Patent Application
20220305559
The invention relates to generating molten droplets from a moving feedstock.
Traditional printing methods can be limited by the material being printed. Moreover, three-dimensional printing techniques can lead to inaccurate distributions of solid materials on a substrate due to physical limitations of the printing method.
In one aspect, a method of generating individual molten droplets from a feed material. The method can include providing a feed material from a feed mechanism, and directing an energy source at or near an end of the feed material to form a liquified region of the feed material to make individual molten droplets. The method can include feeding the feed material at a rate sufficient to break the liquified region into individual droplets. The method can include altering the trajectory of the single droplet with a deflector. The method can include positioning droplets to impinge a target area of a substrate.
In another aspect, a device can include a feed mechanism that advances a feed material at a controlled speed or maintains a desired position of an end of the feed material, an alignment mechanism that determines trajectory and position of the feed material, and an energy source directed toward the end of the feed material to generate molten droplets. The device can include a deflector to modify the trajectory of the molten droplets.
In another aspect, a device can include a printing unit including a feed material feeder, an energy source directed at or near a tip of a feed material passing through the feed material feeder to generate a molten droplet that exits the printing unit, and a stage opposite the printing unit that receives the molten metal droplet to build a part or create a pattern. The feed material can be a wire or ribbon. The feed material can be a metal, an alloy, a composite, a plastic, a rubber, a ceramic, a glass or other material. Preferably, the feed material can be a metal wire.
In another aspect, a method of manufacturing a part can include generating a continuous stream of molten droplets from a feed material without physically contacting a tip of the feed material, while applying energy from an energy source, and depositing the molten droplet on a surface to form a pattern or part. The method can include supplying the feed material at a rate sufficient to break up a molten column of the feed material into a stream of individual droplets. The molten droplet can solidify once delivered to the surface. The solidification can be delayed by applying energy at the time of impact or bonding with the surface can be improved by applying energy at the time of impact.
In another aspect, a method of fabricating a metallic feature on a surface can include generating individual molten droplets, as described herein. The molten droplets can travel through a fluid medium after detaching from the feed material and prior to impacting the surface.
In another aspect, a method of forming a three-dimensional object can include generating individual molten droplets, as described herein. The molten droplets can travel through a fluid medium after detaching from the feed material and prior to impacting a surface to form a portion of the three-dimensional object.
In certain circumstances, the method can include applying multiple energy sources to the moving feed material, so as to control the temperature of the feed material along its length and influence the formation of droplets.
In certain circumstances, the method can include generating a single droplet traveling with a trajectory away from the feed mechanism.
In certain circumstances, sequentially produced molten droplets can be selected to be uniform in size or different in size.
In certain circumstances, the molten droplets can be generated in a controlled environment.
In certain circumstances, the method can include guiding the feed material through an alignment mechanism immediately before directing the energy source to the end of the feed material.
In certain circumstances, sequentially produced molten droplets can have a diameter that is larger than, equal to, or smaller than a diameter of the feed material.
In certain circumstances, the part or pattern can include a metal, ceramic or polymer.
In certain circumstances, the energy source can include an electromagnetic source, a plasma source, an electron beam source, a joule heating source, or an induction source, for example, a laser.
In certain circumstances, the energy source can be constant, modulated, or pulsed, or combinations thereof.
In certain circumstances, the device or method can include at least one droplet deflector in the flight path of the droplet. The deflector can be near an end of the feed material. The deflector can be an electric field, a magnetic field, a vapor propulsion wave or a plasma shock wave.
In certain circumstances, the deflector can include a trajectory modification by electric field deflection, magnetic field deflection, plasma shock wave deflection, vapor propulsion deflection, acoustic or acoustophoretic deflection, gas flow deflection, mechanical deflection, or a combination thereof. For example, the deflector can include a deflection surface. The deflection surface can include a dense or porous surface optionally including a liquid. The deflection surface can include a ceramic, a metal, a polymer or a composite fibrous nanostructure. The deflection surface can include cooling channels and can be flat or curved. The method can include controlling a temperature of the deflection surface.
In certain circumstances, the feed material can be a wire or ribbon. The feed material can include a metal, an alloy, a plastic, a rubber, a ceramic, or a glass. For example, the feed material can be a metal wire. The metal wire can include platinum, gold, silver, copper, palladium, nickel, cobalt or stainless steel. The feed material can include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Ir, Pt, Au, Al, Ga, In, Sn, Pb, As, Sb, Bi, or S. For example, the feed material can be stainless steel, CoCr.
In certain circumstances, the device or method can include a second printing unit, for example, an inkjet printhead.
In certain circumstances, the device can include a three-axis, four-axis, five axis or six-axis control stage. Similarly this number of degrees of freedom may be controlled between the printing unit and the stage.
In certain circumstances, the stage can include a temperature controller.
In certain circumstances, the device or method can include an optical sensor to determine the position or trajectory of the feed material or one or more of the molten droplets. For example, the device or method can include a vision system oriented to view at least one of the stage, the printing unit, or a flight path of the molten droplet.
In certain circumstances, the energy source can include a photonic source, for example, a laser, directing light energy at the tip of the wire.
In certain circumstances, the device can include a second energy source, the second energy source generates heat at the stage or building part to facilitate building the part, for example, by preheating the wire to elevated temperature below material's melting point, by generating a molten surface on the part, by slowing the rate of solidification of the molten droplet, by sintering a portion of the part, or by annealing a portion of the part. The portion of the part can be a small section of the part or the entire part.
