Patent Publication Number: US-2019176397-A1

Title: Laser laying patterning

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from British Patent Application Number 1720597.2 filed 11 Dec. 2017, the entire contents of which are incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure concerns the field of additive layer manufacturing, and more specifically to powder bed fusion. 
     Description of the Related Art 
     Powder bed fusion is a form of additive layer manufacturing (ALM) where thermal energy from a laser or electron beam is used to fuse specific areas of a bed of powder and build up a three-dimensional component. The process typically involves raster passes, with the laser moving successively back and forth across the area/region of interest to melt/fuse the powder as required. 
     Many standard raster strategies are used in industry. However, these existing raster scanning strategies may create undesirable thermal input scenarios in the material. The thermal input and thermal history defines the residual stress in the component, and if not correctly managed may lead to high residual stress possibly leading to component distortion or even failure. The metallurgical response to the thermal input may also induce micro cracking, which is similarly undesirable in its effects and presence. 
     WO 2016/079496 attempts to address certain problems with known techniques by splitting a scanning path into discrete areas that are scanned in multiple passes along the same path. For example, an entire path can be split into a number of segments along its length, which for reference we will successively number 1, 2, 3, 4, 5. A pulsed laser then melts the odd numbered segments (1, 3, 5, etc.) in a first pass, before the even numbered segments (2, 4, etc.) in-between are melted in a second pass. 
     The need for multiple segments may result in a slower processing time. Furthermore, the method suggested in WO 2016/079496 may only be implemented using pulsed laser systems, but many manufacturers prefer to use continuous wave lasers (where energy is output constantly rather than in discrete pulses). 
     SUMMARY 
     According to a first aspect there is provided a method of processing a layer of material in a layer-by-layer additive manufacturing process, wherein successive layers of flowable material are formed and a laser beam is scanned across a selected area of each layer to consolidate the material in said selected area, the method comprising providing a first raster path for a laser beam, the first raster path having a defined spacing between successive passes, and providing at least one further raster path for a laser beam, wherein the or each further raster path has the same defined spacing between successive passes as the first raster path and is offset from the first raster path in the direction of said defined spacing by a distance that is less than said defined spacing, such that the laser beam irradiates the entire selected area in a single pass over the first path and the or each further path. 
     The first raster path may have a first raster path width. The defined spacing may be greater than the first raster path width. The first raster path width may relate to a width of melting when the raster path is followed by a laser beam. The defined spacing may be twice the first raster path width. The defined spacing may be a multiple of the first raster path width. The first raster path and the or each further raster path may be interleaved (for example passes of a further raster path may be located between passes of the first raster path). 
     The laser beam may be a continuous laser beam. The first raster path may be followed by a laser beam performing back and forth scanning. The laser beam may be an intermittent laser beam. The first raster path may be followed by a laser beam performing unidirectional scanning. The first raster path may be followed by a laser beam along passes in one, two or more directions. 
     The flowable material may be powder suitable for use in additive manufacturing, for example a metal powder such as a nickel superalloy powder. 
     The first raster path and the or each further raster path may be followed by a single laser beam, provided by a single source. Alternatively multiple laser beams and/or multiple sources may be used. 
     Alternatively, the first raster path and the or each further raster path may be followed by separate laser beams. For example, a first raster path may be followed by a first laser beam, and a second, further, raster path may be followed by a second separate/distinct laser beam. A time delay may be introduced between the paths so that the second laser beam does not immediately follow the first. 
     The offset distance may be a defined fraction or percentage of the defined spacing. For example, the offset distance may be half, one third, one quarter, one fifth, two thirds, two fifths, three quarters, three fifths or four fifths of the defined spacing. The offset distance may be the defined spacing divided by the number of paths, wherein the number of paths is the number of further raster paths plus one. 
     The method provides benefits when only one further path is used. However, the method may provide a first raster path and at least two further raster paths. The offset between each successive path may be constant, such that the offset from the first path may increase for uniformly for each further path. For example, where two further paths are provided, the total offset for the first further path (second path overall) may be one third of the defined spacing, and the total offset for the second further path (third path overall) may be two thirds of the defined spacing. 
     Additional further raster paths could be provided if desired. With higher numbers of paths it would be possible to not simply increase the offset from the first path with each successive path, but to instead interlace paths to some degree. For example, with three further paths (four paths in total), the offset from the first path for successive paths could be one half, then one quarter, then three quarters, of the defined spacing. 
     Additionally the first raster path and/or a further raster path may be remelted or partially remelted during the step of providing a further raster path. Advantageously remelting or partially remelting may improve the levels of porosity in a component. 
     The method may further comprising the initial step of calculating the defined spacing and the or each offset distance based on one or more of the laser beam diameter, the laser beam power and the properties of the flowable material. The calculations could be based on simulations or experimental data. 
