Abstract:
An additive manufacturing apparatus for building objects by layerwise consolidation of material. The apparatus includes a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers and a flow device for generating a gas flow across at least a part of the working area from a gas inlet to a gas outlet. The gas inlet and gas outlet are arranged to be movable within the build chamber.

Description:
SUMMARY OF INVENTION 
       [0001]    This invention concerns an additive manufacturing apparatus and method. The invention has particular, but not exclusive, application to providing gas flow across a powder bed in a laser solidification apparatus. 
       BACKGROUND 
       [0002]    Additive manufacturing or rapid prototyping methods for producing objects comprise layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam or electron beam. A powder layer is deposited on a powder bed in a build chamber and a laser beam is 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. 
         [0003]    During the melting or sintering process, debris (e.g. condensate, unsolidified particles of powder etc) is produced within the build chamber. It is known to introduce a gas flow through the build chamber in an attempt to remove debris from the chamber in the gas flow. For example, the M280 model of machine produced by EOS GmbH, Munich, Germany comprises a series of gas outlet nozzles located in the build chamber to the rear of the powder bed that pass a flow of gas to a series of exhaust vents that are located in the build chamber at the front of the powder bed. In this manner, a planar layer of gas flow is created at the surface of the powder bed. A similar arrangement is provided in Renishaw&#39;s AM250 and AM125 machines, wherein apertures in the build chamber either side of a powder bed provide substantially planar gas flow across the powder bed. 
         [0004]    A problem with the gas flow generated by such arrangements is that the gas flow may not be sufficient to carry all debris to the exhaust vents and some of the debris may be blown onto powder that has yet to be scanned. This can result in the object being built inaccurately. For example, the debris blown onto the powder can solidify to the object being built resulting in a solid projection from the bed that contacts a wiper blade when the wiper spreads the next layer of powder. This projection can cause damage to the wiper blade, which in turn, can result in subsequent layers of powder having a ridge at a location corresponding to the area of the wiper blade that is damaged. These ridges are created in each layer for the rest of the build, affecting the accuracy with which the object is built. 
         [0005]    Furthermore, it is known to vary the direction in which the laser is scanned for different layers and/or for different portions of a layer, for example, see US2008/0241392 and US2005/0142024. It may be desirable to scan the laser in a particular direction based on gas flow direction, for example as set out in patent applications U.S. 61/791,636 and U.S. 61/774,215, which are incorporated herein by reference. However, with the apparatus described above, it may be necessary to compromise between optimum scan direction based on gas flow direction and the desire to change the scan direction for different layers and/or different portions of a layer. 
         [0006]    U.S. Pat. No. 6,215,093 describes apparatus wherein a nozzle for providing a protective gas stream travels together with the laser beam. 
       SUMMARY OF INVENTION 
       [0007]    According to a first aspect of the invention there is provided additive manufacturing apparatus for building objects by layerwise consolidation of material, the apparatus comprising a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers and a flow device for generating a gas flow across at least a part of the working area from a gas inlet to a gas outlet, the gas inlet and gas outlet arranged to be movable within the build chamber. 
         [0008]    By providing a movable gas inlet and gas outlet, the locations of the gas inlet and outlet can be altered based upon the scan path of the beam across the working area. For example, the locations of the gas inlet and outlet may be changed based on a direction in which a series of successive hatch lines are progressed and/or to move with the progression of the scan such that the gas inlet and gas outlet can be located closer to the impact point of the beam on the material. 
         [0009]    It will be understood that the term “scan” used herein is not limited to continuously running a spot of the high energy beam over a surface but includes a series of separated discrete exposures (or hops). For example, optics may direct the high energy beam to expose a first location to the beam, the beam then turned off and the optics reoriented to direct the energy beam to a second location spaced from the first location when the high energy beam is switched back on. The high energy beam is a beam having sufficient energy to consolidate the material. 
         [0010]    The gas inlet and gas outlet may my movable together such that the relative positions of the gas inlet and gas outlet remain fixed. For example, the gas inlet and outlet may be built as a single movable unit. 
