Patent Publication Number: US-11654628-B2

Title: System, method and apparatus for fluidized bed additive manufacturing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/353,939, filed Mar. 14, 2019, which claims the benefit of U.S. Provisional Application No. 62/643,632, filed on Mar. 15, 2018. The contents of the above-referenced applications are incorporated herein by references in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to additive manufacturing and, in particular, to a system and a method that uses a selectively directed energy beam to sinter powder in a pattern to fabricate an article. 
     BACKGROUND OF THE DISCLOSURE 
     Additive Manufacturing has enormous potential for improving the way articles are made. Of the different additive manufacturing production methods available, laser sintering of powder is considered as one of the most promising of these methods. Laser sintering is a process based on dispensing powder from a hopper, spreading the powder in a smooth layer using a roller and then selectively irradiating the powdered material. As a result of the irradiation, the polymer particle partially melts or melts the surface of the particle. The particles then sinter by adhering to each other coalescing and solidifying thus producing the desired shape of the layer of powder irradiated. The pattern of sintered powder in each layer corresponds to a slice of the article being fabricated. This layer-by-layer process continues by sintering each subsequently applied layer. Repetition of the above steps results in the formation of a laser-sintered article lying in a bed of unused powder. The completed article is then dug out of the powder bed and dusted off. In this way the process can produce complicated three dimensional articles and is suitable for low volume production of high value articles. 
     Although the laser sintering technology described above is well suited for making low volume, high value articles with a complex shape, there are a number of limitations that prevents this technology from being suitable for high volume production of articles:
         The types of powder are limited to those with good flowability such that the roller can lay down a smooth layer. Generally only a few formulations such as, polyamide 11 and polyamide 12, are available. Suitable powder formulations are not only very limited, but also more expensive by at least an order of magnitude than commodity plastics. A larger range of cheaper powders is needed.   The amount by which the powder can be preheated is limited by a caking temperature above which the powder starts to clump and the roller cannot lay down a smooth layer.   Fabrication speed is an issue and is limited by including a step to lay down powder with the roller.   Good temperature control of the most recent layers fabricated is difficult in a quiescent powder bed. Poor temperature control can lead to deformation of the article or curling up of layers at edges and corners.   Heat removal from the fabrication process is inefficient in a quiescent powder bed. The amount of heat supplied by the laser or lasers can limit the speed of fabrication.   It is difficult to fabricate articles with a gradient in composition.   It is cumbersome to remove the completed article from the bed of unused powder.       

