Abstract:
A selective laser solidification apparatus including: a powder bed onto which a powder layer can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a gas flow direction, a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form at least one object and a processing unit for selecting a scanning sequence of the laser beam based on the gas flow direction.

Description:
[0001]    The present invention relates to selective laser solidification and in particular to an improved selective laser melting process and apparatus in which an order in which objects or parts of objects are built is selected based on a direction of gas flow. 
       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 laser 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. In a single build, more than one object can be built, the objects spaced apart in the powder bed. 
         [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 to the rear of the powder bed that pass a flow of gas to a series of exhaust vents that are located 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 at either side of a powder bed provide substantially planar gas flow across the powder bed. It has been found that debris can be blown from one section of an object to another section of the or another object. This can result in non-uniformity and increased porosity of the solidified metal layers generated by the melting process. In particular, debris blown across the powder bed can result in an increase in surface roughness such that pores are formed between adjacent layers. Non-uniformities in a build can result in an object not conforming to the desired design and damage the apparatus. In particular, a wiper blade is typically used to spread each powder layer across the powder bed. Solidified structures that project out of the powder bed can catch on and cause damage to the wiper blade. Damaged wiper blades may result in powder layers with ridges of powder. Accordingly, non-uniformities in a build may be a concern not just for the layer being formed, but powder layers formed thereafter. 
       SUMMARY OF INVENTION 
       [0004]    According to a first aspect of the invention there is provided a selective laser solidification apparatus, comprising; a powder bed onto which a powder layer can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a gas flow direction, a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form one or more objects and a processing unit for selecting a scanning sequence of the laser beam based on the gas flow direction. 
         [0005]    The scanning sequence may be selected such that debris produced by the scan is carried away from areas of the powder layer which are yet to be scanned. In this way, these areas of powder are not disturbed and contaminated by the debris ensuring that, when these areas are solidified, the solidified layer is built to a desired, uniform height. For example, the processing unit may select to scan one area before another area because the area is located downwind in the gas flow direction of the other area. Debris produced in forming the upwind area may be blown onto the already formed downwind area but this debris may be removed by the wiper and, if not removed, covered by the next powder layer to be remelted when forming the next cross-section. Accordingly, scanning areas in this order is less likely to result in non-uniformities in the build. 
         [0006]    The one or more objects may be formed through the solidification of separate islands in at least one powder layer, the processing unit arranged to select an order in which islands are formed based upon the relative location of the islands in the at least one powder layer and the gas flow direction. 
         [0007]    The order in which islands are formed may be selected such that debris produced by forming an island is carried away from areas of the powder layer in which islands are yet to be formed. In this way, these areas of powder are not disturbed and contaminated by the debris ensuring that, when these areas are solidified, the solidified layer is built to a desired, uniform height. For example, the processing unit may select to form at least part of an island before at least part of another island because the at least part of the island is located downwind in the gas flow direction of at least part of the other island. Debris produced in forming the upwind island may be blown onto the already formed downwind island but this debris will be covered by the next powder layer and the debris is likely to be remelted when forming the next cross-section. Accordingly, building the islands in this order is less likely to result in non-uniformities in the build. 
         [0008]    For islands wherein a first island is located wholly downwind of a second island, the processing unit may be arranged to select to form the first island completely before forming the second island. However, if a first island is located to at least partially surround a second island such that parts of the first island are downwind and other parts of the first island are upwind of the second island, the processing unit may be arranged to select to form at least part of the second island in between forming the downwind and upwind parts of the first island. 
         [0009]    The processing unit may determine an order in which areas should be scanned by projecting a debris fallout zone that would be created when solidifying each area and determining whether one or more other areas to be solidified fall within the debris fallout zone, the processing unit selecting to solidify the one or more other areas that fall within the debris fallout zone before solidifying the area from which the debris fallout zone has been projected. The debris fallout zone may be projected as a pair of parallel lines extending in the gas flow direction from outmost edges of the area in a direction perpendicular to the gas flow direction. Such a debris fallout zone may be suitable when the gas flow is a laminar gas flow (Reynolds number less than 2000). However, it may also be desirable to project the debris fallout zone as diverging lines at a slight angle to the gas flow direction to take into account slight turbulence in the gas flow that may cause the debris to be deposited beyond the outmost edges of the area in a direction perpendicular to the gas flow direction. More complex models of the region over which debris may be deposited may be used requiring more complex projections of the debris fallout zone. For example, areas to be solidified that are located close to an edge of the powder bed may be subjected to more turbulent flow because these areas are closer to the sidewalls of a build chamber than areas located centrally in the powder bed. 
