Abstract:
An additive manufacturing apparatus including a chamber, a build platform movable in the chamber such that layers of flowable material can be successively formed across the build platform, a unit for generating an energy beam for solidifying the flowable material, a scanning unit for directing the energy beam onto selected areas of each layer to solidify the material in the selected areas and a getter for absorbing oxygen, nitrogen and/or hydrogen from atmosphere in the chamber.

Description:
FIELD OF INVENTION 
       [0001]    This invention concerns additive manufacturing apparatus and methods in which material layers are solidified in a layer-by-layer manner to form an object. 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 
       [0002]    Selective laser melting (SLM) and selective laser sintering (SLS) apparatus produce objects 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 is then scanned across portions of the powder layer that correspond to a cross-section of the object 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. 
         [0003]    The process is carried out in an inert gas atmosphere because the metal powder is highly reactive with gases, such as oxygen. As described in International patent application No: 2010/007394, it is known to first form a vacuum in a build chamber and then refill the chamber with an inert gas in order to ensure low oxygen content in the resultant atmosphere. For example, using this technique, an oxygen content of the atmosphere in the chamber may be reduced as low as 1000 ppm. Over the course of the build, the oxygen content may drop further due to the remaining oxygen in the chamber being consumed as oxides in the part being formed. The amount of oxygen absorbed will depend on the material being used. For example, titanium is much more able to absorb oxygen than steel or aluminium. It is desirable to have a consistent oxygen content in the atmosphere throughout the build in order to achieve consistent build properties throughout the part. 
         [0004]    It is believed that moisture in the air and the powder leads to hydrogen being absorbed into the material of the part, which results in hydrogen gas porosity in the part. 
         [0005]    The presence of nitrogen in the inert atmosphere during a build may also be undesirable. 
       SUMMARY OF INVENTION 
       [0006]    According to a first aspect of the invention there is provided an additive manufacturing apparatus comprising a chamber, a build platform movable in the chamber such that layers of flowable material can be successively formed across the build platform, a unit for generating an energy beam for solidifying the flowable material, a scanning unit for directing the energy beam onto selected areas of each layer to solidify the material in the selected areas and a getter for absorbing oxygen, nitrogen and/or hydrogen from atmosphere in the chamber. 
         [0007]    The apparatus may comprise a gas recirculation circuit having an inlet and outlet connected to the chamber for recirculating gas through the chamber, wherein the gas recirculation circuit has the getter therein for absorbing oxygen, nitrogen and/or hydrogen from the recirculating gas. 
         [0008]    The getter may allow the levels of oxygen, nitrogen and/or hydrogen to be reduced to lower levels than can be achieved through the conventional methods of degassing the build chamber and backfilling with an inert gas. Furthermore, the getter may ensure levels of oxygen, nitrogen and/or hydrogen remain substantially stable throughout a build. 
         [0009]    The getter may be an oxygen getter, such as a copper based getter. Oxygen is often absorbed by the material during solidification, especially in the case of metals. Providing an oxygen getter may reduce the amount of oxygen absorbed by the material during solidification. In an alternative embodiment, the getter may be a titanium based getter. 
         [0010]    The gas recirculation circuit may comprises valve(s) that can isolate the getter from atmosphere in the chamber. The getter can be permanently damaged through exposure to excessive levels of certain gases. Providing isolation valves allows the getter to be isolated from the atmosphere in the chamber until such gases are reduced to a level that is safe for the getter. 
         [0011]    The chamber may comprise a door through which a part, built using the additive manufacturing apparatus, can be removed from the chamber and the valve(s) may be arranged to isolate the getter from the chamber when the door to the chamber is opened. 
         [0012]    The additive manufacturing apparatus may comprise means for removing unwanted gases absorbed by the getter from the chamber, the valve(s) arranged to isolate the getter from the chamber whilst gases absorbed by the getter remain above a predetermined level. 
         [0013]    The additive manufacturing apparatus may comprise means for regenerating the getter after use. It may desirable to regenerate to getter within the apparatus such that the getter does not need to be regularly removed from the apparatus. The means for regenerating the getter may comprise a heating element for heating gas that flows past the getter. The means for regenerating the oxygen getter may comprise a source of hydrogen gas. 
         [0014]    The apparatus may comprise means for removing moisture, generated from the regeneration of the getter, from the apparatus. Regeneration of the getter may generate steam/water as by-product, which it may be desirable to remove as it may be undesirable for such moisture to be present in the atmosphere of the chamber during a build. 
         [0015]    The additive manufacturing apparatus may comprise a sensor for detecting a characteristic that is indicative of progress in the regeneration of the getter. The sensor may be a temperature sensor for monitoring a temperature of material of the getter, a moisture detector for detecting an amount of moisture in gases leaving the getter and/or a hydrogen sensor for detecting a concentration of hydrogen in gases leaving the getter. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic of a selective laser solidification apparatus according to an embodiment of the invention; and 
           [0017]      FIG. 2  is a schematic of the selective laser solidification apparatus from another side. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    Referring to  FIGS. 1 to 5 , a laser solidification apparatus according to an embodiment of the invention comprises a main chamber  101  having therein partitions  115 ,  116  that define a build chamber  117  and a surface onto which powder can be deposited. A build platform  102  is provided for supporting an object  103  built by selective laser melting powder  104 . The platform  102  can be lowered within the build chamber  117  as successive layers of the object  103  are formed. A build volume available is defined by the extent to which the build platform  102  can be lowered into the build chamber  117 . 
         [0019]    The main chamber  101  comprises a door  170  to allow access to the chamber  101  for removable of parts built using the apparatus. 
         [0020]    Layers of powder  104  are formed as the object  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. 
