Apparatus and method for manufacturing a three-dimensional object

Improved assembly and methods for manufacturing a three-dimensional object are described. The assembly includes a crucible for holding molten metal, an orifice disposed in the bottom of the crucible through which a jet of molten metal can flow towards a movable substrate, and a mechanically oscillating member immersed in the molten metal for controlling the flow of molten metal through the orifice and for breaking the flow of molten metal into the sequence of molten metal drops. As the drops land on the movable substrate, a three-dimensional object is built up. Continuously variable diameter or controllable planar jets may be used as the orifices. In forming drops from the output of a planar jet, the jet is first broken up by the oscillating member into horizontal cylindrical ligaments and the ligaments are then broken up into drops by acoustic energy applied by audio loudspeakers adjacent the falling ligaments. The assembly and methods are useful in the fields of rapid prototyping and materials processing.

BACKGROUND OF INVENTION
 The present invention relates to an apparatus and method for manufacturing
 a three-dimensional object. According to the apparatus and methods of the
 present invention, continuously variable diameter or controllable planar
 jets are used to form three-dimensional objects in an
 environment-controlled processing chamber. The apparatus and methods of
 the present invention are useful in the fields of rapid prototyping and
 materials processing.
 Manufacturing processes utilizing deposition techniques have been developed
 for rapid and flexible prototyping of three-dimensional parts and tooling.
 For example, U.S. Pat. Nos. 5,301,863, 5,301,415, 5,207,371 and 5,286,573
 to Prinz et al. disclose conventional systems and methods for
 manufacturing three-dimensional objects by forming incremental buildup of
 layers on a work surface. In preferred embodiments of these inventions,
 thermal spray or weld deposition techniques are used to deposit object and
 support layers on a work surface.
 Drop generators have also been developed and applied to the rapid
 prototyping of three-dimensional objects. See P. F. Jacobs, Rapid
 Prototyping and Manufacturing, ch. 16 (Society of Manufacturing Engineers,
 Dearborn, Minn. 1992). In a conventional drop generator of this type,
 molten metal is ejected as a uniform laminar liquid jet from a circular
 injector, or nozzle, located at the bottom of a heated reservoir. The
 uniform laminar liquid jet is broken into a series of uniform drops of
 desired size by selecting an injector diameter and varying the frequency
 of external oscillation near the injector or nozzle orifice. The uniform
 drops are then deposited in layers on a substrate surface where they
 solidify to form the desired three-dimensional metal product.
 With this technique, the resulting metal products can be designed to have
 fine, equiaxed micro-structures without manufacturing defects such as
 porosity or alloy segregation. See C. -A. Chen, P. Acquaviva, J. -H. Chun
 and T. Ando, "Effects of Droplet Thermal State on Deposit Microstructure
 in Spray Forming", Scripta Materiala, vol. 34, pp. 689-696 (1996); J. -H.
 Chun and T. Ando, "Thermal Modeling of Deposit Solidification in Uniform
 Droplet Spray Forming," Proceedings of the 1996 NSF Design and
 Manufacturing Grantees Conference, pp.353-354 (Society of Manufacturing
 Engineers 1996). This allows for a rapid one-time process for metal
 forming that does not require expensive and time-consuming post-processing
 of metal products.
 However, despite these advantages, the manufacturing capabilities of
 conventional drop generators remain limited by the relatively small range
 of possible drop sizes. Greater variability in the drop size is desired to
 allow more efficient rapid prototyping by allowing the mass flux to be set
 according to the outline geometry and desired internal micro-structure of
 the product at a given point. Despite the variability of external
 oscillation, the possible range of drop sizes from a conventional drop
 generator is limited by the fixed injector diameter, which is typically
 less than one millimeter. Consequently, the liquid jet of a conventional
 drop generator remains laminar.
 Therefore, a principal object of the present invention is to provide an
 apparatus and method for manufacturing a three-dimensional object
 utilizing a continuously variable diameter liquid jet to create variable
 drop sizes.
 Another object of the present invention is to provide an apparatus and
 method for manufacturing a three-dimensional object utilizing a
 controllable laminar planar liquid jet to form a liquid sheet and create
 arrays of uniform molten metal drops.
 A further object of the present invention is to provide an apparatus and
 method for manufacturing a three-dimensional object utilizing a
 piezoelectric oscillator and piezoelectric transducer circuit for creating
 continuously variable diameter and controllable planar jets.
