Patent Description:
Metal parts may be additively manufactured using MHD printing of molten metal. Such additive manufacturing is described in <CIT>.

In jetting of liquid metal by magnetohydrodynamic (MHD) principles, the pressure to expel the liquid metal is created by Lorentz forces resulting from the interaction of a magnetic field and an electrical current.

In certain additive manufacturing systems using MHD printing of liquid metal, a fluid chamber contains liquid metal and a magnetohydrodynamic force is created in the liquid metal, causing a drop or stream of liquid metal to be expelled from a discharge orifice of a nozzle. This may be accomplished in a continuous stream, segments of continuous stream or a series of droplets. The nozzle is moved relative to a build plate on which the successive layers are deposited, thereby forming a three-dimensional shape.

Jetting techniques can be divided into so-called "continuous jetting" and "drop-on-demand". In continuous jetting, a continuous stream of liquid is issued from the nozzle. This stream may or may not break into droplets before reaching a target via Raleigh breakup. In "drop-on-demand" jetting, drops are issued individually from the nozzle orifice. In one form of drop-on-demand MHD jetting (distinguished from continuous jetting), the magnetic field is held constant while the electric current is varied and/or pulsed to produce droplets. However, the nozzle may fail to produce droplets of a desired speed, size, trajectory, and frequency, or may do so inconsistently, due to factors including inertance in the nozzle throat, unwanted resonance of the liquid inside the nozzle or the nozzle assembly itself, and oscillation of the meniscus of liquid metal at the outlet of the nozzle orifice after a droplet is ejected. While some of these factors may be partially addressed by improvements in nozzle design, there is nonetheless a desire to further increase nozzle performance in the areas mentioned above. Related prior art comprises <CIT> and <CIT>.

The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.

Disclosed is an additive manufacturing system using MHD printing of molten metal and methods for operating the same. In an embodiment, a first current pulse is applied to a liquid metal in a nozzle to eject a droplet from a discharge orifice. A second current pulse is applied to the liquid metal in the nozzle to reduce an amplitude of the oscillations in a meniscus on the discharge orifice. The second current pulse can be either of an opposite or the same polarity as the first current pulse. The strength of the current pulses relative to one another is controlled, either by varying the current of the pulses or duration thereof.

Beneficially, the meniscus of liquid metal formed at the discharge orifice can be controlled. In certain embodiments, this allows the length of a ligament and the resultant snap-back force to be reduced. This may also allow the nozzle to jet liquid metal closer to the surface being built, which increases the tolerances of angle deviation that can be permitted among other advantages. Satellites, or errant additional droplets, may be caused to merge with intended droplets to reduce printing defects.

The foregoing summary, preferred embodiments, and other aspects of the present disclosure will be best understood with reference to a detailed description of specific embodiments, which follows, when read in conjunction with the accompanying drawings, in which:.

Disclosed is a method for the manipulation of the jetting current in the MHD printing of molten metal in additive manufacture of metal parts to improve jetting performance. Several techniques are disclosed, which may be used singly or in combination.

<FIG> is a schematic depiction of an additive manufacturing system <NUM> using MHD printing of liquid metal <NUM> in which the disclosed improvements may be employed. Additive manufacturing system <NUM> can include a nozzle <NUM>, a feeder system <NUM>, and a robotic system <NUM>. In general, the robotic system <NUM> can move the nozzle <NUM> along a controlled pattern within a working volume <NUM> of a build chamber <NUM> as the feeder system <NUM> moves a solid metal <NUM> from a metal supply <NUM> and into the nozzle <NUM>. As described in greater detail below, the solid metal <NUM> can be melted via heater <NUM> in or adjacent to the nozzle <NUM> to form a liquid metal <NUM>' and, through a combination of a magnetic field and an electric current acting on the liquid metal <NUM>' in the nozzle <NUM>, MHD forces can eject the liquid metal <NUM>' from the nozzle <NUM> in a direction toward a build plate <NUM> disposed within the build chamber <NUM>. Through repeated ejection of the liquid metal <NUM>' as the nozzle <NUM> moves along the controlled pattern, an object <NUM> (e.g., a two-dimensional object or a three-dimensional object) can be formed. The object may be formed based on a model <NUM> enacted through a controller <NUM>. In certain embodiments, the object <NUM> can be moved under the nozzle <NUM> (e.g., as the nozzle <NUM> remains stationary). For example, in instances in which the controlled pattern is a three-dimensional pattern, the liquid metal <NUM>' can be ejected from the nozzle <NUM> in successive layers to form the object <NUM> through additive manufacturing. Thus, in general, the feeder system <NUM> can continuously, or substantially continuously, provide build material to the nozzle <NUM> as the nozzle <NUM> ejects the liquid metal <NUM>', which can facilitate the use of the three-dimensional printer <NUM> in a variety of manufacturing applications, including high volume manufacturing of metal parts. As also described in greater detail below, MHD forces can be controlled in the nozzle <NUM> to provide drop-on-demand delivery of the liquid metal <NUM>' at rates ranging from about one liquid metal drop per hour to thousands of liquid metal drops per second and, in certain instances, to deliver a substantially continuous stream of the liquid metal <NUM>'. A sensor or sensors <NUM> may monitor the printing process as discussed further below.