     Also provided is an additive manufacturing apparatus comprising a build table, a source of flowable material, a laser source (or sources, for example in a multilaser or multisource system) and a control unit for controlling the laser source (or sources), wherein, in use, layers of flowable material are successively formed across the build table, the laser source(s) directs a laser beam onto each layer to selectively solidify the material in a defined area, and the control unit directs the laser beam to successively follow a first raster path having a defined spacing between successive passes, and at least one further raster path, the or each further raster path having the same defined spacing between successive passes as the first raster path and being offset from the first raster path in the direction of said defined spacing by a distance that is less than said defined spacing. 
     The apparatus may comprise, for example, a vertically movable build table that can be gradually lowered, and a scraper for moving a suitable powder (such as a metal powder) from a powder reservoir onto the table each time the table is lowered to create a new powder layer. A powder collection area may also be provided to catch any surplus powder as successive layers are formed. 
     The apparatus may further comprise a memory and a processor for accessing the memory and providing instructions to the control unit. 
     The memory may contain a database of raster paths (for example number of paths, direction of paths, timing of paths). A suitable raster path may then simply be selected by a user. The memory may contain information required to operate the claimed method, for example parameters as claimed herein. 
     Alternatively, or additionally, the memory may contain a database of defined spacings and offset distances and the processor may simulate the results of different combinations and defined spacings and offset distances to provide optimum raster paths to the control unit. The simulations may be based on input data, for example relating to properties of the laser, of the flowable material, and/or of the part being produced. 
     Also provided is a system comprising an additive manufacturing apparatus as previously described and computer readable media containing computer readable instructions for the additive manufacturing apparatus. 
     This described method, apparatus and system provide a novel Laser Scanning pattern for an ALM process. The pattern introduces a pause time between adjacent tracks with little or no process time penalty. This homogenises and alters thermal history compared to standard raster scanning strategies, such that cracking susceptible processing zones can be avoided. 
     The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the accompanying Figures, in which: 
         FIG. 1  is a sectional side view of a gas turbine engine; 
         FIG. 2  is an indicative perspective view of an additive manufacturing machine illustrating a typical additive layer manufacturing process; 
         FIG. 3 a    is a schematic views of a known raster scanning pattern; 
         FIG. 3 b    is a schematic views of a known raster scanning pattern; 
         FIG. 3 c    is a schematic views of a known raster scanning pattern; 
         FIG. 3 d    is a schematic views of a known raster scanning pattern; 
         FIG. 3 e    is a schematic views of a known raster scanning pattern; 
         FIG. 3 f    is a schematic views of a known raster scanning pattern; 
         FIG. 4 a    is a schematic view illustrating a scanning strategy of the present invention; 
         FIG. 4 b    is a schematic view illustrating a scanning strategy of the present invention; and 
         FIG. 4 c    is a schematic view illustrating a scanning strategy of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a gas turbine engine is generally indicated at  10 , having a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , an intermediate pressure turbine  18 , a low-pressure turbine  19  and an exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  12  and the exhaust nozzle  20 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high  17 , intermediate  18  and low  19  pressure turbines drive respectively the high pressure compressor  15 , intermediate pressure compressor  14  and fan  13 , each by suitable interconnecting shaft. 
     Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. 
     Additive layer manufacturing is an increasingly common technology which may be used in the manufacture of various engine components. 
       FIG. 2  shows a typical additive layer manufacturing (ALM) process in an additive manufacturing machine. The machine comprises a laser source A, a powder source D, a movable working plate or build table E, a scraper C and a powder collection area F. 
     In use, the build table E is first lowered in the order of 20-200 μm, as indicated by arrow  1 . The scraper C moves across the build table, as shown by arrow  2 , and deposits a fine layer of powder from the powder source D, proportional to the build table E lowering distance. In step  3 , the laser A is used to selectively melt an area of the powder bed, by using a scanning pattern to fill the area to consolidate. By repeating steps  1 - 3  as described, a three-dimensional component B can be produced. 
       FIGS. 3 a  to 3 f    illustrate a number of known scanning patterns. In each case, three successive layers of scanning are shown to illustrate how the path of the laser changes between successive layers during the ALM process. 
       FIG. 3 a    shows Island scanning, where the total area is divided into smaller ‘islands’.  FIGS. 3 b  and 3 c    respectively show basic line scanning and 45° line scanning.  FIG. 3 d    shows 45° rotate scanning, where the laser path angle is changed by 45° between each successive layer. This effectively provides a scanning pattern which alternates between line scanning and 45° scanning.  FIG. 3 e    shows 90° rotate scanning, where the change in laser path angle between successive layers is 90° and  FIG. 3 f    shows 67° rotate scanning. 
