as an external load is:

[0058] Equation 6 provides the viscosity zj0expressed as:

[0059] With Ao the pre-exponential factor (Pa-s-K'1), Q the apparent activation energy of sintering (kJ / mol), R the universal gas constant, Tthe temperature (K).

[0060] A 5-ferrite phase transition in SS316L, in accordance with an embodiment, being around 1316°C (1581 K), implies that in the model, the activation energy changes from Qi = 217.250 kJ / mol to Q2 = 1182.176 kJ / mol and the pre-exponential factor changes from Ao = 2.502 Pa-s-K'1to A0 2 = 4.599e-32 Pa-s-K'1. The grain growth is also modeled with Equation 7:

[0061] In accordance with an embodiment for a sintered part material of SS136L, the identified preexponential term ko = 2.97- 10-22m3s-1, the apparent grain growth activation energy is QG = 164.8 kJ -mol'1and the critical porosity is 6C- 5.20%. The thermal cycle follows a heating rate of 35 K / min from 773.15 K to 1283.15 K and 1.5 K / min from 1283.15 K to 1658.15 K and with a temperature holding of 220 min at 1658.15 K.

[0062] Figures 3A and 3B show the simulated shape of a part sintered under centrifugal conditions in accordance with an embodiment. The intended shape 301 of the part is a hollow cylinder. When simulated being sintered in accordance with an embodiment such that it is affected by centrifugal forces, the simulated finished part 302 does not exhibit deformation.

[0063] In various embodiments, initial sintering simulations that integrate rotational movement into a hollow structure, demonstrate the mitigation of gravity’s influence on the sintered part. Such embodiments reduce the reliance on trial and error. In other embodiments, the simulation plays a crucial role in understanding the key stages of sintering at high temperatures and in determining the most effective rotation speed. Such embodiments provide valuable insights into the sintering process and aid in the optimization of procedures.Centrifugal Sintering Furnace to Apply a Negative Pseudo-Gravitational Force

[0064] In various embodiments, the part's motion can be achieved by affixing the part 108 to a rotating cylindrical frame 102 with a central fixed rod heater 106, as illustrated in Figure 1A. The rotator 110 rotates the cylindrical frame 102 and the affixed part 108 about the central axis 104 of the cylindrical frame during the sintering process. The model described above can be utilized to determine the speed of rotation required to impart a negative pseudo-gravitational force. The requisite negative pseudo-gravitational force is determined to achieve the desired final part geometry, such as through preventing slumping of sintered parts.

[0065] In accordance with various embodiments, the centrifugal force, directed towards the radius of the frame (-z0 in the part’s frame of reference), can be manipulated by adjusting the rotation speed by adjusting the speed of rotator 110 to simulate specificsintering conditions for a part. In accordance with some embodiments, sintering conditions using a centrifugal sintering furnace could replicate specific conditions, such as increased or decreased gravitational forces (or gravity in a pseudo-gravity environment). In accordance with numerous embodiments, the frame’s rotation is tailored to the viscosity of the material. In accordance with such embodiments, when the material’s viscosity is high, such as at lower temperatures, the rotational speed can be set to ensure sufficient grain compression. Conversely, as the viscosity decreases at higher sintering temperatures, the speed can be adjusted to avoid excessive slumping.

[0066] The sintering model, in accordance with various embodiments, can be integrated into COMSOLM Multiphysics software, enabling the application of a positive or negative pseudo-gravitational force on the component in terms of direction and magnitude. Such integration enables the inclusion of centrifugal forces in accordance with various embodiments. With the inclusion of gravity and centrifugal force in the sintering model, it becomes possible to simulate sintering in a centrifugal sintering furnace in accordance with several embodiments.

[0067] In various embodiments, the general expression of the centrifugal acceleration that can be embedded in the model is expressed in Equation 8:

[0068] With a> (rad -s'1) the rotation speed and Rt(m) the radial distance between the central axis 104 and the geometry of the part 108. In addition, the application of gravity in the global cartesian coordinate system can be expressed as a function of the time and rotation speed as provided in Equation 9:

[0069] Therefore, the equilibrium (Equation 5) can be rewritten with the incorporation of the gravitational and centrifugal forces to the model as expressed in Equation 10:

[0070] Utilizing Equation 10 in the model to express gravity as an external load, accounts for the effect of both gravitational and centrifugal forces on shape distortion during sintering in accordance with several embodiments.