     SUMMARY OF THE DISCLOSURE 
     In general terms, the present disclosure can include a system, method, and apparatus that comprises fluidizing a bed of powder and using a directed energy beam to join (e.g., sinter) the particles of the powder in selected target areas. The target areas can be at or near the surface of a structure being fabricated. The pattern of the target areas can correspond to a cross-sectional slice of the model of the article being fabricated. Sintering particles in a pre-determined pattern fabricates portions of the structure being fabricated. Embodiments of the structure are designed to have an open porous structure. The porosity of the structure is sufficiently open so as to allow movement of the particles of the fluidized bed through the pores and to the build surface. 
     Embodiments can use the principle of powder fluidization to deposit powder at the build surface of a structure being fabricated by additive manufacturing. Fluidization is a process whereby a granular powder material is converted from a solid-like state to a fluid-like state. Fluidization can be achieved by passing a fluid (e.g., gas, liquid or supercritical fluid) up through a powder bed. This is generally done in a vessel with a distributor plate. The powder is above the distributor plate and the fluidizing medium is forced up through the distributor plate and through the powder bed. For the powder to be fluidized, the fluid velocity is high enough that the drag forces on the particles of the powder overcome gravitational forces, causing the particles to become suspended and collide with each other. If the particles are small enough, and the velocity is not so high as to form bubbles, then the bed expands smoothly in a homogeneous manner, with the top surface being well defined. When a powder bed of solid particles is thus fluidized it exhibits liquid-like behavior. For example a fluidized bed of powder can fill the volume of a chamber like a liquid so that the surface of the fluidized bed is more or less flat and perpendicular to gravity. Objects can be immersed in a fluidized bed. The liquid-like behavior of a fluidized bed causes objects with higher density than the fluidized bed density to sink. Also, fluidized powder can be transported, for example channeled through pipes. Further, the height of the surface of two connected fluidized bed vessels will tend to equalize. A wide variety of powder types are suitable for fluidization. The particles of the powder can have a wide range of compositions, different shapes, sizes and size distributions. Fluidized beds are also known to provide excellent heat transfer. 
     In current laser sintering machines, the powder is dispensed layer-by-layer by a roller to form a quiescent powder bed on top of the build surface of the article being fabricated. In contrast, embodiments of this disclosure can supply powder to the build surface(s) through pores in the open porous structure of the article being fabricated. Therefore, in general terms, this can be an “inside-out” or “exoskeleton” method for building up an article by using building material supplied from or through the inside of the article. The article being fabricated can be started on a build substrate that is held in place in the fluidized bed. In some versions, the build substrate is more or less horizontal and is porous with holes over at least the region where the fabrication of the base of the article is to start. The holes can be large enough and sufficient in distribution that the fluidized bed can pass through the holes of the substrate. At the start of the fabrication the substrate can be immersed slightly below the surface of the fluidized bed and selectively irradiated with a directed energy beam, such as a laser, so as to sinter the particles of the fluidized bed to the surface of the substrate in selected target areas. The target areas where the laser scans can be determined from a slice taken of a three-dimensional model of the structure in the computer or from a mathematical description of the structure of the article. There are several embodiments of the method to fabricate the porous structure using a fluidized bed to dispense powder. In one embodiment, as the structure is built up with sintered particles, it is submerged into the fluidized bed so that recently built portions of the structure are slightly below the surface of the fluidized bed. In another embodiment, the build substrate or build surface is raised above the fluidized bed, selectively irradiated so as to become tacky or molten at the target areas on the build surface and then submerged into the fluidized bed so as to pick up fresh particles. As well as lowering the structure being fabricated into the fluidized bed, the fluidized bed can be raised by various means. The type of structure fabricated can be sufficiently open and porous so as to allow the fluidizing medium and particles of the fluidized powder to be transported through the structure being fabricated. Because of this open porosity property, fresh powder can continue to be transported to the build surface by the fluidizing medium that flows through the porous structure. 
     A computerized model of the article can be used to direct the energy beam selectively to the target areas of the build surface corresponding to the cross section of the structure being fabricated. For example the boundary of the target area can be made up of scans of the laser focal point and the middle filled in by a raster scan. Alternatively a single point of the laser focal point can be used for the build surface corresponding to the cross-section of a structural element such as a strut. A computerized means can be used to control the height of the fluidized bed surface and the height of the structure relative to the fluidized bed vessel. A computer can control operations including fluidizing the bed, circulating the fluidizing medium, supplying powder to the fluidized bed and controlling temperatures, temperatures and flows. 
     Fluidized beds are excellent mediums for heat transfer, and this property can be used to control the temperature of the powder being brought to the build surface and to moderate the temperature of freshly built portions of the structure. The temperature of the fluidized bed can be controlled so that the powder is close to but below the temperature at which particles become tacky and stick to each other. By controlling the temperature, less energy needs to be supplied to the build surface to heat the particles. By being immersed in the fluidized bed, temperature gradients in the freshly built structure can be moderated, which can help to prevent geometric distortion of the structure. The upward flow of the fluidizing medium efficiently removes heat supplied by the energy beam to the build surface during the sintering process. The heated fluidizing medium can be removed from the top of the fluidization vessel and then can be cooled by a heat exchanger. The cooled fluidization medium stream can then be circulated back through the bottom of the fluidized bed vessel by a pump or a compressor. With efficient heat removal and good control of the temperature of the particles and structure being fabricated and the vessel as a whole, faster build times than existing solutions are possible. 
     During the sintering process, volatile by-products can be produced. The volatiles thus produced are carried away by the fluidizing medium, out of the fluidization vessel and to other areas of the process where a portion of the impurities can be removed by a separation unit. After the purification step, the fluidizing medium can be circulated back to the fluidized bed. 
     This disclosure is not limited to a particular type of powder, but is adaptable to many powder types that are fluidizable. These materials can include plastic, metal, polymer, ceramic powders, powders of composite particles or a mixture of such powders. Powders with a wide range of particle shapes, sizes and size distributions can be fluidized and used. 
     The composition of the powder in the fluidized bed can be varied at a specific rate by removing powder and adding powder of a different type to the fluidized bed and thus changing the composition of the fluidized bed at a specific rate. By doing so, the composition of the article being fabricated can be varied with a specific gradient. 
     The porosity of the structure can take many forms and can be designed so as to allow the easy, generally upward flow of the fluidizing medium and fluidized particles through the pores. All of the pores can be interconnected and can have regular or irregular structure. When the porous structure is immersed in the fluidized bed, the liquid-like properties of the fluidized bed allows it to fill the pores. Wall effects from the pore walls may tend to hinder fluidization, but these effects can be reduced by using larger pore sizes or changing the pore type, orientation or shape. For example, a porous structure with minimal walls can be composed of a three dimensional honeycomb network of struts interconnected at nodes. Each strut can be fabricated by the method of selective irradiation and particles sintering to the build surface. During fabrication of such a structure, the target areas of the build surface represent horizontal slices of a strut or node. The open porous structure can be designed to impart specific properties to the fabricated article. For example, the method can manufacture article with an open porous structure that is light weight and has a high stiffness-to-weight ratio. 
     In a conventional laser sintering process where powder is dispensed by roller, the powder is sintered together layer-by-layer until the completed part is formed. In contrast, embodiments of the disclosure can utilize both layer-by-layer and continuous methods. In the continuous method, portions of the structure can be built up continuously when particles from the fluidized bed are allowed to continuously deposit at build surfaces irradiated by the directed energy beam. 
     In some embodiments, the fabrication process of the porous structure can be continued so as to describe the volume of the intended article. When the article is completed, the article can be raised out of the fluidized bed by a lift mechanism, and/or the fluidized bed can be partially drained to a holding vessel so that the fluidized bed surface is below the base of the article or the build substrate. A number of useful processes can be performed at the end of the fabrication process, including heating and quenching at specific rates or reacting with a reactive gas. The vessel can then be cooled, isolated, purged and opened and the article can be removed from the vessel through a hatch. 
     The cost per part manufactured by the system and method above can be reduced by increasing the number of fluidized bed additive manufacturing units at a factory. In an embodiment, much of the equipment associated with recycling, heating, cooling, cleaning and pressurizing the fluidizing medium and much of the equipment associated with heating and supplying the powder to the fluidized bed is shared by an array of a large number of fluidized bed additive manufacturing units. 
     As can be appreciated from the above general description, the method and system reduces many of the limitations associated with fabrication of articles by methods that use a roller. The “inside out” method of feeding the fresh powder to the build surface has several advantages over the method of depositing powder layer-by-layer with a roller. Various aspects of my fluidized bed additive manufacturing system and method may have one or more of the following advantages:
         A broad range of particle types, compositions, shapes and size distributions can be used.   The efficient heat transfer in a fluidized bed allows the temperature of the fluidized powder to be controlled and temperature gradients in the fabricated structure to be moderated. This can help reduce curling and geometric distortion.   The upward flow of the fluidizing medium carries away heat from the sintering process and out of the fluidizing vessel and can help increase build speed when they would otherwise be limited by cooling.   Powder can be transported to the build surface, without the use of a roller. Eliminating the roller step means that build times are potentially reduced.   Articles with specific gradient properties in their structure can be produced by changing the composition of the powder in the fluidized bed at a specific rate.   After the fabrication is completed, the article does not need to be dug out of a static powder bed. The unused powder can be removed by lowering the height of the fluidized bed surface below that of the base of the article. This can help reduce turnover times.   If a gas is used as the fluidization medium, the ease of fluidization or heat removal can be improved, by pressurizing the gas and so increasing the gas density and heat capacity. A supercritical fluid such as supercritical CO 2  can be used. Metal powders have high density and that can make them difficult to fluidize smoothly. For this reason a liquid or supercritical fluid may be useful as fluidizing medium for metal powders.   The upward flow of the fluidizing medium helps to remove volatiles that can be produced during the sintering process.   Equipment can be shared by multiple fluidized bed additive manufacturing units in an array of fluidized beds.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate some of the embodiments of the system, method and apparatus. 
         FIG.  1    depicts a schematic view of an example of the fluidized bed additive manufacturing system. 
