Patent Publication Number: US-2018043614-A1

Title: Additive manufacturing apparatus and methods

Description:
FIELD OF INVENTION 
     This invention concerns additive manufacturing apparatus and methods in which layers of material are consolidated in a layer-by-layer manner to form a part. The invention has particular, but not exclusive application, to selective laser solidification apparatus, such as selective laser melting (SLM) and selective laser sintering (SLS) apparatus. 
     BACKGROUND 
     Selective laser melting (SLM) and selective laser sintering (SLS) apparatus produce parts through layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam. A powder layer is formed across a powder bed in a build chamber by depositing a heap of powder adjacent to the powder bed and spreading the heap of powder with a wiper across (from one side to another side of) the powder bed to form the layer. A laser beam, introduced through a window in the top of the build chamber, is then scanned across areas of the powder layer that correspond to a cross-section of the part being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required. An example of such a device is disclosed in U.S. Pat. No. 6,042,774. 
     The solidification process is carried out in an inert gas atmosphere, such as an argon or nitrogen atmosphere, as the metal powder is highly reactive. Melting of the powder results in gas-borne particles in the build chamber. These particles include a cloud or fog of nanometre sized particulates formed by material that has re-solidified in the inert atmosphere after being vaporised by the laser. It is undesirable for the gas-borne particles to resettle on the powder bed as this can affect the accuracy of the build. To remove such matter a gas knife of inert gas is generated across the powder bed between a nozzle and an exhaust. The gas collected by the exhaust is passed through a filter to remove the gas-borne particles, the filtered gas recirculated through a gas circuit back to the nozzle. 
     WO2010/007394 discloses a parallel filter arrangement in which the gas flow through the circuit can be switched between either one of two filter assemblies such that the filter element in the other filter assembly can be replaced during a build. 
     During a build, the gas-borne particulates can collect on surfaces of the build chamber, including the window, forming a soot-like covering. The particulates collected on the window and the gas-borne particulates can deflect and/or disperse the laser beam, resulting in an inaccurate build. It is known to provide a gas curtain across the window to mitigate the problem of particulates gathering on the window. Examples of such gas flow devices are disclosed in EP0785838 and EP1998929. 
     It has been found, however, that, even with gas flows across the powder bed and the window, sufficient particulates collect on the window to affect the quality of the build. 
     US2013/0101803A1 discloses the gas of a construction-chamber atmosphere removed by suction and conducted through a tubular component with cooled areas on which the vapours produced during a layer-by-layer production process can condense. The gas is then conducted back into the construction chamber. The gas of the construction-chamber atmosphere is reheated after condensation of the volatile constituents of the polymer before being conducted back into the construction chamber. 
     US2014/0265045 discloses a scrubber to clean and filter air within a build chamber of a laser sintering system. The scrubber comprises an initial cooling section. The cooling section is a serpentine passage that causes relatively hot air in the build chamber to be cooled, such as with a heat sink or fan assembly in thermal communication with the passages in the cooling section. 
     SUMMARY OF INVENTION 
     According to a first aspect of the invention there is provided an additive manufacturing apparatus for building a part by selectively consolidating flowable material in a layer-by-layer building process comprising an inert gas vessel comprising a build chamber, a layering device for depositing layers of material in the build chamber; a scanner for delivering an energy beam to selected areas of each layer to consolidate flowable material of the layer; and a thermal device configured to affect heating and/or cooling of an internal surface of the inert gas vessel to cause particulates to be preferentially deposited at a predetermined location in the vessel desirable for particulate collection as a result of a cooler temperature of the predetermined location, the cooler temperature being lower than an ambient temperature of the inert gas. 
     It has been found that particulates, in particular, nanoparticles created by cooling of the plasma formed during the consolidation process, present in the inert gas deposit on surfaces that are cooler relative to the ambient inert gas temperature. It is believed that by controlling a temperature of an internal surface/temperatures of internal surfaces it is possible to cause the particulates to preferentially deposit onto surfaces at desired locations in the vessel. In this way, the deposition of particulates at undesired locations in the vessel, such as on a laser window, a viewing window, a gas nozzle for delivering gas into the build chamber, a wiper and a doser for delivering powder, may be reduced. 