         [0010]    The apparatus may comprise an interface for the user to identify the one or more objects to be formed. The user may select the locations on a build platform where the one or more objects are to be built. Alternatively, the processing unit may be arranged to select the locations on a build platform for the one or more objects. The processor may select the location for one or more of the objects based upon the debris fallout zone of an object whose location has already been selected. 
         [0011]    According to a second aspect of the invention there is provided a method of selecting a scanning sequence of a laser beam in a selective laser solidification process, in which one or more objects are formed layer-by-layer by, repeatedly, depositing a layer of powder on a powder bed and scanning a laser beam over the deposited powder to selectively solidify at least part of the powder layers, wherein a gas flow is passed over the powder bed in a gas flow direction, the method comprising selecting a scanning sequence of the laser beam based on the gas flow direction. 
         [0012]    The method may be a computer-implemented method. 
         [0013]    According to a third aspect of the invention there is provided a data carrier having instructions stored thereon, the instructions, when executed by a processor, cause the processor to carry out the method of the second aspect of the invention. 
         [0014]    According to a fourth aspect of the invention there is provided apparatus for selecting a scanning sequence of a laser beam in a selective laser solidification process, in which one or more objects are formed layer-by-layer by, repeatedly, depositing a layer of powder on a powder bed and scanning a laser beam over the deposited powder to selectively solidify at least part of the powder layers, wherein a gas flow is passed over the powder bed in a gas flow direction, the apparatus comprising a processing unit, a display and a user input device, the processing unit arranged to receive data on one or more objects, including a location of the objects on a build platform, cause the display to display an image of areas to be solidified, wherein a debris fallout zone is projected from each area, and receive a user input from the user input device of an order in which the areas are to be scanned. 
         [0015]    According to a fifth aspect of the invention there is provided a data carrier for selecting a scanning sequence of a laser beam in a selective laser solidification process, in which one or more objects are formed layer-by-layer by, repeatedly, depositing a layer of powder on a powder bed and scanning a laser beam over the deposited powder to selectively solidify at least part of the powder layers, wherein a gas flow is passed over the powder bed in a gas flow direction, the data carrier having instructions thereon, which, when executed by a processor, cause the processor to receive data on one or more objects, including a location of the objects on a build platform, cause a display to display an image of areas to be solidified, wherein a debris fallout zone is projected from each area, and receive a user input from a user input device of an order in which the areas are to be scanned. 
         [0016]    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 Blu 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 fiber 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 
         [0017]    Embodiments of the invention will now be described, as examples only, with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  is a schematic view of a laser solidification apparatus according to one embodiment of the invention; 
           [0019]      FIG. 2  is a schematic view of the laser solidification apparatus from another side; 
           [0020]      FIG. 3  is a flowchart showing the steps of a method according to the invention; 
           [0021]      FIG. 4  is a plan view of islands to be solidified on a build platform of the apparatus, wherein debris fallout zones have been projected; and 
           [0022]      FIG. 5  is a plan view of islands to be solidified on a build platform of the apparatus, wherein debris fallout zones according to a different embodiment of the invention have been projected. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0023]    Referring to  FIGS. 1 and 2 , a laser solidification apparatus according to an embodiment of the invention comprises a build platform  102  for supporting an object  103  built by selective laser melting powder  104 . The platform  102  can be lowered in the chamber  101  as successive layers of the object  103  are formed. 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 as required by optical module  106  under the control of a computer  130 . The laser enters the build chamber via a window  107 . 
         [0024]    An inlet  112  and outlet  110  are arranged for generating a gas flow across the powder bed formed on the build platform  102 . The inlet  112  and outlet  110  are arranged to 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 outlet  110  to the inlet  112  through a gas recirculation loop  111 . A pump  113  maintains the desired gas pressure at inlet  112  and openings  5 ,  6 . A filter  114  is provided in the recirculation loop  111  to filter from the gas condensate that has become entrapped in the flow. It will be understood that more than one inlet  112  may be provided in the build chamber  101 . Furthermore, rather than extending outside of the build chamber  101 , the recirculation loop  111  may be contained within the build chamber  101 . 
         [0025]    Computer  130  comprises a processor unit  131 , memory  132 , display  133 , user input device  135 , such as a keyboard, touch screen, etc, a data connection to modules of the laser sintering 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 described with reference to  FIGS. 3 to 5 . 