         FIG.  2   a - 2   d    depict views of various stages of fabrication of an example of a porous structure on a porous support. 
         FIG.  3   a - 3   g    depict perspective views of various stages of fabrication of an example of a porous structure on a porous support. 
         FIG.  4    depicts a schematic view of an example of an array of fluidized bed additive manufacturing units. 
         FIG.  5    depicts one or more of the many structures and/or arrangements that may be created by the method. 
         FIG.  6   a - f    depicts of additional structures that may be created by the method. 
         FIG.  7   a - e    depicts additional structures that may be created by the method. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a system and a method of forming an article having a porous three dimensional structure using additive manufacturing are disclosed. The system can use selective laser scanning technology to sinter powder at a build surface. The powder is admitted to the build surface by a fluidized bed. In some versions, the powder at the build surface is selectively sintered to fabricate a portion of a porous structure. The porous structure thus fabricated is the core of the article being built. A skin can be attached to the porous core structure to make the surface continuous instead of porous. 
       FIG.  1    provides a schematic view of an embodiment of the fluidized bed additive manufacturing (FBAM) system  10 . The system  10  comprises a primary vessel  12  that is impermeable fluids and may be insulated. The primary vessel  12  has a top and bottom being vertically spaced and may have a window  104  at the top. The primary vessel  12  defines a chamber and may have hatch  98  which can be opened to provide access to the chamber for inserting and removing articles  96  and cleaning. The hatch  98  is closed during manufacturing. A distributor plate  14  is disposed near the bottom of the primary vessel  12  and may be attached to the primary vessel  12 . The distributor plate  14  extends horizontally across the chamber and has a plurality of holes which may be distributed evenly across the distributor plate  14 . The distributor plate  14  can be a perforated plate or a porous diffusor or other suitable choices that are available. 
     Embodiments of a fluidized bed  16  can be disposed in the chamber above the distributor plate  14  and extends vertically from the distributor plate  14  to a surface  18 . The fluidized bed  16  is dynamic and may contain peaks and troughs. Herein, the surface  18  is referring to a horizontal line below which something would be submerged in the fluidized bed  16 . The fluidized bed  16  can include particles and a medium having a superficial velocity u that is greater than the minimum fluidizing superficial velocity u mf  for the particles but less than a minimum bubbling velocity. In some examples, the superficial velocity may be calculated by dividing the volumetric flow of the medium through the primary vessel by the cross sectional area of the primary vessel  12 . The particles have an average outer dimension d p  of at least about 10 microns and not greater than about 1 mm. In one embodiment, the fluidized bed may have a transparent zone that is at least partially transparent and extends a distance D below the surface  18 . 
     In some embodiments, a lift device  90  is in the chamber and is attached to the primary vessel  12 . The lift device  90  may vertically move a frame  92  attached to the lift device  90  which extends at least partially around the perimeter of the primary vessel  12  and has at least one opening  93  defined by a ledge. A substrate  94  extends across the opening  93  and is received and secured by the ledge. The substrate  94  defines perforations which are larger than d p  such that particles can flow through the perforations and may be of varying size. 
     Embodiments can include a first camera  106  mounted adjacent to the primary vessel  12  and pointed toward the substrate  94  for visually monitoring fabrication of the article. A second camera  108  of the infrared type may be mounted adjacent to the primary vessel  12  for detecting temperature variations within the chamber. The cameras  106 ,  108  monitor the level of the fluidized bed  16  and the state and temperature of the build. The output of the cameras is used by a computer  28  to determine the height of the surface of the fluidized bed  16 . The cameras  106 ,  108  can also be used to determine the height of the article  96  above the substrate  94  when it is lifted above the surface  18  of the fluidized bed  16 . The first camera  106  can be used to monitor for defects in the fabrication and the second camera  108  can be used to detect temperature variations. It can be appreciated that more than two cameras and cameras of different type and resolution can be used. 
     Some embodiments of the system  10  can comprise an energy beam source  100  which emits an energy beam  101  (shown by a dashed line) toward optical components  102 . The energy beam  101  can be a laser or an electron gun. The beam  101  can be manipulated by means of optical components  102 . It can be appreciated that there are many means to manipulate laser beams  101 . Beam  101  can be focused, split into multiple beams, pulsed, selectively directed in different directions, selectively masked and/or otherwise manipulated by the optical components  102 . Optical components  102  comprise a prism or prisms and a mirror assembly for selecting the direction of travel of the laser beam or beams  103 . It can be appreciated that there are many means of directing the aim of a laser beam, here the term aim meaning the propagation direction of the beam. The mirror assembly comprises mirrors and galvanometers coupled to the mirrors to selectively orient them. The movement of the galvanometers is controlled by the computer  28  so that the aim of the laser beam can be directed to scan in the target area on or near the surface  18  of fluidized bed  16  according to different patterns determined by a cross-section of a model of the article  96  to be fabricated. The laser beam or beams  103  may enter the fluidized bed vessel through window  104 . It can be appreciated that in different embodiments, some of the optical components  102  can be placed inside primary vessel  12 . Computer  28  controls the optical components  102  so as to direct the energy beam  101  at selective parts of a target at or near surface  18 . The energy beam  101  may be split into a plurality of energy beams  103  and the plurality of energy beams  103  are directed through the window  104  into the chamber and toward the substrate  94 . 
     Embodiments of a primary resistance heater  56  may be coupled to the primary vessel  12  and may extend into the chamber above the distributor plate  14  for regulating the temperature of the chamber and the fluidized bed  16 . A first temperature monitoring device  54 , which may a thermocouple, thermowell or any suitable temperature monitoring device, is disposed in the chamber for monitoring the temperature of the fluidized bed  16 . 
     In some examples, a first pressure tap  38  may be coupled to the primary vessel  12  toward the top and above the fluidized bed  16  and a second pressure tap  39  may be coupled to the primary vessel  12  directly above the distributor plate  14  for monitoring the pressure and calculating the mass of particles in the fluidized bed  16  based on the difference in pressure between the pressure taps  38 ,  39 . 
     Versions of the system  10  may also comprise a holding vessel  30  containing the medium. The holding vessel  30  may be fluidly connected through a first line  32  to a first valve  34  coupled to the first line  32  that controls flow from the holding vessel  30  and may be fluidly connected to a first flow meter  36  which monitors the flow of the medium from the holding vessel  30  through the first line  32  such that the medium can flow between the holding vessel  30  and the chamber. 
     Embodiments of the system  10  may also comprise a second line  23  which is fluidly connected to the first line  32  and to a second flow meter  26  attached to the second line  23  for monitoring the flow into the chamber and which is coupled to a second valve  24 . The second valve  24  is coupled to the second line  23  which controls flow into the chamber. 
     In one example, the system  10  may also comprise a third line  40  which is fluidly connected to the chamber and to a third valve  41  coupled to the third line  40  for controlling flow of the medium and the particles out of the chamber. The third line  40  may be coupled to a third flow meter for monitoring flow out of the chamber. The third line  40  may also be coupled to a unit  42  for separating particles from the medium. 
     In some versions, the system  10  may also comprise a recycle line  43  coupled to a first cooling device  50  for cooling the medium in the recycle line  43 . The cooling device may be a heat exchanger. The recycle line  43  may also be coupled to a second heat exchanger  52  for heating the medium in recycle line  43  and may be coupled a recycle-valve  53  for controlling flow out of the recycle line  43 . The recycle line  43  may also be fluidly connected to a fourth line  45  for adding medium to the recycle line  43 . 
     Other versions of the system  10  may also comprise a fifth line  48  coupled to a fourth valve  49  and to a reservoir of purge fluid used to purge the primary vessel  12  when the fourth valve  49  and the second valve  24  are open and the recycle-valve  53  valve is closed. 
     Another example of the system  10  may also comprise a sixth line  22  for receiving the medium from the fifth line  48  and the recycle line  43 . The sixth line  22  may be fluidly connected to a pump or compressor  20 . 
     Still another embodiment of the system  10  may also comprise a seventh line  44  which may receive flow from the third line  40  for purification in a separation unit followed by releasing the medium to the atmosphere. The seventh line  44  may be coupled to a seventh valve  46  for controlling flow through the seventh line  44  and to a seventh flow meter  47 . 
     Versions of the system  10  may also comprise a salvage vessel  70  containing additives and particles and the medium. The particles in the salvage vessel  70  may be fluidized. The salvage vessel  70  may be fluidly connected to an eighth line  77 , to an eighth valve  76  coupled to the eighth line  77 , to a ninth line  61 , to a ninth valve  62  coupled to the ninth line  62 , and to the chamber such that particles and the medium can be conveyed from the salvage vessel  70  through the eighth line  77 , the eighth valve  76 , the ninth line  61 , and the ninth valve  62  to the chamber. The system  10  may also comprise a fourth heat exchanger  78  coupled to the salvage vessel  70  for heating particles and the medium inside the salvage vessel  70 . A second temperature monitoring device  79  may be coupled to and extending into the salvage vessel  70  for monitoring the temperature inside the salvage vessel  70 . 
     Examples of the system  10  may also comprise a storage vessel  60  containing some of additives and some the particles. The storage vessel  60  may be fluidly connected to a tenth line  63 , a tenth valve  64  coupled to the tenth line  63 , the ninth line  61 , the ninth valve  62  and the chamber such that particles and the medium may be conveyed from the storage vessel  60  through the tenth line  63 , the tenth valve  64 , the ninth line  61 , and the ninth valve  62  to the chamber. The system  10  may also comprise a storage heating device  68  attached to and extending into the storage vessel  60  for heating particles inside the storage vessel  60 . The system  10  may also comprise a third temperature monitoring device  69  coupled to the storage vessel  60  for monitoring the temperature inside the storage vessel  60 . 
     Other versions of the system  10  may also comprise an eleventh line  71  which is fluidly connected to the chamber and coupled to an eleventh valve  72  and a twelfth valve  74  of the three-way type. The twelfth valve  74  may be coupled to a twelfth line  73  which may be fluidly connected to the salvage vessel  70  such that medium and particles may be conveyed from the chamber, through the eleventh line  71 , the eleventh valve  72 , the twelfth valve  74  and the twelfth line  73  to the salvage vessel  70 . 
     Embodiments of the system  10  may also comprise a thirteenth line  75  coupled to a twelfth valve  74  which may be fluidly connected to the storage vessel  60  such that the medium and some of the particles may be conveyed from the chamber through twelfth valve  74  and the thirteenth line  75  to the storage vessel  60 . 
     In one example, the system  10  may also comprise a fourteenth line  67  fluidly connected to the storage vessel  60  for conveying particles to the storage vessel  60 . 
