Patent Publication Number: US-2015064047-A1

Title: Systems and methods for additive manufacturing of three dimensional structures

Description:
BACKGROUND 
     The present disclosure relates generally to the field of additive manufacturing (also referred to as three dimensional (3D) printing). Additive manufacturing has become more prevalent in recent years as an option not only to rapidly produce prototypes, but also to manufacture final products. While more commonly used to produce polymer objects, current advances have allowed additive manufacturing to also be used to produce metal objects. 
     SUMMARY 
     One embodiment relates to a method of fabricating a three dimensional structure, comprising delivering a metal material to a printing site; and defining a microstructure of the metal material at the printing site by controlling the delivery of heating energy to the printing site; and controlling the delivery of ultrasonic vibrations to the printing site. 
     Another embodiment relates to a method of fabricating a three dimensional structure, comprising delivering a metal material to a printing site; delivering heating energy to the printing site; delivering a vaporizable coolant to the printing site; and defining a microstructure for the metal structure based on providing the heating energy to the metal material at the printing site and vaporizing the vaporizable coolant. 
     Another embodiment relates to a method of fabricating a three dimensional structure, comprising delivering a first metal material to a first printing site; delivering a first amount of heating energy to the first printing site; delivering a first vaporizable coolant to the first printing site; agitating the first printing site; and forming a first portion of a printed metal structure by providing the first amount of heating energy to the first metal material at the first printing site and vaporizing the first vaporizable coolant while agitating the first printing site. 
     Another embodiment relates to a system for fabricating a three dimensional structure, comprising a support for supporting the structure; a material delivery device configured to provide a metal material to a printing site; a heating energy delivery device configured to heat the material at the printing site; and a vibration delivery device configured to provide ultrasonic vibrations to the printing site. 
     Another embodiment relates to a system for fabricating a three dimensional structure, comprising a material delivery device configured to deliver a metal material to a printing site; a heating energy delivery device configured to deliver heating energy to the printing site; a coolant delivery device configured to deliver a vaporizable coolant to the printing site; and an ultrasonic vibration delivery device configured to deliver ultrasonic vibrations to the printing site. 
     Another embodiment relates to a method of forming a three dimensional structure comprising delivering material, heating energy, and vibrations to a first printing site to define a first grain structure at the first printing site; and delivering material, heating energy, and vibrations to a second printing site to define a second grain structure at a second printing site; wherein at least one of the delivered material, heating energy, and vibrations differs between the first and second printing sites to modify the second grain structure relative to the first grain structure. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features descried above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a system for fabricating a three dimensional structure according to one embodiment. 
         FIG. 2  is a schematic side view of a printing device of the system of  FIG. 1  according to one embodiment. 
         FIG. 3  is a schematic side view of a printing device of the system of  FIG. 1  according to another embodiment. 
         FIG. 4  is a schematic side view of a printing device of the system of  FIG. 1  according to another embodiment. 
         FIG. 5  is a schematic view of a microstructure of a three dimensional structure according to one embodiment. 
         FIG. 6  is a block diagram of a control system for a device for fabricating a three dimensional structure according to one embodiment. 
         FIG. 7  is a flowchart of a method of fabricating a three dimensional structure according to one embodiment. 
         FIG. 8  is a flowchart of a method of fabricating a three dimensional structure according to another embodiment. 
         FIG. 9  is a flowchart of a method of fabricating a three dimensional structure according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. 
     Additive manufacturing is a process in which an object is built up layer by layer, with the desired geometry typically being read from a computer file and recreated by extrapolating the geometry into a series of thin layers. The layers may be cut and joined together with a lamination process, formed by selectively curing portions of a substance (e.g., stereolithography, etc.), or formed by transforming a powdered material to a solid mass by melting or otherwise fusing the powdered material together (e.g., selective laser sintering, fused deposition, laser deposition, etc.). Current additive manufacturing processes often do not produce objects with material properties that are suitable for use as a final product. Instead, the objects are often more suitable for display or prototype and proof-of-concept purposes. 
     Referring to the figures generally, systems and methods for fabricating a metal object with an additive manufacturing process are shown. In some embodiments, additive manufacturing is used to form an object by depositing material at various printing sites, or areas, in succession to eventually form a completed object. The additive manufacturing process can be configured such that the delivery of heating energy, material, cooling, and other processing is controlled locally at each individual printing site (or, alternatively, at sub-areas within an individual printing site). The microstructure of the fabricated object may therefore be controlled at each printing site and/or varied between printing sites to achieve a desired grain size, phase concentration, impurity concentration, pinning point distribution, or other characteristic. In this way, the fabricated object can be engineered to have superior material properties relative to objects provided with more conventional processes. 
     Referring now to  FIG. 1 , printing system  10  (e.g., an additive manufacturing system, etc.) configured to fabricate a metal structure is shown according to one embodiment. Printing system  10  includes printing device  12  (e.g., an additive or 3D printing device, etc.) operated by control system  14 . Printing system  10  can form object  18  using digital data, such as a 3D computer-aided design (CAD) model. Control system  14  may receive additional instructions or data from auxiliary system  16 . Auxiliary system  16  may be, for example, an external drive or storage device containing a CAD model and/or other control data. The CAD model may be generated with any suitable CAD program, and may be stored in any suitable digital file format. According to various alternative embodiments, one or both of control system  14  and auxiliary system  16  can be integrated into device  12 . 
