Abstract:
A method for inspecting an additive manufacturing process in which a directed energy source is used to create a weld pool at an exposed build surface of a mass of powdered material, and selectively fuse the powdered material to form a workpiece. The inspection method includes: using a noncontact method to generate an acoustic wave in the build surface; using a noncontact method to measure displacement of the build surface in response to the acoustic wave; and determining at least one sub-surface material property of the workpiece by analyzing the displacement of the build surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to additive manufacturing, and more particularly to apparatus and methods for inspection and process control in additive manufacturing. 
         [0002]    Additive manufacturing is a process in which material is built up layer-by-layer to form a component. Unlike casting processes, additive manufacturing is limited only by the position resolution of the machine and not limited by requirements for providing draft angles, avoiding overhangs, etc. as required by casting. Additive manufacturing is also referred to by terms such as “layered manufacturing,” “reverse machining,” “direct metal laser melting” (DMLM), and “3-D printing.” Such terms are treated as synonyms for purposes of the present invention. 
         [0003]    Prior art additive manufacturing processes typically require a post-build inspection process such as computerized tomography (“CT”) to verify the integrity of the build. While effective, this process requires undesirable extra time and cost. 
         [0004]    Accordingly, there is a need for a real-time inspection process for additive manufacturing. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    This need is addressed by a method of non-contact acoustic in-situ inspection. 
         [0006]    According to one aspect of the technology described herein, a method is provided for inspecting an additive manufacturing process in which a directed energy source is used to create a weld pool at an exposed build surface of a mass of powdered material, and selectively fuse the powdered material to form a workpiece. The method includes: using a noncontact method to generate an acoustic wave in the build surface; using a noncontact method to measure displacement of the build surface in response to the acoustic wave; and determining at least one sub-surface material property of the workpiece by analyzing the displacement of the build surface. 
         [0007]    According to another aspect of the technology described herein, a method of making a workpiece includes: depositing a powdered material so as to define a build surface; directing a build beam from a directed energy source to create a weld pool in the build surface, and selectively fuse the powdered material in a pattern corresponding to a cross-sectional layer of the workpiece; using a noncontact method to generate an acoustic wave in the build surface; using a noncontact method to measure displacement of the build surface in response to the acoustic wave; and determining at least one sub-surface material property of the workpiece by analyzing the displacement of the build surface. 
         [0008]    According to another aspect of the technology described herein, an apparatus is provided for inspecting an additive manufacturing process in which a directed energy source is used to create a weld pool at an exposed build surface of a mass of powdered material, and selectively fuse the powdered material to form a workpiece. The apparatus includes: a noncontact device configured to generate an acoustic wave in the build surface; and a noncontact device configured to measure displacement of the build surface in response to the acoustic wave. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0010]      FIG. 1  is a schematic cross-sectional view of an exemplary additive manufacturing apparatus; 
           [0011]      FIG. 2  is an enlarged view of a portion of  FIG. 1 ; 
           [0012]      FIG. 3  is a schematic cross-sectional view of an alternative additive manufacturing apparatus; 
           [0013]      FIG. 4  is a schematic graph showing a matrix of cells representing inspection data; and 
           [0014]      FIG. 5 . is a schematic graph showing a matrix of cells representing a model of a workpiece. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates schematically an apparatus  10  for carrying out an additive manufacturing method. The basic components are a table  12 , a powder supply  14 , a scraper or recoater  16 , an overflow container  18 , a build platform  20  surrounded by a build chamber  22 , a directed energy source  24 , and a beam steering apparatus  26 , all surrounded by an enclosure  28 . Each of these components will be described in more detail below. 
         [0016]    The table  12  is a rigid structure defining a planar worksurface  30 . The worksurface  30  is coplanar with and defines a virtual workplane. In the illustrated example, it includes a build opening  32  communicating with the build chamber  22  and exposing the build platform  20 , a supply opening  34  communicating with the powder supply  14 , and an overflow opening  36  communicating with the overflow container  18 . 
         [0017]    The recoater  16  is a rigid, laterally-elongated structure that lies on the worksurface  30 . It is connected to an actuator  38  operable to selectively move the recoater  16  along the worksurface  30 . The actuator  38  is depicted schematically in  FIG. 1 , with the understanding devices such as pneumatic or hydraulic cylinders, ballscrew or linear electric actuators, and so forth, may be used for this purpose. 