 Yet another object of the present invention is to provide an apparatus and
 method for manufacturing a three-dimensional object utilizing a position
 controllable platform.
 Further objects, features and advantages of the invention will become
 apparent from the following detailed description taken in conjunction with
 the accompanying figures showing illustrative embodiments of the
 invention.
 SUMMARY OF THE INVENTION
 The present invention is an improved apparatus and method for manufacturing
 a three-dimensional object whereby a sequence of molten metal drops is
 deposited on a substrate to create the three-dimensional object. In
 accordance with a preferred embodiment of the present invention, the
 apparatus includes a crucible for holding molten metal, an orifice
 disposed in the bottom of the crucible through which a jet of molten metal
 flows towards the substrate, and a mechanically oscillating member
 immersed in the molten metal for controlling the flow of molten metal
 through the orifice and for breaking the flow of molten metal into the
 sequence of molten metal drops.
 According to further embodiments of the present invention, the molten metal
 flowing from the crucible orifice is formed to be a continuously variable
 diameter jet or a controllable laminar planar jet. The continuously
 variable diameter jet is useful for producing a wide range of drop sizes.
 Variability in the drop size allows more efficient rapid prototyping since
 the mass flux can be set according to the outline geometry and desired
 internal micro-structure of the product at a given point. The controllable
 laminar planar jet is useful for large-scale drop forming processes.
 The apparatus of the present invention also includes a position
 controllable platform for positioning a traversable substrate element
 whereupon the desired three-dimensional object is formed. Advantageously,
 the position controllable platform is operated and controlled by a
 computer, position controller device, interface and corresponding
 software.
 In accordance with another aspect of the present invention, a method for
 manufacturing a three-dimensional object is provided whereby a sequence of
 molten metal drops is deposited on a substrate to form the
 three-dimensional object. The method for manufacturing a three-dimensional
 object includes the steps of depositing molten metal in a crucible;
 ejecting the molten metal through on orifice disposed in the bottom of a
 crucible to form a jet of molten metal that flows towards the substrate;
 and actuating a member immersed in the molten metal to control the flow of
 molten metal through the orifice and to break up the flow of molten metal
 into the sequence of molten metal drops. According to further aspects of
 the invention, the method utilizes a continuously variable diameter jet or
 a controllable laminar planar jet.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 illustrates a preferred embodiment of an apparatus for manufacturing
 a three-dimensional objects. The apparatus includes a uniform molten drop
 deposition system 20 for use in rapid prototyping and materials
 processing. The uniform molten drop deposition system 20 can be configured
 as a continuously variable diameter jet apparatus 300 as shown in FIG. 3,
 or a controllable planar jet apparatus 400 as shown in FIG. 4.
 As further shown in FIG. 1, the uniform molten drop deposition system 20
 includes a crucible section 14. The crucible section 14 includes a
 crucible 1, preferably a heated crucible, for holding molten metal
 deposited therein, a piezoelectric oscillator 2 for agitating the molten
 metal deposited in the crucible into a sequence of molten metal drops, and
 an orifice 3 disposed in the bottom of the crucible 1 through which a jet
 of the molten metal flows towards a traversable substrate 18. Also
 included as part of the crucible section 14 is a temperature control
 ("T/C") sensor 4 for monitoring the temperature of the molten metal, a
 band heater 5 for heating the crucible 1, and a charge plate 6 for
 controlling the trajectories and preventing agglomeration of the molten
 metal drops. The crucible section 14 may also include an external pressure
 source 17, preferably a conventional pressure source using non-reactive
 gases such as nitrogen or helium, for facilitating the flow of the molten
 metal from the crucible 1.
 In addition, the uniform molten drop deposition system 20 of FIG. 1
 includes a spray chamber section 15 and a deposition chamber section 16.
 The spray chamber section 15 includes a spray chamber 7 through which the
 molten metal passes, and a spray chamber window 8 for monitoring the
 deposition process. The deposition chamber section 16 includes a
 deposition chamber 9 for housing a position controllable platform 11 that
 is used for supporting and positioning the traversable substrate 18. The
 deposition chamber section 16 also includes sensor wires 10, a vacuum/gas
 line 12 and a deposition chamber window 13 for further monitoring of the
 deposition process.
 As further shown in FIG. 1, the traversable substrate element 18 is used to
 build the three-dimensional object in a uniform, incremental manner. The
 resulting metal products are thus fine, equiaxed micro-structures
 relatively free of defects such as porosity or alloy segregation. This
 allows for a rapid metal forming that does not require expensive and
 time-consuming post-processing of the metal products.
