System and method for making a metal object via slurry deposition

Abstract

A metal object printing system includes a nozzle, one or more motors configured to move the nozzle relative to a platform on which the metal object is to be printed, a pump in fluid communication with the nozzle and configured to discharge a metal slurry' through the nozzle, a laser dryer configured to emit a laser beam toward the platform, and a controller operatively connected to the one or more motors, the pump, and the laser dryer and configured to, in accordance with specifications provided from a file for printing the metal object: operate the one or more motors to move the nozzle relative to the platform, operate the pump to discharge the metal slurry' through the nozzle during the relative movement of the nozzle and the platform, and operate the laser dryer to deliver the laser beam toward the metal slurry deposited on the platform.

Description

TITLESystem and Method for Making a Metal Object Via Slurry DepositionBACKGROUND

[0001] Embodiments described herein relate generally to a three-dimensional printing system, and more particularly, to a three-dimensional printing system for high-speed printing of a metal object having high structural integrity.

[0002] The process of building a three-dimensional (3D) object from a computer-aided design (CAD) model or digital 3D model is known as 3D printing or additive manufacturing (AM). 3D printing can be performed via various procedures in which materials are combined, connected, or hardened under the control of a computer, usually layer by layer (such as by fusing plastics, liquids, or powder grains). One of the main advantages of 3D printing is its ability to produce extremely complex structures or geometrical figures, including hollow parts or portions with interior truss structures to reduce weight.

[0003] For printing metal objects, laser bed fusion metal printers are currently the mainstream methodology. However, these types of printers are prohibitively expensive for small and even some medium sized companies.

[0004] A different type of 3D printing process called robocasting is described in U.S. Patent No. 6,027,326. A robocasting system for freeforming a sample includes a moveable platform and a material depositing device having a nozzle through which material is controllably deposited. The depositing device is controllably moved with respect to a platform in three dimensions and may be a syringe used to deposit viscous slurry through the nozzle to form the object on the platform. Material preferably flows rapidly from the nozzle for even distribution and rapid solidification upon striking a surface. As multiple layers are applied, they must flow together sufficiently to form a unitary' structure, but not so much as to distort the shape of the structure. The deposited structure should be hardened relatively quickly into a final form that is free of voids, otherwise lower layers of the printed model will begin to sag under the weight of the upper layers.

[0005] The proposed slurry’ composition includes a mixture of a volatile solvent, particles of material which are insoluble in the solvent, and a small amount of non-volatile organic binder and / or dispersant. The volume percentage of particles in the liquid is sufficiently7high that the slurry' is pseudoplastic. Although it is still capable of flow through a nozzle, it becomes hardupon deposition and rapid partial drying of the volatile solvent, which causes the volume percentage of particles to increase and the material to be dilatant. According to U.S. Patent No. 6,027,326, the bead of slurry7at room temperature was deposited at a printing speed of only 5-12 mm / sec onto a heated platform, which is used to evaporate the volatile water to change the slurry7to the dilatant stage, where the part was solid, but not dry. It has been found that for deposition speeds greater than 5 mm / sec, the platform heating does not provide adequate energy to evaporate the solvent, resulting in distorted object parts due to sagging of the material. Since higher printing speeds increase productivity, the limits imposed by the platform heating process prevent this type of printing system from being practical for high throughput operation.

[0006] The slurry viscosity7must also be tailored during processing for optimal performance. During dispensing, the slurry will experience high shear conditions while flowing through the nozzle and as the moving platform interacts with the dispensing slurry7. However, once the slurry7is clear of the nozzle, the slurry experiences a shear rate near zero. Therefore, in order to control the shape of dispensed beads, the slurry rheology must be extremely pseudoplastic so that the material can flow smoothly during dispensing but then solidify in place once shear stresses are removed (similar to paint). If the slurry is too fluid, beads will spread uncontrollably. If the slurry is too viscous, beads lay down like rope and maintain rounded tops. When the proper rheology is obtained, beads yield nearly rectangular cross sections with relatively straight walls and flattened tops. Another potential issue is separation of solvent from the slurry under high shear stress. The high stress will occur when highly viscous paste is being pushed from a cartridge through a flexible hose or syringe pump because significant force must be applied.

[0007] Another 3D printing technique is clay paste printing, which ty pically utilizes a fused deposition modeling (FDM) delta printer in conjunction with a clay extruder. An example of this type of printer includes a print head and paste delivery system. The print head may utilize a screw pump with a 1-2 mm inner diameter (ID) nozzle attached to the outlet of the screw pump, which may be driven by a geared stepper motor controlled by the printer control board. The clay paste is loaded in a cartridge that is connected to the print head via a flexible hose. The clay paste from the cartridge is fed into the flexible tubing either by applying compressed air or some mechanical force to the piston in the cartridge.

[0008] It is desirable to provide a 3D metal printing system that is capable of fast printing speeds while providing good structural integrity of the printed object.BRIEF SUMMARY

[0009] Briefly stated, one embodiment comprises a metal object printing system including a nozzle, one or more motors configured to move the nozzle relative to a platform on which the metal object is to be printed, a pump in fluid communication with the nozzle and configured to discharge a metal slurry through the nozzle, a laser dryer configured to emit a laser beam toward the platform, and a controller operatively connected to the one or more motors, the pump, and the laser dryer and configured to, in accordance with specifications provided from a file for printing the metal object: operate the one or more motors to move the nozzle relative to the platform, operate the pump to discharge the metal slurry through the nozzle during the relative movement of the nozzle and the platform, and operate the laser dryer to deliver the laser beam toward the metal slurry deposited on the platform.

[0010] In one aspect, at least a portion of the laser dryer is positioned proximate the nozzle for relative movement therewith with respect to the platform. In a further aspect, the controller is configured to operate the laser dryer to deliver the laser beam toward the metal slurry during the relative movement of the nozzle and the platform. In a still further aspect, the laser dry er includes a laser source connected to a beam expander, and the laser beam is emitted from the beam expander. In a still further aspect, the beam expander is positioned proximate the nozzle and the beam expander is configured to move relative to the laser source.