In certain circumstances, the wire feed can include a mechanism capable of moving the wire at a speed of 0.001 to 20 m/s.
In certain circumstances, the device can also include a vision system oriented to view the stage. The vision system can also be oriented to view one or more of the printing unit, and space between the printing unit and the stage. For example, the vision system can be oriented to view at least one of the stage, the printing unit, or a flight path of the molten droplet.
In certain circumstances, the printing unit includes a deflector in a flight path of the molten droplet that directs the molten droplet to the stage.
In certain circumstances, the molten droplet can solidify once delivered to the surface.
In certain circumstances, the method can include applying a material to the stage from a second printing unit.
Other aspects, embodiments, and features will be apparent from the following description, the drawings, and the claims.
On-demand production, especially for parts with complex geometries and/or high-value material requirements, would be significant to many industries. Additive manufacturing (AM) processes broadly aim to enable this; however, state-of-the-art methods cannot achieve the dimensional resolution and surface finish required for precision applications such as dental implants and jewelry, unless extensive manual post-processing is applied. Production of metal components with customized and/or complex geometries is a longstanding manufacturing challenge. Current processes (including additive methods) can be highly labor intensive for small volumes of precision components or can require high capital investment for large volumes. For example, dental laboratories and jewelry making exemplify markets that produce products primarily of this type (small, detail-oriented and individually tailored and/or designed). A key value proposition in advancing the approach to making products in these technology areas relates to automating customized production. Both exemplary industries face similar challenges in producing customized items for individual clients and delivery of value to the customer can be highly time-sensitive and design-driven.
Three main methods are used today for metal 3D printing and additive manufacturing: powder bed fusion where a part is built from successive layers of powder molten by a laser or electron beam, direct energy deposition where material is build up by feeding a wire into a molten pool of metal created by a laser beam; and binder jetting where parts are made by ink jetting a binder fluid onto successive layers of powder followed by de-powdering and sintering.
Additionally, metal patterns or 3D structures can be formed by depositing a liquid molten metal directly onto a substrate. Known methods to eject droplets from reservoirs of molten metal through a small nozzle have proven challenging because of the corrosive nature of molten metals, thermal management issues, the inability to create molten droplets with varying temperature, and nozzle clogging due to oxide formation.
Molten metal printing from a feed material, such as a ribbon or a wire, as described here, advances metal printing technology with unexpected advantages. The system and method of molten metal printing from a wire consists of a process to generate a stream of metal droplets and optionally a way to modify the droplet flight path that enables printing of 2D patterns or 3D structures directly from molten material. Advantageously, the molten material does not come into contact with a crucible material or a nozzle, which reduces the likelihood of creating contamination and increases the lifetime of the printing unit by reducing wear and tear.
Moreover, the approach of generating molten metal from a wire can allow the temperature of each droplet to be controlled individually. In certain circumstances, individual droplet temperature control can be independent of droplet generation mechanism. Temperature control can be used with any drop generation mechanism. Generating and depositing individual droplets and controlling the temperature of each drop by heating during droplet formation or during flight can improve pattern or part accuracy and metallurgical properties compared to state-of-the-art technologies.
As described herein, exemplifying a wire as a feedstock or feed material, a method to generate individual molten droplets from a wire feedstock can include a wire feed mechanism, and liquefying the feed material with an energy source directed at or near an end of the feed material. The wire may be fed fast enough to break the liquefied region into individual droplets. In other circumstances, the wire may be heated to generate a single droplet travelling with a trajectory away from the wire. At least one deflector can be used to affect the speed and direction of the droplet in the vicinity of the end of the wire. The deflector can be located within a few centimeters or a few millimeters of the end of the wire or the target surface.
As described herein, a device to repeatedly generate molten droplets from a wire feedstock with controlled velocity and position can include a wire feed mechanism that advances a wire at a controlled speed and/or maintains a desired position of free end. The device can include a wire alignment mechanism that determines trajectory and position of the wire, both with respect to an absolute coordinate system and in relation to the energy source. The device can also include an energy source applied at or near the tip of the wire to generate molten droplets.
Sequentially produced droplets may be uniform in size, or different in size; may be larger, equal, or smaller than wire diameter. The details can be method specific. For example, the energy source can be modulated for each individual segment of the wire that ultimately breaks up into a droplet. This may produce individual droplets wherein the size and temperature of each depends on the particular modulation of the energy source. The energy source modulation may also be a duty cycle, for example a repeating on-off sequence; this periodic heating may create a spatially periodic distribution of temperature and surface tension along the liquefied portion of the wire, which influences the subsequent breakup into droplets. Alternatively, a constant energy source and feed rate may exhibit a multi-mode capillary instability, resulting in a periodic sequence of droplets (e.g., large-small-large-small- . . . ). A deflector may then, for instance, selectively deflect only the “small” droplets to a target substrate; the larger ones sent to a waste collection bin.
In another example, the amount and rate of heating, and location of heating relative to the end of the feed wire, can determine the size of the droplet that is deflected away.