     According to one embodiment, printing device  12  includes frame  20  and delivery device  22  movable relative to frame  20  via positioning system  24 . Printing device  12  may include a multitude of delivery devices configured to deliver a material (e.g., a powdered metal, a metal wire, a liquid metal, etc.) to form object  18 , as well as heating energy, cooling, agitation, or other means of manipulating the material, as described in more detail below. Printing device  12  forms object  18  by delivering and manipulating the material at successive printing sites  19  (e.g. printing zones or areas, work zones, delivery zones, fabrication zones, etc.). Object  18  is formed in interior  38  of printing device  12 . Interior  38  is defined by sidewalls  36 . According to one embodiment, interior  38  can be a sealed interior, which can have characteristics different from the characteristics of the surrounding environment. For example, the temperature, pressure, or other characteristics (e.g., composition of atmospheric gases, etc.) of interior  38  can be controlled to facilitate improved fabrication of object  18 . According to one embodiment, interior  38  can be maintained at a partial vacuum or in an atmosphere of an inert gas (e.g., argon, helium, etc.). 
     According to one embodiment, positioning system  24  is configured to position delivery device  22  using, for example, coordinates provided to printing device  12  from control system  14 . Positioning system  24  can use a Cartesian coordinate system, with delivery device  22  movable via carriage system  26 . Carriage system  26  includes a rail oriented in the X direction and a rail in the Y direction (providing X-Y horizontal movement), and a vertical adjustment member  28  (providing Z direction vertical movement). 
     Object  18  is supported by object support platform  30 , which is in turn supported by frame  20 . Platform  30  can be coupled to frame  20  via positioning system  32 . Positioning system  32  can be configured to position support platform  30  using coordinates provided to printing device  12  from control system  14 . According to one embodiment, support platform  30  is movable relative to frame  20  on a horizontal X-Y plane through carriage system  34 . According to a further embodiment, support platform  30  is further movable in a vertical direction using a vertical adjustment member (e.g., a vertical adjustment member similar to vertical adjustment member  28 , etc.). According to another embodiment, support platform  30  may not be movable and may instead be rigidly coupled to frame  20 . 
     According to another embodiment, rather than using an X-Y-Z Cartesian coordinate system, positioning systems  24  and  32  may use an alternative coordinate system, such as a cylindrical coordinate system, to position delivery device  22  and/or support platform  30 . According to other embodiments, positioning systems  24  and  32  may be configured to tilt or rotate support platform  30  or delivery device  22  about any of the X, Y, Z, or another positioning axis. 
     Positioning systems  24 ,  32  are configured to properly position object  18  relative to delivery device  22  during the 3D printing of object  18 . Object  18  is formed through an additive process, with material being selectively added to object  18  by delivery device  22  at printing site  19 . The added material is joined together or fused with material in neighboring printing sites (e.g., material below the printing site  19  on another plane, material surrounding the printing site  19  on the same plane, etc.) to form a solid object. 
     Referring now to  FIG. 2 , a portion of printing device  50  is shown according to one embodiment as a laser deposition device. Printing device  50  can be incorporated into a 3D printing system such as system  10  or a similar system. Printing device  50  forms object  52  supported by platform  54 . Object  52  is formed from material  56  delivered from first delivery device  58  to printing site  60 . Material  56  is melted by heating energy  62  delivered to printing site  60  from first delivery device  58  (e.g., via a laser, etc.). According to one embodiment, material  56  can be melted by heating energy  62  (e.g., heat or energy provided as radiant energy, etc.) and manipulated during and/or after the application of heating energy  62  with coolant  64  delivered to the printing site  60  from second delivery device  66 , and/or energy such as ultrasonic vibrations generated by a transducer shown as agitation device  68 . The manipulation of material  56  during formation at printing site  60  enables the controlled formation of a desired microstructure of material  56 . 
     According to one embodiment, material  56  is or includes a powdered metal material (e.g., tool steel, stainless steel (e.g., 420, 316, 304, etc.), nickel alloys, cobalt alloys, titanium alloys, etc.). The powdered metal is supplied to first delivery device  58  from a supply (e.g., hopper, feeder, bin, etc.). The powdered metal is ejected from first delivery device  58  from one or more nozzles  59 . Multiple nozzles  59  can be angled relative to one another to focus the streams of material  56  at printing site  60  spaced a distance away from first delivery device  58 . The flow rate of material  56  and the speed at which first delivery device  58  is moved relative to object  52  can be controlled to achieve a desired thickness for each printing site and/or layer of material forming object  52 . 