     Versions of the aforementioned system  10  may be used to perform the method of fluidized bed additive manufacturing. The particles in the primary vessel  12  may be fluidized by means of a medium (liquid, gas or supercritical fluid) that passes up through the distributor plate  14 . Pump or compressor  20  forces the fluidization medium through lines  22 ,  23  into the base of the primary vessel  12 . The flow of the medium into the primary vessel  12  may be controlled by opening and closing the second valve  24 . The second flow meter  26  monitors the flow through the second line  23 . The second valve  24  and second flow meter  26  are connected to computer  28 , which can control the flow through the second line  23  by opening or closing the second valve  24 . Fresh medium can be admitted from the holding vessel  30  through first line  32 . The flow of the medium can be controlled by computer  28  by opening or closing the first valve  34  and monitoring the flow using the first flow meter  36 . The medium in the primary vessel  12  can be pressurized if it is a gas or converted to a supercritical fluid. The medium becomes a supercritical medium when the temperature and pressure in the primary vessel  12  are adjusted to be greater than the critical temperature and critical pressure, respectively, of the medium. The medium may also be near critical. Supercritical and near supercritical fluids may be advantageous due to their high heat capacities. The pressure in the primary vessel  12  may be monitored using the first pressure taps  38  located high up in the primary vessel  12 , above the surface  18  of the fluidized bed  16 . The second pressure tap  39  is located low in the fluidized bed  16  slightly above the level of the distributor plate  14 . The difference in pressure between pressure taps  38  and  39  can be used to determine the mass of particles in the fluidized bed. 
     In some embodiments, the medium flows out of the primary vessel  12  through third line  40 . Particles entrained in the medium flowing out of the primary vessel  12  can be removed by the unit  42 . The unit  42  may be a trap, a filter or a cyclone. All or part of the medium can be circulated back to the fluidized bed  16  by lines  43 ,  22  and  23  or can be sent through the seventh line  44  to be purified in a separation unit, collected, released to atmosphere or sent to an atmospheric flare. Purified medium can be returned to the recycle line  43  through the fourth line  45 . The flow of medium through the seventh line  44  can be controlled by opening and closing the seventh valve  46  with the flow measured by the seventh flow meter  47 . The fifth line  48  allows a purge fluid to enter the system  10  when the fourth valve  49  is opened. The recirculating medium in the recycle line  43  can be cooled by the first heat exchanger  50  and heated by the second heat exchanger  52 . Heating may be needed prior to the start of the fabrication to raise the temperature of the fluidized bed but below the sticky temperature at which particles adhere to each other. Cooling of the recirculating medium may be needed to remove heat supplied to the primary vessel  12  by the energy beam. As can be appreciated there are different embodiments of the lay-outs of the different streams and units such as heat exchangers, storage vessels, valves, lines and compressors of the system that would work. The layout in  FIG.  1    is one example. 
     It can be appreciated that appropriate outputs from different components of the layout can be fed back to computer  28 , and that the computer can take appropriate actions according to well established control practices by sending signals to open or close valves, or to control a heating unit or compressor or other such actions with other types of units. Therefore specific connections between computer  28  and the other units in  FIG.  1    are very prolific but are not shown because these connections would render the drawing difficult to read. 
     The temperature of the fluidized bed  16  can be monitored by thermocouple or thermowell  54  connected to computer  28 . An amount of heat can be supplied by the second heat exchanger  52  to heat the recirculating medium in the recycle line  43  and an amount of heat can be removed from the recirculating medium in the recycle line  43  by the first heat exchanger  50 . The amount of heating and cooling is adjusted by computer  28  to achieve a desired temperature in the fluidized bed. The fluidized bed temperature can be adjusted further by using heat exchanger or a resistance heater  56  in the fluidized bed  16 . A pressurized gas can be used as the medium in the system  10  and the method. Pressurizing the gas increases the heat capacity of the gas and therefore allows for more heat removal from the fluidized bed. A higher density gas can also make the fluidized bed  16  more expandable when the fluidizing velocity is increased. Especially if the particles have high density such as with metal particles, then it may be advantageous to use a supercritical medium or a liquid as the medium. 
     In some embodiments, particles are held in the storage vessel  60  and can be conveyed through the ninth line  61  and the tenth line  63  to the primary vessel  12 . The flow of particles can be controlled by computer  28  by opening or closing valves  62  and  64 . It can be appreciated that there is a wide range of equipment options available for conveying particles, including but not limited to pneumatic conveying and gravity assisted conveying. The particles in the storage vessel  60  can be heated by the storage heating device  68  which comprises an electrical resistance heater, a heat exchanger or other heating device. The third temperature monitoring device  69  is connected to the computer which controls the amount of heat supplied to storage vessel  60  so that the temperature of the particles in storage vessel  60  can be controlled. The third temperature monitoring device  69  may be a thermocouple, thermowell or any other suitable temperature monitoring device. The particles in the storage vessel  60  can be in a fluidized bed, in which case, the line supplying the fluidizing medium is not shown in the figure. Fresh particles fed through fourteenth line  67  are used to replenish the particles in the storage vessel  60 . 
     In addition, particles can be removed from the primary vessel  12  to the salvage vessel  70  through eleventh line  71  by opening the eleventh valve  72  and lining up the 3-way twelfth valve  74  to the salvage vessel  70  through the twelfth line  73 . Alternatively, the 3-way valve twelfth valve  74  can be lined up to feed particles back to the storage vessel  60  through thirteenth line  75 . The particles in salvage vessel  70  can be heated by the fourth heating device  78  which comprises an electrical resistance heater, a heat exchanger or other heating device. The second temperature monitoring device  79 , which may be a thermocouple, thermowell or suitable temperature monitoring device, is connected to the computer which controls amount of heat supplied to the salvage vessel  70  by the fourth heating device  78  thus controlling the temperature of the particles. The particle bed in the salvage vessel  70  can be a fluidized bed. Particles can be returned to the primary vessel  12  by conveying the particles through the eighth line  77  by opening the eighth valve  76 . 
     Versions can include a moveable lift device  90  that may be controlled by the computer  28  and can raise or lower the support frame  92  in the primary vessel  12 . Support frame  92  comprises a rigid frame and supports a perforated build substrate  94 . The support frame  92  is such that the fluidized bed  16  can easily pass through the opening of the frame  92 . The frame  92  can be rectangular but as can be appreciated, many other geometries are suitable, including extending mostly across the vessel  12 . The opening in the frame is such that the frame  92  does not block off portions of the build substrate  94  where fabrication is to start. The article  96  is fabricated on the build substrate  94 . Build substrate  94  has holes or pores, can be a perforated plate and can be composed of many types materials including the same or a similar type of material as the particles. The size of the holes in the perforated plate  94  can be at least large enough to allow particles from the fluidized bed  16  to pass through easily. Therefore the holes can be at least the average particle size d p  of the particles making up the fluidized particles. The number of holes can be sufficient in number and distribution such that holes are present at least in the vicinity where the fabrication is to occur. The holes may not be needed or desired in the area where no fabrication is to occur. In this embodiment the support frame is horizontal but it can take on different geometries depending on the geometry of the base of the article being fabricated. Hatch  98  in the primary vessel  12  is closed during operation of the system  10  and can be opened to remove the completed article  96  or to access, maintain or clean the interior of the primary vessel  12  or lift mechanism  90 . 
     In some embodiments, the cameras  106 ,  108  can monitor the level of the fluidized bed  16  and the state and temperature of the build. The output of the cameras  106 ,  108  is used by the computer  28  to determine the height of the surface  18  of the fluidized bed  16 . The cameras  106 ,  108  can also be used to determine the height of the article  96  above the build substrate  94  when it is lifted above the surface  18  of the fluidized bed  16 . Visuals from the first cameras  106  can be used to monitor for defects in the fabrication and infrared information from the second camera  108  can be used to detect temperature variations. It can be appreciated that more than two cameras and cameras of different type and resolution can be used. 
     To operate some embodiments of the system  10 , valves  24 ,  41  and  53  are opened and pump or compressor  20  is turned on so as to circulate medium through the primary vessel  12  and the lines  40 ,  43 ,  22  and  23 . Particles are heated to a desired temperature in the storage vessel  60 . Valves  62  and  64  are opened and the particles are conveyed into the primary vessel  12  where it may be fluidized by the medium forced up through the distributer plate  14 . The amount of particles conveyed is sufficient such that when the particles are fluidized, the height of the surface  18  of the fluidized bed  16  is slightly greater than the height of the top of the perforated build substrate  94 . The mass of particles in the fluidized bed  16  can be determined from the difference in pressure between pressure taps  38  and  39 . Cameras  106  and  108  can be used to determine the height of the surface  18  of the fluidized bed  16  relative to the perforated build substrate  94 . For example the cameras  106  and  108  can be used to determine when the top surface of the build substrate  94  is level with the surface  18  of the fluidized bed  16 . The build substrate  94  has sufficient number and size of openings so that the fluidized bed  16  can envelope and go through the build substrate  94 . The cameras  106 ,  108  are also used to determine the location of the holes or openings in perforated substrate  94 . Lift  90  is then lowered by a small amount such that the substrate  94  is immersed in the fluidized bed  16  such that particles from the fluidized bed  16  covers the substrate  94 . Laser  100  is turned on and beam  101  or beams  103  fuse the particles to substrate  94  in select locations. The target areas where the build-up of the article  96  starts are on the solid surface between the holes in the perforated surface so as not to block the openings. Computer  28  manipulates optical components  102  to direct the aim of the beam or beams  103  to selectively scan over and sinter particles at the target areas of the substrate  94 . The target areas lie within a horizontal cross section of the base of the three dimensional structure  96  to be fabricated. As sintered particles builds up in the target area forming a bottom portion of structure  96 , the structure increases in height, lift  90  lowers frame  92 , substrate  94  and partially fabricated structure  96  so that it remains immersed slightly below surface  18  of the fluidized bed  16 . The fused particles is cooled by up-flowing medium through the holes, and is positioned for additional particles to be transported by the fluidized bed  16  through the article  96  to the build surface. As the article  96  builds up and is lowered, the computer  28  changes the aim of beam or beams  103  to new target areas that correspond to the next cross-sectional slice of the article  96  to be fabricated. The beams  103  then selectively sinters particles at the new target areas. There are many methods of controlling the height of the structure being fabricated relative to the surface of the fluidized bed. One example is as follows. Computer  28  analyses images from cameras  106  and  108  to control the vertical position of lift  90  such that the surface of the article  96  is slightly below the surface of the fluidized bed  18 . This can be done for example by raising the top of the article  96  above the surface  18  of the fluidized bed to correlate the top with the surface  18  of the fluidized bed with the cameras  106 ,  108 , then lowering the article  96  by a small amount into the fluidized bed  16 . 
     The article  96  can be designed in such a way that it is largely porous and that the pores are continuous and connect to the openings in the perforated plate. The pores can also be interconnected and has an open porosity such that the pores are large enough relative to the particles that they can enter and be transported through the pores by the action of the fluidizing medium. 
     The porosity of the article  96  is the fraction of void space in the article. The void space is defined by the plurality of pores. The volume fraction of solid material in the article  96  is the density p of the article  96  divided by the bulk density ρ s  of the solid material used to manufacture the article  96 . The porosity is one minus the volume fraction of solid material, as shown below.
 