         [0011]    Alternatively, the gas inlet is movable separately from the gas outlet. The gas inlet and gas outlet may be movable such that the distance between the gas inlet and gas outlet can be varied. In particular, the gas inlet and gas outlet may be movable such that the distance between the gas inlet and gas outlet can be less than a width of a working area, such as defined by a build platform, in which an object is built. In this way, the gas inlet and gas outlet can be located closer together than for nozzles fixed either side of the build platform such that a more uniform gas flow may be achieved and debris ejected from the area being consolidated are more likely to be captured by the gas flow and carried to the gas outlet. The apparatus may comprise a gas flow device for controlling the gas flow through the inlet and/or outlet based upon the distance between the gas inlet and gas outlet. 
         [0012]    The gas inlet and gas outlet may be mounted on an assembly for moving the inlet and outlet along at least one linear axis and, additionally, may be mounted on an assembly for rotating the inlet and outlet about at least one rotary axis. Rotating the gas inlet and gas outlet may allow one to change the direction of gas flow based upon the scan direction. The movement may be controlled by a computer at the object is built. 
         [0013]    The gas inlet and/or gas outlet may comprise an elongate aperture that extends across an entire width of the working area, the gas inlet and/or gas outlet movable in a linear direction perpendicular to a longitudinal axis of the aperture. Such an arrangement may move in only one linear direction as the aperture provides flow across the entire width of the working area. 
         [0014]    However, in another embodiment, the gas inlet and/or gas outlet may comprise an aperture that extends across less than a width of the working area, the gas inlet and/or gas outlet movable in a linear direction perpendicular to a gas flow direction from the gas inlet/into the gas outlet. In this way, a smaller gas inlet or outlet can be provided whilst full coverage of the working area may still be achieved through movement of gas inlet and/or outlet in a direction perpendicular to the gas flow direction. A smaller gas inlet and/or outlet may be beneficial as it may result in a lighter unit that can be moved more quickly over the working area than a larger unit that extends across the entire working area. 
         [0015]    Preferably, the apparatus is a selective laser solidification, such as melting (SLM) or sintering (SLS), apparatus, wherein powder layers are successively deposited across the working area in the build chamber and a laser beam is scanned across portions of each powder layer that correspond to a cross-section of the object being constructed to consolidate the portions of the powder. 
         [0016]    The apparatus may further comprise a wiper for spreading powder across the working area. The wiper may be mounted to move with at least one of the gas inlet and gas outlet. In this way, the powder may be simultaneously spread across the working area with movement of the gas inlet and/or gas outlet. The wiper (which is mounted to move with at least one of the gas inlet and gas outlet) may be movable from an extended position, in which the wiper engages the powder, to a retracted position, in which the wiper is held clear of the powder. In this way, the gas inlet and/or gas outlet can be moved both with and without spreading powder by moving the wiper between the extended and retracted positions. 
         [0017]    The apparatus may further comprise a probe for measuring geometry of the object being built, the probe mounted to move on an axis common with the gas inlet and/or gas outlet. The gas inlet and/or gas outlet that is mounted on a common axis with the probe may be arranged to move in a first direction, such as a first linear direction, and the probe is arranged to move in the first direction and in a further direction, such as a further linear direction perpendicular to the first linear direction. The probe may be a contact probe, such as a scanning or touch probe, or a non-contact probe, such as a video probe. 
         [0018]    The gas inlet is for propelling gas into the build chamber and the gas outlet is for drawing gas from the build chamber. The apparatus may comprise a controller for controlling a rate of flow at which gas is propelled into the build chamber from the gas inlet based upon a location of the gas inlet and/or gas outlet in the build chamber. The apparatus may comprise a controller for controlling a rate of flow at which gas is drawn from the build chamber through the gas outlet based upon a location of the gas inlet and/or gas outlet in the build chamber. 
         [0019]    According to a second aspect of the invention there is provided an additive manufacturing method for building objects by layerwise consolidation of material, the method comprising depositing material in a working area in a build chamber, scanning a high energy beam across the working area to consolidate the material in layers and operating a flow device for generating a gas flow across at least a part of the working area from a gas inlet to a gas outlet comprising moving the gas inlet and the gas outlet during building of the object. 
         [0020]    The method may comprise moving the gas inlet and gas outlet to vary the distance between the gas inlet and gas outlet. 
         [0021]    The method may comprise moving the gas inlet and gas outlet to alter the direction of gas flow across the working area. 