[0071] An exemplary embodiment, depicted in Figures 4A to 4E, illustrates the comparison of a part’s simulated shape (Figure 4E) when sintered without rotation (Figure 4B) and when sintered in a rotating furnace (Figure 4D). The furnace was set to rotate at a speed of 1 revolution per minute. Due to this slow rotation speed, the centrifugal force computation is negligible, and consequently, the variation of gravity’s effect on the part’s geometry becomes the only factor influencing the part’s deformation at high temperatures.

[0072] Figure 4A shows a hollow component sintered under the same conditions as those simulated. When sintered under the same conditions as the simulation, the component shows significant slumping, which aligns with the predictions made by the simulation. The simulation's computational results compare the original configuration of the component as affixed and after sintering. The resulting slumping of the components is compared in Figures 4B and 4D. Both the fixed and rotated parts can achieve a relative density of up to 0.99. However, the fixed part exhibits greater slumping, as shown at the top of Figure 4E. The observed slumping is corroborated by the graphs that plot the evolution of five different diameters of the component overtime during the sintering process. Figure 4B depicts the fixed part, and Figure 4D depicts the rotated part. The final deformation ratio is computed by dividing the smallest resulting diameter by the largest diameter, specifically diameters 1 and 5, as shown in Figure 4C. The deformation ratio is 0.86. Whereas the component sintered in the rotating furnace shown in Figure 5C has a deformation ratio of 0.99.

[0073] In many embodiments, low revolution at high temperature, where the viscosity of the material is the lowest, but the density is high, allows a homogenization of the distribution of the gravity load on the part leads to a more uniform shape retention. Sintering in a rotating furnace imparts a negative pseudo-gravitational force to ensure the part is subject to a uniform force. In several embodiments, the exact speed needed will depend on the geometry, the material being sintered and the thermal cycle. In manyembodiments, the model is configured to estimate the needed speed. The rotational speed may change during the sintering process to accommodate the change in material viscosity during the sintering process in accordance with the modeled sintering parameters in accordance with many embodiments.

[0074] As depicted in the exemplary embodiment shown in Figure 5A, a hollow cylindrical part, sintered under the same conditions as those simulated, exhibits significant slumping. Although slumping due to gravity is typically avoided, this demonstration serves to highlight the efficiency of the centrifugal sintering furnace, in accordance with several embodiments, in accurately replicating gravity conditions under pseudo-gravity sintering. The comparison between computational results (original fixed configuration and centrifugal sintering furnace) is shown in Figures 5B and 5D.

[0075] The embodiments of parts in Figure 4E and Figure 5E achieve a relative density of up to 0.99 and exhibit significant slumping. This observation is confirmed by the graphs plotting the evolution of five different diameters as a function of sintering time (Figure 5B for the fixed part and Figure 5D for the part in the centrifugal furnace without gravity). The final deformation ratio is calculated by dividing the smallest diameter by the largest diameter, specifically diameters 1 and 5, as shown in Figure 5C. This results in a deformation ratio of 0.86 for both configurations. Sintering in a centrifugal rotating furnace, in accordance with the conditions of the exemplary embodiment, such as the rotation speed, makes it possible to replicate sintering conditions experienced under Earth's gravity.

[0076] The addition of gravity and centrifugal components to sintering models in accordance with various embodiments permits the simulation of sintering in a rotating furnace. The inclusion of these components in the simulation further allows the simulation of pseudo-gravity, where the only force applied to the part is the centrifugal force from the furnace. An exemplary embodiment depicted in Figures 5A through 5E shows the simulated shape of a part sintered with rotation configured to replicate the gravitational force a part would experience if freely sintered under Earth’s gravitational forces. The rotation speed required to simulate the effect of gravity during sintering in a rotating furnace under pseudo-gravity can be calculated using Equation 11 :“ = jl (11)

[0077] In the embodiment shown in Figure 5D RL= 0.2 m and, the calculated rotational speed is 1.1 rev / s.Centrifugal Sintering to Impart a Positive Pseudo-Gravitational Force

[0078] The centrifugal sintering furnace of many embodiments may be useful for sintering in the context of low gravity, for example in space, on the Moon or on Mars. In low gravity conditions, the lack of buoyancy forces prevents pore removal, and as pores coalesce there is distortion from swelling. Additionally, there is not sufficient grain compression, leading to incomplete densification. For these reasons, while gravity does not contribute to distortion, the effects of removing gravity are just as detrimental to shape fidelity after sintering.