Now with reference to Figures 2A-D which depict the nozzle of the printer of <FIG>. The nozzle can include a housing <NUM>, one or more magnets <NUM>, and electrodes <NUM>. The housing <NUM> can define at least a portion of a fluid chamber <NUM> having an inlet region <NUM> and a discharge region <NUM>. The one or more magnets <NUM> can be supported on the housing <NUM> or otherwise in a fixed position relative to the housing <NUM> with a magnetic field "M" generated by the one or more magnets <NUM> directed through the housing <NUM>. In particular, the magnetic field can be directed through the housing <NUM> in a direction intersecting the liquid metal <NUM>' as the liquid metal <NUM>' moves from the inlet region <NUM> to the discharge region <NUM>. Also, or instead, the electrodes <NUM> can be supported on the housing <NUM> to define at least a portion of a firing chamber <NUM> within the fluid chamber <NUM>, between the inlet region <NUM> and the discharge region <NUM>. In use, the feeder system <NUM> can engage the solid metal <NUM> and, additionally or alternatively, can direct the solid metal <NUM> into the inlet region <NUM> of the fluid chamber <NUM> as the liquid metal <NUM>' is ejected through the discharge orifice <NUM> through MHD forces generated using the one or more magnets <NUM> and the electrodes <NUM>. A heater <NUM> may be employed to heat the housing <NUM> and the fluid chamber <NUM> to melt the solid metal <NUM>. A discard tray <NUM> is located in proximity to the build plate and the nozzle may deposit droplets in it during a testing or calibration step.

In certain implementations, an electric power source <NUM> can be in electrical communication with the electrodes <NUM> and can be controlled to produce an electric current "I" flowing between the electrodes <NUM>. In particular, the electric current "I" can intersect the magnetic field "M" in the liquid metal <NUM>' in the firing chamber <NUM>. It should be understood that the result of this intersection is an MHD force (also known as a Lorentz force) on the liquid metal <NUM>' at the intersection of the magnetic field "M" and the electric current "I". Because the direction of the MHD force obeys the right-hand rule, the one or more magnets <NUM> and the electrodes <NUM> can be oriented relative to one another to exert the MHD force on the liquid metal <NUM>' in a predictable direction, such as a direction that can move the liquid metal <NUM>' toward the discharge region <NUM>. The MHD force on the liquid metal <NUM>' is of the type known as a body force, as it acts in a distributed manner on the liquid metal <NUM>' wherever both the electric current "I" is flowing and the magnetic field "M" is present. The aggregation of this body force creates a pressure which can lead to ejection of the liquid metal <NUM>'. It should be appreciated that orienting the magnetic field "M" and the electric current substantially perpendicular to one another and substantially perpendicular to a direction of travel of the liquid metal <NUM>' from the inlet region <NUM> to the discharge region <NUM> can result in the most efficient use of the electric current "I" to eject the liquid metal <NUM>' through the use of MHD force.

In use, the electrical power source <NUM> can be controlled to pulse the electric current "I" flowing between the electrodes <NUM>. The pulsation can produce a corresponding pulsation in the MHD force applied to the liquid metal <NUM>' in the firing chamber <NUM>. If the impulse of the pulsation is sufficient, the pulsation of the MHD force on the liquid metal <NUM>' in the firing chamber <NUM> can eject a corresponding droplet from the discharge region <NUM>.