         [0018]    The powder supply  14  comprises a supply container  40  underlying and communicating with the supply opening  34 , and an elevator  42 . The elevator  42  is a plate-like structure that is vertically slidable within the supply container  40 . It is connected to an actuator  44  operable to selectively move the elevator  42  up or down. The actuator  44  is depicted schematically in  FIG. 1 , with the understanding that devices such as pneumatic or hydraulic cylinders, ballscrew or linear electric actuators, and so forth, may be used for this purpose. When the elevator  42  is lowered, a supply of powder “P” of a desired composition (for example, metallic, ceramic, and/or organic powder) may be loaded into the supply container  40 . When the elevator  42  is raised, it exposes the powder P above the worksurface  30 . 
         [0019]    The build platform  20  is a plate-like structure that is vertically slidable below the build opening  32 . It is connected to an actuator  46  operable to selectively move the build platform  20  up or down. The actuator  46  is depicted schematically in  FIG. 1 , with the understanding that devices such as pneumatic or hydraulic cylinders, ballscrew or linear electric actuators, and so forth, may be used for this purpose. When the build platform  20  is lowered into the build chamber  22  during a build process, the build chamber  22  and the build platform  20  collectively surround and support a mass of powder P along with any components being built. This mass of powder is generally referred to as a “powder bed”, and this specific category of additive manufacturing process may be referred to as a “powder bed process”. 
         [0020]    The overflow container  18  underlies and communicates with the overflow opening  36 , and serves as a repository for excess powder P. 
         [0021]    The directed energy source  24  may comprise any known device operable to generate a beam of suitable power and other operating characteristics to melt and fuse the metallic powder during the build process, described in more detail below. For example, the directed energy source  24  may be a laser. Other directed-energy sources such as electron beam guns are suitable alternatives to a laser. 
         [0022]    The beam steering apparatus  26  may include one or more mirrors, prisms, magnetic fields, and/or lenses and provided with suitable actuators, and arranged so that a beam “B” from the directed energy source  24  can be focused to a desired spot size and steered to a desired position in plane coincident with the worksurface  30 . For purposes of convenient description, this plane may be referred to as an X-Y plane, and a direction perpendicular to the X-Y plane is denoted as a Z-direction (X, Y, and Z being three mutually perpendicular directions). The beam B may be referred to herein as a “build beam”. 
         [0023]    The enclosure  28  serves to isolate and protect the other components of the apparatus  10 . It may be provided with a flow of an appropriate shielding gas “G”, for example nitrogen, argon, or other gases or gas mixtures. The gas G may be provided as a static pressurized volume or as a dynamic flow. The enclosure  28  may be provided with inlet and outlet ports  48 ,  50  respectively for this purpose. 
         [0024]    The basic build process for a workpiece W using the apparatus described above is as follows. The build platform  20  is moved to an initial high position. The build platform  20  is lowered below the worksurface  30  by a selected layer increment. The layer increment affects the speed of the additive manufacturing process and the resolution of the workpiece W. As an example, the layer increment may be about 10 to 50 micrometers (0.0003 to 0.002in.). Powder “P” is then deposited over the build platform  20  for example, the elevator  42  of the supply container  40  may be raised to push powder through the supply opening  34 , exposing it above the worksurface  30 . The recoater  16  is moved across the worksurface to spread the raised powder P horizontally over the build platform  20 . Any excess powder P drops through the overflow opening  36  into the overflow container  18  as the recoater  16  passes from left to right. Subsequently, the recoater  16  may be moved back to a starting position. The leveled powder P may be referred to as a “build layer”  52  and the exposed upper surface thereof may be referred to as a “build surface”  54  (see  FIG. 2 ). 
         [0025]    The directed energy source  24  is used to melt a two-dimensional cross-section or layer of the workpiece W being built. The directed energy source  24  emits a beam “B” and the beam steering apparatus  26  is used to steer the focal spot “S” of the build beam B over the exposed powder surface in an appropriate pattern. A small portion of exposed layer of the powder P surrounding the focal spot S, referred to herein as a “weld pool”  56  (best seen in  FIG. 2 ) is heated by the build beam B to a temperature allowing it to sinter, melt, and/or flow, and consequently consolidate. As an example, the weld pool  56  may be on the order of 100 micrometers (0.004 in.) wide. This step may be referred to as fusing the powder P. 