     The thermal device may comprise an (active) cooling device for cooling the internal surface. The cooling device may comprise a Peltier device, a heat exchanger through which coolant is pumped, a refrigeration unit and/or other suitable device for cooling a surface. 
     The cooling device may be arranged to cool an internal surface of the build chamber. The apparatus may comprise a laser for generating a laser beam and the build chamber may comprise a window through which the laser beam is directed by the scanner, wherein the cooling device is arranged to cool an internal surface of the build chamber that is remote from the window. 
     The cooling device may be arranged to cool a surface of a collection member movable in the vessel relative to a wiper for wiping particulates off the collection member into a collection bin. For example, the collection member may be an annular member mounted for rotation such that the collection member is continuously moved past a wiper, such as a brush, for wiping particulates collected on the annular member into a collection bin. Alternatively, the collection member may be a surface of the build chamber, a wiper being moved across the surface to wipe the particulates into the collection bin. In this way, the surface for the collection of particulates is regularly renewed for efficient particulate collection. 
     The inert gas vessel may comprise a gas flow circuit for generating an inert gas flow through the build chamber and the cooling device may be arranged to cool an internal surface of the gas flow circuit. The gas flow circuit may comprise a filter for filtering particles from the gas flow and the cooling device may cool an internal surface located upstream of the filter. The cooling device may be arranged to cool an internal surface of the build chamber located in the vicinity of a gas outlet of the gas circuit from the build chamber. The cooling device may comprise a Peltier device, a heat pipe and/or other suitable device for cooling a surface. 
     The gas circuit may comprise a heater located downstream for heating the cooled inert gas that has passed through the filter before reintroduction of the inert gas onto the build chamber. Alternatively, cooled inert gas may be reintroduced to the build chamber to maintain an ambient temperature of the inert gas below a temperature of internal surfaces of the build chamber. 
     The thermal device may comprise a heater for heating the internal surface above an ambient temperature of the inert gas. Heating of the internal surface above the ambient temperature may cause particulates to be deposited at a location in the inert gas vessel away from the heated internal surface. The heater may comprise a Peltier device, a radiant heater, a heating element, a heat pipe and/or other suitable device for heating a surface. 
     The apparatus may comprise a laser for generating a laser beam and the build chamber may comprise a window through which the laser beam is directed by the scanner, wherein the internal surface heated by the heater is a surface surrounding the window. The inert gas vessel may comprise a gas flow circuit for generating an inert gas flow through the build chamber and the internal surface heated by the heater may be a nozzle of the gas circuit for directing the gas flow into the build chamber. The build chamber may comprise a door comprising a viewing window and the internal surface heated by the heater may be an internal surface surrounding the viewing window. 
     The thermal device may comprise thermally insulative and/or conductive material for affecting the conduction of heat through walls of the inert gas vessel such that particulates preferentially deposit at the predetermined location. During additive manufacturing processes, such as selective laser melting and selective laser sintering, the temperature within the inert gas vessel is higher than that of the external environment such that heat is typically conducted to the external environment through the walls of the vessel. By appropriately arranging insulative and/or highly conductive material in and/or around the inert gas vessel, it may be possible to produce a temperature difference between different locations in the vessel such that particulates preferentially deposit at desired locations in the vessel. For example, thermally insulative material may be provided around a laser window and/or viewing window. A retainer for holding the laser window and/or viewing window in place in the build chamber may be made of insulative material. A gas circuit may comprise conductive material such that internal surfaces of the gas circuit are cooler than other internal surfaces of the inert gas vessel. 
     According to a second aspect of the invention there is provided a method of removing particulates from an inert gas atmosphere provided in a vessel in a layer-by layer additive manufacturing process, wherein a part is built by selectively consolidating flowable material in layers, the method comprising providing a relatively cool internal surface in the inert gas vessel having a temperature lower than an ambient temperature of the inert gas to cause particulates in the inert gas atmosphere to preferentially deposit on to the internal surface, the internal surface located at a desirable location in the vessel for particulate collection. 