 The uniform molten drop deposition system 20 can therefore be used to
 manufacture three-dimensional multi-material structures in layers without
 the need for special mandrels or tooling. In addition, the uniform molten
 drop deposition system 20 of the present invention can be used to
 manufacture three-dimensional parts and tooling directly from computerized
 virtual models. "Slicing" algorithms can be used in conjunction with the
 computer models to operate the uniform molten drop deposition system 20
 according to the desired shapes of the three-dimensional objects.
 FIG. 2 shows a side elevation view of a three-dimensional object formed by
 a uniform molten drop deposition system such as the one shown in FIG. 1.
 As shown in FIG. 2, semi-molten metal drops 101, each characterized by a
 drop diameter d and drop pitch p, are deposited layer-by-layer on a
 solidified metal substrate 103 to form a three-dimensional metal object
 100. When completed, the three-dimensional metal object 100 is thus
 comprised of a plurality of metal layers 102, each layer being
 characterized by a layer height h.sub.l. The aggregate of the metal layers
 102 forming the solidified metal substrate 103 is further characterized by
 a workpiece height h.sub.w. The three-dimensional metal object 100 can be
 built layer-by-layer as instructed, for example, by a computer-aided
 design ("CAD"), process-model integrated computer.
 In addition, a low-melting-temperature complementary material (not shown)
 can be deposited along with the molten metal to form a support structure
 for the molten metal during formation of more complex metal structures.
 Layers of this complementary material are deposited in the same manner as
 the metal layers 101. Because of the lower melting temperature of the
 complementary material as compared to the melting temperature of the
 metal, the complementary material can be removed immediately after
 completion of the formation process thus yielding the desired
 three-dimensional metal object.
 FIG. 3 shows an apparatus 300 for manufacturing a three dimensional object
 utilizing a continuously variable diameter jet according to a preferred
 embodiment of the present invention. As shown in FIG. 3, the continuously
 variable diameter jet apparatus 300 includes a piezoelectric oscillator
 302, which includes a piezoelectric transducer 309 for agitating molten
 metal, a pintle 303 for cooperating with the piezoelectric oscillator 302,
 a motion control unit 301 for positioning the pintle 303 and for
 controlling the position of a position controllable platform, a heated
 crucible 305, and a conical nozzle orifice 304 through which the molten
 metal flows. The heated crucible 305 is a reservoir for liquid metal 310,
 which the apparatus 300 uses to produce a continuously variable diameter
 liquid jet 306 and drops 307 of various sizes. The drops 307 are then
 deposited layer-by-layer on a traversable substrate element 308.
 Accordingly, the apparatus of FIG. 3 can be operated to produce a wide
 variety of drop sizes.
 As further shown in FIG. 3, the pintle 303 is responsive to the motion
 control unit 301 and thus the effective size of the orifice, or injector
 diameter, is variable. When the pintle 303 is completely withdrawn from
 the conical nozzle orifice 304, there is no flow constriction and the
 effective injector diameter is that of the conical nozzle orifice 304
 itself. When the pintle 303 is inserted into the conical nozzle orifice
 304, the effective flow area is reduced and the flow exiting the conical
 nozzle orifice 304 follows the contour of the pintle 303 to form the
 liquid jet 306 having a diameter smaller than that of the conical nozzle
 orifice 304. Thus, the apparatus 300 is effective for yielding molten
 metal liquid jets with variable size diameters, and consequently, molten
 metal drops of different sizes.
 Proper insertion of the pintle 303 in a downward direction towards the
 conical nozzle orifice 304 can reduce the diameter of the continuously
 variable diameter liquid jet 306 by a factor of three. In addition, with
 further optimization of the pintle 303 and the conical nozzle orifice 304
 structures, and with optimization of liquid jet flow rate controls, the
 diameter of the liquid jet 306 can be reduced by a factor of five. This
 corresponds to a factor of 25 change in flow area and hence mass flux.
 Since it is the thermal state of the drop that directly affects the
 deposited micro-structure, the metal product property is a very sensitive
 function of the drop size. Thus, the ability to vary the drop diameter
 from 50 to 250 .mu.m, for example, is a significant enhancement over the
 current technology.
 Advantageously, the variability of the drop size allows (1) formation of
 the micro-structure of the metal product from the deposited liquid metal
 to be controlled in a continuous process, and (2) the mass flux of the
 liquid metal to be directly controlled for optimum deposition of metal
 depending upon the outline geometry of the product at a given point.