     While in some embodiments, material  56  is delivered to printing site  60  as a metal powder, according to other embodiments, material  56  may be a metal delivered in another form. For example, material  56  may be a solid metal delivered as a wire fed from first delivery device  58  to printing site  60 , or alternatively, may be a liquid metal delivered as a liquid metal stream or jet from first delivery device  58  to printing site  60 . Material  56  can be delivered in various other forms according to various alternative embodiments. 
     According to further embodiments, material  56  may be provided by material streams from several different supplies. For example, a primary material may be mixed with additives, such as particles configured to act as catalysts for nucleation, or grain refiners configured to retard the growth of dendritic grains. In other embodiments, material  56  may be provided by different elemental streams, with the flow rate of the different streams varied during the fabrication of object  52  to achieve different alloy compositions in different portions of object  52 . 
     According to one embodiment, first delivery device  58  is configured to provide a first material at a first printing site and a second material at a second printing site. For example, the amount of material, the rate of deposition of material, the composition of the material, or other parameters may be varied between the first material delivered to the first printing site and the second material delivered to the second printing site. Varying various parameters associated with the delivery of material to different printing sites enables variation of the microstructure (e.g., grain structure, etc.) between printing sites. 
     Referring further to  FIG. 2 , in one embodiment, heating energy  62  is provided by a laser directed at printing site  60  by first delivery device  58 . The laser may be any suitable laser capable of providing the heating energy needed to melt material  56 . For example, the laser may be a fiber laser with an optical fiber doped with a rare-earth element (e.g., erbium, ytterbium, neodymium, etc.), another type of solid-state laser, or a gas laser. In some embodiments, first delivery device  58  may further provide a volume (e.g., envelope, sleeve, etc.) of a shielding gas surrounding the laser and/or material  56  that differs from the atmosphere in the interior of the printing device to provide a more favorable environment for the fabrication of object  52  (e.g., to limit oxidation, etc.). 
     The laser provided by first delivery device  58  is configured to generate a “melt pool” of molten material, and provide precise control of the size and depth of the melt pool. As such, a relatively narrow heat-affected zone surrounds the melt pool, thereby minimizing thermal distortion of the portions of object  52  surrounding printing site  60 . Printing site  60  can be heated by any method or combination of methods, including through the use of a laser. According to other embodiments, heating energy  62  can be provided via first delivery device  58  in another form, such as an electron beam or a micro-arc. According to still another embodiment, heating energy  62  can be delivered to printing site  60  through conduction by local resistance heating of object  52 , by thermal conduction from a (small) heat source contacting printing site  60 , or by controlling the temperature of material  56  delivered to printing site  60 . 
     In one embodiment, first delivery device  58  focuses heating energy  62  at printing site  60  (e.g., with a lens, etc.), and material  56  melts to form a melt pool. First delivery device  58  forms a bead of material (e.g., a weld bead, etc.) as it is moved relative to platform  54 . The bead includes material deposited at successive printing sites, and forms a layer of solidified material on the X-Y plane. The bead may be a continuous bead of material, or alternatively, a non-continuous bead of material. Successive layers of material are fused together to form object  52 . By controlling the delivery of heating energy  62  to printing site  60 , the melting of material  56  can be controlled. Although controlling only the delivery of heating energy  62  provides some control over the post-melting solidification of material  56  and of the resulting microstructure of object  52 , more precise control is possible, as in conventional metal formation, by also controlling the quenching (i.e., the cooling) of the molten metal. 
     According to one embodiment, first delivery device  58  is configured to vary the heating energy provided to first and second printing sites. For example, the amount of heating energy, the intensity of heating energy, the delivery method, or other parameters may be varied between the first and second printing sites. Varying various parameters associated with the delivery of heating energy to different printing sites enables variation of the microstructure (e.g., grain structure, etc.) between printing sites. 
     In one embodiment, the microstructure of object  52  is further controlled through cooling of the melt pool (e.g., material  56 ) at printing site  60 . For example, a coolant  64  can be locally delivered by second delivery device  66 . Second delivery device  66  can be any mechanism suitable for the delivery of coolant  64  to printing site  60 . Coolant  64  can be delivered directly to printing site  60 , or alternatively, can be delivered indirectly to printing site  60  by, for example, cooling areas of object  52  surrounding printing site  60  (i.e., neighboring printing sites) or by cooling platform  54  supporting object  52 . By locally cooling printing site  60 , the microstructure of different portions of object  52  can be individually controlled and/or varied. 
     The material at printing site  60  is locally cooled in a controlled manner, either through the direct or indirect cooling of the material. Local, controlled cooling allows the quenching of the material to be controlled to a greater degree than, for example, bulk cooling object  52 , or allowing object  52  to cool slowly to room temperature. Controlling the quenching of the material enables for the controlled formation of a desired microstructure (e.g., the transformation of austenite to martensite in steel, etc.) in object  52 . In some embodiments, the delivery of coolant  64  can be delayed to allow the material to remain at an elevated temperature for a period of time. 
