Abstract:
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.

Description:
This is a divisional of application Ser. No. 09/010,923 filed Jan. 22, 1998, U.S. Pat. No. 6,216,765 and claims benefit of Provisional Application Ser. No. 60/052,427 filed Jul. 14, 1997. 
    
    
     This invention was made with support from the National Science Foundation under contract numbers DMI-9696062 XAA-009. Accordingly, the U.S. government may have rights in the disclosed invention. 
    
    
     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 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, Mich. 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 Stale on Deposit Microstructure in Spray Forming”,  Scrilpta 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 principle 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 by 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
     FIG. 1 is a sectional view of a preferred embodiment of an apparatus for manufacturing a three-dimensional object; 
     FIG. 2 is a side elevation view of a three dimensional object formed by the apparatus of FIG. 1; 
     FIG. 3 is a diagrammatic illustration of an apparatus for manufacturing a three dimensional object utilizing a continuously variable diameter jet according to a preferred embodiment of the present invention; 
     FIG. 4 is a further diagrammatic illustration of an apparatus for manufacturing a three dimensional object utilizing a controllable planar jet according to a preferred embodiment of the present invention; and 
     FIG. 5 is a schematic circuit diagram for a piezoelectric transducer circuit according to a preferred embodiment of the present invention. 
    
    
     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 spray a 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 l . The aggregate of the metal layers  102  forming the solidified metal substrate  103  is further characterized by a workpiece height h 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. 
     It has been shown that 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 μm, 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 since 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 PA08A 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.