         [0022]    The method may comprise altering a rate of flow at which gas is propelled into the build chamber from the gas inlet based upon a location of the gas inlet and/or gas outlet in the build chamber. The method may comprise controlling a rate of flow at which gas is drawn from the build chamber through the gas outlet based upon a location of the gas inlet and/or gas outlet in the build chamber. 
         [0023]    The method may comprise moving the gas inlet and gas outlet whilst scanning the material with the high energy beam. The inlet and outlet may be moved based upon the scan path of the high energy beam. For example, the gas inlet and gas outlet may be moved to track an impact point of the high energy beam with the material/in the working area. 
         [0024]    The method may be a selective laser solidification method and may comprise successively depositing layers or powder across the working area and scanning the high energy beam across portions of each powder layer that correspond to a cross-section of the object being constructed to consolidate the portions of the powder. 
         [0025]    According to a third aspect of the invention there is provided a data carrier having instructions thereon, the instructions, when executed by a processor, causing the processor to control an additive manufacturing apparatus according to the first aspect of the invention to carry out the method of the second aspect of the invention. 
         [0026]    According to a fourth aspect of the invention there is provided additive manufacturing apparatus for building objects by layerwise consolidation of material, the apparatus comprising a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers, a first flow device for propelling gas into a volume above the working area and a second flow device for drawing gas from the volume so as to generate a gas flow between the first and second flow devices, at least one of the first flow device and second flow device arranged to be movable within the build chamber, and a control unit for controlling scanning of the material with the high energy beam in accordance with a predetermined scanning plan and controlling movement of the first and/or second flow device based upon the scanning plan. 
         [0027]    According to a fifth aspect of the invention there is provided an additive manufacturing method for building objects by layerwise consolidation of material, the method comprising depositing material in a working area in a build chamber, scanning a high energy beam across the working area to consolidate the material in layers in accordance with a predetermined scanning plan and operating a first flow device for propelling gas into a volume above the working area, which includes the material being consolidated with the high energy beam, and a second flow device for drawing gas from the volume to generate a gas flow between the first and second flow devices, and further comprising moving at least one of the first and second flow devices within the build chamber during building of the object based upon the scanning plan. 
         [0028]    According to a sixth aspect of the invention there is provided a data carrier having instructions thereon, the instructions, when executed by a processor, causing the processor to control an additive manufacturing apparatus according to the fourth aspect of the invention to carry out the method of the fifth aspect of the invention. 
         [0029]    According to a seventh aspect of the invention there is provided additive manufacturing apparatus for building objects by layerwise consolidation of material, the apparatus comprising a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers, a first flow device for propelling gas into a volume above the working area and a second flow device for drawing gas from the volume so as to generate a gas flow between the first and second flow devices, the first and second flow devices arranged to be movable within the build chamber. 
         [0030]    According to a eighth aspect of the invention there is provided an additive manufacturing method for building objects by layerwise consolidation of material, the method comprising depositing material in a working area in a build chamber, scanning a high energy beam across the working area to consolidate the material in layers and operating a first flow device for propelling gas into a volume above the working area, which includes the material being consolidated with the high energy beam, and a second flow device for drawing gas from the volume to generate a gas flow between the first and second flow devices, further comprising moving the first and second flow devices within the build chamber during building of the object. 
         [0031]    According to a ninth aspect of the invention there is provided a data carrier having instructions thereon, the instructions, when executed by a processor, causing the processor to control an additive manufacturing apparatus according to the seventh aspect of the invention to carry out the method of the eighth aspect of the invention. 
         [0032]    According to a tenth aspect of the invention there is provided additive manufacturing apparatus for building objects by layerwise consolidation of material, the apparatus comprising a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers, a probe for measuring geometry of the object being built, a first flow device for propelling gas into a volume above the working area and a second flow device for drawing gas from the volume so as to generate a gas flow between the first and second flow devices, wherein at least one of the first flow device and second flow device is arranged to be movable within the build chamber and the probe is mounted to move on an axis common with the gas inlet and/or gas outlet. 
         [0033]    According to an eleventh aspect of the invention there is provided additive manufacturing apparatus for building objects by layerwise consolidation of material, the apparatus comprising a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers, a first flow device for propelling gas into a volume above the working area, a second flow device for drawing gas from the volume so as to generate a gas flow between the first and second flow devices, at least one of the first flow device and second flow device arranged to be movable within the build chamber, and a wiper for spreading powder across the working area, the wiper mounted to move with at least one of the first flow device and the second flow device. 