     Printing site  60  can be locally cooled by thermal conduction to a small cooling probe such as a thermoelectric cooler, a heat pipe, a mini-cooling loop, or the like. In other embodiments, printing site  60  can be cooled by applying coolant  64  which absorbs energy from printing site  60 . Coolant  64  can respond to the absorbed heat by increasing its temperature (i.e., via its specific heat) and/or by undergoing a phase change (i.e., via latent heat). A vaporizable liquid coolant provides an effective embodiment of coolant  64 , because vaporization of the liquid provides an efficient way to absorb heat and because the coolant is directly removed from the site as it vaporizes, without leaving residuals. Coolant  64  can be a liquid with a relatively low boiling point, or alternatively, a liquid with a relatively high boiling point, with coolant  64  chosen such that the boiling point of coolant  64  corresponds to a desired quench temperature and/or cooling rate for the material at printing site  60 . Coolant  64  can be or include water, alcohol, an oil, a solvent, or a liquid metal, including, but not limited to, sodium, sodium-potassium alloy, sodium-lithium alloy, lithium, or a mixture of liquid metals. In some embodiments, the boiling point of coolant  64  can be varied by controlling the pressure of the interior of printing device  50 , by modifying the composition of coolant  64 , etc. 
     According to one embodiment, delivery device  66  is configured to deliver a high-speed stream of coolant  64  in the form of a vaporizable liquid to printing site  60 . According to another embodiment, delivery device  66  is an atomizer configured to deliver liquid coolant  64  as a mist. According to yet another embodiment, delivery device  66  is or includes a device such as a wick, brush, or tube that directs a low-speed stream of a liquid coolant to printing site  60 . According to a further embodiment, delivery device  66  can be a fan configured to direct a stream of coolant  64  in the form of a gas (e.g., air, an inert gas, etc.) at printing site  60  to cool material by convection. According to various alternative embodiments, combinations of one or more coolant delivery devices can be used to deliver coolant  64 . 
     In one embodiment, coolant  64  is not delivered directly to material to printing site  60 , but rather is provided as a part of a heat pipe or similar system incorporated into or separate from delivery device  66 . A heat pipe can include a casing with a first end proximate the melt pool at printing site  60  (e.g., on the surface of the object  52 ). The first end of the heat pipe absorbs heat through the walls of the casing and vaporizes a liquefied coolant contained within the casing. The vaporized coolant releases latent heat at a second end and condenses back to a liquid. One or both ends of the heat pipe can include features such as a heat sink to facilitate the transfer of heat between the outside environment and the coolant contained within the heat pipe. The coolant contained within the heat pipe can be chosen to achieve a preferred heat transfer from printing site  60 . The internal pressure of the heat pipe can also be chosen and/or varied to control the phase changes of the coolant and further control the heat transfer from the printing site. 
     According to one embodiment, coolant  64  can be delivered continuously to printing site  60 . Alternatively, coolant  64  can be delivered intermittently (e.g., in a digital manner, etc.) to printing site  60  to achieve a desired microstructure. Various coolants, delivery devices, and delivery durations may be utilized for object  52  to form a metallic structure with varied microstructures. In some embodiments, delivery device  66  can be operated based on feedback data collected from sensors monitoring the fabrication of object  52 , as described in more detail below. 
     Printing device  50  may further include a system for removing coolant (e.g., vaporized or heated liquid coolant) from the surface of object  52  or the interior of printing device  50 , after the coolant has been utilized to cool printing site  60 . For example, printing device  50  may include a gas circulation system (e.g., incorporated into delivery device  66  or another component of the printing system) configured to remove gas from the interior of printing device  50  through an outlet duct and introduce gas to the interior of the printing device through an inlet duct. After being removed from the interior of printing device  50 , the gas may be scrubbed, cooled or otherwise processed and returned back to the interior of printing device  50 . Printing device  50  may include multiple inlet and outlet ducts such that the ducts can be opened, closed, or reversed to advantageously control the movement of gas within the interior of printing device  50  and across the surface of object  52 , including proximate printing site  60 . 
     According to one embodiment, delivery device  66  is configured to provide a first coolant to a first printing site and a second coolant to a second printing site. For example, the type of coolant, the amount of coolant, the timing or rate of delivery of coolant, the predefined delivery temperature of the coolant, the composition of the coolant, or other parameters may be varied between the first and second printing sites. In some embodiments, the delivery of heating energy (e.g., from delivery device  58 ) may be interrupted during the delivery of the coolant. In some embodiments, the delivery of coolant can be nonsimultaneous with the delivery of heating energy at a site. In some embodiments, the delivery of coolant can begin after the delivery of heat energy begins at a site. In some embodiments the delivery of coolant can continue after the delivery of heat energy has stopped at a site. Varying various parameters associated with the delivery of coolant to different printing sites enables variation of the microstructure (e.g., grain structure, etc.) between printing sites. 