[0011] In another aspect, the controller is configured to control an energy of the laser beam emitted by the laser dryer based on at least one of printing speed, beam size, or composition of the metal slurry.

[0012] In yet another aspect, the laser dryer is configured to output the laser beam with a wavelength between about 450 nm and about 1000 nm.

[0013] In still another aspect, the laser dry er is configured to output the laser beam with a power density' of between about 1 W / cm2and about 500 W / cm2

[0014] In yet another aspect, the system further includes a cartridge enclosure, configured to receive a cartridge housing the metal slurry to be supplied to the nozzle. The cartridge enclosure is in fluid communication with the nozzle by tubing. In a further aspect, the cartridge enclosure is connectable to a pressure source configured to expel the metal slurry from the cartridge into the tubing. In a still further aspect, the controller is operatively connected to the pressure source.

[0015] In still another aspect, the pump is a lobe pump.

[0016] Another embodiment comprises a method of panting a metal object using a system including a nozzle, one or more motors configured to move the nozzle relative to a platform on which the metal object is to be printed, a pump in fluid communication with the nozzle, a laser dryer, and a controller operatively connected to the one or more motors, the pump, and the laser dryer. The method includes, in accordance with specifications provided from a file for printing the metal object and read by the controller: operating, by the controller in accordance with the specifications, the one or more motors to move the nozzle relative to the platform, operating, by the controller in accordance with the specifications, the pump to discharge metal slurry through the nozzle during relative movement of the nozzle and the platform, and operating, by the controller, the laser dryer to deliver a laser beam toward the metal slurry deposited on the platform.

[0017] In one aspect, the controller operates the laser dryer to deliver the laser beam toward the deposited metal slurry during relative movement of the nozzle and the platform. In a further aspect, the method further includes oven drying the metal object. In a still further aspect, the oven drying is performed at a temperature of about 80 °C for between about 1 hour and about 10 hours. In a still further aspect, the method further includes, after the oven dry ing step, performing thermal decomposition on the metal object to remove volatile material. In a still further aspect, the thermal decomposition includes heating the metal object at a rate of about 3 °C per minute up to a temperature of about 550 °C, which is held for about one hour. In a still further aspect, the method further includes, after the thermal decomposition step, sintering the metal object. In a still further aspect, the sintering is performed as a micro wave sintering process. In a still further aspect, the sintering includes heating the metal object at a rate of about 10 °C per minute up to a temperature of about 1250 °C, which is held for about two hours.

[0018] In another aspect, the laser dryer outputs the laser beam with a wavelength between about 450 nm and about 1000 nm.

[0019] In yet another aspect, the laser dry er outputs the laser beam with a power density7of between about 1 W / cm2and about 500 W / cm2.

[0020] Yet another embodiment comprises a method of manufacturing a metal slurry7for use in printing a metal object. The method includes providing, in solution, a powder of metal or metal oxide, forming, by a first spray drying process performed on a first portion of the powder, a first set of particles having a first median particle size, and forming, by a second spray drying process performed on a second portion of the powder, a second set of particles having a second median particle size. A ratio of the second median particle size to the first median particle size isbetween about 1.5 and about 3. The method further includes mixing the first set of particles and the second set of particles with a slu - solvent and a slurry binder. A ratio of the first set of particles to the second set of particles is between about 10: 1 and about 3:2.

[0021] In one aspect, the powder is a metal oxide powder. In a further aspect, the method further includes forming the metal oxide powder by alloying a starting metal powder with an oxide. In a still further aspect, the alloying is performed using an acoustic mixer. In a still further aspect, the alloying is performed by operating the acoustic mixer for four mixing periods each lasting about ten minutes, and providing a break period lasting about five minutes at a conclusion of each of the mixing periods. In a still further aspect, the oxide is Y2O3. In a still further aspect, the starting metal powder is grade 304L stainless steel.

[0022] In another aspect, the first median particle size is between about 10 pm and about 50 pm. In still another aspect, the second median particle size is between about 20 pm and about 100 pm.

[0023] In yet another aspect, the slurry binder is provided in amount between about 1-12% wt.

[0024] In still another aspect, the solution is formed by a combination of a solution solvent and a solution binder. The solution is provided in an amount of between 1-5% wt.

[0025] In yet another aspect, the powder is water-atomized stainless steel grade 316 particles with a median particle size of between about 1 pm and about 50 pm.

[0026] Another embodiment comprises a metal slurry formed by the example methods described above.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0027] The following detailed description of preferred embodiments will be better understood when read in conjunction with the appended drawings. For the purpose of illustration, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

[0028] In the drawings:

[0029] Fig. 1 is a schematic block diagram of a metal object printing system in accordance with an example embodiment;

[0030] Fig. 2 is a side elevational view of an example nozzle, pump, and laser dryer for use in the system of Fig. 1;

[0031] Fig. 3 is a side elevational cross-sectional view of an example cartridge, cartridge enclosure, and tubing for use in the system of Fig. 1 ;

[0032] Fig. 4 is a side elevational cross-sectional view of an example pump for use in the system of Fig. 1;

[0033] Fig. 5 is a flow diagram illustrating an example process for manufacturing slurry for use with the system of Fig. 1;

[0034] Fig. 6 is a flow diagram illustrating an example method of using the system of Fig. 1; and

[0035] Fig. 7 is a schematic block diagram of a portion of a metal object printing system in accordance with a second example embodiment.DETAILED DESCRIPTION

[0036] Certain terminology is used in the following description for convenience only and is not limiting. The words "‘right7’, “left”, “lower”, and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the device and designated parts thereof. The terminology includes the above-listed words, derivatives thereof, and words of similar import. Additionally, the words “a” and “an”, as used in the claims and in the corresponding portions of the specification, mean “at least one.”