         [0034]    According to a twelfth aspect of the invention there is provided an additive manufacturing method for building objects by layerwise consolidation of material, the method comprising depositing material in a working area in a build chamber, moving a wiper for spreading powder across the working area, scanning a high energy beam across the working area to consolidate the material in layers and operating a first flow device for propelling gas into a volume above the working area, which includes the material being consolidated with the high energy beam, and a second flow device for drawing gas from the volume to generate a gas flow between the first and second flow devices, the method further comprising moving at least one of the first and second flow devices during building of the object simultaneously with moving the wiper. 
         [0035]    According to a thirteenth aspect of the invention there is provided a data carrier having instructions thereon, the instructions, when executed by a processor, causing the processor to control an additive manufacturing apparatus according to the eleventh aspect of the invention to carry out the method of the twelfth aspect of the invention. 
         [0036]    According to a fourteenth aspect of the invention there is provided an additive manufacturing apparatus for building objects by layerwise consolidation of material, the apparatus comprising a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers, an optical module for directing the high energy beam on to the working area, a flow device for drawing gas from a volume above the working area so as to generate a gas flow across the working area, the flow device arranged to be movable within the build chamber, and a controller for controlling movement of the gas flow device and the optical module such that the flow device is moved during scanning of the working area with the high energy beam. 
         [0037]    A gas outlet that draws gas from the build chamber may create a sufficient flow in the vicinity of the gas outlet such that debris created by solidification of the material is sufficiently removed. Accordingly, confining solidification to be within the vicinity of the outlet may allow greater freedom in the placement of the inlet because laminar flow between the inlet and outlet may no longer be required. For example, the inlet my propel gas in a direction that is not parallel with the working area and/or may have a fixed location within the build chamber. 
         [0038]    The additive manufacturing apparatus may further comprise a guide for guiding gas drawn from the build chamber into the gas outlet, the guide movable within the build chamber. The guide may be movable together with the gas outlet. The guide may facilitate a desired circulation of gas within the build chamber. 
         [0039]    The controller may be arranged to control the optical module to direct the high energy beam to a location between the guide and the gas outlet. 
         [0040]    According to a fifteenth aspect of the invention there is provided an additive manufacturing apparatus for building objects by layerwise consolidation of material, the apparatus comprising a build chamber containing a working area, a high energy beam for consolidating material deposited in the working area in layers, an optical module for directing the high energy beam on to the working area, a flow device for propelling gas into and/or drawing gas from a volume above the working area so as to generate a gas flow across the working area, the flow device arranged to be movable within the build chamber, and a controller for controlling movement of the gas flow device and the optical module such that the flow device can be moved separately from movement of the high energy beam across the working area. 
         [0041]    The flow device may be moved on a guide, such as a rail or track, by a first motor, the controller arranged for controlling the first motor to adjust the position of the flow device. The optical module may comprise an optical element, such as a lens or mirror, for directing the high energy beam to the desired location in the working area, the optical element mounted on an assembly for movement and a second motor for moving the optical element in the assembly, the controller arranged for controlling the second motor to adjust the position of the optical element. The controller may be arranged to control the first motor to move the flow device whilst the optical element remains stationary. The controller may be arranged to control the second motor to move the optical element whilst the flow device remains stationary. 
         [0042]    This may be desirable for certain scanning strategies and/or operative conditions of the apparatus, wherein movement of the flow device and optical element together is not required. For example, when carrying out a border scan of the object, it may be desirable for the flow device to be fixed at a specified location for the duration of the border scan. Furthermore, in-between the scanning of layers it may be desirable to move the flow device to a location outside of the working area such that a wiper or the like can pass over the working area. 