In certain implementations, the pulsed electric current "I" can be driven in a manner to control the shape of a droplet of the liquid metal <NUM>' exiting the nozzle <NUM>. In particular, because the electric current "I" interacts with the magnetic field "M" according to the right-hand rule, a change in direction (polarity) of the electric current "I" across the firing chamber <NUM> can change the direction of the MHD force on the liquid metal <NUM>' along an axis extending between the inlet region <NUM> and the discharge region <NUM>. Thus, for example, by reversing the polarity of the electric current "I" relative to the polarity associated with ejection of the liquid metal <NUM>', the electric current "I" can exert a pullback force on the liquid metal <NUM>' in the fluid chamber <NUM>.

Each pulse can be shaped with a pre-charge that applies a small, pullback force (opposite the direction of ejection of the liquid metal <NUM>' from the discharge region <NUM>) before creating an ejection drive signal to propel one or more droplets of the liquid metal <NUM>' from the nozzle <NUM>. In response to this pre-charge, the liquid metal <NUM>' can be drawn up slightly with respect to the discharge region <NUM>. Drawing the liquid metal <NUM>' slightly up toward the discharge orifice in this way can provide numerous advantageous, including providing a path in which a bolus of the liquid metal <NUM>' can accelerate for cleaner separation from the discharge orifice as the bolus of the liquid metal is expelled from the discharge orifice, resulting in a droplet with a more well-behaved (e.g., stable) shape during travel. Similarly, the retracting motion can effectively spring load a forward surface of the liquid metal <NUM>' by drawing against surface tension of the liquid metal <NUM>' along the discharge region <NUM>. As the liquid metal <NUM>' is then subjected to an MHD force to eject the liquid metal <NUM>', the forces of surface tension can help to accelerate the liquid metal <NUM>' toward ejection from the discharge region <NUM>.

Further, or instead, each pulse can be shaped to have a small pullback force following the end of the pulse. In such instances, because the pullback force is opposite a direction of travel of the liquid metal <NUM>' being ejected from the discharge region <NUM>, the small pullback force following the end of the pulse can facilitate clean separation of the liquid metal <NUM>' along the discharge region <NUM> from an exiting droplet of the liquid metal <NUM>'. Thus, in some implementations, the drive signal produced by the electrical power source <NUM> can include a wavelet with a pullback signal to pre-charge the liquid metal <NUM>', an ejection signal to expel a droplet of the liquid metal, and a pullback signal to separate an exiting droplet of the liquid metal <NUM>' from the liquid metal <NUM>' along the discharge region <NUM>. Additionally, or alternatively, the drive signal produced by the electrical power source <NUM> can include one or more dwells between portions of each pulse.

As used herein, the term "liquid metal" shall be understood to include metals and metal alloys in liquid form and, additionally or alternatively, includes any fluid containing metals and metal alloys in liquid form, unless otherwise specified or made clear by the context. Metals suitable for use with the disclosure include aluminum and aluminum alloys, copper and copper alloys, silver and silver alloys, gold and gold alloys, platinum and platinum alloys, iron and iron alloys, and nickel and nickel alloys.

<FIG> depict the formation of a droplet during the MHD printing process. In <FIG>, a meniscus <NUM> can be seen wetted to a nozzle stem face <NUM>. In <FIG>, an MHD force <NUM> acts to increase the size of the meniscus <NUM> to form a droplet. In <FIG>, the droplet <NUM> can be seen having separated from the meniscus. Finally in <FIG> the droplet <NUM> can be seen in flight towards a build plate or object being manufactured.

<FIG> depict conditions that can occur during MHD printing. A droplet <NUM> is ejected from nozzle <NUM> to build a layer upon object <NUM>. However, a long ligament <NUM> can form behind droplet <NUM>. If the droplet <NUM> contacts the object being printed before the ligament detaches from the meniscus, and has had sufficient time to coalesce with the droplet <NUM>, it can freeze into a stalagmite-like feature. Such structures can severely disrupt the printing of subsequent layers and the achieved print quality. This can limit how close the nozzle can be to the build surface during printing because sufficient clearance must be made to accommodate such ligaments. Further, long ligaments may result in satellites <NUM> that can be flung to angles off of the intended jetting path, creating undesirable defects.