[0037] It should also be understood that the terms “about,” “approximately,” “generally,” “substantially” and like terms, used herein when referring to a dimension or characteristic of a component, indicate that the described dimension / characteristic is not a strict boundary or parameter and does not exclude minor variations therefrom that are functionally similar. At a minimum, such references that include a numerical parameter would include variations that, using mathematical and industrial principles accepted in the art (e.g., rounding, measurement or other systematic errors, manufacturing tolerances, etc.), would not vary the least significant digit.

[0038] Referring to the drawings in detail, wherein like reference numerals indicate like elements throughout, Fig. 1 shows an example embodiment of a metal object printing system 10. The system 10 may use a nozzle 12 to build a metal object (not shown) layer by layer on aplatform 14. The nozzle 12 may have an internal diameter of between about 0. 1 mm to about 1 mm, although other diameters may be used based on the specifications of the object to be printed. The system 10 may be provided with a controller 16 that may be programmed to cause movement between the nozzle 12 and the platform 14 relative to one another in the X, Y. and Z directions to allow the building of the metal object. The controller 16 may be a microcontroller unit (MCU), a central processing unit (CPU), a microprocessor, an application specific controller (ASIC), a programmable logic array (PL A), combinations thereof, or the like. The controller 16 may include or be coupled to a memory7(not shown) that may store code or software for carry ing out processes described herein and / or carrying out other operations. The controller 16 mayreceive a 3D printing file (such as STL, OBJ, 3MF, FBX, VRML, AMF, X3G files, or the like) and parse the file in order to maneuver the nozzle 12 and platform 14 with respect to each other to create the object specified by the file. The controller 16 may include other functionality7as well. It should be further appreciated that although controller 16 is referred to in this example as a single component, the controller 16 may include a plurality of individual devices, with control functions divided among the individual devices. The controller 16 may' be wired or wirelessly connected to components of the system 10 necessary' for carry ing out the operations and processes described herein.

[0039] To implement the required relative motion, at least one of the nozzle 12 or the platform 14 may be equipped with one or more motors 18, 20, respectively, which may be operated by the controller 16. For example, the nozzle 12 may be moved by two motors 18, one controlling motion in the X-direction with the other controlling motion in the Y-direction, and the platform 14 may be moved by a motor 20 that controls motion in the Z-direction (e.g., vertical). How'ever, any permutation of motors 18, 20 that enables the nozzle 12 and platform 14 relative to one another may be utilized, including embodiments where one of the nozzle 12 or the platform 14 is entirely stationary (z.e., no motor is required) and the other of the nozzle 12 or the platform 14 is movable in all three directions. The motor(s) 18. 20 may be stepper motors, although other conventional types of motors may be used as well in keeping with the spirit and scope of the invention.

[0040] The nozzle 12 may be supported by a frame 22 of the system 10, which can be any type of supporting structure including, but not limited to, walls, wires, poles, panels, plates, combinations thereof, or the like. The motor(s) 18 may be operatively attached to the nozzle 12 and move the nozzle 12 along one or more structures of the frame 22. Alternatively, the motor(s) 18 may be configured to move one or more components of the frame 22 to cause motion of the nozzle 12 in one or more directions. The frame 22 may also be connected to orcontain the platform 14. In some embodiments, the frame 22 may form a sealable chamber within which the nozzle 12 and the platform 14 may be movable relative to one another. In other embodiments, the frame may allow for the passage of ambient air within the work area containing the nozzle 12 and platform 14.

[0041] The platform 14 may include a heater (not shown) for heating the platform during a printing operation. The heat may provide some assistance in evaporating water from the deposited material, although the main source for this function is preferably a laser dryer 46, which will be described in more detail further below.

[0042] The controller 16 may be in communication with one or more user interfaces 50, which may include a display screen, a keyboard, mouse, touchpad, buttons, dials, touchscreen, dials, combinations thereof, or the like. User interfaces 50 may be attached to the frame 22, other components of the system 10, or may be separately mounted or supported. User interfaces 50 may further be connected to the controller 16 or other components via wires or may operate wirelessly. The controller 16 may also have a communication module (not shown) to allow for external communication, such as over a network (which may be a LAN, WAN, the Internet, cellular network, or other type of wired and / or wireless communication network) to external devices, such as desktops, tablets, mobile devices, laptops, servers, or the like. For example, the communication module may provide or include one or more Ethernet, USB, IEEE 1394 ports, or the like, and / or circuitry for communicating via wireless protocols, such as BLUETOOTH. WIFI, ZIGBEE, Z-WAVE, 3G, 4G. or 5G cellular, infrared, or the like.

[0043] Material may be fed to the nozzle 12 from a cartridge 24. Fig. 3 shows a crosssection of an example cartridge 24, which may take the form of a cylindrical syringe or like type of container. The cartridge 24 may have an opening 26 at one end thereof through which material held within the cartridge 24 may be passed toward the nozzle 12 via a tubing 28 or like fluid connector. The tubing 28 is preferably flexible but may, in some embodiments, be fixed in position. A plunger 30 may be provided within the cartridge 24 and be selectively movable therein for forcing the material through the opening 26. In the example shown in Fig. 3, the cartridge 24 is selectively insertable into a cartridge enclosure 32 that may retain the cartndge 24 in an operative position during use. The cartridge enclosure 32 may attach to or be integrated with the frame 22 or another structural support (not shown) of the system 10, may be freestanding, or the like. The cartridge enclosure 32 may include an outlet 34 that may be aligned with the opening 26 of the cartridge 24 when the cartridge 24 is inserted therein and provide fluid communication between the opening 26 and the tubing 28. In the embodimentshown, the outlet 34 is in the form of a protruding spout to which the tubing 28 may be attached. In the example shown in Fig. 3, an end of the cartridge enclosure 32 opposite to the outlet 34 may be closed using a cap 36, which is shown with a threaded engagement to the cartridge enclosure 32. However, other methods of attaching the cap 36 may be used as well.