         [0026]    The build platform  20  is moved vertically downward by the layer increment, and another layer of powder P is applied in a similar thickness. The directed energy source  24  again emits a build beam B and the beam steering apparatus  26  is used to steer the focal spot S of the build beam B over the exposed powder surface in an appropriate pattern. The exposed layer of the powder P is heated by the build beam B to a temperature allowing it to melt, flow, and consolidate both within the top layer and with the lower, previously-solidified layer. 
         [0027]    This cycle of moving the build platform  20 , applying powder P, and then directed energy melting the powder P is repeated until the entire workpiece W is complete. 
         [0028]      FIG. 2  shows in more detail a workpiece W being constructed in a powder bed of the type described above. The exemplary workpiece W includes a pair of spaced-apart vertical walls  58 ,  60  interconnected by horizontal wall  62 . A cavity  64  is present between the vertical walls  58 ,  60  and is filled with powder P; additional powder P is present between the vertical walls  58 ,  60  and the side walls of the build chamber  22 . The workpiece W is shown as having an exemplary defect  66  (specifically, a void) disposed therein. Nonlimiting examples of types of defects that can be detected using this method include pores, cracks, and density variations. This particular defect  66  is located below the build surface  54  and thus would not be detectable by a surface inspection method. 
         [0029]    A noncontact acoustic inspection process may be incorporated into the build process described above. Generally stated, the inspection process includes using a noncontact method to generate an acoustic wave in the build surface  54 , and using a noncontact method to monitor the return signal. In general, this type of inspection process may be referred to as “laser ultrasonic inspection”. 
         [0030]    Any noncontact means of generating an acoustic wave in the build layer  52  may be used. For example, the action of the build beam B in generating the weld pool  56  inherently generates an acoustic wave radiating out from the weld pool  56 . The return signal from this acoustic wave may be monitored. 
         [0031]    Means are provided for monitoring the return signal. In the example shown in  FIGS. 1 and 2 , a low-power continuous-wave (“CW”) or pulsed monitoring laser  68 , or other suitable device, is positioned so as to be able to direct a monitoring beam M at the build surface  54 . The monitoring beam M may be directed through the same beam steering apparatus  26  as the build beam B. Deflection of the build surface  54  (caused by the arrival of the return signal at the build surface  54 ) in turn cause changes in the index of refraction of the gas “G” above the build surface  54 , which ultimately affect the path of the monitoring beam M. The variations in the monitoring beam&#39;s path can be sensed by a photodetector  70 . This type of acoustic detection is commonly referred to as gas-coupled laser acoustic detection or “GCLAD”. Numerous other types of interferometers are known and could be substituted for the monitoring laser  68  and photodetector  70 . In this example, the return signal of the monitoring beam M is directed to the photodetector  70  through a beam splitter  72 ; the specific hardware used to focus and/or direct the monitoring beam M is not critical to the present invention. 
         [0032]    Alternatively, the build beam B could be modulated to a reduced power level relative to that used to fuse the powder P, and directed to the build surface  54  to generate an acoustic wave. For example, the power level of the build beam B could be alternated between high and low levels during the building of a layer. Alternatively, a complete layer of powder P could be fused, and then the build beam B could be rescanned over the build surface  54  at a reduced power level to generate an acoustic wave, before applying the next layer increment of powder P. 
         [0033]    Alternatively, as shown in  FIG. 3 , a probe beam PB (generated by a separate probe laser  74  or other suitable radiant energy device) may be used specifically for that purpose. As shown in  FIG. 4 , the probe beam PB may optionally be directed through the same beam steering apparatus  26  as the build beam B. 
         [0034]    The operation of the apparatus  10  including the inspection devices (e.g. probe laser  74 , monitoring laser  68 , and/or photodetector  70 ) may be controlled by software running on one or more processors embodied in one or more computers, represented generically in  FIG. 1  by controller  75 . The same controller  75  may be used to retrieve and analyze sensor data, for statistical analysis, statistical process control, and for feedback control. 
         [0035]    The method described above generates information about the round-trip time of an acoustic wave from the build surface  54  to a sub-surface structure and back, which, knowing the speed of sound of the material in question, can be used to determine one or more sub-surface material properties, such as the thickness and/or density of material underlying the build surface  54 . 