The energy source can include one or more of the following: an electromagnetic source, a plasma source, an electron beam source, a joule heating source, an induction source, a convective source or a conductive source. The energy output can be modulated or pulsed or both. Each individual droplet can be heated to a different temperature. For example, the energy source can include a laser. The duration and intensity of exposure of each droplet to the energy source can be controlled so as to achieve a desired droplet temperature. In certain circumstances, the duration and intensity can be modulated for each droplet.
The feed material may be any cross section geometry. For example, the feed material can be a “wire”, in which perpendicular cross-section dimensions are substantially similar in size. In another example, the feed material can be a “ribbon”, in which perpendicular cross-section dimensions can be substantially different in size.
The feed material can include any solid material which is liquefied by the energy source, such as a metal, a metal alloy, a plastic, a rubber, a ceramic, a composite or a glass.
In certain circumstances, the feed material can be pre-heated by an additional energy source up to, but not over, the melting temperature.
The droplet trajectory can be modified with a deflector. The deflector can be a solid surface, oriented so that the droplet bounces off of it travelling in a desired direction. For example, the deflector can be actuated to change its orientation, and thereby the bounce direction, individually for each droplet. The deflection surface may be flat or curved.
The temperature of the deflection surface can be controlled.
The deflection surface can include a dense or porous surface optionally including a fluid; The fluid may be replenishable or circulating, for example, through cooling chambers or cooling channels.
The texture of the surface can be smooth or rough. For example, the roughness length scale can be small compared to the droplet length scale.
The deflector can be an electric or magnetic field subject on the droplet to impart a force in a desired direction. For example, a gradient electric field may deflect an uncharged droplet. In another example, a charged droplet may be deflected by an electric or magnetic field (according to the Lorentz force).
In certain circumstances, the deflector may be vapor propulsion or a plasma shock wave by superheating the droplet on one side. This can create a vapor plume that imparts momentum on the droplet.
In general, the deflector may modify the droplets trajectory by: electrostatic deflection, plasma shock wave deflection, vapor propulsion deflection, acoustic or acoustophoretic deflection, drag deflection, mechanical deflection, or a combination thereof.
Another important aspect of the device in method can involve delivery of the feed material to the energy source. In certain circumstances, alignment of the feed material via an alignment mechanism can utilize a mechanical constraint transverse to the wire feed direction. This can be accomplished by a rigid planar or curved surface, for example, a v-groove). Other factors that can influence the alignment of the feed material can include the bending stiffness of the feed material, inertia or centripetal acceleration of the feed material, or electric or magnetic fields to impart controlled forces on the feed material. The electric or magnetic fields can require a closed loop control system to sense the feed material position and change the strength of the field to maintain the feed material's position.
The wire may be aligned to intersect an energy source, for example, a laser.
Once the molten droplet is formed, the droplet may be directed towards a target surface in order to print a desired pattern or part. The target surface may be an arbitrarily large planar or contoured surface. The target surface may be fixed to a multi-degree of freedom stage, which may be actuated to change its position or orientation with respect to the incoming droplet. The target surface may be a metal, ceramic, polymer, glass. The molten droplet may solidify once delivered to the surface.
The droplets can be combined at the target to form a pattern or part. The pattern or prat can be formed of one or more materials.
A 2D pattern or 3D part may be built droplet-by-droplet.
In order to build a pattern or part, the thermal state of the particle and target substrate upon impact may be controlled. The particle temperature can be determined by the heating method described above. A portion of the target surface, pattern or part can be laser heated, softened, or melted before the impact of the molten droplet.
In certain circumstances, a second print unit may be included to print a multi-material part. The second printing unit may be an additional wire-fed droplet generator or an inkjet printhead.
The droplet generation device and target substrate can be housed inside an enclosure with environmental control. This configuration can allow the temperature of deposition to be controlled with heaters/coolers, and temperature sensors. The gas composition in the housing can be controlled via flow inlet/outlet ports with flow sensors or chemical sensors inside the enclosure. The gas composition can include air, an inert gas, a reducing gas, water vapor, or combination/percentage thereof. The gas pressure in the housing can be controlled via flow pumps and a pressure sensor. The enclosure can be maintained under reduced pressure, atmospheric pressure, or elevated pressure.
The device can include a vision system oriented to view at least one of the stage, the printing unit, or a flight path of the molten droplet(s). The vision system can provide feedback during the building of a pattern or part.
Referring to
Referring to
The system and methods described here can have one or more of the following advantages or features.
In an exemplary embodiment, a metal wire can be fed through a laser beam and subsequently melts. At sufficiently high wire velocities and appropriate laser energy input, the molten column formed by the wire breaks up into a stream of individual droplets that is then directed towards a deflecting surface mounted on a galvanometer or other rotatable element. The position of droplet landing on the substrate can be controlled by the angle of the deflecting surface, and therefore a pattern of metal droplets or a 3D part is formed on a substrate by deposition of a plurality of droplets with position control.
For example,
As generally described, the methods and systems described herein can create a system to print dots, lines, planar patterns, or three-dimensional structures from drops of molten material created within a printing unit. The system can include the following components:
The material feed mechanism can take feed material from a supply, i.e, for instance by unwinding a wire from a spool. The material feed mechanism, optionally, can substantially straighten the wire to remove residual bending. The material feed mechanism, optionally, can pre-heat the feed material from the storage temperature to below the melting temperature of the material. The mechanism can feed the material “into” the heat source with high special precision, i.e. feed a wire through the center of a laser beam. The feed rate can be, for example, 0.1 to 50 meters per second, and may vary according to the wire material, diameter, and/or other considerations. The heat source can include a laser. The power of the laser can be between 10 and 50000 Watts, for example 80 to 500 Watts in one exemplary embodiment. The laser wavelength can be in the infrared or visible, for example 10.6 micron, 1064 nm, 532 nm or ˜450 nm; and ideally equal to the maximum absorption wavelength of the feed material.