[0044] In this configuration, the cartridge enclosure 32 may be a reusable component of the system 10 and receive replaceable cartridges 24. In some other embodiments, the cartridge enclosure 32 and cap 36 may come preassembled with the cartridge 24 and the combined components may be replaceable as a single unit. In still other embodiments, the cartridge enclosure 32 may be omitted altogether and the cartridge 24 itself may be connected to the tubing 28, either directly or indirectly. In some of the embodiments described above, the cartridge 24 may be for a single use and can be discarded or recycled thereafter. In other embodiments, the cartridge 24 may be refillable.

[0045] To move the plunger 30 within the cartridge 24 to expel the material into the tubing 28, the cartridge 24 may be connected to a pressure source 38 (Fig. 1), such as a compressed air supply. In Fig. 3, the cap 36 includes a nipple 40 to which a compressed air supply 38 may be connected such that the compressed air may act upon the plunger 30 during operation. However, in embodiments without a cartridge enclosure 32, the compressed air supply 38 may be connected directly to the cartridge 24 or indirectly through other attachments (not shown). In some embodiments, rather than a compressed air supply, the pressure source 38 may be a mechanical force generator, such as a stepper motor or the like, acting upon a rod (not shown) or the like in contact with the plunger 30. As shown in Fig. 1, the pressure source 38 may be operated by the controller 16, although independent operation of the pressure source 38 may be utilized as well.

[0046] Referring again to Fig. 1, extrusion at the nozzle 12 may be facilitated by a pump 42, w hich may preferably be a lobe pump although other types of pumps w ith similar material handling capabilities may be used as well. Previous printing systems have used syringe or screw pumps, which generate significant shear stress due to the dependence of the extruded material quantity on pump inlet pressure. In contrast, the extrusion volume of a lobe pump 42, such as the one shown in Fig. 4, depends only on the internal volume of the pump cavity 43. Vane pumps are similarly unsuitable for this type of application as their efficiency decreases with higher viscosities and the pumps are prone to damaging wear when used with particle slurries. A lobe pump 42 is therefore more suitable to operate with the type of slurry used to print metal objects and to handle the material viscosities that allow for discharge without utilizing pressuresthat render the slurries nonuniform. The lobes 44 of the pump cause a specific amount of material to discharge through each revolution, allowing the output of the pump 42 to be mechanically varied by varying the speed of a motor (not show n) of the pump 42. A stepper motor may be used to operate the pump 42, although other types of motors may be used as well. To increase motor torque, a gear reducer (not shown) may be used, preferably made of plastic to minimize w eight. It is preferred to use a lightweight motor and pump 42 to allow? for faster printing speeds. The pump 42 and / or its motor may be connected to the controller 16 for operation thereof. The tubing 28 may connect to the pump 42 either directly or indirectly via a suction port 45. Although Fig. 4 shows the pump 42 discharging directly to the nozzle 12, other indirect arrangements may be used as well. Another type of pump that can be used is a progressive cavity pump, known as Moineau pump. Similar to lobe pumps, the extrusion volume of the progressive cavity pump depends only on revolution of the pump stator and can be used for pumping sludges and heavy pastes.

[0047] Referring again to Fig. 1, to enable faster printing speeds, a laser dryer 46 may be included as part of the system 10 and at least a portion thereof may be installed near to the nozzle 12 for travel therewith. Fig. 2 shows one example where the laser dry' er 46 includes a laser source 47, which may be a fiber laser, although other types of solid state lasers, gas lasers, or the like may be used as well. The laser source 47 may connect to a beam expander 48, shown in Fig. 2 as mounted to the pump 42 housing proximate the nozzle 12, using a fiber optic cable 49. The laser source 47 may be stationary, either freestanding or attached to the frame 22 or other supporting surface of the system 10, and the beam expander 48 may deliver the laser beam (with or without further conditioning, as necessary) toward deposited material as the beam expander 48 travels with the nozzle 12 during operation. However, in other embodiments, all necessary' components of the laser dry er 46 may be attached to or mounted proximate the pump 42 and / or nozzle 12 for travel therewith.

[0048] In alternative embodiments, such as the one shown in Fig. 7, the laser dryer 46' may be stationarily mounted relative to the nozzle 12' (as w ell as pump 42' and optional motor 18'). For example, in Fig. 7, the laser dryer 46' is mounted to the frame 22'. In such embodiments, the laser dryer 46' may be used to dry large sections of material deposited on the platform 14' at once. For example, after the nozzle 12' has deposited a layer of material, the laser dryer 46' may illuminate the entire layer (with the nozzle 12' and pump 42' moved to prevent beam obstruction, if necessary) before the nozzle 12' begins to deposit the next layer. For particularly large layers, the nozzle 12' may pause to allow for a laser drying cycle before continuing to print the layer. In some embodiments, the motor(s) 20' for the platform 14' may periodically move the platform 14'to the laser dryer 46' to perform a drying cycle. The upshot is there are multiple ways to implement laser drying during the printing process to provide the advantages described herein.

[0049] The laser source 47 may operate in a wavelength range between about 450 nm to about 1000 nm. The power output of the laser source 47 may be in the range of about 250 W to about 3000 W. The spot size of the beam may be between about 10 mm to about 20 mm, although other spot sizes may be used as well. For example, when the laser beam is stationary and configured to illuminate the entire printed layer, such as in the embodiment of Fig. 7, the spot size may be a rectangular area with a size of 200x250 mm, or the like. Spot size should be chosen to minimize areas of the beam that fall on locations other than the material to be heated (e.g., the platform or the like) so as not to waste energy. The controller 16 may be operatively connected to the laser dryer 46 to control the energy delivery, which can be based on printing speed, beam size, material composition, and / or other parameters. In some embodiments, for example, the power density of the beam may be between about 1 W / cm2to about 500 W / cm2Using the laser dryer 46 enables focused and controlled drying of the material as it is deposited to change the material to a dilatant state.

