Patent Publication Number: US-2017361377-A1

Title: Neutralization of Reactive Metal Condensate in Additive Manufacturing

Description:
FIELD OF THE INVENTION 
     This invention relates to additive manufacturing (AM) and, more particularly, to neutralization of condensate in an AM system. 
     BACKGROUND OF THE INVENTION 
     AM enables fabrication of three-dimensional objects from a digital model or another electronic data source through additive processes in which successive layers of material are laid down. A laser beam or electron beam is used to fuse a previously-leveled powder surface layer into a thin sheet of solid material. A further layer of powder is applied on top of the previously-fused thin sheet and the process is repeated until a three-dimensional object is built layer-by-layer. This process is known as, for example, powder bed fusion (PBF), laser selective melting, or direct laser metal sintering. The process may be applied to metals, plastics, or other materials that can be fused together. 
     The AM process may be contained in a chamber filled with an inert gas to prevent unwanted chemical reactions or the oxidation of molten metal. This inert gas may be, for example, argon. During the layer fusion process of, for example, a metal, the surface of the melt pool overheats and vaporizes. Vaporized material cools and condenses within the inert atmosphere into nanometer-sized dust, referred to herein as “condensate.” These condensate particles may be approximately 10 nm to 100 μm in diameter or longest dimension. This condensate is initially suspended in the inert gas within the chamber. Condensate in the chamber may settle on a surface or remain suspended in the inert gas as a smoke. This poses a safety hazard because the condensate may be highly reactive in the presence of oxygen, which can result in a severe risk of explosion or fire. 
       FIG. 1  is a schematic of an AM system  100  of the prior art. The AM system  100  includes a build chamber  101 , which is where the layer fusion process occurs. To reduce the risk to personnel, convection currents of the inert gas can direct the condensate out of the build chamber  101 . The inert gas and suspended condensate (represented by the solid arrow) can be pumped through a filter  102  to capture the solid material. Inert gas with less condensate or even no condensate (represented by the dashed arrow) is then pumped to the build chamber  101  using the circulating fan  103 . Large quantities of condensate can be captured in the filter  102 , but the filter  102  may not capture all of the condensate and not all the condensate may be directed out of the build chamber  101 . Thus, while some condensate is directed toward the filter  102 , a significant portion of the condensate may accumulate in and around the chamber  101 . 
     Settled condensate may build up on the walls of the chamber  101 , the transparent window through which the laser beam is directed, or the object being manufactured. The laser beam may be obscured and the AM process may be interrupted or degraded if condensate settles on the transparent window. This can result in the production of poor-quality parts. For example, an object may take 10 to 200 hours to build in an AM system. However, the transparent window may be obscured after only approximately five hours of use due to deposits that have formed. It may be necessary to pause and clean the system if the transparent window is obscured. 
     Any condensate build-up on the object being manufactured can impact the quality or properties of this object. For example, condensate may reduce fidelity or impact the shape, dimensions, or physical properties of the object being manufactured. The condensate build-up may even ruin the object being manufactured. Thus, there may be a maximum build time that can be performed before the chamber  101  and transparent window need to be cleaned due to the presence of the condensate. This may render AM unsuitable for fabricating large or geometrically-complex objects. 
     Condensate build-up on the walls of the chamber  101  can be a fire risk, which presents a safety issue for operators during manual cleaning. Some materials in the condensate may be highly-reactive in air, which may lead to spontaneous ignition if enough condensate has accumulated and the chamber  101  is opened for cleaning or maintenance. For example, titanium or aluminum condensate can be formed during laser processing. Titanium or aluminum dust is a fire hazard and, when exposed to air, may pose an explosion hazard. Fires or explosions can occur while cleaning the chamber  101 , especially in vacuum cleaners or other cleaning equipment used during the cleaning process. Serious accidents have occurred while cleaning condensate or handling filters  102  contaminated with condensate. 
     Contaminated filters  102  must be manually removed during replacement and disposed of in a specialized facility for hazardous waste. Changing or handling the filter  102  increases the risk of spontaneous combustion and serious accidents. Several injuries have been reported during filter cleaning or replacement. To reduce fire risk with the filter  102 , the dimensions of the filter  102  are kept small. However, this limits the maximum build time during the AM process before the filter  102  must be replaced. Furthermore, not all the condensate is collected in the filter  102  and the explosion or fire risk in the chamber  101  remains. 
     Therefore, what is needed is a system and method of condensate neutralization during AM and, more particularly, neutralization of condensate during AM process that converts the condensate to a safe form. 