φ=1−ρ/ρ s  
 
     In some embodiments, the porosity is greater than 50%. In some embodiments, the porosity is greater than 90%, 99%, 99.9% and 99.99%. The porosity affects various properties of the article  96  such as the specific stiffness (stiffness to density ratio). For example, a lightweight article  96  with high specific stiffness would also have high porosity. Near solid articles  96  have low porosity such as 50% to 90%. 
     There are several benefits of these embodiments including that the fluidized bed  16  transports particles to the build surface through the pores. The fluidized bed  16  regulates the temperature of the newly built part of the article  96 . The fluidized bed  16  also provides buoyancy and so supports fragile members of the article  96  as long as the action of the fluidized bed  16  is not too vigorous. Additionally, the medium from the fluidized bed  16  can carry away volatiles from the sintering process. 
     The method may manufacture articles  96  that have high stiffness to density ratios. For lightweight porous material, the stiffness decreases with decreasing density p of the material. Young&#39;s modulus (E) is a measure of stiffness. E s  is the Young&#39;s modulus of the solid material. Lightweight materials such as foams and aerogels that are stochastic are known to decreases according to the below relationship.
 
 E/E   s ˜(ρ/ρ s ) 3  
 
     As shown by the below relationship, the method may manufacture articles  96  with a stiffness greater than stochastic materials.
 
 E/E   s &gt;φ/ρ s ) 3  
 
     In some embodiments the stiffness of the article  96  follows the below relationship. For example, the specific stiffness, E/□, of a titanium article  96  which has an E s  of 112.5 GPa and a bulk density ρ s  of 4500 kg/m 3  may be equal to or below 25×10 6  m 2 /s 2 .
 
 E/□≤E   s /ρ s  
 
     In some embodiments, the stiffness of the article  96  follows the below relationship.
 
 E /□≥(ρ/ρ) 2   E   s /ρ s  
 
     For example, the specific stiffness of a polystyrene article  96  which has a porosity φ of 0.9, an E s  of 3.2. GPa and a bulk density ρ s  of 1000 kg/m3 may be between 3.2×10 6  m 2 /s 2  and 0.032×10 6  m 2 /s 2 .
 
Polystyrene: 3.2×10 6  m 2 /s 2   &lt;E/ρ&lt; 0.032×10 6  m 2 /s 2  
 
     In addition to raising or lowering the lift  90 , the surface  18  may be raised or lowered by many methods. Examples of such methods include raising the surface  18  through bed expansion by increasing the flow of fluidizing medium through the second line  23  by opening the second valve  24 . An additional example may be reducing the flow of fluidizing medium into the primary vessel  12  by closing the second valve  24  to lower the surface  18 . The computer  28  uses input on the level of the fluidized bed surface  18  from the cameras  106 ,  108  or other level detection device  110  in combination with medium flow data to determine the amount to open or close the second valve  24 . The fluidized bed  16  is achieved when the superficial velocity of the medium u fulfills the following:
 
 u≥u   mf  
 
where u mf  is the minimum fluidization superficial velocity. The superficial velocity is also less than the bubbling velocity. A broad range of small particle sizes can meet the above criterion but generally are such that d p  is greater than about 10 micron. A wide variety of particle compositions, shapes and size distributions can be used and are suitable to meet the velocity criterion above. Broadly speaking it is desirable to have an expandable bed and by the term expandable it is meant the ability to expand and contract the fluidized bed vertically by adjusting the velocity. Polymer particles with d p &lt;about 200 microns can be suitable. For metals, particle density is generally high and smaller particle sizes than 200 micron may be suitable. In addition to decreasing particle size, the fluidized bed  16  can be made more expandable by using a more dense fluidizing medium such as a pressurized gas a supercritical medium or a liquid; by decreasing the particle size of the fluidized particles; or by increasing the viscosity of the fluid.
 