[0050] It is preferred to minimize a weight of the components travelling for extrusion (e.g. , pump 42, laser dryer 46, any motors, etc.) in order to achieve high printing speeds. In one embodiment, the pump 42 alone weighs less than 150 g, and the overall weight of components travelling with the pump 42, including the beam expander 48 of the laser dryer 46, weighs less than 500 g. This enables printing speeds of up to about 150 mm / sec.

[0051] In some embodiments, it may be necessary' to print removable support structures under certain sections of an object, such as beneath cantilevers or bridges, where the deposited slurry does not have the platform 14 or a previously deposited layer of the object to rest upon. In one embodiment, for example, a second extruder (not shown) may be provided that may move relative to the platform 14 along with the nozzle 12 under control of the motor(s) 18, 20 and utilize the laser dryer 46. Alternatively, such a second extruder may be equipped for independent movement relative to the platform 14 and / or utilize its own laser dryer (not shown). In some embodiments, the second extruder may be removable from the system 10 when support structures are not necessary' for printing an object, but in some embodiments the second extruder may not be removable but simply remains dormant for printing such objects. The second extruder may build up support structures layer by layer intermittently with printing of the object by the nozzle 12. although in some circumstances, support structures may be deposited on theplatform 14 ahead of object printing, as desired. Support structures may be removed from the object upon completion of printing.

[0052] The second extruder may be fed a paste containing preceramic particles, such as clay, alumina, or the like from a separate cartridge (not shown) or other feeding system operated by the controller 16. In an alternative embodiment, the second extruder may be an FDM extruder that prints the support structures from a thermoplastic FDM filament. This embodiment has the advantage that FDM extruders are much cheaper than the paste extruder described above and can be very light, which enables keeping the component weight low for fast printing speeds, even with the need for printing temporary support structures.

[0053] The composition of the material also affects fast printing of strong and undistorted objects. It is preferred to use a material which includes fine metal or metal oxide powders in a water-based slurry containing a small amount of organic binder. The viscosity of the material depends on the percentage of solids, their morphology, and particle size distribution of the powder. A high solid content is desirable to reduce dry ing time, shrinkage, and porosity7. Longer drying times cause slower print speeds and more shrinkage. Higher porosity of the material results in longer sintering times and lower part density, negatively impacting the object’s tensile strength. In a comparative example, a stainless steel SS316 slurry with D50 = 40 gm and 10% liquid content requires nearly 300 psi of air pressure to be moved through a 1 meter long tubing having a 10 cm ID. These levels of pressure can result in the separation of the solvent and powder, which causes viscosity changes as well as variable binder content, ultimately producing non-homogenous objects with variable density. Finer particles are desirable to achieve high sintering densities but can also excessively increase the viscosity. Particles with less than a 5 pm diameter are also difficult to work with due to agglomeration. It is preferred to use a material that includes a bi-modal particle distribution where smaller particles fill gaps between larger particles.

[0054] An example method 500 of manufacturing a slurry for use in metal 3D printing will now be described with reference to Fig. 5. At step 502, a metal powder or metal oxide powder may be provided in solution from which particles of two different sizes will be produced. In one example embodiment, a metal powder may be provided as water-atomized grade 316 stainless steel with particles having a D50 between about 0.05 pm and about 10 pm, and the solution may be a combination of a solvent (e.g., water or alcohol or the like) and a binder (such as sodium silicate (Na2SiOs), polyvinylpyrrolidone (PVP), polyvinyl butyral (PVB). or the like, although other binders, such as those described in detail below for the finished material contained withinthe cartridge, may be used as well) in an amount of between about 1-5% wt. A metal powder dispersant may also be included to reduce agglomeration. Such dispersants may be, for example, anionic, cationic, or non-ionic surfactants, polymeric (e.g., polyacrylic, polyphosphonate, or the like) dispersants, steric acid dispersants, or the like. However, other starting powders, sizes, and solutions may be used as well depending on the materials of construction of the part to be printed.

[0055] At step 504, a set of first particles may be produced by spray drying from a first portion of the metal or metal oxide powder in solution. For example, the spray dryer may be a Mini Spray Dryer B-290 available from BUCHI Corporation. The first particles may have a D50=DI, where Di is preferably between about 10 pm and about 50 pm. However, other diameters may be used instead, depending upon particular characteristics of the system 10, including tubing 28 length and / or diameter, the pump 42 flow rate, the type of pressure source 38, and the like. At step 506, a set of second particles may be produced by spray drying from a second portion of the metal or metal oxide powder in solution. The second particles may have a DSO=D2, where a ratio of D2 / D1 is preferably about 2. In this example, D2 is preferably between about 20 pm and about 100 pm, depending upon the corresponding Di value. The second particles may be formed larger by starting from larger particles from the original powder range than for the first particles, for example. Notably, the spray drying process described herein may increase a particle’s starting diameter size by about 20X maximum, although variations are also possible. While the preferred D2 / D1 ratio is given above as about 2. similar ratios may be used, such as between about 1.5 and about 3, depending on the desired characteristics of the slurry. Spray drying to obtain the particles for the slurry significantly improves the flowability of the particles while maintaining low viscosity of the slurry overall. The spray drying also enables the use of irregular particle shapes instead of expensive, engineered spherical powders.