[0079] Due to the independent effect of centrifugal and gravitational forces, it is possible to simulate different sintered parts rotated at different speeds. With a higher rotating speed, the part is subject to an additional centrifugal force that would not be negligible anymore. Hence, the part is subjected to a unidirectional pressure that is directed radially in the cylindrical coordinate system.

[0080] The rotation speed needed to simulate the effect of the gravity, during sintering in a rotating furnace under microgravity, can be calculated by equating the centrifugal and gravitational forces given be Equations 8 and 9. The speed of rotation is then expressed in Equation 11.

[0081] The exemplary embodiment shown in Figure 6A through Figure 6E provides a comparison of a simulated part rotated at a high speed of rotation and a low speed of rotation in accordance with various embodiments. Figure 6A provides a schematic of the location of the five diameters of a hollow cylindrical part to determine shape deformation of a part sintered in accordance with an embodiment. Figure 6D provides the simulated final shape of a cylinder sintered under a high revolution speed of (3.6rev / sec). When rotated at a high speed, the evolution of five different diameters of the componentovertime during the sintering process is shown in Figure 6E, resulting in a deformation ratio of 0.33. Graphs of the evolution of the five diameters for a fixed part (Figure 6B), a part rotated at 1 revolution per hour (Figure 6C), a part rotated at 3.6 revolutions per second (Figure 6D), and a part rotated at 1.1 revolution per second (Figure 6F) are provided. Figure 6D provides the part rotated at 3.6 rev / sec has a significantly higher distortion than the fixed sample of Figure 6B, the sample of Figure 6C rotated at 1 rev / 60 minutes, and the sample of Figure 6F rotated at 1.1 revolution / second.

[0082] The R, = 0.2 m in the embodiment of Figure 6F for Equation 11 of the model to determine a rotation speed of 1.1 rev / second to present a no-gravity computation in accordance with an exemplary embodiment. A similar shape distortion is observed in Figure 6B and Figure 6F. In accordance with an embodiment, controlling the rotation speed of the centrifugal sintering furnace can replicate the effect of Earth’s gravity in low or no gravity environments.

[0083] Several embodiments may be configured to optimize the rotation speed, apply sufficient grain compression to initiate the sintering at the early stages of sintering, and then slightly reduce this rotation speed at high temperatures / low viscosity of the material to avoid the slumping due to centrifugal forces, with respect to the surrounding gravitational environment. In several embodiments, the rotation during sintering can also be used to change the shape of the sintering part.

[0084] Revolution at high temperatures, where the viscosity of a material is low but has high density, can result in the adjustment of load the part experiences during sintering, which can result in better homogenization of the part, the reduction of slumping and leads to more uniform shape retention. The rotational speed may change during the sintering process to accommodate the change in material viscosity during the sintering process in accordance with the modeled sintering parameters in accordance with many embodiments. Additionally, the exemplary simulations show the benefits of sintering in a rotating furnace to augment the effect of gravity on the sintering of parts. The exact speed required depends on the geometry, the material being sintered, and the thermal cycle and the simulation provides the ability to calculate the needed speed. If used in tandem with preliminary FEM simulation of densification, rotational furnace sintering, and simulationprovide the ability to counteract the distortion that occurs during the final stages of traditional high temperature sintering.DOCTRINE OF EQUIVALENTS

[0085] This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.

[0086] As used herein, the singular terms “a,” “an,” and “the,” may include plural referents unless the context clearly dictates otherwise. Reference to an object in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.”

[0087] As used herein, the terms “approximately” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ± 10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1 %, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

[0088] Additionally, amounts, ratios, and other numerical values may sometimes be presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-rangeis explicitly specified. Where ranges are described, the range should be understood to include the endpoints of the ranges, and the endpoints of such ranges are also contemplated to stand on their own as inventive, individual data points and to form the endpoints of other ranges. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, sub-ranges such as about 1 to about 10, about 10 to about 50, about 20 to about 100, about 100 to about 200, and so forth, and related ranges such as greater than about 1 or less than about 200.