Material feed can provide material either on demand (i.e. on/off, advancing material step wise at a constant or variable frequency), at variable speed, or at constant speed. The speed may be balanced to match the growth rate of the pattern or part.
The feed material can be any metal or alloy, provided the material may be liquefied by the energy source. The feed material, optionally, can also be a composite containing a metal/alloy and non-metallic particles, for example, a metal/alloy mixed with ceramic nanoparticles or microparticles or mixtures thereof. The feed material can have a defined cross-section geometry. The feed material can be amorphous or crystalline or a mixture thereof.
The cross-section geometry can be round (wire), rectangular (ribbon) or arbitrary shape (oval, rectangular with rounded edges, or other shape). The feed material cross section can be constant over the entire length of the feed material. Alternatively, the material cross section can change over the length of the feed material. The change can be a regular change or irregular change. For example, the feed material can be a wire with indentations at regular intervals.
In certain embodiments, the feed material can have a thickness across its diameter of about 1 to 10,000 microns, for example, less than 1,000 microns, less than 100 microns, or less than 50 microns. The molten droplets created from the feed material can have a size that is larger than, equal to, or smaller than the thickness of the feed material. In certain circumstances, the molten droplets can be monomodal distribution of sizes and substantially the same size. In other circumstances, the molten droplets can be a bimodal distribution of sizes, one distribution of sizes that is larger than the thickness of the feed material and another distribution of sizes that is smaller than the thickness of the feed material. The two size distributions can be separated during the execution of the method to deliver the larger distribution to one target and the smaller distribution to another target. The molten droplets can have a size of 500 microns, 300 microns, 200 microns, 150 microns, 100 microns, 50 microns, 20 microns, or 10 microns.
Another important parameter for the system and method described herein includes material heating and droplet formation. The literature describes many ways to make molten material droplets. These methods are generally based on a heated reservoir holding the molten material connected to a nozzle opening. In those methods, the reservoir is pressurized and a molten material stream exits the nozzle and breaks up into individual droplets due to Rayleigh-Plateau instability. Depending on the different embodiments, the pressure can be generated with a gas, an electromagnetic force, a vibrating piezo-element or a combination thereof several challenges exist with the current techniques: the reservoir and nozzle materials can oxidize or corrode in contract with the surrounding atmosphere or molten metal; the droplets all have the same temperature after ejection; molten materials are corrosive and impurities can leach from the reservoir materials into the molten material; and impurities or oxides inside and on the surface of the molten metal often lead to nozzle clogging and consequently reliability issues. There are also thermal management issues associated with maintaining a molten reservoir of high melting point materials.
The system and method described herein can have advantages over the previous methods. One approach under the system and method to generate molten material droplets is contactless, in which case the melt does not contact a surface. In other words, using the system and method described herein creates a situation in which no hot molten material comes into contact with any material other than the surrounding gas. The surrounding atmosphere can be ambient, inert, or it can be reducing to decrease contamination of the droplet through surface contact and/or oxidation of the molten droplet, or it can be reactive if desired to modify the characteristics of the droplet and/or the surface upon which printing is performed.
In certain circumstances, the material feeder can transport the material “into” a heat source that heats the material above the melting point. At low feed material velocities, the material can melt and a droplet of molten material hanging from the tip of the material can be formed due to surface tension forces balling up the molten material. At some point, for droplet diameters in the millimeter range, the molten drop will detach due to gravitational forces overcoming surface tension forces. When the feed velocity is low such that the molten material balls up, it is not possible to generate droplets with diameter of the same order as the wire diameter. Advantageously, by using the method described herein, the feed rate of the material is fast enough through the heat source such that a molten “column” or jet of liquid material is formed. The molten jet can remain stable for some time after exiting the heat source but will eventually break up into individual droplets due to Rayleigh-Plateau (RP) instability. The continuous section of the molten jet of wire must be at least as long as the wavelength of the fastest growing unstable mode, and this constrains the minimum required feed rate and thermal power. This wavelength defines the size of the droplets and is determined by the wire's surface tension, viscosity, and density. These material properties are a function of the wire's thermodynamic state, in particular the wire's temperature, and therefore the amount of heating also determines the droplet size. A schematic illustration is shown in
Based on the material properties of metals such as platinum, gold, silver, copper, nickel, stainless steel and others, as well as anticipated wire sizes in the range of 5-500 microns, the required wire feed rates to achieve the described phenomenon will typically be between 50 and 1 m/s, respectively.
In one example, it was possible to demonstrate the continuous formation of ˜100 micron diameter platinum droplets from 50 micron diameter platinum wire in the lab using a laser beam as a heat source (exemplary embodiment).
The expected phenomenon can also be validated by moving a laser at constant velocity over a stationary wire, instead of moving the wire through the laser beam. An example of an experiment for this case is shown in
The wire can be preheated before it is fed into the laser melting region.