[0056] It is possible that during the drying process of printing, the escape of binder and / or solvent may increase porosity, which may increase the surface area of the part and lowers oxidation and corrosion resistance. Therefore, in some embodiments, it may be advantageous to combine oxides, particularly rare earth oxides, with the metal material, which can strengthen the final printed object, enhance oxidation and corrosion resistance, and improve operating temperature range. This technique may be particularly applicable to Ni-based and Fe-based alloys, although it may be employed with other types of metals as well. Examples of rare earth oxides which may be added include Y2O3, La2C>3, Ce2C>3, and the like. The rare earth oxide may be dispersed as particles in the range of about 100 nm to about 5 gm in size in the solvent with the metal particles to be spray dried in an amount of about 0.5% to about 3.0% wt. (preferablybetween about 0.75% and about 1.5% wt.). The rare earth oxide may coat the metal particles in the spray drying process, providing an even distribution of the oxide particles on the surfaces of the metal particles. Prior techniques for combining oxides with metal alloys involved high energy mill coating, which is an expensive undertaking and does not provide the type of even distribution obtained using the above-described spray-drying technique.

[0057] At step 508, the first and second particles may be mixed together with a solvent and a binder to produce a metal paste. The solvent used for making the paste should be different than the solvent used to form the particles - if the same solvent is used for both steps, the solvent for the paste may dissolve the particles. Thus, the binder used earlier in the process should be insoluble in the solvent used to make the paste. For example, if water is the solvent used earlier in the process, then alcohol should be used in step 508 as the solvent. In this example, an alcohol-soluble binder, such as PVB or the like, may then be used as the binder in step 508. In another example, if alcohol is the solvent used earlier, then water should be used in step 508 as the solvent, and the binder may be a water-soluble binder, such as PVP, sodium silicate, or the like. However, other types of solvents and binders may be used as well. A ratio of first particles to second particles may be between about 10: 1 to about 3:2. The binder may be between about 1-12% wt., preferably between about 1-4% wt. The mixing may be performed in a planetary vacuum mixer, as this type of mixer will advantageously produce the slurry free of air, although other types of mixers may be used as well.

[0058] In some embodiments, prior to step 502 of placing the powder in solution, a step 501 may be performed wherein a starting metal powder may be alloyed with an oxide material for oxide dispersion strengthening. As one example, 100 g of starting metal powder, such as grade 304L stainless steel 304L powder, may be placed in a container for an acoustic mixer, such as the LabRAM II available from RESODYN ACOUSTIC MIXERS, INC. Approximately 1% (in this case equal to 1g) of oxide, such as Y2O3 or the like, may also placed in the container. Although specific amounts and materials are recited, other amounts and / or materials may be utilized, as desired. The container may be filled with argon, helium, hydrogen, or a like gas to protect the starting metal powder from oxidation.

[0059] It is found that best alloying results from milling the materials for a total time of about one hour but performed in approximately ten minute mixing interv als with breaks lasting about five minute in between to cool the mixture. Mixing for periods longer than about ten minutes can lead to overheating and melting of the container. Shorter mixing times may lead to incomplete welding of the oxide to the stainless steel powder. Overall mixing times of longerthan about forb' minutes do not appear to improve the properties of the final product. In cases where two sizes of the powder are used to obtain better packing results, both sizes may be alloyed together in the manner described above. The resulting metal oxide powder may be placed in the solution at step 502, and the remainder of the process described above may be performed to obtain the slurry.

[0060] While described above and in connection with Fig. 5 as producing two separate spray-dried particle groups that are subsequently mixed together, variations to the method of manufacturing the slurry can be made in keeping with the invention. For example, prior to spray drying, powders of two different sizes in appropriate ratios may be mixed together with a solvent (e.g., water or the like), a binder (e.g., PVP, PVB, or the like), and optionally an oxide in, if desired. The spray drying process may then be implemented on the powder mixture to produce the first and second particles in the appropriate D2 / D1 ratio via the spray-drying process. The resulting spray-dried particles may then be mixed with water and the slurry binder (e.g., dextrose, glucose, cellulose, starch, or the like) in the vacuum mixer.

[0061] In some embodiments where a lower density of the object to be printed is acceptable, powders with a single size (as opposed to bi-modal in the examples described above) may be spray-dried and mixed with the solvent and the binder (and optionally an oxide, if desired) to produce the slurry. This procedure provides the benefit of reduced cost, although, again, with the trade-off of lower density in the finished, printed object.

[0062] Although the preferred method of preparation for the slurry involves spray drying, as described above, other methods may be used as well to create a paste or slurry compatible with the system 10 described herein. For example, the process may include mixing a metal or metal oxide powder in spherical particle form (with single or bi-modal particle sizes between about 5 pm and about 100 pm) with a solvent (e.g., water or the like, in an amount of between about 35% vol. to about 45% vol.), a binder (e.g., PVP, PVB, dextrose, glucose, cellulose, starch, or the like, in an amount between about 3% and about 10% by weight of the solvent), and an oxide (e.g., dispersed as particles in the range of about 100 nm to about 5 pm in size, in an amount between about 0.5% to about 3.0% wt., preferably between about 0.75% and about 1.5% wt.) in a vacuum mixer or other type of mixer.

[0063] Referring now to Fig. 6, an example method 600 for 3D printing a metal object using the system 10 described above is shown. A metal slurry, such as the one described above, maybe present in the cartridge 24, which may be installed for connection with the nozzle 12. The controller 16 may receive a file for printing the object, such as through the user interface 50, overa network, retrieval from memory, or the like. The controller 16 therefore operates motor(s) 18, 20 to provide relative movement between the nozzle 12 and the platform 14 in accordance with the file specifications. At step 602, the controller 16 will cause the slurry to be deposited from the nozzle 12 onto the platform 14 during the relative movement. In this example, the deposition may occur in ambient atmosphere, although in some embodiments where the frame 22 creates a chamber for the printing process, other types of atmospheres, including vacuum, may be present. The platform may be heated to a temperature of between about 40 °C and about 80 °C.