     In some examples, based on the amount of particles removed from the fluidized bed  16  by being incorporated into a portion of the article  96 , the computer  28  opens the tenth valve  64  and admits more particles from the storage vessel  60  to replenish the particles in fluidized bed  16  and maintain the surface of the fluidized bed  18  above the article  96 . As the fabrication process continues, the heat from the laser heats up the fluidizing medium. In order to maintain the desired temperature of the fluidized bed  16  as monitored by thermocouple  54 , cooling device  50  is used to remove heat from the fluidizing medium. Heat exchanger  52  may also remove heat from the fluidizing medium. 
     Undesirable volatiles can be released by the sintering process. These volatiles are carried away by the fluidizing fluid. As the volatiles build up in the fluid, they can be removed by purifying a portion of the medium through the seventh line  44  by opening the seventh valve  46  to a purification unit which can include a condenser or a distillation column or suitable separation unit. Once purified, the medium can be returned through the fourth line  45 . 
     In one version, t process of adding material to the structure being fabricated by selectively scanning and maintaining the top level of the structure slightly below that of the fluidized bed continues until fabrication of the final shape of the structure is complete. 
     In some embodiments, once the fabrication of the article  96  is complete, the laser  100  is turned away or off, and lift  90  is repositioned for removal of the article  96  through hatch  98 . To cool the part in a controlled manner, the temperature of the fluidized bed  16  is lowered by using cooling unit  50  and monitoring temperature with thermowell  54 . It may be necessary to cool the article  96  in a controlled manner to avoid geometric distortion. The valves  72 ,  74  are opened to remove particles to salvage vessel  70  and lower the level  18  of the fluidized bed  16  below the base of the fabricated part  96 . The seventh valve  46  can be opened to vent out the system. The system can be purged by feeding purge medium through the fifth line  48  by opening the fourth valve  49  and closing the recycle-valve  53 . Once the system is vented and cooled, valves  24  and  41  are closed to isolate the primary vessel  12 , hatch  98  is opened and the fabricated part  96  is removed. A new build substrate  94  is attached to the support frame  92  and the hatch  98  is closed. The system is then readied for the next fabrication by opening valves  24 ,  41  and  53 , closing the valves  49  and  46  and allowing fluidizing medium to enter the primary vessel  12  through the lines  22  and  23 . Particles from the previous build in the salvage vessel  70  are conveyed to the primary vessel  12  through lines  77  and  61  and by opening valves  76  and  62 . 
     Turning now to examples of the fabricated article  96 ,  FIG.  2   a - 2   d    depict various stages of the fabrication process.  FIG.  2   a    depicts a section of the perforated build substrate  94  as seen from directly above. The direction of the medium velocity may be perpendicular to the perforated substrate  94  and can easily pass through the holes. The build substrate  94  is attached to the support frame  92  and can be raised and lowered using lift device  90  controlled by computer  28 . The perforated plate  94  is lowered into the fluidized bed  16 , so that a small portion of the fluidized bed  16  is above the perforated plate  94 . 
       FIG.  2   b    depicts the perforated surface as shown at an angle from above with the laser aimed at the surface of the substrate  94  adjacent to some holes. The direction of fluidizing medium flow is upwards through the holes. The arrow indicates the upward direction of the velocity through one of the holes. The fluidized bed flows up through the holes in porous substrate  94 . The size of the holes is sufficiently large so that particles can easily pass through the holes under the action of the fluidizing medium. The substrate  94  is lowered slightly below the fluidized bed surface  18 , the action of the fluidized medium spreads the particles over the submerged substrate  94  and laser beam or beams  103  sinters particles to the substrate  94  in select locations. In this embodiment the computer  28 , directs the aim of the laser to scan over the area of the perforated substrate  94  adjacent to the holes and builds the material in an upwards direction in such a way as to maintain a pathways to the hole structure of the perforated substrate  94 . In this way a portion of the porous structure  96  is fabricated on the substrate  94  as depicted in  FIG.  2   c   . As can be seen, the pores in the article  96  fabricated by this method continue to allow the fluidizing medium and fluidized particles to pass through it. The medium in the fluidized bed  16  flows up through the pores and transports particles through the pores. The article  96  is lowered slightly below the fluidized bed surface  18 , the action of the fluidized bed  16  spreads the particles over the thus submerged article  96 . The laser then sinters particles to the top surface of the new structure in select locations to fabricate further portions of the structure.  FIG.  2   d    depicts a further portion thus fabricated. The process is continued until the structure is completed according to the model of the article in the computer. 
     Turning now to another example of the fabricated porous structure,  FIG.  3   a - 3   g    depict perspective views of various stages of the fabrication process of an open three dimensional honeycomb network structure. The structure is depicted very schematically as struts and nodes but may take on many forms.  FIG.  3   a    depicts a perspective view of a portion of the perforated fabrication substrate  94 . The process of fabrication is similar to that described above in  FIG.  2   a - 2   d    except that the pore structure is more open. In the example of  FIG.  3   a - 3   g    the pores are composed of unit cells defined by struts connected at nodes in a tetrahedral geometry. In  FIG.  3   b    the laser beam scans the target areas to form a portion of multiple predetermined unit cells. The article  96  is started by building up vertical struts  130  from the surface of the perforated substrate  94  as shown in  FIGS.  3   b  and  3   c   . The tops of the vertical struts form nodes  132  from which angled struts  134  are built up to connect at nodes  136  as depicted in  FIGS.  3   d  and  3   e   . From nodes  136 , vertical struts  138  are built up as depicted in  FIG.  3   f    This process is continued to form a larger portion of the porous article  96  as depicted in  FIG.  3   g   . The porous article  96  is interconnected and has pores large enough that the medium can pass through the structure and continue to transport particles within the article  96 . The process is further continued until the entire volume of the article  96  being fabricated is filled in with the porous structure made up of interconnected struts. 
     Embodiments of the unit cells are composed of struts that can have a cross-sectional diameter as small as the size of a single particle or larger. In some embodiments, the size of the particles are as small as 10 microns in diameter. Particles smaller than 10 microns in diameter may begin cohering and may have difficulty smoothly fluidizing. As shown in  FIG.  2   , the length of the struts, or any structure forming the article  96 , may be as long as the article  96 . In some embodiments, the length of the each strut is greater than quadruple the particle diameter. In some embodiments, the width of the pores are greater than about 20 microns. 
     Embodiments of the unit cells are surface based, being composed of surfaces that can have a cross-sectional diameter as small as the size of a single particle or larger. 
     EXAMPLE EMBODIMENTS 
     Embodiments of the system  10  and the method of the present disclosure are included below but are not limited to those included below. 
     As can be appreciated it is not necessary for the porous structure to be built of unit cells that have tetrahedral geometry. The unit cells can have many different designs that give the structure a selected porosity. By varying the design of the unit cells, the orientation, shape and size of pores can be varied. For example, the unit cells can have dodecahedral, octahedral as well as many other types of shapes. The unit cells may take many forms both regular and irregular in size and orientation of struts. Irregular geometries may especially be needed to describe the surfaces of the article. The orientation, size, shape and composition of the pores may be varied depending on various needs for the properties of the final fabricated part. 
     The article  96  may comprise microlattice material that has repeating cells defined by trusses (truss-lattice) or surfaces (surface based unit cell lattice). Microlattice materials can be lighter than air with densities less than 1 kg/m 3 . Surface based unit cells are optimized over truss based unit cells for higher specific stiffness. Examples of surface based unit cell lattices are shown in  FIGS.  5  and  6   . The article  96  may comprise triply periodic minimal surface (TPMS) lattices or any other repeating structure. The article  96  may comprises a microlattice that is irregular. Such an irregular microlattice may have a Young&#39;s modulus greater than E s /ρ s . 
     The structure of the article  96  may vary over its volume such as by including include different unit cells at different locations in the article  96 . For example, the article  96  may transition from truss based units cell to surface based until cells over its length. Repeating unit cells are not necessary. The article may have a continually changing structure, such as varying pore sizes and truss thicknesses. 
     As can be appreciated there can be different embodiments of the method. One embodiment is an iterative method. In this embodiment, the top surface of the portion of the structure under fabrication is raised up by lift  90  above the fluidized bed surface  18 , selectively heated above its sticky temperature or partially melted by the laser and then immersed below surface  18  in the bed where particles from the fluidized bed impinge on the hot tacky surface and stick. The structure with the particles sticking to the build surface can then be raised again above the fluidized bed and selectively heated by the laser, and sintered or fused to the build surface. The process is then continued with repeated dunking and irradiation of the structure so that successive layers of particles build up on the surface that is irradiated. 
     Another embodiment is a continuous method. In this mode, the structure is fabricated by a continuous process. The build surface is held slightly below the surface of the fluidized bed. The build surface is irradiated to maintain a sufficiently high temperature so that it is partially melted or has sufficient tackiness for particles that collide with the surface to adhere and thus allow accretion to occur. As fresh structure builds up at the build surface, the structure is continuously lowered or the surface of the fluidized bed continuously raised or both so that the top of the build surface remains slightly below the fluidized bed surface. Multiple laser beams can be used to irradiate multiple build surfaces simultaneously to trace out the pattern of the porous structure as the article is being lowered into the fluidized bed. 
     In another embodiment of the method, the particle bed is fluidized intermittently. In this embodiment, the structure being fabricated is submerged in the fluidized bed to a certain depth. The fluidization is then stopped by reducing the velocity of the fluidization medium below the fluidization velocity by closing second valve  24 . The depth to which the structure is submerged is such that when fluidization is stopped, the structure is covered by a thin layer of particles. Selective laser sintering can then take place on the quiescent layer of particles sitting on the surface of the structure so as to fabricate a cross-sectional slice of the structure. After sintering, the fluidizing medium velocity is increased to above u mf  to re-fluidize the bed, the structure is submerged once again and the process is repeated thus building up successive layers of the structure. 
     Many types of particles are suitable. Examples of the type of particles that can be used can have the following properties:
         The particles can be fused or sintered by an energy beam such as a laser or an electron gun. Examples of such particles include polymers, metals, ceramics, composites and mixtures thereof. Examples of polymers include thermoplastic polymers such as but not limited to polyamides, polypropylene and polyethylene as well as thermosetting polymers. Examples of metals include but are not limited to titanium and its alloys, aluminum, stainless steel, cobalt chrome alloys, and other metals. The particles can also be a mixture of one or more metal particles types with one or more polymer particles types. The particles can also be composed of particles comprising a mixed composition of one or more metals and one or more polymers.   The particles can be fluidized. This means that for example the particles are not too small that interparticle forces become relatively large enough to make fluidization difficult.   The particle size distribution can be such that most particles can easily pass through pores of the structure under fabrication.       