     BRIEF SUMMARY OF THE INVENTION 
     Metal condensate can be neutralized by converting it to a safe compound or a safe form of the metal. Titanium is one example of a metal condensate that can be converted. In an example, reactive titanium nano-scale condensate is transformed into an inert macro-scale titanium deposit. This can be a closed process. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of an AM system of the prior art; 
         FIG. 2  represents a chemical reaction process that can be used in an AM system in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic block diagram of an AM system using a halide gas in accordance with an embodiment of the present invention; 
         FIG. 4  is a flow diagram representing a process of using the AM system of  FIG. 3 ; 
         FIG. 5  is a schematic block diagram of an AM system using a halide gas in accordance with another embodiment of the present invention; 
         FIG. 6  is a flow diagram representing a process of using the AM system of  FIG. 5 ; 
         FIG. 7  is a schematic block diagram of an AM system using a halide gas in accordance with another embodiment of the present invention; and 
         FIG. 8  is a flow diagram representing a process of using the AM system of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although claimed subject matter will be described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this invention. 
     Various structural, logical, process step, and electronic changes may be made without departing from the scope of the invention. Accordingly, the scope of the invention is defined only by reference to the appended claims. 
       FIG. 2  represents a chemical reaction process that can be used in an AM system embodying the present invention. A solid halide is vaporized in step  120 . 
     The gaseous halide that is formed is contacted with the metal condensate. While titanium is specifically disclosed as the metal condensate, aluminum, vanadium, or other metals also may be used. This causes a reaction in step  121 . A gaseous metal halide compound is formed in step  122 . A filament is used to separate the metal from the halide and reverse the reaction in step  123 . The gaseous halide is then cooled and solidifies in step  124 . 
     For example, the solid halide may be a solid material containing iodine. The I(s) can be vaporized by raising its temperature above, for example, approximately 150° C. The pressure in the AM system also may be reduced. Solid titanium condensate reacts with the I 2 (g) to form TiI 4 (g) using the following reaction: 
       Ti(s)+2I 2 (g)→TiI 4 (g)
 
     The filament may be controlled to approximately 1400° C. to separate the TiI 4 (g) using the following reaction: 
       TiI 4 (g)→Ti(s)+2I 2 (g)
 
     When cooled, the I 2 (g) will solidify into the I(s). For example, the I 2 (g) may be cooled below 150° C. to form I(s). During solidification, the pressure in the AM system may be increased. 
     The particular halide or gaseous halide species can be selected based on, for example, the metal in the condensate, the AM process parameters, the component being manufactured, or the desired condensate removal level. Instead of I 2 , other halides like F 2 , Br 2 , or Cl 2  may be used. A mixture of different halides or one or more halides with other gaseous species also may be used. The temperature to melt the solid halide material, condense the gaseous halide material, dissociate the gaseous metal halide compound, heat components in the AM system, or cool components in the AM system may vary based on the vaporization temperature and condensation temperature of the halide, halides, or gas mixture that is used. 
       FIG. 3  is a schematic of an embodiment of an AM system  200  using a halide gas. In  FIG. 3 , the solid arrows represent inert gas and suspended condensate and the dashed arrows represent inert gas with less condensate or even no condensate. By less condensate, the flows represented by the dashed arrows have less condensate than the flows represented by the solid arrows. 
     The build chamber  201 , where the AM process occurs, is upstream of the filter  202  and circulating fan  204 . The build chamber  201  may use, for example, a laser beam or electron beam. Between the build chamber  201  and the filter  202  are valves V 1  and V 4 . Between the build chamber  201  and the circulating fan  204  are valves V 1  and V 2 . 
     Downstream of the circulating fan  204  is the halide vessel  205  and dissociation chamber  206 . The dissociation chamber  206  and filter  202  are connected to the build chamber  201  through valve V 3  and fan  203 . 
     The halide vessel  205  is connected to the build chamber  201  and has a halide gas source that is configured to generate a gaseous halide. The halide vessel  205  includes a heating system to raise the temperature of and vaporize a solid halide material. This generates a gaseous halide, such as I 2 . 
     The dissociation chamber  206  has a replaceable filament, which may be fabricated of tungsten or other materials. The filament in the dissociation chamber  206  may be placed in a flowpath of the gaseous halide or gaseous metal halide compound. 
     The embodiment of  FIG. 3  may be used to clean the filter  202 .  FIG. 4  is a diagram representing a process of using the embodiment of  FIG. 3 . In step  300 , valves V 1  and V 3  are closed and valves V 2  and V 4  are opened. The filter  202  and halide vessel  205  are heated to vaporize the solid halide material and form a gaseous halide in step  301 . The filament in the dissociation chamber  206  is heated in step  302 . The gaseous halide is circulated between valve V 1  and V 3 . The gaseous halide reacts with the metal condensate to form a gaseous metal halide compound in step  303 . The gaseous metal halide compound is circulated to the dissociation chamber  206  and the metal dissociates onto the filament in step  304 . There, the metal may collect, coalesce, or otherwise be deposited on the filament. Once the cleaning is finished, the halide vessel  205  is cooled in step  305  to cause the gaseous halide to condense. Valves V 1 , V 3 , and V 4  are then opened and valve V 2  is closed in step  306 . 