[0064] As the nozzle 12 deposits material on the platform 14, at step 604. the controller 16 will cause the laser dryer 46 to illuminate the deposited material during relative motion. The light from the laser dry er 46 may provide sufficient evaporation of the volatile water to change the material to a dilatant stage, where the deposited material is solid, but not dry'. As a result, the dilatant material can withstand the weight of subsequently applied layers of material without sagging and while maintaining straight walls and flat tops. The solidification of the deposited material through the contemporaneous illumination by the laser dryer 46 further enables the relative movement of the nozzle 12 and the platform 14 to be significantly increased in speed, allowing faster printing. In this particular example, the print speed may be about 50 mm / sec, although, as mentioned above, print speeds as fast as about 150 mm / sec may be achievable using the invention described herein.

[0065] After material deposition and solidification in the system 10 is complete, the object may, at step 606, be oven dried to evaporate any remaining water. In this example, the object may be dried at a temperature of about 80 °C for between about 1 hour to about 10 hours. However, other temperatures and times may be utilized, as needed for evaporating the remaining water. In addition, other drying methods may be used as well. At step 608, volatile material (e.g., any binder or solvent) may be removed from the object by thermal decomposition. In this example, the object may be heated at a rate of about 3 °C per minute up to a temperature of about 550 °C, which may be held for about one hour, although other times and temperatures may be utilized depending on the nature and amounts of the volatile material to be removed, the object material, and the like. At step 610, the object may then be sintered by heating at a rate of about 10 °C per minute up to a temperature of about 1250 °C, which may be held for about two hours, although other times and temperatures for sintering may be utilized, as needed. Inert gases, such as nitrogen or argon, should be used to prevent oxidation, and in some instances it is desirable to add hydrogen to inert gas to create a reducing atmosphere in the furnace.

[0066] In general, powder sintering is a thermal treatment process that binds metal powder particles together to create a solid, dense piece. The process involves heating the powder to just below its melting point, which allows the particles to fuse together without melting. Sintering may be performed in an inert atmosphere (such as nitrogen, argon, or the like), pure hydrogen or a reforming gas (e.g.. a mixture of H2 and an inert gas). Both inductive and resistance heaters or furnaces may be used for the powder sintering process described herein; these devices are well known in the art and the specific parameters used for the sintering may be determined by routine experimentation. However, to further reduce manufacturing cost and improve densification, it is preferable to use a microwave oven for sintering. In contrast to traditional heating methods, microwave sintering of powders offers certain advantages, such as energy efficiency, environmental friendliness, higher densification, and a fine grain size due to the faster heating rates and the lower sintering temperature. (See, for example, Janney et al., Ceramic Powder Science II, 21(18), 919-924 (1988)); Matli et al., Metals, 6(7). 143 (2016). Additionally, uniform volumetric heating and smaller and equiaxed pores in the sintered green compacts and fine microstructure are added advantages of microwave sintering. According to Rajkumar et al., (J. Mater. Proc. Technol., 209, 5601-5605 (2009), microwave sintering should result in improved mechanical properties and better product performance.

[0067] The basic difference between traditional and microwave sintering processes lies in the heating mechanism. (See, e.g., Agarwal, Trans. Ind. Ceram. Soc., 65, 129-144 (2006). In conventional heating, the specimen (i.e., particle) is heated using heating elements like silicon rods. The heat is then transferred to the specimen from the outer to the inner surface of the specimen either by conduction, convection or radiation. This poses issues of non-uniform heating, thereby creating thermal gradients, which in turn results in internal stresses in the specimen.

[0068] In contrast, in a microw ave sintering process, the heat transfers from the inner to the outer surface of the specimen. The most recent development in microw ave applications is in sintering of metal pow ders, a surprising application, in view of the fact that bulk metals reflect microwaves. However, reflection by a metal occurs only when it is in a solid, nonporous form and is exposed to microwaves at room temperature. Metals in the form of powder will absorb microwaves at room temperature and will be heated very effectively and rapidly. Because of the different heating mechanism, microwave sintering promises a more efficient (as high as about 80-90% volumetric heat generation efficiency) and economic densification approach with energy savings of about 90% over conventional sintering. (See https : / / onlinelibrary . wiley . com / doi / epdf / 10. 1002 / adem.202302065).

[0069] In the micro wave sintering process, 2.45 GHz and 915 MHz are the commonly used microwave frequencies, with a suitable frequency range being about 2.45 GHZ and 890- 950 MHz. The specimen is usually housed in an insulated alumina chamber or tube. The insulation of the chamber ensures that the heat generated within the specimen does not escape and is effectively utilized. The temperature generated in the specimen can be measured using, e.g., an infrared (IR) sensor, optical pyrometer, and / or sheathed thermocouple, which is placed close enough to the surface of the specimen. The sintering chamber is appropriately equipped to provide the desired inert atmosphere by passing an inert gas, such as Ar or N2, and is capable of achieving temperatures up to 1600 °C and is typically operated at a temperature range of about 200 °C to about 1600 °C. The microwave sintering apparatus or furnace normally includes four parts, namely: (a) a microwave generator or magnetron; (b) a susceptor; (c) a heating system, and (d) a programmable controller.

[0070] Another advantage for microwave sintering is it allows simultaneous reduction of sintering temperature and time, which results in less grain growth, which is desirable to obtain parts with better mechanical properties.

[0071] Those skilled in the art will recognize that boundaries between the above-described operations are merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Further, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

[0072] While specific and distinct embodiments have been shown in the drawings, various individual elements or combinations of elements from the different embodiments may be combined with one another while in keeping with the spirit and scope of the invention. Thus, an individual feature described herein only with respect to one embodiment should not be construed as being incompatible with other embodiments described herein or otherwise encompassed by the invention.

[0073] It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined herein.