     In addition to particles that are used for building the article, additives such as anti-oxidants for polymers can be mixed into the fluidized bed so that the additives are incorporated into the structure. 
     If a laser is used for the energy beam, then a CO 2  laser is one example because of the power, availability and cost, but any type of laser that can sinter or fuse the particles is suitable. Lasers that are appropriate emit radiation with a wavelength that is sufficiently absorbed by the particles material to allow sintering to take place. Fabrication can take place in multiple locations on the surface by splitting the laser beam into a multitude of beams each selectively directed at a target on the surface. Alternatively beams from multiple lasers can be used. The beam can be focused so as to concentrate power in a small target area. In certain cases where the relative sintering power of the laser is strong, the beam can be passed through a mask so that a large cross-sectional area can be fabricated at the same time. The laser can be directed by different methods including by oscillating the beam aim with an oscillating mirror. A pulsing beam or a continuous beam can be used. 
     Many different types of fluids can be used that are suitable for both fluidization and heat removal. These fluids can be gases, liquids or supercritical fluids. Non-limiting examples of gases are air, nitrogen, carbon dioxide, helium, neon, argon. The fluidizing gas can be pressurized. It may be advantageous for heat removal or fluidization quality to use a pressurized gas. Instead of a gas as fluidizing medium it may be advantageous to use a supercritical medium such as but not limited to supercritical propane, ethane or carbon dioxide. The fluidizing medium can be either inert or reactive to the structure. For some applications a reactive gas such as methane or hydrogen can be used when the particles is a metal. For particles with high density such as metals it can be advantageous to use a fluidizing medium with higher density than a gas such as a supercritical medium or liquid. 
     In one embodiment, the fluidizing medium is not recycled. Gas from the holding vessel  30  or air is used for fluidization in in the primary vessel  12  and collected or vented to the atmosphere through lines  40  and  44 . 
     An alternative example of the layout in  FIG.  1    has different particle types in multiple vessels such as storage vessel  60  from which it is possible to feed different particles types to the fluidized bed in the primary vessel  12 . By feeding different types of particles during different phases of the fabrication, it is possible to change the composition of the particles of the fluidized bed to a different particles type. Since the composition of the fabricated structure is dependent on the composition of the particles in the bed, as the fluidized bed particles composition changes, the composition of particles being sintered changes and the composition of the fabricated structure changes. If the composition of particles in the fluidized bed is changed during fabrication then the article  96  will have a gradient of compositions. In this way an article can be fabricated with varying properties at different locations in its volume. The composition can be changed either gradually or rapidly, in some versions. If the composition of the fluidized bed is changed rapidly, then the material composition of the article may change over the length scale of a single particle. If the composition of the fluidized bed is changed gradually, then the length scale of change in the article would be larger, for example, 1 cm. This type of article can be useful for a number of different applications including mechanical and optical. For example, the material of the article  96  may be varied between a stiff materials, such as metals, to a rubbery material, such as certain polymers, in different locations of the article thus giving it special mechanical properties. Examples of polymers that are rubbery include elastomers like polyisoprene, polybutadiene, styrene-butadiene rubber (copolymer of styrene and butadiene), EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), and ethylene vinyl acetate (EVA). 
     It can be appreciated that the equipment that services an FBAM unit, including particles holding and medium holding vessels, instrumentations, valves, heat exchangers, units for purification and pumps (or compressors) in the layout in  FIG.  1    can add considerable cost per article fabricated. In an embodiment, much of this equipment is shared over multiple FBAM units such as FBAM unit  10  depicted in  FIG.  1    each comprising a vessel much like the primary vessel  12  fitted with a distributor plate, laser, lift, cameras and individual temperature control. Each FBAM unit is connected to a computer or multiple computers that control the additive manufacturing and fluidized bed process in each vessel. The multiple FBAM units can be configured in an array and serviced by a shared cooler, compressor, and particles handling system. An example of an array  140  of four FBAM units  141 ,  142 ,  143  and  144  is depicted in  FIG.  4   . As can be appreciated, many more than four FBAM units can be configured in a vast array including hundreds, thousands or more FBAM units. For each additional FBAM unit in the array, the particles handling, storage, compressor, purification and heating units can be sized accordingly to be able to handle the additional particles and medium requirements. Note that in  FIG.  4   , the valves in the lines are not shown but are appropriately placed, particularly so that the fluidization in each FBAM unit can be controlled separately, and particles can be added and removed to each FBAM unit independently. Each FBAM unit can be individually isolated from the medium flow by closing valves similarly placed to valves  24  and  41  in  FIG.  1    on each FBAM unit  141 ,  142 ,  143  and  144 . 
     The sintering or fusing process in each FBAM unit generates volatiles that would build up in the recirculating medium stream. In the example depicted in  FIG.  4   , a purification unit partially separates volatiles from stream  44  in separation unit  150 . Volatiles separated by unit  150  are removed through line  152 . Purified medium is returned to recirculation recycle line  43  through the fourth line  45 . 
     OTHER EMBODIMENTS 
     It can be appreciated from the foregoing that while certain examples of this disclosure have been depicted and described, they do not limit the embodiments and modifications that can be made without departing from the spirit and scope of this disclosure. Other embodiments can include one or more of the following versions. 
     A method of fabrication comprising: 
     fluidizing particles with a medium to form a fluidized bed having a surface  18 , 
     additively manufacturing an article comprising the particles, the article having an open porosity, 
     forming a plurality of pores in the article that define fluid paths through the article, and 
     flowing the particles and the medium through the fluid paths while the article and the fluid paths are being formed. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises additively manufacturing the article with a non-stochastic structure. 
     The method of any of these embodiments further comprising simultaneously flowing the particles and the medium through all of the pores in the article. 
     The method of any of these embodiments wherein additively manufacturing further comprises flowing produced volatiles away from the article. 
     The method of any of these embodiments further comprising cooling the fluidized bed and the article while additively manufacturing the article. 
     The method of any of these embodiments further comprising: 
     heating the particles at a target on the article, and 
     stabilizing the article by flowing the particles and the medium adjacent the target. 
     The method of any of these embodiments wherein forming the plurality of pores comprises forming interconnected pores. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises additively manufacturing a plurality of struts converging at nodes comprising the article, such that the struts and nodes define the pores. 
     The method of any of these embodiments further comprising filling some of the pores with the particles to further form the struts and nodes. 
     The method of any of these embodiments further comprising: 
     providing the article with a volume, and 
     filling a majority of the volume with the particles and the medium. 
     The method of any of these embodiments further comprising substantially filling an entirety of the volume with the particles and the medium. 
     The method of any of these embodiments wherein the plurality of pores comprises a majority of the volume. 
     The method of any of these embodiments wherein flowing the particles and the medium through the fluid paths comprises pumping the particles and medium through the fluid paths. 
     The method of any of these embodiments wherein flowing the particles and the medium through the fluid paths comprises enveloping the article within the fluidized bed by moving the article. 
     The method of any of these embodiments wherein flowing the particles and the medium through the fluid paths comprises enveloping the article within the fluidized bed by changing the height of the fluidized bed. 
     The method of any of these embodiments wherein flowing the particles and the medium through the fluid paths comprises changing a pressure of the medium to change a level of the surface. 
     The method of any of these embodiments further comprising: 
     additively manufacturing a top on the article, and further comprising: 
     leveling the top of the article with the surface  18  of the fluidized bed such that the fluidized bed envelopes the top and the top is adjacent the surface  18  of the fluidized bed; 
     continuously lowering the article into the fluidized bed; or 
     continuously raising the surface of the fluidized bed. 
     The method of any of these embodiments further comprising: 
     supporting the article on a substrate defining a plurality of channels extending through the substrate, and 
     flowing particles and the medium through all of the channels in the substrate. 
     The method of any of these embodiments further comprising binding the article to the substrate. 
     The method of any of these embodiments wherein the substrate is a mesh. 
     The method of any of these embodiments further comprising: 
     defining the particles with an average diameter dp, and defining the channels with a minimum diameter of at least about twice the average diameter dp of the particles. 
     The method of any of these embodiments wherein the channels have varying diameters. 
     The method of any of these embodiments wherein the fluid paths have a minimum diameter of at least about twice the average diameter dp of the particles. 
     The method of any of these embodiments 8 further comprising forming the substrate from a same material as the particles. 
     The method of any of these embodiments further comprising pressurizing the medium to increase a heat capacity of the medium and thereby increasing a rate of heat dissipation. 
     The method of any of these embodiments further comprising: 
     providing the particles as first particles and second particles wherein the first particles and the second particles comprise different materials, 
     temporally varying a ratio of the first particles to the second particles in the fluidized bed, 
     additively manufacturing the article having a spatially varied material composition of the first particles and the second particles. 