The dynamical process of Rayleigh-Plateau breakup of the molten column may produce a sequence of uniform size droplets, or a repeating sequence of different size droplets depending on system parameters. For example, the breakup may produce a large main droplet followed by a smaller satellite droplet; these again may or may not coalesce during transit in the droplet stream. For the case where the droplets do not coalesce, the embodiment described here enables selection of which droplets are printed towards the substrate and which are captured within the print head. This enables, for instance, a “small droplet, high-resolution” mode and “large droplet, low-resolution” mode for the same printing unit and system parameters depending on which size droplet is captured.
Another important component of the system and method is the heat source. The energy source for providing the thermal input can be one of the following:
In the system and methods described herein, there are several important additions to the above concepts that can be implemented. Examples include:
Additional properties/embodiments of the energy source(s) or stream break-up can include the following:
Another important parameter for the system and method described herein includes control of the temperature difference between the droplet and the substrate at the location of the droplet impact. If the temperature difference is too large, the droplet can bounce off the surface. If the temperature difference is too small, the bonding between the substrate and the droplet can be poor. The surface temperature of the substrate can vary in space and time during printing and, in order to obtain good adhesion, it is advantageous to control the temperature difference between the droplet and the substrate by adjusting the droplet temperature or substrate temperature or local substrate temperature or any combination thereof. The following features can be important:
Described above is a method to generate a stream of molten metal droplets by melting a wire with a heat source. In another configuration, instead of generating a continuous stream of molten droplets from a wire, a single droplet can be generated on demand from a wire. A schematic of the process is shown in
By exerting an additional force on the droplet, the droplet can be prevented from coalescing with the remaining wire and it can be directed towards a substrate.
There are many possible ways to exert a force on the droplet, some of which are described in more detail below (e.g., electrostatic, plasma shock wave, vapor propulsion, acoustic/acoustophoretic, drag, mechanical)—all of these methods are applicable for directing a single drop towards a substrate. For a proof of concept, we demonstrate using vapor propulsion via laser heating to exert a force on a detached droplet.
Initially, a sufficient length of the wire was melted such that surface tension causes an individual droplet to detach as previously described. The droplet was then initially traveling upwards with some momentum due to the dynamics of the detachment. The laser was then left powered on, such that the droplet continued to be heated to the point where some material started to be evaporated on the surface. The vapor generated created a pressure that then accelerated the droplet to the right. The same principle could be used to direct the droplet downward by irradiating the top of the droplet with a laser that comes down at an angle or is annular at the wire. Further the substrate could be positioned at any desired angle below or beside the wire to achieve the desired deposition. Additionally, while in this case the laser was on at a constant power for the duration of the experiment, the laser could also have an arbitrary power profile and/or a modulated intensity profile, for example initially having a relatively lower constant power to melt and detach the droplet, followed by a higher power pulse which is optimized to achieve the desired amount of vapor on the surface for proper acceleration and deposition, while minimizing the amount of vapor generated. A varying power profile could also be used to aid droplet detachment. Additionally to the laser having an arbitrary power profile or modulated intensity profile the laser can also be pulsed to generate the desired amount of vapor. The laser heating that is used for melting the tip, aiding droplet detachment, and generating vapor can all originate from the same laser or different lasers. Additionally to use the laser(s) to generate a vapor to exert a force on the droplet, the laser(s) can also be used to generate a plasma that can exert a force on the droplet and propel it towards the substrate, see also details below. As an example a continuous wave laser beam with or without modulation can be used to generate the droplet and a second pulsed beam can be used to generate a vapor cloud or plasma shock wave to propel the droplet towards the substrate.
Further detail is given below on different methods for modifying the trajectories of droplets, all of which can be applicable to the drop-on-demand case. However, it is valuable to clarify some of the possible configurations for the cases of electrostatic and mechanical force generation, as the configuration for the drop-on-demand case may be slightly different from the case of a continuous droplet stream. In particular, examples include
The resulting droplet stream can be deposited directly onto a substrate by moving the stream in relation to the substrate, for example in a 3-axis or 5-axis system, described below.
Alternatively, when the wire translation speed required to match the droplet breakup frequency exceeds the motion capability of state-of-the-art motion stages (<<1-5 m/s), resulting in “pile up” of droplets, another mechanism can be used to manipulate the droplet trajectory that is not limited by the motion capability limitation. Such trajectory manipulations may be implemented in any case, regardless of the droplet speed emanating from the printing unit.
Several approaches for drop trajectory manipulation can be used to direct the molten droplets. The first approach can be electrostatic or magnetostatic manipulation. The material feed can be held at one potential and a charging electrode surrounding the material feed during Rayleigh-Plateau breakup can be held at another potential, resulting in a charged droplet after jet break-up. The flight path of the charged droplet can then be controlled by passing the droplet through an electrical field, i.e., between two charged plates, see
A second approach can include a plasma shock wave. For example, a laser beam with sufficiently high intensity impinging on a molten metal droplet can result in evaporation of material from the surface. Strong laser absorption in the vaporized material and plasma creation can result in a plasma shock wave that propels the droplet away from the origin of the shock wave. The existence of a plasma and/or plasma shock wave can propel the molten droplet in a controlled way towards a substrate. Using the plasma shockwave together with the droplet on demand generation can provide unique control of droplet generation and deposition. The process is shown schematically in
A third approach can include vapor propulsion. For example, a laser with sufficiently high intensity can impinge on a molten metal droplet, which, in turn, can generate vapor on the surface of the droplet that creates a pressure gradient and propels the droplet away from the impinging laser beam. In this circumstance, a force on the droplet can be created by evaporating material whereas for the plasma shock wave it is generated by rapidly expanding gas due to the plasma generation. The laser can be the same laser used to melt the material and generate the droplets, or it can be a different laser.