         [0043]    The data carrier of the above aspects of the invention may be a suitable medium for providing a machine with instructions such as non-transient data carrier, for example a floppy disk, a CD ROM, a DVD ROM/RAM (including −R/−RW and +R/+RW), an HD DVD, a BIu Ray™ disc, a memory (such as a Memory Stick™, an SD card, a compact flash card, or the like), a disc drive (such as a hard disk drive), a tape, any magneto/optical storage, or a transient data carrier, such as a signal on a wire or fibre optic or a wireless signal, for example a signals sent over a wired or wireless network (such as an Internet download, an FTP transfer, or the like). 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  is a schematic view of an additive manufacturing apparatus according to one embodiment of the invention; 
           [0045]      FIG. 2  is a schematic view of the additive manufacturing apparatus of  FIG. 1  from another side; 
           [0046]      FIG. 3  is a plan view of a gas flow device of the apparatus during building of a core region of the object; 
           [0047]      FIG. 4 a    is a plan view of the gas flow device during building of a peripheral region of the object; 
           [0048]      FIG. 4 b    is a plan view of the gas flow device during deposition of a powder layer using a wiper; 
           [0049]      FIG. 5 a    is a perspective view of another embodiment of the invention comprising a fixed length gas recirculation loop; 
           [0050]      FIG. 5 b    is a plan view of another embodiment of a fixed length gas recirculation loop; 
           [0051]      FIGS. 6 a  and 6 b    show a further embodiment of a gas flow device according to the invention comprising a metrology device for measuring an attribute of the object being built; 
           [0052]      FIG. 7  is a plan view of a gas flow device according to another embodiment of the invention; 
           [0053]      FIG. 8 a    is a plan view of a gas flow device according to another embodiment of the invention; 
           [0054]      FIG. 8 b    is a plan view of a gas flow device according to another embodiment of the invention; 
           [0055]      FIG. 8 c    is a plan view of a gas flow device according to another embodiment of the invention; 
           [0056]      FIG. 9  is a plan view of a gas flow device according to another embodiment of the invention; 
           [0057]      FIG. 10  shows additive manufacturing apparatus according to an embodiment of the invention having a gas flow device with an enlarged gas outlet; 
           [0058]      FIG. 11 a    is a perspective view of a gas flow device according to another embodiment of the invention; 
           [0059]      FIG. 11 b    is a perspective view of a modification to the gas flow device of  FIG. 11 , wherein the wiper blade is retractable; 
           [0060]      FIG. 12  is a schematic view of an additive manufacturing apparatus comprising the flow device shown in  FIG. 11 ; 
           [0061]      FIG. 13  is a schematic view of the additive manufacturing apparatus shown in  FIG. 12  from a different side; 
           [0062]      FIG. 14  is a perspective view of a gas flow device according to another embodiment of the invention; and 
           [0063]      FIG. 15  is a perspective view of a gas flow device according to yet another embodiment of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0064]    Referring to  FIGS. 1 and 2 , a laser solidification apparatus according to an embodiment of the invention comprises a build chamber  101  having therein partitions  114 ,  115  that define a build volume  116  and a surface onto which powder can be deposited. A build platform  102  defines a working area in which an object  103  is built by selective laser melting powder  104 . The platform  102  can be lowered within the build volume  116  using mechanism  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 volume  116 . Layers of powder  104  are formed as the object  103  is built by dispensing apparatus  108  and a wiper  109 . For example, the dispensing apparatus  109  may be apparatus as described in WO2010/007396. A laser module  105  generates a laser for melting the powder  104 , the laser directed onto the powder bed  104  as required by optical module  106  under the control of a computer  160 . The laser enters the chamber  101  via a window  107 . 
         [0065]    A gas flow device comprises a movable gas nozzle  112  comprising a gas inlet  112   a  and a movable gas exhaust  110  comprising a gas outlet  110   a . The gas nozzle  112  and gas exhaust  110  are movable for generating a gas flow across a part or whole of the powder bed  104  formed on the build platform  102 . The gas inlet  112   a  and gas outlet  110   a  produce a laminar flow having a flow direction from the inlet to the outlet, as indicated by arrows  118 . Gas is re-circulated from the exhaust  110  to the nozzle  112  through a gas recirculation loop  111  that is also located within the chamber  116 . A pump  113  maintains the desired gas pressure at gas inlet  112  and gas outlet  110 . A filter  119  is provided in the recirculation loop  111  to filter from the gas condensate that has become entrapped in the flow. The recirculation unit  111  has means for changing the length of the gas recirculation loop with changes in the relative distance between the gas inlet  112   a  and gas outlet  110   a . In  FIGS. 1 and 2 , bellows  111   a  are used to make the gas recirculation loop  111  adaptable to the changes in position of the nozzle  112  and exhaust  110 . 