     In some embodiments, the microstructure of object  52  is further controlled by subjecting the material at printing site  60  to agitation, such as by sound waves (e.g., acoustic waves, ultrasonic waves, etc.). According to one embodiment, the waves are generated by agitation device  68  (e.g., agitator, wave generator, etc.) and directed at object  52 . Agitation device  68  is positioned and configured to direct the waves to printing site  60  to induce local vibration in object  52 . According to various alternative embodiments, agitation device  68  can be a piezoelectric transducer, a magnetostrictive transducer, a surface acoustic wave (SAW) generator, a bulk acoustic wave (BAW) generator, or a standing wave field generator (e.g., an ultrasonic wave field generator, etc.). According to one embodiment, as shown in  FIG. 2 , agitation device  68  can be positioned remote from printing site  60  and can be configured such that the waves are steered to or focused at printing site  60 . Wave generation and steering and/or focusing can utilize a coherent array of wave generators (e.g., with phase and/or amplitude control of each); phase conjugation can be used to help control such remote wave delivery. According to another embodiment, agitation device  68  can be positioned proximate to printing site  60 . 
     In one embodiment, agitation device  68  provides ultrasonic vibrations to printing site  60 . Ultrasonic vibrations applied to a solidifying metal or alloy can decrease the size of the grains, increase the soundness of the grains, and/or decrease the occurrence of dendritic grain formation in the material. When molten material in the melt pool at printing site  60  is near the melting point (for a pure metal) or liquidus temperature (for an alloy), ultrasonic waves can influence the formation of solid nuclei, which leads to the corresponding formation of grains in the solidifying material, the grains being increased in number and decreased in size. 
     The amplitude and frequency of the waves produced by agitation device  68  can be controlled to produce grains of a desired size. According to one embodiment, agitation device  68  is configured to produce waves with a frequency selected based on a desired microstructure (e.g., grain size, etc.). The frequency produced by agitation device  68  can be maintained at a constant level for the entire fabrication process, or alternatively, can be altered to facilitate the growth of grains of different desired sizes in different portions of object  52 . The wavelength of the waves produced by agitation device  68  can also be configured to produce grains of a desired size. In some embodiments, the delivery of waves can be nonsimultaneous with the delivery of heating energy at a site. In some embodiments, the delivery of waves can begin after the delivery of heat energy begins at a site. In some embodiments, the delivery of waves can continue after the delivery of heat energy has stopped at a site, e.g., to perform ultrasonic peening. In some embodiments, agitation device  68  may be operated based on feedback data collected from sensors monitoring the fabrication of object  52 , as described in more detail below. 
     According to one embodiment, agitation device  68  is configured to provide differing waves to induce different vibrations at first and second printing sites. For example, the amplitude, wavelength, or other parameters associated with the delivery of the waves may be varied between the first and second printing sites. Varying various parameters associated with providing vibrations to different printing sites enables variation of the microstructure (e.g., grain structure, etc.) between printing sites. 
     According to one embodiment, the microstructure of object  52  can be further controlled by subjecting the material in the melt pool at printing site  60  to other processing or conditions. For example, magnet  69  can be provided proximate to printing site  60 . Magnet  69  produces a magnetic field that passes through printing site  60 . For magnetic materials (e.g., many steel alloys) the magnetic field influences the grain formation as the material in the melt pool solidifies and cools. Magnet  69  can be a permanent magnet generating a constant magnetic field, or may be a variable magnet (e.g., an electromagnet) that can be controlled to produce a variable magnetic field. 
     Referring further to  FIG. 2 , in some embodiments, printing site  60  is monitored to provide feedback data to printing device  12 . The data may then be utilized by printing device  12  to control the printing process to achieve the desired microstructure in object  52 . According to one embodiment, printing device  50  may include image monitoring device  70 , and one or more sensors  72  to collect data from printing site  60 . 
     In one embodiment, image monitoring device  70  (e.g., an image capturing device, etc.) is configured to monitor the microstructure of object  52 . Image monitoring device  70  can be an optical microscope, an electron microscope, an x-ray microscope, etc. Optical microscopes can be used to examine relatively large microstructures, while electron microscopes and x-ray microscopes can be used to examine relatively small images (e.g., features or structures smaller than approximately one half micron). Image monitoring device  70  may include multiple devices, allowing the microstructure of object  52  to be examined at different scales simultaneously. Image monitoring device  70  captures an image (e.g., a still image or a video) of object  52 . The image may be transferred to an analysis device and be utilized to collect data concerning the microstructure at printing site  60 , such as an average grain size, or the formation of various phases of the material. According to one embodiment, image monitoring device  70  captures images of object  52  after the material at printing site  60  has solidified. Image monitoring device  70  may therefore be configured to capture images of an area trailing the current printing site  60 . Image monitoring device  70  may be configured to collect further visual data, such as by capturing an image of a portion of object  52  surrounding the current printing site  60 . The additional image data may be utilized, for example, to monitor the heat-induced distortions in the microstructure of object  52  surrounding printing site  60 , as caused by the heating energy provided to create the melt pool at printing site  60 . 
     Sensors  72  may be configured to collect a wide variety of data concerning the portions of object  52  at printing site  60 . According to one embodiment, sensor  72  can be or include a thermometer configured to monitor the temperature of printing site  60  or the portion of object  52  surrounding printing site  60 . For example, sensor  72  may be a contact thermometer, such as a thermocouple in direct contact with object  52 , or may be a non-contact thermometer, such as an infrared thermometer that is disposed away from object  52 . Sensor  72  may be an array of multiple thermometers configured to monitor the temperature at several locations at and/or surrounding printing site  60 . According to another embodiment, sensor  72  can be or include a vibration transducer configured to monitor the longitudinal or shear waves produced by agitation device  68 . According to other embodiments, sensor  72  may include multiple types of sensors that operate together to monitor multiple phenomena related to the solidification of the material forming object  52 . 