Another approach can include an acoustic or acoustophoretic approach. For example, a pressure wave in a gas surrounding a droplet can exert a force on a droplet that can be used to modify the droplet flight path.
Another approach can include a drag approach. For example, a gas stream flowing past a droplet can exert a force on a droplet that can be used to modify the droplet flight path and/or assist in detaching the droplet from the tip of the molten wire.
Another approach to trajectory control can include mechanical deflection. This can include mechanically deflecting the molten material droplet off a solid surface that can be located in the flight path, i.e., a form of mechanical mirror. This configuration is shown schematically in
Whether a liquid droplet bounces off a surface or sticks can be determined by many factors. Important factors include the wettability of the surface by the liquid (surface energies of liquid and solid), surface roughness, droplet temperature, surface temperature, thermal properties and stability of the impingement surface (i.e. melting/evaporation of the surface during impact, heat transfer between the particle and substrate during impact), surface impurities such as dust, surface oxides, adsorbed species etc., droplet size, droplet speed. Small molten metal droplets are often observed to bounce from a solid surface unless there is interfacial freezing (i.e., the droplet partially or completely solidifies while spreading) or the substrate melts under the spreading droplet or the droplets wets the surface well. It is, therefore, advantageous to use a material for the deflecting surface that has one or more or all of the below properties. The reflective material can have a higher melting point than the impinging droplet. The reflective material can be non-reactive to the atmosphere surrounding it and the molten impinging droplet. The reflective material can be substantially not wettable by the impinging droplet.
In this example, the deflection surface can be dense or porous. The deflection surface can be actively cooled or heated. It can also be advantageous to use a material with low thermal conductivity or thermal diffusivity to reduce cooling of the impinging metal droplet during contact.
In another embodiment of the deflection surface, an additional liquid at the deflector surface can be included. The liquid can be a continuous or discontinuous thin film on the surface, a liquid infused into or on top of a porous body forming the surface, channels filled with liquid in the surface or a combination thereof. The presence of the liquid can help prevent thermal damage to the surface and prevent sticking of the droplet to the surface. Examples of porous surfaces including porous ceramics (e.g., sintered or compacted powder, anodic aluminum oxide), porous metals (e. g., sintered or compacted powder), anodized metals (e.g., anodized aluminum), or carbon nanotube films (e.g., a ceramic-coated or uncoated carbon nanotube forest)
The concept was demonstrated by impinging molten platinum droplets onto a porous alumina substrate at an angle of ˜45 degrees. Ten molten Pt drops were deposited at various positions onto the dry material and all ten droplets were observed to stick to the surface. The substrate was then wetted with water and ten more droplets were deposited and all of them bounced off the wetted surface.
More specifically, in
The images show melting of the surface when porous substrate is used as is. The contrasting surface image shows no melting of surface when pores are filled during droplet impact.
The surface roughness of the deflector surface can be adjusted from mirror finish to very rough. The porous structure can be made of a ceramic, metal, polymer or composite. The porous structure can also be made of fibrous nanostructures such as carbon nanotubes (CNTs), optionally coated with another material such as a ceramic (e.g., alumina). Such a surface can have low effective contact area with the impinging droplet, minimizing heat transfer, while being mechanically robust and porous, thus possibly improving supply of gas or liquid to the surface.
A thin vapor film between the droplet and the reflecting surface can be created by using a thermally unstable material for the reflecting surface that will decompose or pyrolyse during the impingement droplet, creating a vapor layer at the boundary.
In another embodiment the deflecting surface can be a single or multi-layer metallic or ceramic plate with cooling channels on the back side that can be actively cooled by circulating coolant through the cooling channels during droplet impingement. The structure can be a MEMS structure where the deflecting surface is a single or multi-layer thin metal or metal oxide film that can be supported by a silicon structure with etched cooling channels.
A deflector assembly can be constructed by using galvanometers similar to those used for guiding laser beams, here attaching a deflector surface rather than an optical mirror. This approach has several key advantages: the positioning of the mirror surface can be much faster than traditional motion stages and droplet stream velocities relative to a substrate, which can be greater than 10 m/s. The final droplet landing location can also be adjusted “digitally”, i.e. the drop landing pattern can be chosen arbitrarily for each droplet whereas electrostatic deflection results in a continuous “sweeping” pattern as described above. A deflector assembly can also be constructed by using a rotating polygon deflector surface. Moreover, the contour of the surface can be flat, or curved in a manner to refocus the droplets at a fixed distance after bouncing off the mirror (for example, a parabolic surface profile) in order to compensate for trajectory deviations of the droplet stream. The deflector surface can also be curved to allow particles with an angular variation from the ideal flight path to be focused back onto a single deposition spot, similar to a mirror focusing light. This concept is schematically shown in
In certain circumstances, molten droplets experience cooling and will ultimately freeze while moving from the printing unit to the substrate. For very small droplets the travelled distance until freezing can be in the millimeter range. In one embodiment, the droplets can be deflected one or multiple times before freezing, partially or fully solidify during flight and then can be molten partially or fully again by an energy source such as a laser before impact on the substrate.