     The particulates preferential deposit on the internal surface because the internal surface is cooler than other internal surfaces of the vessel. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an additive manufacturing apparatus according to one embodiment of the invention; 
         FIG. 2  is a schematic diagram of the additive manufacturing apparatus from another side; 
         FIG. 3  is a schematic diagram of an additive manufacturing apparatus according to another embodiment of the invention; 
         FIG. 4  is a schematic diagram of a particulate collection device for use in an additive manufacturing apparatus; and 
         FIG. 5  is a schematic diagram of an additive manufacturing apparatus according to another embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to  FIGS. 1 and 2 , an additive manufacturing apparatus according to an embodiment of the invention comprises an inert gas vessel  100  comprising build chamber  101  and a gas circuit  160 . 
     The build chamber  101  has partitions  115 ,  116  therein that define a build cylinder  117  and a surface onto which powder can be deposited. A build platform  102  is provided for supporting a part  103  built by selective laser melting powder  104 . The platform  102  can be lowered within the build cylinder  117  as successive layers of the part  103  are formed. A build volume available is defined by the extent to which the build platform  102  can be lowered into the build cylinder  117 . The build cylinder  117  and build platform  102  may have any suitable cross-sectional shape, such as circular, rectangular and square. 
     Partitions  115 ,  116  and the build platform  102  split the build chamber  101  into an upper chamber  120  and a lower chamber  121 . Seals (not shown) around the build platform  102  prevent powder from entering into the lower chamber  121 . A gas connection, such as a one-way valve, may be provided between the upper and lower chambers  120 ,  121  to allow gas to flow from the lower chamber  121  to the upper chamber  120 . The lower chamber  121  may be kept at a slight over-pressure relative to the upper chamber  120 . 
     Layers of powder  104  are formed as the part  103  is built by dispensing apparatus  108  and an elongate wiper  109 . For example, the dispensing apparatus  108  may be apparatus as described in WO2010/007396. 
     A laser module  105  generates a laser for melting the powder  104 , the laser directed as required by optical scanner  106  under the control of a computer  130 . The laser enters the chamber  101  via a window  107 . 
     The optical scanner  106  comprises steering optics, in this embodiment, two movable mirrors  106   a,    106   b  for directing the laser beam to the desired location on the powder bed  104  and focussing optics, in this embodiment a pair of movable lenses  106   c,    106   d,  for adjusting a focal length of the laser beam. Motors (not shown) drive movement of the mirrors  106   a  and lenses  106   b,    106   c,  the motors controlled by processor  131 . 
     A computer  130  controls modules of the additive manufacturing apparatus, including the thermal devices such as the cooling devices and heaters, as described below . . . Computer  130  comprises the processor unit  131 , memory  132 , display  133 , user input device  134 , such as a keyboard, touch screen, etc., a data connection to the modules. Stored on memory  132  is a computer program that instructs the processing unit to carry out the method as now described. 
     The gas circuit  160  comprises a gas nozzle  140  and a gas exhaust  141  for generating a gas flow  142  through the chamber  101  across the build platform  102 . The gas flow  142  acts as a gas knife carrying gas-borne particles created by the melting of the powder with the laser away from the build area. The gas circuit comprises a further gas nozzle integrated into a retainer ring  161  for generating a gas flow  148  across the laser window  107 . This gas flow may help to prevent particulates from collecting on the laser window  107 , which in turn could affect the quality of the laser beam  118  delivered through the laser window  107 . 
     A pump  170  drives the circulation of inert gas through gas circuit  160 . 
     A vent  143  provides a means for venting/removing gas from the chambers  120 ,  121 . A backfill inlet  145  provides an inlet for backfilling the chambers  120 ,  121  with inert gas. The lower chamber  121  may comprise a further inlet  146  for maintaining the lower chamber  121  at an overpressure relative to the upper chamber  120 . 
     The gas flow circuit comprises filter assemblies  180 ,  181  connected in parallel within the gas circuit to filter particulates within the recirculated gas. Each filter assembly  180 ,  181  comprises a filter housing  182 ,  183 , a filter element  184 ,  185  located in the filter housing  182 ,  183  and manually operated valves  186 ,  187 ,  188 ,  189  for opening and closing gas inlet and gas outlet, respectively. Each filter assembly  180 ,  182  is detachable from the gas circuit for replacement of the filter element  182 ,  183 , as is described in WO2010/026396. 