     Referring now to  FIG. 3 , a portion of printing device  80  is shown according to one embodiment as a laser deposition device. Printing device  80  forms object  82  supported by a platform  84  in a manner similar to the printing device  50  shown and discussed with respect to  FIG. 2 . Object  82  is formed from a material  86  delivered from a first delivery device  87  to printing site  90 . Material  86  is melted by heating energy  92  delivered to printing site  90  from a second delivery device  89 . According to one embodiment, material  86  is manipulated during and/or after the application of heating energy  92  with substances such as coolant  94  delivered to printing site  90  from third delivery device  97 , energy such as vibrations generated by agitation device  98 , or by a magnetic field generated by magnet  99 . The delivery of material  86  and heating energy  92  via separate delivery devices  87  and  89  may advantageously provide for the improved melting of material  86  and/or creation of a melt pool at printing site  90 . 
     According to one embodiment, the additive manufacturing system is configured to provide, or define, different microstructure at or within different portions of an object. For example, as discussed in greater detail below, one or more image capture devices, sensors, etc. may be configured to provide feedback regarding the formation of an object, and in response, one or more parameters associated with the delivery of material, heating energy, vibrations, coolant, etc. can be varied between printing sites. 
     After being formed with a printing process, the fabricated object may be subjected to further processing, such as heat treating (e.g., annealing, tempering, etc.) and the like. Such post-printing processing enables further altering of the microstructure and the mechanical properties of the material beyond what may be possible during the 3D printing process. 
     Referring now to  FIG. 4 , a schematic top view of a portion of printed metal object  100  is shown according to one embodiment. Material from delivery device  102  is melted and solidified at printing site  104 . In one embodiment, as delivery device  102  is moved relative to object  100 , bead  106  of solidified material is formed on the surface of object  100  (the surface of the object being material printed at previous print sites and/or in previous layers). By controlling the delivery of material and heating energy to printing site  104 , as well as the delivery of ultrasonic or acoustic waves and the rate of cooling through the delivery of a coolant, the mechanical properties of object  100  can be controlled. The heat affected zone  108  can be minimized by providing heating energy to printing site  104  in the form of a laser or an electron beam. Locally controlling the heating energy, material, agitation, cooling, and other factors, as opposed to subjecting the entirety of object  100  to “bulk” conditions (e.g., with a bulk cooling process, etc.), allows the mechanical properties of object  100  to be varied between different areas/printing sites of object  100 . Mechanical properties may be further controlled within different portions of printing site  104  (e.g., local cooling or quenching of material may generate a microstructure in center  110  of printing site  104  that is different than the microstructure at the periphery of printing site  104 ). 
     Referring now to  FIG. 5 , an example microstructure of an object formed by the printing devices disclosed herein is shown. A desired microstructure is created by locally controlling the heating energy, material, agitation, cooling, and other factors as the material is printed, thereby allowing the fabricated object to have desired mechanical properties (e.g., strength, toughness, ductility, hardness, etc.). According to one embodiment, the microstructure is configured to have a relatively small grain structure including a multitude of small grains  120  (e.g., crystallites) separated by grain boundaries  122 . Grain boundaries  122  represent disconnects between crystal lattices of neighboring grains  120 , and impede the movement of dislocations through the material. A fine grain structure increases the number of grain boundaries  122 , and increases the yield strength of the material. A large grain structure, conversely, lowers the yield strength of the material, but increases ductility and electrical and thermal conductivity. Local control of heating energy, material, agitation, cooling, and other factors allows the grain structure to be written, or printed, as desired, allowing different portions of the manufactured object to have different mechanical properties. The local control of the printing process may also be used to vary the mechanical properties of the material in other ways, such as by varying the presence and/or concentrations of different phases of the material, the presence and/or concentration of dislocations, pinning points, impurities, etc. 
     Referring now to  FIG. 6 , a schematic block diagram of printing system  130  is shown according to one embodiment. Printing system  130  includes 3D printing device  132  operated by control system  134 . Printing device  132  forms an object using digital data, such as a 3D computer-aided design (CAD) model. Printing device  132  can be the same or similar to any of the other printing devices discussed herein. Furthermore, printing system  130  may include one or more auxiliary systems  136  (e.g., computer systems, etc.). 
     According to one embodiment, control system  134  includes processor  140  and memory  142 . Processor  140  may be implemented on a chip, integrated circuit, circuit board, etc., as a general purpose processor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGAs), a group of processing components, or other suitable electronic processing components. Memory  142  can be or include one or more devices (e.g., RAM, ROM, Flash memory, hard disk storage, etc.) for storing data and/or computer code for completing and/or facilitating the various processes described herein. Memory  142  can be or include non-transient volatile memory or non-volatile memory or non-transitory computer readable storage media. Memory  142  can include database components, object code components, script components, or any other type of information structure for supporting the various activities and information structures described herein. Memory  142  can be communicably connected to the processor and include computer code or instructions executable by the processor for executing one or more processes described herein. Control system  134  can include one or more modules configured to use data and code stored in memory  142  to execute a process via processor  140 . 