         [0066]    Computer  160  comprises a processor unit  161 , memory  162 , display  163 , user input device  164 , such as a keyboard, touch screen, etc, a data connection to modules of the laser melting apparatus, such as optical module  106 , laser module  105  and motors (not shown) that drive movement of the dispensing apparatus  108 , wiper  109 , build platform  102 , nozzle  112  and exhaust  110 . An external data connection  165  provides for the uploading of scanning instructions to the computer  160 . The laser unit  105 , optical unit  106 , movable gas inlet  112  and gas outlet  110  of the flow device and movement of build platform  102  are controlled by the computer  160  based upon the scanning instructions. 
         [0067]    A door  125  (shown in  FIG. 3 ) is provided in the chamber  101  for removing the object therefrom on completion of the build. 
         [0068]    Referring to  FIG. 3 , the nozzle  112  and exhaust  110  are mounted on a guide  120 , such as a rail or a track, to be movable along a linear axis. In use, the relative position of the nozzle  112  and exhaust  110  is altered based upon an area of the powder bed  104  being scanned and a scanning strategy being used. For example, in  FIG. 3 , a stripe scanning strategy is being used wherein an inner region of a section  121  of an object  103  is scanned by raster scans  122  that are progressed across the section  121  within a series of striped regions  123 . The nozzle  112  and exhaust  110  are moved to follow the laser beam as it makes the scan across the section  121 . The gas inlet  112   a  of the nozzle  112  and the gas outlet  110   a  of the exhaust  110  are located within the periphery of the powder bed  104  and, preferably, are maintained substantially a constant distance apart during the scanning of the stripes. 
         [0069]    After the scanning of the stripes  123 , a border scan  124  may be carried out around the outside of the section  121 . This is shown in  FIG. 4 a   . During the border scan  124 , the exhaust  110  and nozzle  112  may be located far enough apart such that the entire border scan can be completed without further movement of the nozzle  112  and exhaust  110 . 
         [0070]    Once a section (layer)  121  of the object has been completed, a further layer of powder is deposited on the powder bed  104 . To do this, a wiper  109  moves across the powder bed  104  to spread a fresh layer of powder. In this embodiment, in order for the wiper  109  to pass over the powder bed  104  the nozzle  112  and exhaust  110  must be sufficiently far apart to allow the wiper to pass therebetween. In  FIG. 4 b   , the wiper  109  is shown spreading powder, the position of the wiper  109  during consolidation of the powder shown in dotted lines. 
         [0071]    In this embodiment, the nozzle  112  and exhaust  110  have a curved outer surface to reduce disturbance of the gas in the build chamber  101  as the nozzle  112  and exhaust  110  move during consolidation of the powder using the laser beam. 
         [0072]    The gas-circulation loop  111 , nozzle  112  and exhaust  110  may be arranged to switch the direction of gas flow dependent upon the scanning direction. In such a scenario, the nozzle  112  would thus become the exhaust and the exhaust  110  the nozzle. 
         [0073]      FIGS. 5 a  and 5 b    show alternative embodiments of the recirculation loop having a fixed length to avoid pumping effects for changes in the distance between the nozzle  112  and exhaust  110 . In this embodiment, the recirculation loop  111  comprises tubes  180  to  183  for transporting the gas connected to rotary joints  185  to  190  that allow the tubes  180  to  183  to move with movement of the nozzle  112  and exhaust  110 . Rotary joints  185  and  190  are fixed relative to the build chamber and rotary joints  187  and  188  are fixed to the nozzle  110  and exhaust  112 , respectively. Rotary joints  186  and  189  “float” within the build chamber 
         [0074]    In  FIG. 5 a    the tubes  180  to  183  move in a horizontally plane with the rotary axes (shown in dotted lines) of the rotary joints  185  to  190  aligned vertically. In  FIG. 5 b   , the tubes  180  to  183  move in a vertical plane with the rotary axes of the rotary joints  185  to  190  aligned horizontally. The tubes  180  to  183  may be connected to filters and a pump of the recirculation loop located outside of the build chamber via an outlet/inlet located at the rotary joints  185 ,  190  fixed to the build chamber. 
         [0075]    The arrangements shown in  FIGS. 5 a  and 5 b    allow the nozzle  110  and exhaust  112  to move relative to each other whilst ensuring that the length of the recirculation loop  111  remains constant to avoid pumping effects that could occur with a bellows or telescopic tubing arrangement. Locating of the pump and filters external to the build chamber allows the filter to be replaced and maintenance of the pump without having to gain access to the build chamber, which may compromise the integrity of the inert gas atmosphere contained in the build chamber. An advantage of the embodiment of  FIG. 5 b    is that gravity as well as gas flow will drive the debris to the outlet at joint  185 , which may prevent clogging of the tubes. 