Now with reference to <FIG>, described is an embodiment method. In step <NUM>, when printing, a first current pulse is passed through the liquid metal in the nozzle of the additive manufacturing system, ejecting a droplet of liquid metal. In step <NUM>, a second current pulse is passed through the liquid metal in the nozzle, being one of a lower current or a lower duration. There is a time pause between the first current pulse and second current pulse. The second current pulse may be of the same polarity as the jetting pulse or may be of opposite plurality. In certain instances, both may be employed. The second pulse is timed so as to reduce the amplitude of the oscillations in the meniscus and/or assist in causing a ligament to detach from or be withdrawn into the meniscus. In step <NUM>, if the jetting is not complete, that is subsequent additional jetting is desired, the steps of applying a first and second current pulse are repeated until jetting is no longer desired (i.e., the nozzle will be moved without jetting or the print is done, etc.).

<FIG> depict the conditions of <FIG> as ameliorated through the use of a second negative pulse. Droplet <NUM> is again deposited from nozzle <NUM> to the build surface <NUM>. The second negative pulse causes the ligament <NUM> to snap off earlier than it otherwise would, with part of the ligament <NUM> returning to and settling in the meniscus of molten metal normally residing on the face of the nozzle, and the other part of the ligament being incorporated into the droplet <NUM>. Any satellites <NUM> formed tend to impact the top of the droplet <NUM> and form part of it.

<FIG> depicts a method of adjusting the droplet speed during MHD printing. In step <NUM>, the speed of a droplet is measured during jetting. The measured speed is compared to a desired speed. If the speed needs to be increased, the jetting pulse duration may be increased. If the droplet speed needs to be decreased, the jetting pulse duration may be decreased. It may be desirable to increase the velocity of the ejected droplet. For example, faster drops will experience a smaller "flight time" between nozzle and substrate, which can reduce the opportunity for oxidation if the jetted material is a reactive metal such as aluminum. However, as jetting energy is increased to produce higher velocities, additional unwanted droplets (known as 'satellites' to the art) tend to be produced. These satellites can exit the nozzle in essentially random directions, delivering unwanted mateiral to the drop target. To increase the rate of material delivery, it may be desirable to increase the frequency of droplet ejection. In drop-on-demand jetting, this is nominally achieved simply by increasing the frequency of the pulses used to jet each droplet. However, the maximum usable frequency can be limited by residual vibration in the nozzle or of the meniscus after a drop is ejected.

The techniques described are generally suited for use in a system that has two independently adjustable power supplies and associated capacitor banks. The power supplies and banks are electrically oriented such that one is positive and one is negative, relative to a common ground potential. Switching devices such as metal-oxide-semiconductor field effect transistors (MOSFETs) or other devices and their associated drive circuitry are used to selectively apply positive or negative voltage to the output, thus causing a positive or negative current to flow through the load, which is the molten metal. In certain embodiments, a current-limiting resistor is added in series with the load to prevent excessive current flow while allowing full voltage to be applied at the beginning of the pulse, with the effect being a shortening of the rise time of the current pulse. The MOSFETs or other switching devices may be switched by a waveform generator or other pulse-generating circuitry. It will be understood by those in the art to which the present disclosure pertains that the techniques described are not limited to only the described electrical system.

There are certain methods to produce pulses. In certain embodiments, the impulse delivered to the liquid metal is adjusted by altering the voltage applied to the liquid metal during the jetting pulse, which in turn alters the current passing through it and thus the resulting Lorentz jetting force applied during the pulse.

In another embodiment, the duration of the jetting pulse is altered to adjust the impulse delivered to the liquid metal, and thus the speed of the resulting drops. Unlike the technique of altering the applied voltage, the pulse duration can be adjusted instantly, without for example requiring a change to power supply settings and the subsequent delay for the supply to stabilize at the new setting. In this way, the speed of each drop may be individually controlled. When combined with a means to measure drop speed, this technique can be used to maintain a constant drop speed despite changing nozzle conditions as shown in <FIG>.

<FIG> depicts the application of a jetting pulse and an associated meniscus position without the application of a second current pulse. As is apparent, the meniscus can continue to oscillate for a significant period of time, disrupting the ability to continue printing.