     Control system  134  further includes input device  144  and output device  146 . Input device  144  can be a mouse, keyboard, trackball, touchscreen or any other device that allows a user to input instructions to control system  134 . Input device  144  can be used, for example, in combination with a graphical user interface to allow a user to control various parameters associated with the operation and monitoring of printing device  132  or auxiliary systems  136 . Output device  146  can be a visual output device, such as a monitor (e.g., a CRT monitor, LCD monitor, LED monitor, etc.), an audio device, or another device. 
     Control system  134  can receive additional instructions or data from auxiliary system  136 . Auxiliary system  136  can be, for example, an external drive or storage device containing the CAD model and other control data. The CAD model can be generated with any suitable CAD program and can be stored in any suitable digital file format. The geometry of the CAD model is analyzed and divided into a multitude of slices, layers, or portions that correspond to portions to be printed by printing device  132 . 
     As shown in  FIG. 6 , printing device  132  includes positioning system  150 , material delivery system  152 , heating energy delivery system  154 , coolant delivery system  156 , and agitation system  158 . Positioning system  150  controls the positions of the delivery devices relative to the platform on which the object is fabricated, and can be or include any of the positioning systems discussed herein. Positioning system  150  controls the delivery devices to form a bead of material in a desired path on the X-Y plane. In some embodiments, multiple passes of the delivery device in the X-Y (horizontal) plane forms a slice or layer of the object as defined by the CAD model. Movement of the delivery devices in the Z (vertical) direction positions the delivery devices to form successive layers. According to one embodiment, positioning system  150  may further control the position of the platform on which the object is formed, either in addition to or instead of controlling the position of the delivery devices. Positioning system  150  may further control the orientation of the delivery devices and/or the platform through rotation about one or more axis (e.g., the X-axis, Y-axis, Z-axis, etc.). 
     Material delivery system  152  controls the delivery of material from a supply to the printing site via a material delivery device, and can include any of the material delivery devices discussed herein. Material delivery system  152  can, for example, control the flow rate of a powdered or a liquid metal or the feed rate for a solid wire to the printing site. Material delivery system  152  can control the delivery ratio of two or more materials to a printing site to alter the composition of the material of different portions of the fabricated object. As such, different materials can be delivered to different printing sites of an object. 
     Heating energy delivery system  154  controls the delivery of heating energy to the printing site via a heating energy delivery device, and can include any of the heating energy delivery devices discussed herein. Heating energy delivery system  154  can control the operation of a laser, including focusing the laser at the printing site and controlling the power output of the laser. The heating energy delivery system  154  can operate the laser to provide continuous heating energy to the printing site, or can activate and deactivate the laser to provide intermittent heating energy to the printing site. According to other embodiments, heating energy delivery system  154  can be configured to control an electron beam or another heating energy delivery device, such as resistance heater configured to supply an electrical voltage applied to the object to heat the printing site by resistance heating. 
     Coolant delivery system  156  controls the delivery of coolant (e.g., a liquid or gas coolant) to the printing site via a coolant delivery device to reduce the temperature of the material at a desired rate, and can include any of the coolant delivery devices discussed herein. For example, coolant delivery system  156  can control the flow rate of a high pressure stream of a liquid coolant directed at the printing site or at a portion of the fabricated object proximate to the printing site. Coolant delivery system  156  can vary the type of coolant delivered or the rate/amount of coolant delivered depending on the material used and the desired cooling time. In some embodiments, the delivery of coolant can be delayed to allow the material to remain at an elevated temperature for a period of time. 
     Agitation system  158  controls the generation and delivery of sound energy to the printing site. Agitation system  158  can operate an agitation device (e.g., agitator, wave generator, etc.) to generate ultrasonic or acoustic waves at a desired amplitude and frequency, and can include any of the agitation devices discussed herein. Agitation system  158  can be configured to continuously generate waves, or alternatively, can be configured to engage and disengage the agitation device to intermittently generate waves. 
     Printing device  132  can further include other systems  159 . Other systems  159  can be utilized to, for example, control a magnet (e.g., an electromagnet) to generate a desired magnetic field at the printing site, or any other suitable device. Furthermore, it should be noted that according to various alternative embodiments, one or both of systems  156 ,  158  may be omitted. 