         [0076]      FIGS. 6 a  and 6 b    show a further embodiment of the invention, wherein metrology apparatus, in this embodiment a scanning or touch probe  130 , is mounted to the nozzle  112  such that it can move in a linear direction along the nozzle  112  (as indicated by the arrows A) and in a vertical direction (as indicated by arrows B). In use, the scanning or touch probe  130  can be moved to measure a hybrid blank or an object being built. The metrology apparatus could be used for initial set-up of the apparatus, for in-process control or for measuring the object post-production. However, piggy-backing off the back of the nozzle  112  reduces the number of axes that are required compared to providing a separate set of axes for the measurement probe. It will be understood that the probe  130  could also be mounted in a like manner on the exhaust  110 . 
         [0077]    In this embodiment, the bellows arrangement for the recirculation loop  111  has been replaced with telescopic tubes  127 . 
         [0078]      FIG. 7  shows a further embodiment, wherein the nozzle  112  and exhaust  110  extend across a partial width of the powder bed  104 , with the nozzle  112  and exhaust  110  each mounted for motion along two perpendicular axes. 
         [0079]      FIG. 8 a    shows a further embodiment, wherein the apparatus comprises multiple pairs of nozzles  112  and exhausts  110 , each nozzle  112  and exhaust  110  mounted for motion along two perpendicular axes. Each pair of nozzle  112  and exhaust  110  is arranged to cover a different portion of the build platform  102 . An extent of movement of the nozzle  112  and exhaust  110  of each pair in a direction perpendicular to the gas flow direction is limited to less than the entire width of the build platform  102  and, in this embodiment, is limited to half of the width of the build platform  102 . Such an arrangement may be useful when the object is to be formed by consolidation of powder simultaneously with two or more laser beams, such as disclosed in DE102005014483 A1 or GB1310276.9. 
         [0080]      FIG. 8 b    shows a variation on an apparatus comprising multiple pairs of nozzles  112  and exhausts  110 . In this embodiment, the pairs of nozzles  112  and exhausts  110  are mounted on a common guide  120 . Each pair may be arranged to cover a different area of the build platform  104  or may be arranged such that common areas on the build platform can be covered by either pair. The nozzle  112  and exhaust  110  of each pair can move over an entire extent of the build platform  102  in a direction perpendicular to the gas flow direction. 
         [0081]    In  FIG. 8 c   , the nozzle  112  only extends across a partial width of the powder bed  104  whereas the exhaust  110  extends over a wider region, in this embodiment an entire width, of the build platform  102 . It may be desirable to focus the stream of gas from the inlet to a region where the laser beam impacts the powder bed whereas it may be desirable for the exhaust to extend over a much larger region as debris from consolidation of the powder may spread over a larger region of the powder bed than the volume into which gas is directed by the nozzle  112 . 
         [0082]      FIG. 9  shows a system, wherein the nozzle  112  and exhaust  110  system are mounted for rotary movement about the build platform  104 . In this embodiment, the build platform  104  comprises a round upper surface that defines a working area. The nozzle  112  and exhaust  110  are mounted in a frame  170  for independent movement in a linear direction, the frame  170  rotatable in a guide  120  to rotate the nozzle  112  and exhaust  110  around the build platform  104 . In this way, both the distance between the nozzle  112  and exhaust  110  can be adjusted together with the direction in which flow is generated across the build platform  104 . The gas flow direction may be altered by rotating the nozzle  112  and exhaust  110  as the scanning direction is altered for each layer. For example, the scan direction may be rotated by a set amount between consecutive layers, the flow direction also being rotated by a corresponding amount. The flow direction may be arranged to be parallel to a scan direction or stripe formation direction. An example of scanning of layers in stripes is disclosed in EP1993812. 
         [0083]    Also, as shown in  FIG. 10 , the outlet 110   a  provided by the exhaust  110  may have a greater vertical height than the inlet  112   a  provided by the nozzle  112 . This may prevent condensate from being blown over the exhaust by any turbulence in the gas flow facilitating collection of the splashes generated by the SLM process. 