         [0026]    Referring to  FIG. 3 , geometric data of objects to be built, such as in the form of an STL file, are received  201  by the computer  130 , for example over the external data connection  135 . The processing unit  131  receives  202  information on the location of the objects on the build platform  102 . This location information may already be defined in the STL or the user may the select, using the user input device  135 , where each object should be located on the build platform  102 . For each layer, the processing unit  131  identifies areas of the layer that are to be solidified and determines  203  an order in which these areas should be scanned by the laser beam. An example of how this may be done is shown in  FIG. 4 . 
         [0027]      FIG. 4  shows five separate areas (islands)  122  to  126  to be solidified for a particular layer. For each island  122  to  126 , the processing unit projects a debris fallout zone  122   a  to  126   a  in a gas flow direction from the island. The processing unit  131  then determines, for each island  122  to  126 , if any other island falls within the debris fallout zone. If so, the processing unit selects to form this other island before forming the island for which the debris fallout zone was determined. For example, in  FIG. 4 , islands  125  and  126  fall within the fallout zone of island  122  and therefore, are selected to be scanned before island  122 . Island  126  also falls within the fallout zone of island  125  and therefore, should be formed before island  125 . 
         [0028]    Rather than restricting ordering of the build to a complete island, the processing unit  131  may be arranged to select to form, in between forming different parts of the island, at least part of another island.  FIG. 4  illustrates two examples of this. In the first example, island  123  is completely surrounded by island  124 . Accordingly, island  124  comprises parts that are both upwind and downwind of island  123 . In such a scenario, the processing unit  131  selects to process the part of the island that is located downwind of island  123  before scanning island  123  and then scans the part of the island  124  that is upwind of island  123 . The part of island  124  that is neither upwind nor downwind of island  123  may be scanned before or after island  123  and the selection of the scanning order of these parts may be based on other criteria, such as scan speed. The different parts of island  124  are illustrated by the dotted lines. In the second example, rather than scanning all of island  125  after scanning island  126 , the part of island  125  that is not upwind of island  126  may be scanned before island  126 . There may be reasons for scanning part of island  125  ahead of island  126 , such as to optimize scan speed, variations in material composition and/or focal position. 
         [0029]    In this embodiment, the processing unit  131  carries out this process for each layer. However, in another embodiment, rather than calculating a scanning order for each layer, it may be possible to determine an order for multiple layers from a single analysis. For example, a fallout zone could be determined from a footprint of each object on the build platform  102 , the order being determined based upon whether other objects fall within a debris fallout zone calculated based on this footprint. Even though for some layers the debris fallout zone may be smaller than that calculated from the footprint, such a method may provide a reasonable generalization that reduces the amount of processing required in determining an order in which the parts should be built. 
         [0030]    The selected order of scanning the parts may be displayed to the user and the user may be able to change the order. The user can then activate the build to cause the processing unit to control  204  the optical module  106  and laser module  105  to scan the powder layers to form the islands in the selected order. 
         [0031]    In the embodiment shown in  FIG. 4 , the debris fallout zones are projected by extending straight lines in the gas flow direction from edges of the islands. However, other projections of the fallout zones could be used. Two examples are shown in  FIG. 5 . For island  127 , a fallout zone  127   a  is projected as diverging straight lines at a slight angle to the gas flow direction to take into account slight turbulence in the gas flow that may cause the debris to be deposited beyond the outmost edges of the island in a direction perpendicular to the gas flow direction. A similar principal is embodied by island  128  and fallout zone  128   a , where an initially curved border to the fallout zone is used to model that locally debris may be thrown out by the impact of the laser beam on the powder layer but further from the island the debris is more likely to be carried away along a straighter path by the gas flow. 
         [0032]    In a further embodiment, rather than the processing unit selecting the order in which islands are scanned, a user may select an order in which islands are built. This may be achieved by the processing unit  131  causing the display  133  to display images similar to those shown in  FIGS. 4 and 5  so that the user can select the order islands are scanned based upon this visualisation of the fallout zones. The processing unit  131  then receives user inputs from the user input device of the order in which islands should be scanned. 
         [0033]    It will be understood that in the above description, the islands may come together in earlier or later layers so as to form a single object or may remain separate so as to form one or more separate objects. 
         [0034]    It will be understood that alterations and modifications may be made to the invention without departing from the scope of the invention as defined herein. For example, the invention could be applied to a single island, wherein it is desirable to scan a downwind part of the island ahead of scanning an upwind part of the island.