     To clean the build chamber  201 , either all valves V 1 , V 2 , V 3 , and V 4  in  FIG. 3  are opened or only valve V 4  is closed and valves V 1 , V 2 , and V 3  are opened. This enables the gaseous halide to circulate to the build chamber  201 . 
     When the filter  202  is sufficiently cleaned and condensate has been deposited on the filament in the dissociation chamber  206 , the halide gas is transported back to the halide vessel  205  to be solidified. The solid halide material can remain in solid form until needed for another cleaning cycle. Other parts of the AM system  200  may be kept at an elevated temperature to prevent or reduce solidification other than in the halide vessel  205 . 
     The filament in the dissociation chamber  206 , which may be a tungsten filament, is periodically replaced. The used filament may be discarded. In the example of titanium condensate, the titanium metal adheres to the filament and is inert in the presence of oxygen. Thus, the filament is safe to handle and dispose of normally. 
       FIG. 5  is a schematic of another embodiment of an AM system  400  using a halide gas. In  FIG. 5 , the solid arrows represent inert gas and suspended condensate and the dashed arrows represent inert gas with less condensate or even no condensate. By less condensate, the flows represented by the dashed arrows have less condensate than the flows represented by the solid arrows. 
     The build chamber  401 , where the AM process occurs, may use, for example, a laser beam or electron beam. The build chamber  401  is upstream of the filter  402  and circulating fan  404 . Between the build chamber  401  and the filter  402  is valve V 1 . Between the build chamber  401  and the circulating fan  404  is valve V 2 . 
     Downstream of the circulating fan  404  is the halide vessel  405  and dissociation chamber  406 . The dissociation chamber  406  and filter  402  are connected to the build chamber  401  through valve V 3  and fan  403 . The halide vessel  405  may be similar to the halide vessel  205  of  FIG. 3 . The dissociation chamber  406  may be similar to the dissociation chamber  206  of  FIG. 3 . 
     Downstream of the dissociation chamber  406  between this dissociation chamber  406  and the valve V 3  is a heater  407 . The gaseous halide may need to remain hot (such as ≧350° C.) in order for the titanium iodide reaction to occur. The heater  407  helps maintain the gaseous halide at this temperature. 
     The heater  407  also may be attached to or disposed in the build chamber  401 . Gaseous halide leaving the halide vessel  405  is hot. If the pipes in the AM system  400  are lagged the gaseous halide will remain hot. The build chamber  401  may include the heater  407  to maintain the gaseous halide at the desired temperature. 
     The embodiment of  FIG. 5  may be used to clean the build chamber  401 .  FIG. 6  is a diagram representing a process of using the embodiment of  FIG. 5 . In step  500 , valve V 1  is closed and valves V 2  and V 3  are opened. The build chamber  401  and halide vessel  405  are heated to vaporize the solid halide material and form a gaseous halide in step  501 . The filament in the dissociation chamber  406  is heated in step  502 . The gaseous halide is circulated and reacts with the metal condensate to form a gaseous metal halide compound in step  503 . The gaseous metal halide compound is circulated to the dissociation chamber  406  and the metal dissociates onto the filament in step  504 . There, the metal may collect, coalesce, or otherwise be deposited on the filament. Once the cleaning is finished, the halide vessel  405  is cooled in step  505  to cause the gaseous halide to condense. Valve V 1  is then opened and valve V 2  or valves V 2  and V 3  are closed in step  506 . 
       FIG. 7  is a schematic of another embodiment of an AM system  600  using a halide gas. In  FIG. 7 , the solid arrows represent inert gas and suspended condensate and the dashed arrows represent inert gas with less condensate or even no condensate. By less condensate, the flows represented by the dashed arrows have less condensate than the flows represented by the solid arrows. 
     The build chamber  601 , where the AM process occurs, may use, for example, a laser beam or electron beam. The build chamber  601  is upstream of the filter  602  and circulating fan  604 . Downstream of the filter  602  are valves V 1 , V 2 , and V 3 . V 1  is between the filter  602  and the fan  603 . V 2  is between the filter  602  and circulating fan  604 . V 3  is between the heater  607  and the fan  603 . 