         [0084]      FIGS. 11 to 13  show an alternative embodiment of a flow device  131  wherein the nozzle  112  and exhaust  110  are formed as a single movable unit  131  with a fixed distance between the gas inlet  112   a  and gas outlet  110   a . In this embodiment, the wiper  109  is fixed to the unit  131  and the powder spread across the powder bed  104  simultaneously with movement of the unit  131 . In  FIG. 11 a    the wiper  109  is fixed in relation to the nozzle  112  and exhaust  110 . However, in  FIG. 11 b   , the wiper  109  is movable from an extended position  109   a  in which the wiper engages the powder for spreading the powder across the build platform  104  and a retracted position  109   b  in which the unit  131  can move over the build platform  104  without the wiper  109  engaging the powder. 
         [0085]    The optical unit  106  is controlled to direct the laser beam  133  into the gap between the gas inlet  112   a  and gas outlet  110   a  to consolidate powder therebetween. In use, the unit  131  is moved along the guide  120  (by suitable motors (not shown)) to traverse the powder bed, the laser beam  133  being directed by the optical module  106  to scan between the gap as the unit  131  traverses the powder bed. Switching of the laser beam  133  on and off as the laser beam scans across the gap allows areas of the powder bed  104  to be selectively consolidated. The embodiment shown in  FIG. 11 b    with a retractable wiper  109  may allow the unit  131  to traverse over the build platform  104  two or more times before spreading of the next layer of powder. For example, a retractable wiper may be beneficial if adjacent areas of the powder are to be scanned by the laser beam during separate traverses of the powder bed by the flow device  131  in order to manage heating of the powder bed. For example, a stripe pattern, such as shown in  FIG. 3 , may be used as a scanning strategy for forming the part, with adjacent stripes scanned during different traverses of the powder bed by the flow device  131 . 
         [0086]      FIG. 14  shows a movable flow device  141  according to another embodiment of the invention. In this embodiment, the flow device  141  comprises a gas outlet  110   a  for drawing gas from the chamber  101  that is located in the vicinity of the powder bed  104  and a gas inlet  112   a  for propelling gas into the chamber  101  that is located (relative to the outlet  110   a ) remote from the powder bed  104 . In this embodiment, the inlet  112   a  propels gas upwards into the chamber  101 . The action of the sucking of gas into outlet  110   a  and the propelling of gas out of gas inlet  110   a  may generate a circulation of inert gas in the chamber  101  in the vicinity of the flow device  141 . Housed within the flow device  141  is a filter not shown) for filtering particles from the gas flow before the gas is propelled back into the build chamber through inlet  112   a.    
         [0087]    In use, the optical unit  106  is controlled to direct the laser beam  133  to a location close to the gas outlet  110   a  such that condensate generated by the consolidation of powder  104  is removed in the gas flow generated by the outlet  112   a . The unit  141  is moved along the guide  120  (by suitable motors (not shown)) to traverse the powder bed  104 , the laser beam  133  being directed by the optical module  106  to scan just behind or in front of the gas outlet  110   a  as the unit  131  traverses the powder bed  104 . Switching of the laser beam  133  on and off as the laser beam scans the powder bed allows areas of the powder bed  104  to be selectively consolidated. 
         [0088]      FIG. 15  is a flow device  151  like that shown in  FIG. 14 , but with a gas flow guide  152  added to direct the flow of gas to the gas outlet  110   a . The gas flow guide  152  is mounted to move with the flow device  151  and may be connected to the flow device  151  so as to move therewith. The flow guide  152  may have an appropriate shape, such as a scoop shape or planar surface, which directs gas from an upper region in the chamber  101  to a lower region adjacent the outlet  110   a.    
         [0089]    In a further embodiment (not shown), rather than the inlet  112   a  of the flow device being movable with the outlet  110   a , the inlet  112   a  may be located at a fixed location within the chamber  101 . 
         [0090]    It will be understood that alterations and modifications can be made to the above described embodiments without departing from the scope of the invention as described herein. In particular, features described with reference to one embodiment may be combined with features described with reference to another embodiment. For example, the flow devices described with reference to  FIGS. 11 to 15  may extend across an entire width of the powder bed  104  or may extend across a partial width of the powder bed and be mounted to move in two perpendicular directions, as shown in  FIGS. 7 to 9 .