In another embodiment, liquid material droplets are generated and the distance between the molten droplet generator and the deflector surface is chosen to be large enough such that the droplets partially or completely solidify before being deflected. The particles can then be molten again in flight by an energy source, such as a laser beam, before impacting the build substrate. Optionally, multiple deflector surfaces can be used to deflect the solidified particles. Optionally, the particles can also be reflected one or multiple additional times between partially or fully solidifying and final deposition. Optionally, instead of re-melting the droplet, the substrate can be molten locally and the solid particle can be deposited into the meltpool on the surface of the substrate. Optionally, both the droplet and the surface can be heated or molten or both.
The deflector, optionally, may have an orientation that allows the droplets or particles to be reflected away from the substrate, thereby allowing selection of which droplets in the droplet stream are printed towards the substrate. A droplet or particle waste collection system can be implemented, or particles can be printed onto a waste area.
The overall print system can include of any of the above described components together with a 3-axis (x-y-z), 5-axis (x-y-z-a-b) or 6-axis (x-y-z-a-b-c) motion system for positioning the printing unit or the stage, or both, any number of control units, computers, vision systems (IR, visible, or UV, for example), sensors, or other components.
Examples of sensors can include any of photodiodes, pyrometers, IR/VIS/UV detectors, IR/VIS/UV cameras, X-ray detectors, ultrasonic detectors or mechanical force detectors. Multiple sensors can be used to detect the presence, velocity, velocity vector, temperature, diameter, volume, shape, circumference, outline, color, reflectance, emissivity, surface morphology, either momentarily or over time. The sensors can be arranged in a single or multiple locations.
In one example sensing the presence of a droplet can be performed by a light source illuminating the droplet in flight and a photodiode recording the intensity of the light source while the droplet passes through the beam of light. The reduction of the recorded light intensity, i. e. the shadow of the droplet, can be used to detect the presence of a droplet passing through the light beam. Additionally, a velocity of the droplet can be calculated from the intensity variation over time.
In another example, a high-speed camera, either in the infrared or visible spectrum, can be used to detect the location, velocity vector, shape or other properties as mentioned above of the droplet.
Using sensor data and optionally some physical models, i. e. for atmospheric drag or atmospheric cooling or radiative cooling, can be used to make predictions about the droplet flight path or temperature variation on the flight path. The data can further be used to, for example, modulate one or more power sources, to trigger other sensors or used in a feedback control loop.
The pressure inside the housing of the printer can be controlled to be at ambient pressure, higher than ambient pressure or lower than ambient pressure. Lowering the atmospheric pressure of the fluid that the droplets are travelling in can reduce drag forces and can reduce slowing down on the droplets in flight.
The atmosphere the molten droplets are exposed to can be controlled. The majority of liquid molten materials strongly react with oxygen and/or moisture in air and an atmospheric control chamber can be included to use vacuum, inert gas(es) or reducing gas(es).
Alternatively, the material feed stock may be housed within a reducing liquid. The high-speed motion of the material can entrain a fine viscous coating of the reducing liquid around the material as it transits through the printing unit and to the heat source, thereby preventing any reaction with oxygen and/or moisture in a standard room air atmosphere. Additionally, if the reducing liquid is capable of removing surface oxide formation from the surface of the material within a short amount of time or upon heating, then only a section of the traversing material needs to be coated before passing through the heat source, rather than storing the material stock in a reducing liquid.
The system can be a stand-alone unit or can be retrofit into an existing computer numerical control (CNC) machine or use it together with an existing 3D printer/additive manufacturing equipment, i.e., printing metal onto polymers or into metal parts being printed or manufactured by another method.
State-of-the-art powder-based 3D printing works by spreading a thin layer of powder, sintering/melting the powder with a scanning heat source such as a laser or electron beam and then repeating these steps to form a part inside the powder bed. The system and method described herein can be used to selectively add molten material droplets in any pattern to a powder bed process by either depositing molten droplets onto the freshly spread powder bed or onto the powder after passage of the heat source, either at a location with powder only or sintered or molten parts inside the powder bed. The powder can be a polymer, ceramic or metal and the molten material added can be any metal (same or different than the one used in the powder process). The droplet may optionally be allowed to solidify before impinging onto the powder bed, or upon impingement on the powder bed. The droplet, if molten when impinging, may infiltrate the powder bed.
The method to form a pattern or part (or other object) can use the system described above which can deposit individual droplets or particles, print patterns such as lines, grids, images, or arbitrary patterns as well as print three dimensional structures. In a generic print situation, the substrate might have a varying surface temperature both in x-y-z as well as over time due to in stationary heat transfer. Controlling the droplet temperature can be beneficial such that a specific difference between the droplet temperature and substrate temperature is maintained or the difference can be adjusted for each drop individually to manage heat input from the droplet into the substrate or part. Additionally, controlling the temperature between the droplet and the substrate can be beneficial to improve adhesion. The temperature difference between droplet and substrate can be chosen such that the thermal energy of the molten droplet can be sufficient to re-melt the substrate, resulting in good adhesion. An optimum temperature difference can be found to minimize the additional heat input into the substrate by additional heating and to maximize the adhesion between the droplet and the substrate.