     Downstream of the circulating fan  604  is the halide vessel  605  and dissociation chamber  606 . The halide vessel  605  may be similar to the halide vessel  205  of  FIG. 3 . The dissociation chamber  606  may be similar to the dissociation chamber  206  of  FIG. 3 . The heater  607  may be similar to the heater  407  in  FIG. 5 . 
     The embodiment of  FIG. 7  may be used to clean the build chamber  601  and filter  602 .  FIG. 8  is a diagram representing a process of using the embodiment of  FIG. 7 . In step  700 , valve V 1  is closed and valves V 2  and V 3  are opened. The build chamber  601  and halide vessel  605  are heated to vaporize the solid halide material and form a gaseous halide in step  701 . The filament in the dissociation chamber  606  is heated in step  702 . The gaseous halide is circulated and reacts with the metal condensate to form a gaseous metal halide compound in step  703 . The gaseous metal halide compound is circulated to the dissociation chamber  606  and the metal dissociates onto the filament in step  704 . There, the metal may collect, coalesce, or otherwise be deposited on the filament. Once the cleaning is finished, the halide vessel  605  is cooled in step  705  to cause the gaseous halide to condense. Valve V 1  is then opened and valves V 2  and V 3  are closed in step  706 . 
     The halide vessel can be located upstream or downstream of the build chamber. In an alternate embodiment, the halide vessel is located upstream of the build chamber and the dissociation chamber is located downstream of the build chamber. Of course, other designs are possible. 
     The circuit illustrated in  FIG. 3 ,  FIG. 5 , or  FIG. 7  may be kept at an elevated temperature when the halide gas solidifies in the dissociation chamber  206 ,  406 , or  606 . This prevents the solid halide from depositing elsewhere in the AM system  200 ,  400 , or  600 . Heaters may be located in the various chambers or gas lines to prevent such solidification. However, the temperature of the gaseous halide may be controlled within a particular range to prevent the gaseous halide from excessively corroding the wall materials. 
     The gaseous halide may circulate in the build chamber  201 ,  401 , or  601  during the build process or in between steps of the build process. The effect on the component being manufactured or the powder in the build chamber  201 ,  401 , or  601  may be negligible, may be controlled, or may be compensated for. 
     In yet another embodiment, electrostatic or other methods known to those skilled in the art may be used to cause the metal halide compound or condensate to converge on a heated wire, such as the filament in the dissociation chamber  206 ,  406 , or  606 . 
     Condensate that did not react with the gaseous halide may still melt in the dissociation chamber  206 ,  406 , or  606  and form a deposit after the molten metal cools. 
     The cleaning process disclosed herein may be separate from the build process in the build chamber  201 ,  401 , or  601 . The cleaning process also may be used during the build process in the build chamber  201 ,  401 , or  601  if the impact to the component being manufactured is negligible, controlled, or compensated for. 
     The cleaning process disclosed herein may be a closed cycle or an open cycle. An open cycle may be possible depending on the nature of the halide and reaction products. 
     Components manufactured using an AM process may have a surface with partially-fused or loose powder particles on it. These particles become detached during assembly and use, and may cause subsequent problems during component operation. Such particles may be small and may be of large relative surface area. For example, these particles may be approximately 15 μm to 45 μm in dimension. When exposed to the gaseous halide, the particles may be consumed in a reaction and removed from the component. The surface of the component may be rendered free of the loose or partially-attached particles without subsequent bead blasting, HF treatments, or other particle removal steps. Thus, this cleaning process also can be used during or as an additional step to the AM process to clean the component being manufactured. 
     In an alternate embodiment, a separate cleaning chamber is located in the AM system. This cleaning chamber may be connected to the dissociation chamber and halide vessel. Additional valves and gas lines are used to flow the halide gas to this cleaning chamber. A finished or incomplete part may be moved from the build chamber to the cleaning chamber to remove particles. Instead of a cleaning chamber connected to the AM system, a separate standalone cleaning booth with a dissociation chamber and halide vessel also may be used. 
     The safety of an AM process is improved using embodiments disclosed herein due to elimination or reduction of dangerous condensate. Personnel may not need to handle filters contaminated with condensate or clean contaminated build chambers. Manufacturing costs are lowered due to the elimination of hazardous waste materials. The build chamber and transparent window will remain cleaner for a longer period, which will enable longer build times, improve system uptime, and reduce the necessary preventative maintenance. The laser optics will remain cleaner for a longer period, which will provide improved laser beam quality and improved product quality, fidelity, and consistency. Parts produced using the AM process may be cleaner or have fewer undesired particles attached to surfaces. These benefits can be provided without impacting the quality of the components being manufactured using the AM process. 
     The following are sample claims that are presented for illustrative purposes and are not intended to be limiting.