     Each of the scanning strategies above involves ‘rastering’ (moving the laser back and forth) across the scanning area, from one side of the melting area to the other, with each path being immediately adjacent a previous path. In some methods, such as that shown in  3   a , the area is subdivided into sub regions or islands, and each sub region is scanned with its own raster strategy. The successive paths, however, are still immediately adjacent. Each raster scan is scanned on its own deposited layer. 
     The scanning strategy used has a bearing on the thermal response of the material, which in turn influences defect mechanisms and can ultimately determine if certain materials can be processed. Existing raster scanning strategies, such as those shown in  FIGS. 3 a  to 3 f   , can create undesirable thermal input scenarios in the material. The thermal input and thermal history defines the residual stress in the component. If residual stress is too high, then it can cause component distortion and, in the case of some alloys, can induce cracking and complete component failure. The metallurgical response to the thermal input can also induce micro cracking, which is similarly undesirable in its effects and presence. 
     This application focuses on improving the laser scanning strategy aspect of the process. More specifically, during the laser melting process a pattern of laser movement is introduced to alter the thermal input into the component and reduce the potential cracking problem. The raster strategy is modified to produce a different thermal history, which has implications and advantages for processing, for example, nickel superalloys. 
     Fundamentally, the scanning area is divided into two or more separate scanning lengths, or tracks, that are separated by a set distance (hatch spacing). The laser follows each of these paths in order, therefore travelling multiple times across the component, in order to fully melt the area. 
       FIGS. 4 a  to 4 c    illustrate this process. The active path across a component B in each Figure is shown by the three broken arrows  24 , with previous paths shown as solid arrows  26 ,  28  in  FIGS. 4 b  and 4 c   . The hatch spacing  30  is constant between stages. The stages are explained below. 
       FIG. 4 a    shows the initial melting strategy whereby the laser is scanned in a raster strategy across the scanning area following a path  24   a  with extremely large hatch spacing  30 , from left to right. 
     Once the first pass has been completed, the scanning strategy is performed again across the whole component B, as shown in  FIG. 4 b   . The hatch spacing  30  is the same as before, but in stage  2  an offset  32  is provided in the x-axis (perpendicular to the scanning direction) compared to stage  1 . This offset  32  serves to space the active path  24   b  of stage  2  from the previous path  26  followed in stage  1 . It will be understood that the initial direction of the active path  24   b  in the second stage is determined simply based on the side of the component B where the laser source finished at the end of the first pass. Processing delays are therefore minimised. 
     Stage  2  is then repeated with another offset  34  relative to the first scan path  26 , as illustrated in  FIG. 4 c   . The offset  34  is greater than that used for the second scan path  28 , so that the active path  24   c  is also spaced from the second scan path  28 . Once again, the initial direction of the active path  24   c  is determined based on the finishing position of the laser source at the end of the second pass. 
     It can be seen in the images above that the final part is made of several scan tracks that are adjacent and equally spaced apart across the width of the component scanning area. As illustrated, three passes are used, with the first offset  32  being one third of the hatch pacing  30  and the second offset  34  being two thirds of the hatch spacing  30 . The relative offset between each successive pass is therefore one third of the hatch spacing  30 , and the third active path  24   c  in  FIG. 4 c    fills the gaps that remain between the previous scanning vectors  26 ,  28 . It should be understood that different numbers of stages/passes may be required depending on the chosen hatch spacing  30  and offsets  32 ,  34 , but that the process should simply be repeated until no more gaps remain between the scanning vectors. 
     Following the methodology as described allows a time for the laser tracks to cool significantly compared to prior conventional scanning strategies. This is a significant alteration to the thermal input. In a conventional raster scan, the laser travels left to right by a constant distance in a single continuous path. As a result, the previous track heating can influence the next track as they are immediately next to each other and are scanned in sequence. The method described above effectively adds a pause between adjacent tracks allowing them to cool, but has little or no penalty on process times because the total distance travelled with the laser active is substantially unchanged. 
     The effective pause times are beneficial because they contributing to reducing cracking behaviour in materials, for example in nickel superalloys, processed by selective laser melting. There are many cracking mechanisms that nickel superalloys exhibit that are in part determined by the thermal input. By introducing a pause time the thermal history is altered, allowing thermal processing zones where ductility dip or segregation cracking occur to be avoided. 
     The pause times also allow the thermal history to be more homogenous on the component B. The heating of the previous track now does not affect the next track, leading to more homogenous properties over the scanned layer. This increases the repeatability and reliability of the structural integrity of the scan. 
     The pause time is introduced with little or no penalty on processing time, as the laser A travels and creates a track further along the component B. 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. 
     For example, although described in relation to a whole component/area, the same technique could be extrapolated to island scanning. The islands can be considered as individual areas, with each island being scanned using the strategy described in the application. 
     Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.