Claims

CLAIMSWe claim:

1. A metal obj ect printing system comprising: a nozzle; one or more motors configured to move the nozzle relative to a platform on which the metal object is to be printed; a pump in fluid communication with the nozzle and configured to discharge a metal slurry through the nozzle; a laser dryer configured to emit a laser beam toward the platform; and a controller operatively connected to the one or more motors, the pump, and the laser dryer and configured to, in accordance with specifications provided from a file for printing the metal object: operate the one or more motors to move the nozzle relative to the platform, operate the pump to discharge the metal slurry through the nozzle during the relative movement of the nozzle and the platform, and operate the laser dry er to deliver the laser beam toward the metal slurry deposited on the platform.

2. The system of claim 1, wherein at least a portion of the laser dry' er is positioned proximate the nozzle for relative movement therewith with respect to the platform.

3. The system of claim 2, wherein the controller is configured to operate the laser dryer to deliver the laser beam toward the metal slurry' during the relative movement of the nozzle and the platform.

4. The system of any of claims 1-3, wherein the laser dryer includes a laser source connected to a beam expander, the laser beam being emitted from the beam expander.

5. The system of claim 4, wherein the beam expander is positioned proximate the nozzle and the beam expander is configured to move relative to the laser source.

6. The system of any of the preceding claims, wherein the controller is configured to control an energy of the laser beam emitted by the laser dryer based on at least one of printing speed, beam size, or composition of the metal slurry.

7. The system of any of the preceding claims, wherein the laser dryer is configured to output the laser beam with a wavelength between about 450 nm and about 1000 nm.

8. The system of any of the preceding claims, wherein the laser dryer is configured to output the laser beam with a power density of between about 1 W / cm2and about 500 W / cm2.

9. The system of any of the preceding claims, further comprising a cartridge enclosure, configured to receive a cartridge housing the metal slurry’ to be supplied to the nozzle, the cartridge enclosure being in fluid communication with the nozzle by tubing.

10. The system of claim 9, wherein the cartridge enclosure is connectable to a pressure source configured to expel the metal slurry from the cartridge into the tubing.11 . The system of claim 10, wherein the controller is operatively connected to the pressure source.

12. The system of any of the preceding claims, wherein the pump is a lobe pump.

13. A method of printing a metal object using a system including a nozzle, one or more motors configured to move the nozzle relative to a platform on which the metal object is to be printed, a pump in fluid communication with the nozzle, a laser dry er, and a controller operatively connected to the one or more motors, the pump, and the laser dryer, the method comprising, in accordance with specifications provided from a file for printing the metal object and read by the controller: operating, by the controller in accordance with the specifications, the one or more motors to move the nozzle relative to the platform; operating, by the controller in accordance with the specifications, the pump to discharge metal slurry' through the nozzle during relative movement of the nozzle and the platform; and operating, by the controller, the laser dryer to deliver a laser beam toward the metal slurry' deposited on the platform.

14. The method of claim 13, wherein the controller operates the laser dryer to deliver the laser beam tow ard the deposited metal slurry' during relative movement of the nozzle and the platform.

15. The method of claim 13 or 14, further comprising oven drying the metal object.

16. The method of claim 15, wherein the oven drying is performed at a temperature of about 80 °C for between about 1 hour and about 10 hours.

17. The method of claim 15 or 16, further comprising, after the oven drying step, performing thermal decomposition on the metal object to remove volatile material.

18. The method of claim 17, wherein the thermal decomposition includes heating the metal object at a rate of about 3 °C per minute up to a temperature of about 550 °C, which is held for about one hour.

19. The method of claim 17 or 18, further comprising, after the thermal decomposition step, sintering the metal object.

20. The method of claim 19, wherein the sintering is performed as a micro wave sintering process.

21. The method of claim 19, wherein the sintering includes heating the metal object at a rate of about 10 °C per minute up to a temperature of about 1250 °C, which is held for about two hours.

22. The method of any of claims 13-21, wherein the laser dryer outputs the laser beam with a wavelength between about 450 nm and about 1000 nm.

23. The method of any of claims 13-22, wherein the laser dryer outputs the laser beam with a power density of between about 1 W / cm2and about 500 W / cm224. A method of manufacturing a metal slurry7for use in printing a metal object, the method comprising: providing, in solution, a powder of metal or metal oxide; forming, by7a first spray drying process performed on a first portion of the powder, a first set of particles having a first median particle size;forming, by a second spray drying process performed on a second portion of the powder, a second set of particles having a second median particle size, a ratio of the second median particle size to the first median particle size being between about 1.5 and about 3; and mixing the first set of particles and the second set of particles with a slurry' solvent and a slurry binder, a ratio of the first set of particles to the second set of particles being between about 10: 1 and about 3:2.

25. The method of claim 24, wherein the powder is a metal oxide powder.

26. The method of claim 25, further comprising forming the metal oxide powder by alloying a starting metal powder with an oxide.

27. The method of claim 26, wherein the alloying is performed using an acoustic mixer.

28. The method of claim 27, wherein the alloying is performed by operating the acoustic mixer for four mixing periods each lasting about ten minutes, and providing a break period lasting about five minutes at a conclusion of each of the mixing periods.

29. The method of any of claims 26-28, wherein the oxide is Y2O3.

30. The method of any of claims 26-29, yvherein the starting metal powder is grade 304L stainless steel.

31. The method of any of claims 24-30, yvherein the first median particle size is between about 10 pm and about 50 pm.

32. The method of any of claims 24-31, wherein the second median particle size is between about 20 pm and about 100 pm.

33. The method of any of claims 24-32, yvherein the slurry binder is provided in amount between about 1-12% wt.

34. The method of any of claims 24-33, wherein the solution is formed by a combination of a solution solvent and a solution binder, the solution being provided in an amount of between 1-5% wt.

35. The method of any of claims 24-34, wherein the powder is water-atomized stainless steel grade 316 particles with a median particle size of between about 1 pm and about 50 pm.

36. A metal slurry formed by the method of any of claims 24-35.

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