     The apparatus comprises thermal devices for affecting the heating and/or cooling of an internal surface of the inert gas vessel  100  to cause particulates to be preferentially deposited at a predetermined location in the vessel  100  desirable for particulate collection. 
     A first thermal device is a polymer retainer ring  161  for retaining the laser window  107  in place. The polymer material insulates the internal surface of the ring from the colder environment surrounding the build chamber  101 . Other internal surfaces of the build chamber  101  are provided with a good thermal coupling to the surrounding environment. For example, walls  162  of the build chamber  101  may be made of material that has good thermal conductivity, such as a metal. Accordingly, during a build, the internal surfaces of the build chamber walls  162  may be cooler than the internal surfaces of the retainer ring  161  and laser window  107  such that particulates preferentially collect on surfaces of the build chamber walls  162  rather than the retainer ring or laser window  107 . 
     Further thermal devices in the form of insulation  165  may also be provided around the inlet nozzle for inert gas and the viewing window  163  in the door  149  to ensure that internal surfaces of the inlet nozzle and viewing window  163  remain at a higher temperature than other internal surfaces of the vessel  100  such that the particulates preferentially deposit on the other internal surfaces. 
     The gas flow circuit further comprises a thermal device for controlling the temperature of internal surfaces of the gas circuit. In  FIGS. 1 and 2 , the thermal device is a cooling device  164  for cooling the filter housings  182 ,  183 . The cooling device  164  is arranged to cool internal surfaces of each housing  182 ,  183  that are exposed to gas flow that has yet to pass through the filter elements  184 ,  185 . The colder internal surface of the filter housings  182 ,  183  encourage particulates in this gas flow to be deposited on the internal surfaces of the housings  182 ,  183 . The housings  182 ,  183  may comprise web like structures (not shown) to provide an increased surface area for the collection of particulates. 
     The cooling device  164  may be a refrigeration unit for cooling a coolant, which in turn flows through heat exchange conduits to cool the housings  182 ,  183 . 
     Flooding of the housings  182 ,  183  with water during changing of the filter element cleans particulates from the internal surfaces of the housing  182 ,  183 . As a result, the internal surfaces of the filter housing  182 ,  183  are desirable locations in vessel  100  for the deposition of particulates. 
     The gas circuit may further comprise a heater  167  for heating gas downstream of the filter elements such that the inert gas delivered into the build chamber  101  is close to or above the ambient temperature of the inert gas in the build chamber  101 . This may help to prevent the build-up of deposits around the inlet nozzle. 
       FIG. 3  shows apparatus according to another embodiment of the invention. Features of this embodiment that are the same or similar to features of the embodiment described with reference to  FIGS. 1 and 2  have been given the same reference numerals but in the series  200 . 
     The embodiment shown in  FIG. 3  differs from that shown in  FIGS. 1 and 2  in that the retainer ring  261  is a metal retainer ring thermally coupled to a heater  271 . The heater  271  heats the retainer ring  261  such that an internal surface of the retainer ring  261  exposed to the inert gas in the vessel  200  is heated to a temperature above the ambient temperature of the inert gas. Heating of the retainer ring  261  may limit or prevent altogether particulates being deposited on the retainer ring  261  and the laser window  207 . 
       FIG. 3  also differs from the embodiment shown in  FIGS. 1 and 2  in that the heater  167  is omitted, such that cooled inert gas is delivered into the build chamber  201 . 
       FIG. 4  shows a particulate collection device  300  that may be located in the build chamber shown in  FIGS. 1 to 3 . The device  300  comprises an elongate annular member  301  having an outer surface  302  for the collection of particulates. The annular member  301  is mounted to a spindle  307  which is itself mounted on a frame  303  to allow rotation of the member  301 . The spindle  307  has a formation (not shown) for connecting the spindle to a motor (not shown) for driving rotation of the member  301 . A wiper, in this embodiment a brush  304 , is mounted on the frame  303  so as to engage the outer surface  302  of the member  301  as the member  301  is rotated. The brush  304  extends along the length of the elongate member  301 . The brush  304  removes particulates from the outer surface  302  of the member  301  as the member is rotated. A cooling device  309  is provided to cool the annular member  301  to below an ambient temperature of inert gas in a build chamber of an additive manufacturing apparatus, such as those shown in  FIGS. 1 to 3 . 