         [0036]    During the inspection process, the surface location where the acoustic wave is induced (e.g. the probe beam focal spot) determines the X-Y location of the thickness/or density measurement on the build surface  54 . The focal spot position of the monitoring beam M is not critical; stated another way, the acoustic wave travel path does not have to be directly normal to the build surface  54 . Where the monitoring beam focal spot is spaced-away from the focal spot of the probe beam PB (or other source), appropriate computations can be used to determine the actual thickness based on the acoustic return data. For example, measurements may be taken on a plate of known composition and thickness with different relative positions of the probe beam PB and monitoring beam M. 
         [0037]    The probe beam PB can be scanned over the build surface  54  in an X-Y or raster pattern to build up information about the workpiece W. For example,  FIG. 4  shows a small portion of the build surface  54  represented as a grid of cells  76 . Each cell  76  is filled with a hatch pattern representative of the measured material thickness, where a denser hatch pattern indicates greater material thickness. There is a first group  78  of cells corresponding to one of the vertical walls  58 , a second group  80  of cells corresponding to the horizontal wall  62 , and a third group  82  of cells corresponding to the second vertical wall  60 . A fourth group  84  of cells corresponds to the location of the defect  66 . 
         [0038]    The hatch pattern seen in  FIG. 4  is merely for purposes of description. In use, any type of convenient representation may be used to present the thickness data, such as varied patterns, colors, or brightness levels. The data could also be represented by numerical or textual data. Furthermore, the size, type, and arrangement of cells  76  may vary to suit a particular application. 
         [0039]    This inspection method generates information about the thickness of material underlying the surface. Distinct changes in material thickness can be an indication of the presence of a defect  66 . However, the component may also include discrete thickness changes. In the example workpiece W, there is a discrete thickness change at the transition between the vertical walls  58 ,  60  and the horizontal wall  62 . Depending on the geometry of the workpiece W and the defects, the inspection method may not be able to distinguish such intentional features from defects. 
         [0040]    Therefore, for best results, the real-time measured data may be compared with a model of a known-good component. For example, the model could include information about the expected material thickness for each X-Y location within each layer.  FIG. 5  illustrates a small portion of the build surface  54  represented as a grid of cells  86 . Each cell  86  is filled with a hatch pattern representative of the measured material thickness, where a denser hatch pattern indicates greater material thickness. There is a first group  88  of cells corresponding to one of the vertical walls  58 , a second group  90  of cells corresponding to the horizontal wall  62  and a third group  92  of cells corresponding to the second vertical wall  60 . Comparison of  FIG. 5  with  FIG. 4  clearly shows that the fourth group  84  of cells  76  shown in  FIG. 4  is unexpected and most likely a defect. 
         [0041]    The inspection method described above may be implemented for various purposes in an additive manufacturing process. For example, an inspection could be made during the fusing of each build layer  52 , or immediately after each build layer  52  is complete, or after several build layers  52  have been completed. This allows confirmation that each layer or group of layers has been built correctly and is free of defects. 
         [0042]    If a workpiece is discovered to have a defect, the build process could be abandoned. Alternatively, if a defect is discovered, the apparatus  10  could be used to repair the defect, by directing the build beam B to the workpiece W over the defect, creating a weld pool which remelts the material and permits it to flow into and fill the defect. 
         [0043]    The inspection process described above may be used to provide real-time feedback which can be used to modify the additive build process. For example, if the inspection process determines that the build is creating defects, one or more process parameters such as laser power, scanning speed, gas flow, etc. may be changed to restore performance to nominal or eliminate the source of defects. 
         [0044]    The inspection process described above may also be used as part of a plan of statistical process control. In particular, the inspection process could be used to identify sources of variation in the process. Process parameters could then be altered in subsequent builds to reduce or eliminate sources of variation. 
         [0045]    The data from the acoustic inspection apparatus may be used in conjunction with one or more other process sensors such as photodiodes, pyrometers, acoustic sensors, cameras, or spectrometers. The information from the process sensor may be used as an additional data source for statistical process control or feedback control of the process, as described above. A generic sensor  94  is shown schematically in  FIG. 1 . 
         [0046]    The process described herein has several advantages over the prior art. In particular, it allows mapping the thickness of overhanging layers in an additive structure, and detection of variations from nominal with high precision. This has the potential to eliminate post-build quality control processes such as CT scanning currently used in the prior art. 
         [0047]    The foregoing has described an apparatus and method for non-contact acoustic inspection of an additive manufacturing process. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0048]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0049]    The invention is not restricted to the details of the foregoing embodiment(s). The invention extends any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying potential points of novelty, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.