     Printing device  132  further includes a monitoring system  160  for monitoring the operation of the other systems of printing device  132  and the object fabricated by printing device  132 . Monitoring system  160  can be configured to visually monitor the printing site and the portions of the object surrounding the printing site. Monitoring system  160  can adjust the focus and/or magnification of a monitoring device (e.g., an optical microscope, electron microscope, etc.) to obtain an image of the microstructure of the material. In one embodiment, monitoring system  160  is configured to collect other data, such as pressure data (e.g., to monitor ultrasonic vibrations) and temperature, with a variety of sensors. The sensors can be positioned on the surface of the fabricated object or away from the object. The sensors are configured to collect data from the printing site, a portion of the object near the printing site, an area of the object away from the printing site, or the interior of the printing device. Data collected by monitoring system  160  is used to provide feedback on the formation of the object. The data can be used to adjust the parameters of one of the other systems (e.g., positioning system  150 , material delivery system  152 , heating energy delivery system  154 , coolant delivery system  156 , agitation system  158  or other systems  159 ) to adjust the microstructure of the object. The adjustments can be initiated automatically (e.g., by processor  140 ) or alternatively can be initiated manually (e.g., by a user with input device  144 ). For example, in one embodiment, processor  140  receives inputs from monitoring system  160  (e.g., temperature data, pressure data, etc.), and provides control signals to one or more of systems  150 ,  152 ,  154 ,  156 ,  158 , and  159  based on the inputs. 
     Referring now to  FIG. 7 , method  170  of fabricating a 3D metal structure with an additive manufacturing system is shown according to one embodiment. A material (e.g., material  56  or material  86 ) is delivered to a printing site ( 172 ). According to various embodiments, the amount, location, type, etc. of material provided can be controlled, and can vary within and between printing sites. Heating energy (e.g., heating energy  62  or heating energy  92 ) is delivered to the printing site ( 174 ). As discussed above, heating energy can be provided in a variety of ways, and the amount of heating energy and other parameters can be varied within and between printing sites. The printing site is agitated (e.g., by way of ultrasonic or acoustic waves generated by agitation device  68  or agitation device  98 ) ( 176 ). For example, various types of ultrasonic waves can be continuously and/or intermittently provided, and various characteristics of the waves (e.g., frequency, amplitude, etc.) can be varied within and between printing sites. The resulting properties of the fabricated metal structure are then monitored and the data is utilized to adjust the control parameters for the delivery of material, heating energy, and agitation to the printing site, or alternatively, to a subsequently printed portion of the printing site or a subsequently printed printing site ( 178 ). The process can then continue for subsequent printing sites until the object is formed. 
     Referring now to  FIG. 8 , method  180  of fabricating a 3D metal structure using an additive manufacturing system is shown according to another embodiment. Material (e.g., material  56  or material  86 ) is delivered to a printing site ( 182 ). Heating energy (e.g., heating energy  62  or heating energy  92 ) is delivered to the printing site ( 184 ). The delivery of material and/or heating energy to the printing site can be controlled in a manner similar to that discussed with respect of  FIG. 7 . A coolant (e.g., coolant  64  or coolant  94 ) is delivered to the printing site ( 186 ). The amount, location, type etc. of coolant provided can be varied within and between printing sites. The resulting properties of the fabricated metal structure are then monitored and the data is utilized to adjust the control parameters for the delivery of material, heating energy, and coolant to the printing site, or alternatively, to a subsequently printed portion of the printing site or a subsequently printed printing site ( 188 ). 
     Referring now to  FIG. 9 , method  190  of fabricating a 3D metal structure using an additive manufacturing system is shown according to another embodiment. Material (e.g., material  56  or material  86 ) is delivered to a printing site ( 192 ). Heating energy (e.g., heating energy  62  or heating energy  92 ) is delivered to the printing site ( 194 ). The printing site is agitated (e.g., by way of ultrasonic or acoustic waves generated by agitation device  68  or agitation device  98 ) ( 195 ). A coolant (e.g., coolant  64  or coolant  94 ) is delivered to the printing site ( 196 ). Other process parameters, such as the delivery of a magnetic field, etc. to the printing site can further be controlled ( 197 ). The resulting properties of the fabricated metal structure are then monitored and the data is utilized to adjust the control parameters for the delivery of material, heating energy, agitation, coolant and other processes ( 198 ). The method illustrated in  FIG. 9  may control the delivery of material, heating energy, agitation, coolant, or other processes in a manner similar to that discussed with respect of  FIGS. 7 and 8 . 
     While the systems and methods described herein relate to the fabrication of a metal part with laser deposition or similar technology, the local control of printing variables, along with monitoring and feedback systems, may be useful for other additive manufacturing processes involving metals or non-metals. For example, a selective laser sintering process may be utilized to form an object, and the process can be monitored to detect the size and concentration of pores in the fabricated object. This data may then be utilized to control, for example, the power output of the laser to achieve a desired final product. The systems and methods disclosed herein may be used in combination with other fabrication techniques according to various other alternative embodiments. 
     The present disclosure contemplates methods, systems, and program products on any machine-readable media for accomplishing various operations. The embodiments of the present disclosure may be implemented using existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose, or by a hardwired system. Embodiments within the scope of the present disclosure include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a machine, the machine properly views the connection as a machine-readable medium. Thus, any such connection is properly termed a machine-readable medium. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions. 
     Although the figures may show a specific order of method steps, the order of the steps may differ from what is depicted. Also two or more steps may be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.