In certain embodiments, the "jetting pulse," that is a pulse that ejects a drop is preceeded by a pulse of the opposite electrical polarity as the jetting pulse (a "negative pulse"). Being of opposite polarity, the negative pulse tends to retract liquid (and thus the liquid meniscus) into the nozzle throat instead of ejecting it. The resultant drop size may be manipulated by adjusting the product of current and pulse duration, with larger values resulting in greater retraction of the meniscus and smaller drop sizes. In certain instances, drop volume reductions of up to <NUM>% were observed. The amount of energy to be delivered by the pulse and the time between the negative pulse and the jetting pulse required to produce these effects depend on the properties of the liquid being jetted and the geometry of the nozzle. In one embodiment, the negative pulse is <NUM> in duration, at approximately 200A, while the positive pulse is <NUM> in duration, at approximately 700A, and a delay of <NUM> between the negative and positive pulses.

In another embodiment, the drop size can be increased by using a second negative pulse delivered after the positive pulse, which alters the timing of the droplet breakoff from the material ejected from the nozzle, creating a satellite droplet which eventually rejoins the main droplet, resulting in an overall increase in volume per drop. In this way, the same nozzle can deliver droplets of adjustable size.

In another embodiment, a positive pulse is delivered to begin material ejection. Soon after material begins exiting the nozzle, a substantially equal negative pulse is delivered, with several desirable effects. First, the breakoff from the nozzle surface occurs before the ligament can separate from the main drop, eliminating a satellite which would otherwise occur and thus allowing higher droplet velocities for a given nozzle and material. Second, the resulting higher-speed ligament and droplet reduce the influence of any wetted material on the face of the nozzle, and thus allow a given nozzle to shoot straighter, i.e. with a drop trajectory closer to the intended travel path along the axis of the orifice axis. Finally, the meniscus is disturbed less than with typical positive-only pulses, resulting in a faster vibration damping, thus enabling higher-frequency jetting.

In another embodiment and with reference to <FIG>, oscillation in the meniscus after a drop is ejected is reduced by applying a negative "stabilizing pulse" after the drop is ejected, with the effect of removing energy from the oscillatory system formed by the mass of the liquid in the nozzle and the compliance of the meniscus at the nozzle exit. The pulse is applied at the zero-crossing point where the meniscus is about to transition from concave to convex. In an alternate embodiment, the negative pulse is replaced by a second positive pulse, applied at the zero-crossing point when the meniscus is about to transition from convex to concave, as shown in <FIG>. In one embodiment, multiple such pulses can be applied at sucessive zero-crossing points. A potential benefit of this is that the system stabilizes more quickly after a drop ejection, allowing another drop to be ejected more quickly, and thus the jetting frequency to be increased.

When a drop is ejected, a quantity of material exits the nozzle office at some velocity. This creates a so-called 'ligament' of material. Eventually, a droplet breaks off from the ligament, and the remaining material 'snaps back' toward the nozzle face. Since this material has kinetic energy, it will "splash" against the nozzle face, which can cause undesired conditions such as increased vibration or unwanted spreading of the material across the face. In another embodiment, a negative pulse immediately after the drop ejection is used to "absorb" this splashing material and draw it back into the nozzle orifice.

In another embodiment, applicable to continuous jetting, a positive pulse is used to initiate a constant stream (accomplished with a lower constant jetting current), and a negative pulse is used to cleanly turn off the constant stream, as shown in <FIG>.

In another embodiment, a negative bias current, applied constantly, is used to pull the meniscus back and hold it back in the quiescent state, to provide similar benefits to the negative pulse described above, and also to clear liquid from the face of the nozzle over time.

In another embodiment, positive pulses with energy higher than the normal jetting pulse are used to clean the throat or outlet of the nozzle. For example, a normal jetting pulse could be 700A for <NUM>, while the effective cleaning pulses for the same nozzle might be <NUM> pulses at <NUM>, each pulse consisting of a 700A for <NUM>.

Claim 1:
A method for additively manufacturing metal parts using magnetohydrodynamic (MHD) printing of liquid metal, comprising the steps of:
ejecting a plurality of droplets of liquid metal from the discharge orifice (<NUM>) of a nozzle (<NUM>; <NUM>; <NUM>) to form successive layers of an object (<NUM>; <NUM>);
the ejection of each droplet (<NUM>'; <NUM>; <NUM>; <NUM>) includes applying a first current pulse to the liquid metal in the nozzle (<NUM>; <NUM>; <NUM>) sufficient to eject the droplet (<NUM>'; <NUM>; <NUM>; <NUM>);
characterised by applying a second current pulse to the liquid metal in the nozzle (<NUM>; <NUM>; <NUM>) thereby reducing an amplitude of an oscillation in the meniscus (<NUM>).