     The method of any of these embodiments wherein the particles have an average outer dimension of at least about 10 microns and not greater than about 1 mm. 
     The method of any of these embodiments wherein the particles comprise a metal. 
     The method of any of these embodiments wherein the particles comprise a polymer. 
     The method of any of these embodiments wherein the polymer comprises a material selected from the group consisting of polyamide 6, polyamide 12, polypropylene, polyethylene, polyethylene terephthalate, polybutylene terephthalate, polyoxymethylene, polystyrene, poly(methyl methacrylate) or any combination thereof. 
     The method of any of these embodiments wherein additively manufacturing further comprises visually monitoring the additive manufacturing within a transparent zone of the fluidized bed, the transparent zone extending a distance D below the surface  18  of the fluidized bed. 
     The method of any of these embodiments further comprising increasing the distance D of the transparent zone by increasing a velocity of the medium of the fluidized bed. 
     The method of any of these embodiments further comprising maintaining the particles within a vessel, and continuously circulating the medium into the vessel, through the particles and out of the vessel. 
     The method of any of these embodiments further comprising sealing the medium and the particles within a closed system. 
     The method of any of these embodiments further comprising focusing a plurality of energy beams onto a target on the article to concentrate the power of the energy beams. 
     The method of any of these embodiments wherein the energy beam is a laser beam. 
     The method of any of these embodiments wherein the energy beam is an electron beam. 
     The method of any of these embodiments where the medium is selected from the group consisting of air, nitrogen, carbon dioxide and an inert gas, or any combination thereof. 
     The method of any of these embodiments wherein the medium is a supercritical fluid. 
     The method of any of these embodiments wherein fluidizing the particles with the medium to form the fluidized bed further comprises: 
     conveying the particles into a primary vessel  12  having a top and a bottom and defining a chamber and conveying the particles above a distributor plate  14  extending horizontally across the chamber near the bottom of the primary vessel  12 , the distributor plate having a plurality of holes, and 
     pumping the medium upward through the holes in the distributor plate  14  such that a superficial velocity of the medium is greater than a minimum fluidization superficial velocity u mf . 
     The method of any of these embodiments further comprising: 
     additively manufacturing a first horizontal cross section of the article within the fluidized bed and adjacent the surface  18  of the fluidized bed, and 
     additively manufacturing one or more additional horizontal cross sections on top of the first horizontal cross section wherein all of the horizontal cross sections define the article. 
     The method of any of these embodiments wherein additively manufacturing comprises sintering the particles, with the particles having a sintering window defined as a temperature range from a crystallization temperature to a melting temperature. 
     The method of any of these embodiments further comprising depositing particles on the article by reducing the velocity of the medium to below the minimum fluidization superficial velocity u mf . 
     The method of any of these embodiments further comprising: 
     wherein the article comprises a top and a bottom, and 
     heating the top to maintain the temperature of the top in the sintering window and cooling the bottom to below the sintering window to create a temperature gradient within the article. 
     The method of any of these embodiments wherein sintering the particles comprises emitting an energy beam at a target on the article. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising a microlattice that is irregular. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising a material that has a Young&#39;s modulus E s  and a density ρ s , 
     additively manufacturing the article with a Young&#39;s modulus E and a density p that follow the relationship: E/□≥E s /ρ s . 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising a material that has a Young&#39;s modulus E s  and a density ρ s , 
     additively manufacturing the article with a Young&#39;s modulus E and a density p that follow the relationship: E/□≥(ρ/ρ s ) 2  E s /ρ s . 
     The method of any of these embodiments wherein additively manufacturing an article further comprises additively manufacturing the article the article a Young&#39;s modulus E and a p that follow the relationship: E/□≤E s /ρ s . 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 50%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 60%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 70%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 80%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 90%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 95%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 99%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 99.9%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of at least about 99.99%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 50%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 60%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 70%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 80%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 90%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 95%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 99%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 99.9%. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article with a porosity φ of not greater than 99.99%. 
     The method of any of these embodiments wherein the pores have a minimum diameter of at least about 20 microns. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising particles with average outer dimension d p , and 
     additively manufacturing the struts having a length that is quadruple the average outer dimension d p . 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising a microlattice. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising repeating unit cells that are surface based. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising repeating unit cells that are surface based. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising triply periodic minimal surface lattices. 
     The method of any of these embodiments wherein additively manufacturing the article further comprises: 
     additively manufacturing the article comprising repeating unit cells that are truss based. 
     A system  10  for fabricating an article, the system comprising: 
     a primary vessel  12  being impermeable to fluids and insulated and having a vertical axis Z and a horizontal axis X perpendicular to the vertical axis Z and a top and bottom being vertically spaced apart, 
     the primary vessel  12  including a window  104  at the top, 
     the primary vessel  12  defining a chamber  13  and having a hatch  98  which can be opened to provide access to the chamber, 
     a distributor plate  14  disposed in the primary vessel  12  and extending horizontally across the chamber, the distributor plate having a plurality of holes, and 
     a fluidized bed disposed in the chamber and having a surface  18  above the distributor plate  14 , the fluidized bed being expandable and having particles, the particles having an average outer dimension d p  that is at least about 10 microns and not greater than about 1 mm, the fluidized bed having a medium with a superficial velocity u that is greater than a minimum fluidizing superficial velocity u mf  of the particles. 
     The system  10  of any of these embodiments wherein the fluidized bed has a transparent zone that is at least partially transparent and which extends a distance D below the surface  18  and the article is formed in the transparent zone. 
     The system  10  of any of these embodiments further comprising: 
     a lift device  90  in the chamber  13  and attached to the primary vessel  12  being capable of moving vertically, and 
     a frame attached  92  to the lift device  90  and being vertically movable by the lift device  90  and extending at least partially around the perimeter of the primary vessel  12  and having openings  93 . 
     The system  10  of any of these embodiments further comprising: 
     a substrate  94  disposed in the chamber and defining channels that are of varying size and that are larger than average outer dimension d p  of the particles such that the particles can flow through the channels, holes and openings. 
     The system  10  of any of these embodiments further comprising: 
     a first camera  106  mounted adjacent the primary vessel  12  and directed toward the surface  18  for visually monitoring fabrication of the article. 
     The system  10  of any of these embodiments further comprising: 
     a second camera  108  of an infrared type mounted adjacent the primary vessel  12 . 
     The system  10  of any of these embodiments further comprising an energy beam source  100 . 
     The system  10  of any of these embodiments further comprising, for the energy beam source, optical components  102  comprising at least one prism and at least one mirror assembly comprising at least one mirror coupled to at least one galvanometer. 
     The system  10  of any of these embodiments further comprising: 
     a first heat exchanger  56  coupled to the primary vessel  12 , and 
     a first temperature monitoring device  54  disposed in the chamber. 
     The system  10  of any of these embodiments further comprising a first pressure tap  38  coupled to the primary vessel  12  toward the top and above the fluidized bed and a second pressure tap  39  coupled to the primary vessel  12  above the distributor plate  14 . 
     The system  10  of any of these embodiments further comprising a recycle line  43  coupled to the first heat exchanger  50  and a second heat exchanger  52 . 
     The system  10  of any of these embodiments further comprising a holding vessel  30  containing the medium and being fluidly connected to the chamber. 
     The system  10  of any of these embodiments further comprising 
     a salvage vessel  70  containing some of the particles and the medium and being fluidly connected to the chamber such that particles and the medium and can be conveyed from the salvage vessel  70  to the chamber, 
     a third heat exchanger  78  coupled to and extending into the salvage vessel  70 , 
     a second temperature monitoring device  79  coupled to and extending into the salvage vessel  70 , 
     a storage vessel  60  containing some of the particles and being fluidly connected to the chamber such that the particles and the medium can flow from the storage vessel  60  to the chamber, 
     a storage heat exchanger  68  coupled to and extending into the storage vessel  60 , and 
     a third temperature monitoring device  69  coupled to and extending into the storage vessel  60 . 
     The system  10  of any of these embodiments further comprising a recycle line  43  coupled to the first heat exchanger  50  and the second heat exchanger  52 , and a pump  20  fluidly connected to the chamber  13 . 
     The system  10  of any of these embodiments further comprising a computer  28  configured to receive information regarding temperature, pressure and visuals for the system, and to control the pump  20  based on the information received.