 With the continuously variable diameter jet apparatus 300 of the present
 invention, for example, large-scale features of a metal product can be
 optimally formed using large metal drops, while fine features can be
 optimally formed using small drops for accurate representation of the
 designed geometry. In a preferred embodiment according to FIG. 3, the
 liquid metal 310 is ejected by applying a weak back pressure to the liquid
 metal 310 in the heated crucible 305 via an inert gas pressure source such
 as the external pressure source 17 of FIG. 1. As required, the pintle 303
 itself can serve as the oscillator to exert the external force required to
 break up the liquid jet 306 by directly connecting it to a piezoelectric
 oscillator 302. Alternatively, a separate oscillating plate (not shown)
 can be positioned near the conical nozzle orifice 304 to exert the force
 required to break up the liquid jet 306. The geometry of the pintle 303
 and the parameters of the applied oscillation are selected to provide
 optimum operation.
 Accordingly, feature-dependent drop size production is realized by the
 motion control circuit 301. In a further aspect of the present invention,
 the motion control circuit 301 provides the controls for producing a
 pre-programmed sequence of drops 307 with variable drop sizes. The motion
 control circuit 301 also controls the position of the traversable
 substrate element 308 such that the drops 307 are deposited in the desired
 locations. In addition, by applying an electrical field via a device
 similar to the charge plate 6 in FIG. 1, the drops 307 can be electrically
 charged to control trajectories and prevent agglomeration.
 FIG. 4 shows an apparatus 400 for manufacturing a three-dimensional object
 utilizing a controllable laminar planar jet according to a preferred
 embodiment of the present invention. As shown in FIG. 4, the controllable
 laminar planar jet apparatus 400 includes a piezoelectric oscillator 401,
 a heated crucible 420, a planar nozzle 440, preferably a rectangular
 contoured slit nozzle, loudspeakers 403, a function generator/audio
 amplifier 406 and a piezoelectric transducer circuit 500. The heated
 crucible 420 is used as a reservoir for liquid metal 430, which passes
 through the planar nozzle 440 as laminar planar liquid jet 402 and then is
 broken into horizontally cylindrical ligaments 499 via external pressure
 oscillation. Each of the horizontally cylindrical ligaments 499 are then
 agitated into an array of uniform drops 404 by the external pressure
 oscillation. The array of uniform drops 404 are then deposited onto a
 traversable substrate element 405.
 In forming the desired three-dimensional object, pressure oscillation is
 provided by the piezoelectric oscillator 401, which is driven by the
 piezoelectric transducer circuit 500 of FIG. 5, and loud speakers 403,
 which are driven by the frequency generator/audio amplifier 406. Drop
 sizes are precisely controlled by selecting the proper piezoelectric
 oscillation and audio signal parameters.
 The break-up of the laminar planar liquid jet 402 is made possible by the
 formation and amplification of surface disturbance waves. See N.
 Dombrowski and W. R. Johns, "The Aerodynamic Instability and
 Disintegration of Viscous Liquid Sheets", Chemical Engineering Science,
 Vol. 18, pp. 203-214 (1963). The resulting aerodynamic interaction between
 the laminar planar liquid jet 402 and the surface disturbance waves thus
 results in drop formation from the liquid surface. Via the piezoelectric
 oscillator 401 and the loudspeakers 403, the desired wavelength and wave
 orientation are selected as required to generate linear arrays of drops of
 uniform size.
 In a further aspect of the present invention, the loudspeakers 403 include
 two speakers each with a 50 W output and a frequency response between
 0.028 and 20 kHz. To enhance the acoustic excitation of the laminar planar
 liquid jet 402, an acoustic chamber (not shown) is provided around the
 laminar planar liquid jet 402. The excitation signal for the loudspeakers
 403 is generated by the function generator/audio amplifier 406. The
 generated signal is nominally a 0.00001 to 1 MHZ signal amplified to 75 W
 (max) by the audio amplifier of 406.
 Thus, the advantages of the controllable planar jet apparatus 400 are: (1)
 an ability to produce a large number of drops, and hence large mass flux;
 (2) an ability to spray arrays of metal drops such that metal layers are
 rapidly formed by one scan or sweep; and (3) applicability to high-speed,
 large scale manufacturing processes involving liquid metals.
 The piezoelectric oscillator of FIGS. 3 and 4 includes a piezoelectric
 transducer and a piezoelectric transducer circuit 500 as shown in FIG. 5.