In certain circumstances, multiple “printing units” can be used to increase the throughput of the system. Multiple printing units of different materials can be used to print multi-material parts or patterns, for example multiple wires of the same or different materials can be fed into the different printing units. The material jetting of multiple units can be actuated independently or in synchrony. Multiple printing units with different droplet sizes can be used to print parts with varying voxel sizes/local resolutions. In certain circumstances one laser can be used as heat source for multiple printing unit by splitting the laser into multiple beams or by switching the beam between multiple printing units.
Under certain circumstances, it can be challenging to deposit material at a high volumetric rate because heat cannot be conducted away from the printed part fast enough to ensure solidification before more material is deposited, resulting in distorted parts. In one example, one printing unit can print a thin closed perimeter of one material that always solidifies independent of the deposition rate and another printing unit can print a second material with lower melting point than the first material into the contour at high deposition volumes, forming a molten pool of the second material inside the perimeter of the solidified first material. The printed part can resemble a core-shell structure.
In another embodiment, any 3D printing technology (laser or ebeam powder bed fusion, direct energy deposit, binder jetting or similar) can be used to print a thin shell of a first material. A second material is then placed into the shell. The thermal properties of the second material are selected such that the shell does not melt upon filling with the second material. The second material can, for example, have a lower melting point than the first metal. The second material also can have the same or a slightly higher melting point than the first material and melting of the shell can then be prevented by natural or forced cooling of the surface of the shell. The second material can be cast into the shell or can be printed into the shell as liquid droplets. The second material can be molten or partially molten. The filling can occur during printing the shell, immediately after printing the shell while it is still hot or after cooling of the shell. The shell can optionally be re-heated before placing the second material into it. The shell can have one or multiple separate cavities. Multiple separate cavities can be filled with the same second material or with multiple different materials. The shell can have an arbitrary shape, can have different shapes or can contain intricate parts of printed material itself. The inside of the shell can be structured with features protruding from the surface that allow the shell to mechanically interlock with the material on the inside, for example when printing dissimilar materials that do not form a chemical bond between their surfaces.
In certain circumstances, the build stage, substrate or defined spots on the surface of the part can be heated. For example, heating the entire build stage, substrate, part, or combinations thereof, can be advantageous to reduce stress in the built part, i.e. the part can be kept at an elevated temperature during the entire 3D printing process and is then slowly cooled down after printing is finished. In one embodiment, the entire build volume, stage or part or only a fraction of the substrate, build stage or part are heated during the deposition process to temperatures of 0.1 to 0.99 times the melting temperature (in degrees Celsius) of the material to be deposited.
In another embodiment, a laser beam can be used to selectively heat a small portion of the substrate immediately before, during or after the impact of a single droplet to a temperature below or above the melting point of the material to be deposited. Heating a small area of the substrate approximately the size of the droplet at the impact location shortly before droplet impact is especially beneficial to enable good fusion of the impinging droplet with the substrate. For example, the droplet fuses with a small molten part of the substrate (liquid droplet impinging in liquid meltpool on the substrate surface) or part of the substrate can be re-melted by the impinging droplet, resulting in good metallurgical bonding. For example, one beam can be directed at the wire and one directed at the substrate. See
In certain circumstances, a laser beam can be used to selectively heat the droplet or an area surrounding the droplet or both after impact in order to control the cooling or solidification rate or both of the heat affected zone. Controlling the cooling and solidification times can be beneficial for controlling the microstructure or mechanical properties or both of the material, i. e. adjust grain size, grain orientation, diffusion of atoms in alloys, residual stresses or degree of crystallinity going from amorphous to fully crystalline.
In another example, a laser beam can be used to selectively planarize portions of or the entire surface of the printed part by momentarily heating a thin layer of the part to a temperature above the melting point, letting the molten thin film flow to even out roughness and letting the molten thin film cool below the solidification temperature.
In certain circumstances, support structures can be necessary to mechanically support parts to be build. When the support structures are monolithic to the part, significant time and effort is needed to remove these during post processing of the parts. In the present invention, the adhesion of a molten particle to the substrate can be controlled in the above mentioned process by adjusting the temperature difference between the impinging molten droplet and the substrate. If this temperature difference is chosen such that re-melting of the substrate occurs after droplet impact, the droplet adheres firmly to the substrate. If the temperature difference is chosen such that no re-melting of the substrate surface occurs, the solidified particle adheres poorly to the substrate. This behavior can be used to create single or multiple particle layers with low adhesion that can be inserted between support structures and the printed part to facilitate separation during post-processing.
Non-limiting examples of applications for the system and method are described below:
The system and method can be used to print droplets within the 1-5000 μm size range, and to print droplets in single (two dimensional) or multiple (three dimensional) layers with controlled arrangements. The system and method can be used to manufacture parts of various sizes, for example, parts from tens of microns in size, to hundreds of microns in size, to millimeters in size, to centimeters in size, to decameters in size, to meters in size. For example, the part can be 10-1000 microns, 1-10 millimeters, 1-10 centimeters, 1-10 decimeters, or 1-10 meters in size.
Other embodiments are within the scope of the following claims.
This application claims priority to U.S. Provisional Patent Application No. 62/858,944, filed Jun. 7, 2019, which is incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2020/036525 | 6/7/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62858944 | Jun 2019 | US |