Claims

WHAT IS CLAIMED IS:1 . A centrifugal sintering furnace comprising: a cylindrical frame wherein the cylindrical frame comprises a central axis and an internal cavity, wherein the internal cavity is defined by an interior surface of the cylindrical frame; an at least one heater configured to heat the internal cavity of the cylindrical frame; a fixture point configured to affix a part to the interior surface of the cylindrical frame; a rotator configured to rotate the cylindrical frame about the central axis of the cylindrical frame; and a controller configured to control a speed and a direction of rotation of the cylindrical frame wherein the rotation of the cylindrical frame exerts a pre-determined amount of centrifugal force on the part such that the part achieves a desired final geometry.

2. The centrifugal sintering furnace of claim 1 , wherein the at least one heater is configured about the central axis of the cylindrical frame.

3. The centrifugal sintering furnace of claim 1 , wherein the fixture point comprises a clamp.

4. The centrifugal sintering furnace of claim 1 , wherein the rotator comprises at least two rollers in connection with an outer surface of the cylindrical frame.

5. The centrifugal sintering furnace of claim 1 , wherein the rotator is configured to rotate the cylindrical frame about the central axis such that the affixed part is also rotated about the central axis of the cylindrical frame.

6. The centrifugal sintering furnace of claim 1 , wherein the part is formed comprising additive manufacturing processes.

7. The centrifugal sintering furnace of claim 1 , wherein the pre-determined centrifugal force imparts a pseudo-gravitational force.

8. The centrifugal sintering furnace of claim 7, wherein the pseudo-gravitational force is a negative force such that the part experiences reduced slumping due to gravity.

9. The centrifugal sintering furnace of claim 7, wherein the pseudo-gravitational force is a positive force such that the part experiences grain compression and densification.

10. The centrifugal sintering furnace of claim 1 , wherein the pre-determined centrifugal force is determined by a sintering model embedded in an FEM software based on the continuum theory of sintering comprising: the desired geometry of a finished part; a sintering data for the part material and the desired geometry; an external load during sintering as expressed by a gravitational force and a centrifugal force; and a rotation speed expressed by a centrifugal acceleration and the gravitational force.11 . A process of sintering comprising: attaching a cylindrical frame to a rotator wherein the cylindrical frame comprises a central axis and an internal cavity further comprising an interior surface; attaching a part to the interior surface of the cylindrical frame’s internal cavity; heating the internal cavity of the cylindrical frame such that the internal cavity has a consistent temperature; rotating the cylindrical frame about its central axis; and controlling the rotation of the cylindrical frame such that the rotation imparts a centrifugal force on the part.

12. The process of claim 11 , further comprising modelling a desired final shape of the part to determine a rotation parameters comprising:determining a desired final geometry of a finished part; determining a sintering data for the finished part and the desired geometry; determining an external load during sintering as expressed by a gravitational force and a centrifugal force; and determining a rotation speed during sintering as expressed by a centrifugal acceleration and the gravitational force.

13. The process of claim 11 , wherein the attaching a part to the interior surface of the cylindrical frame’s internal cavity comprises clamping the part at a fixture point.

14. The process of claim 11 , wherein the rotation of the cylindrical frame comprises rotating rollers in connection with an external surface of the cylindrical frame.

15. The process of claim 11 , further comprising forming the part to be sintered comprising additive manufacturing processes.

16. The process of claim 11 , wherein the heating the internal cavity of the cylindrical frame comprises heating the internal cavity of the cylindrical frame to a uniform temperature.

17. The process of claim 11 , wherein the rotation of the cylindrical frame about the central axis also rotates the part about the central axis of the cylindrical frame.

18. The process of claim 11 , wherein the rotation imparts a pseudo-gravitational force.

19. The process of claim 18, wherein the pseudo-gravitational force comprises a negative force such that the part does not experience slumping due to gravity.

20. The process of claim 18, wherein the pseudo-gravitational force comprises a positive force such that the part experiences grain compression and densification.

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