     In use, the device may be placed in the build chamber, such as close to an exhaust outlet for inert gas and above a collection bin  400  for particulates. During a build, the annular member  301  is cooled and rotated such that particulates in the inert atmosphere preferentially deposit on the surface  302  of the annular member  301 . The brush  304  removes the particulates from the annular member  301  causing the particulates to collect in the collection bin  400  located below the device. 
       FIG. 5  shows a further embodiment of the invention. Features of this embodiment that are the same or similar to features of the embodiments described with reference to  FIGS. 1 to 3  have been given the same reference numerals but in the series  400 . This embodiment differs from the embodiments shown in  FIGS. 1 to 3  in that two the cooling devices  464   a,    464   b,  one  464   a  for cooling and capturing particles before the inert gas enters into the filter assemblies  482 ,  483  and a second downstream of the pump  470  for cooling gas heated by the pump  470 . The cooling devices  464   a,    464   b  defines at least one serpentine passageway for the gas, the walls of the passageway(s) cooled by appropriate means, for example a coolant. Internal surfaces of the passageway may comprise spikes or rods that act as cold fingers or anticontaminators (similar to the devices used in electron microscopy) filled with a coolant. Like the embodiment shown in  FIG. 3 , there is no active heater for heating gas that passes through the gas recirculation loop. 
     Furthermore, the pump  470  is provided upstream of the cooling device  464 . This may be advantageous as the specifications for the pump  470  are not limited by the need to pass cooled gas therethrough. 
     In use of any of the above described embodiments, the inert gas may be cooled and passed into the build chamber  101 ,  201 ,  401  without being heated by a heater, to cool the gas within the chamber  101  to a temperature below that of internal surfaces of the build chamber  101 ,  201 ,  401 . (In the first embodiment, the computer  130  may deactivate the heater  167  such that the cooled inert gas passes into the build chamber  101 ). This may reduce a capacity of the inert gas in the build chamber  101 ,  201 ,  401  to hold vaporised material, such a vaporised metal material, produced during the additive building process. Accordingly, less vaporised material will migrate to critical surfaces, such as window  107 ,  207 ,  407 , which are desirably maintained free of condensate. Furthermore, the metal vapour held within the gas is less likely to condense onto the internal surfaces of the build chamber because the surfaces are at a higher temperature (as the walls of the build chamber are in thermal communication with the external environment which is at a higher temperature) than the temperature of the inert gas. In particular, heater  271 ,  471  may be used to heat the retainer ring  261 ,  461  around the window  207 ,  407  to elevate a temperature of an internal surface of the window  207 ,  407  above a temperature of the inert gas in the build chamber  201 ,  401 . The cooled gas  142 ,  242 ,  442  acts as a cooled gas blanket/curtain thermally isolating critical surfaces, such as the window  107 ,  207 ,  407  and viewing window  163  of door  165  from the heated powder bed  104 ,  204 ,  404  and solidified material of the object  103 ,  203 ,  204 . 
     The gas flows  142 ,  242 ,  442  may generate a temperature inversion layer within the build chamber  101 ,  201 ,  401  wherein a layer of warmer gas is trapped above the gas flow  142 ,  242 ,  442  of the cooled gas. The temperature inversion may act to trap vaporised material below the layer of warm gas where the particulates are removed by the gas knife  142 ,  242 ,  442 . 
     The cooled inert gas delivered into the build chamber may be less than 20 degrees and preferably between 0 and 10 degrees. 
     Furthermore, at the end of the build, cooled inert gas continues to be recirculated/is recirculated to cool the build chamber  101 ,  201 ,  401  and the object built using the additive build process. This may reduce the time between the end of the build and when the build chamber and object have cooled sufficiently to allow the build chamber door to be opened and the object removed from the build chamber  101 ,  201 ,  401 . 
     It will be understood that alterations and modifications may be made to the embodiments as described herein without departing from the invention as defined in the claims.