         [0021]    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 . 
         [0022]    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 . 
         [0023]    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 modules of the laser melting unit, such as optical module  106  and laser module  105 , and an external data connection  135 . Stored on memory  132  is a computer program that instructs the processing unit to carry out the method as now described. 
         [0024]    Processor receives via external connection  135  geometric data describing scan paths to take in solidifying areas of powder in each powder layer. To build a part, the processor controls the scanner  106  to direct the laser beam in accordance with the scan paths defined in the geometric data. In this embodiment, to perform a scan along a scan path, the laser  105  and scanner  106  are synchronised to expose a series of discrete points along the scan path to the laser beam. For each scan path, a point distance, point exposure time and spot size is defined. In an alternative embodiment, the spot may be continuously scanned along the scan path. In such an embodiment, rather than defining a point distance and exposure time, a velocity of the laser spot may be specified for each scan path. 
         [0025]    The apparatus further comprises a gas recirculation circuit  150  and an inlet  160  for backfilling the chamber  101  with inert gas, such as nitrogen or argon. 
         [0026]    Gas recirculation circuit  150  comprises an inlet  151  and outlet  152  connected with the main chamber  101  and a pump  153  for recirculating gas through the gas recirculation circuit  150  and main chamber  101  to generate a gas knife. K, across the powder bed  104  for removing condensate generated during the melting process. The pump  153  is also connectable to a degassing valve  161  to allow the pump  153  to degas chamber  101  to a rough vacuum. The gas recirculation circuit  150  further comprises a filter  154  for removing particles from the recirculating gas and an oxygen getter  155 . 
         [0027]    In this embodiment, the oxygen getter  155  comprises a copper based oxygen getter, such as the catalyst GetterMax 133 or 233 supplied by Research Catalysts Inc. The recirculated gas is pumped past the material of the oxygen getter  155  as it is recirculated. The copper based oxygen getter absorbs oxygen through the formation of copper oxide. 
         [0028]    The recirculation circuit  150  comprises a heating element  162  for heating gas transported in the recirculation circuit  150  and a valve  163  connectable to a source  164  of hydrogen gas, the valve  163  controllable to control a quantity of hydrogen gas allowed into the recirculation circuit  150 . One or more temperature monitors, such as thermocouples  165 , monitor the temperature of the oxygen getter  155 , a moisture content sensor  166  monitors the moisture content of gas leaving the oxygen getter  155  and a hydrogen gas sensor  167  monitors the quantity of hydrogen in the gas leaving the oxygen getter  155 . A means  168  may be provided for condensing and removing water from the oxygen getter  155 . 
         [0029]    In use, before a build, the chamber  101  is degased to a vacuum and then backfilled via inlet  160  with an inert atmosphere, such as argon or nitrogen. The build is carried out under the inert atmosphere with gas being recirculated via the recirculation circuit  150  during the build to form gas knife, K. Oxygen remaining in the atmosphere is absorbed by the oxygen getter  155 . To this end, the inert gas may be recirculated for a set period of time before the build commences such that oxygen in the atmosphere is reduced to a desired low level through absorption of the oxygen by the getter  155  before the build commences. 
         [0030]    To regenerate the oxygen getter, for example at the end of each build, the oxygen getter  155  must be activated by reduction. In one embodiment, reduction of the getter may be carried out external to the apparatus by removal of the oxygen getter from the apparatus. However, in this embodiment, activation of the oxygen getter is carried out in the gas recirculation circuit  150 . 
         [0031]    Regeneration of the oxygen getter  155  may comprise heating an inert, hydrogen free gas, such as nitrogen or argon, flowing though the getter  155  with the heating element  162  such that the material bed of the oxygen getter  155  is at a desired temperature, such as between 175 to 180° C. The temperature of the oxygen getter  155  may be monitored by the thermocouples  165 . 
         [0032]    Once the catalyst has been heated for a predetermined length of time, such as 2 hours, hydrogen is introduced into the gas in the recirculation circuit through valve  163 , whilst maintaining the temperature of the gas flowing onto the getter  155  at the desired temperature. The temperature of the oxygen getter  155  will increase as the hydrogen in the gas reacts with the oxygen. The temperature rise in the oxygen getter  155  may be monitored using the thermocouple  165 . If the temperature of the oxygen getter  155  exceeds a predetermined value, such as 225° C., the gas flowing through the oxygen getter  155  is switched back to hydrogen free gas. 
         [0033]    Completion of the activation process may be determined from any one or more of the thermocouple  165 , the moisture sensor  166  and the hydrogen gas sensor  167 . Completion of the activation process may be determined from:
       a) A stable temperature of the material bed of the oxygen getter;   b) Stoppage of the formation of water; and/or   c) The concentration of hydrogen in the gas leaving the oxygen getter  155  equalling the concentration of hydrogen in the gas entering the oxygen getter  155 .       
 
         [0037]    Water generated as a result of the activation process may be removed by means  168 . 
         [0038]    After activation, the recirculation circuit  150  may be purged with the hydrogen free inert gas, such as argon or nitrogen, to ensure the recirculation circuit  150  is free from hydrogen. 
         [0039]    The recirculation circuit  150  further comprise valves  156  and  157  for isolating the gas recirculation circuit  150  from the build chamber  101  when door  170  is opened. Sensors (not shown) may be provided for detecting if the door is opened such that the valves  156 ,  157  can be automatically activated by the processor when opening of the door is detected. With the oxygen getter in the activated state, exposure to air can cause the oxygen getter to heat up sufficiently to permanently damage the oxygen getter  155 . 
         [0040]    In another embodiment, in addition to or instead of the oxygen getter, the apparatus comprises a hydrogen or nitrogen getter.