 The frequency of oscillation for the piezoelectric oscillator is chosen
 according to the size and the volumetric flow rate of the drops to be
 generated. The drop size varies with the frequency of oscillation, e.g., a
 smaller size drop can be obtained by increasing the oscillation frequency
 and vice versa.
 FIG. 5 is a circuit schematic for the piezoelectric transducer circuit 500.
 The piezoelectric transducer circuit includes a function generator (not
 shown) supplying a sine or square wave waveform voltage to input terminal
 501, current limiting resistors 502 and 511-513, zener diodes 503 and 504,
 capacitors 505, 506, 508 and 509 for filtering the input voltage into the
 amplifier 507, a potentiometer 510, an amplifier 507 for amplifying the
 signal supplied by the function generator 501, fly-back diodes 514 and
 515, a transformer 516 for stepping up the signal amplified by amplifier
 507, a power supply 517 and shielded BNC cables (not shown). The
 piezoelectric transducer circuit 500 is used to drive the piezoelectric
 transducers of FIGS. 3 and 4, which are in turn used to produce both
 continuously variable diameter and controllable laminar planar liquid jets
 used to form the molten metal drops.
 In a further preferred embodiment of the piezoelectric transducer circuit
 500, the amplifier 507 has a constant output voltage response between the
 frequencies 0.5 to 25 kHz, i.e., the output voltage does not change with
 frequency when operated within the frequency range of 0.5 to 25 kHz. The
 amplifier 507 combined with the transformer 516 is capable of providing an
 output signal having a maximum peak-to-peak voltage of 800 V, although a
 peak-to-peak voltage level less than 800 V is required to excite the
 piezoelectric transducer 521. In a preferred embodiment of the present
 invention, the amplifier 507 is an Apex 8A operational amplifier and
 the transformer 516 is a 1:10 step up transformer.
 Also, the piezoelectric transducer circuit 500 of FIG. 5 is designed to
 protect the amplifier 507 from dangerous over-current conditions. Fly-back
 diodes 514 and 515 placed between the amplifier 507 and the transformer
 516 prevent current from flowing into the amplifier 507 when a polarity
 reversal occurs. The zener diodes 503 and 504 placed at the amplifier 507
 input protect the amplifier 507 by clipping over-currents that may be
 harmful to the amplifier 507. In addition, as shown in FIG. 5, current
 limiting resistors 502 and 511-513 prevent the amplifier 507 from drawing
 additional current.
 As will be understood, mechanical deformation of the piezoelectric
 transducer results when the piezoelectric transducer circuit output signal
 is applied, the degree of mechanical deformation depending on the
 magnitude of the applied signal. The deformation is used to oscillate the
 shaft (not shown) at a resonant frequency required to break the laminar
 planar or variable diameter liquid jets into horizontally cylindrical
 ligaments 499 (FIG. 4) or continuously variable diameter jets 306 (FIG.
 3), and subsequently into uniform drops 307 or 404 (FIGS. 3 and 4,
 respectively). In a preferred embodiment of the present invention, the
 material comprising the piezoelectric transducer is lead zirconate
 titanate manufactured by PiezoKinetics, Inc., of Ellefonte, Pa.
 Referring again to FIG. 2, the preferred embodiments of the present
 invention also include a position controllable platform 11 centered around
 a motorized stage (not shown) capable of translating in all three
 Cartesian axes with micro-meter resolution. The motorized stage is
 controlled by a computer (not shown), a position controller device (not
 shown), and an appropriate interface (not shown) between the computer and
 the position controller device (not shown).
 In a further preferred embodiment of the present invention, position
 controllable platform 11 is controlled by a desktop PC via an RS-232
 interface. Inputs from the PC to the position controller device are
 governed by a software package which accepts three dimensional data in
 CAD-type formats, slices the data into appropriate cross sections and then
 properly positions traversable substrate element 18 beneath the jets (not
 shown) to form each cross section. The completed workpiece is formed by
 building the cross sections from the bottom up in sequential order. The
 system is highly flexible in that a multitude of software packages can be
 used to design and store the desired forms. Additionally, designs can be
 transmitted electronically to the system allowing for remote operation and
 resource networking.
 Although the present invention has been described in connection with
 particular embodiments thereof, it is to be understood that such
 embodiments are susceptible of modification and variation without
 departing from the inventive concept disclosed. All such modifications and
 variations, therefore, are intended to be included within the spirit and
 scope of the appended claims.