Abstract:
A method of monitoring an additive manufacturing process in which a layer of powdered material is deposited in a recoating process so as to define a build surface, and a directed energy source is used to create a weld pool in the build surface and selectively fuse the powdered material to form a workpiece. The method includes: measuring a vibration signal profile generated by the recoating process; and controlling at least one aspect of the additive manufacturing process in response to the measured vibration signal profile.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to additive manufacturing, and more particularly to apparatus and methods for providing real-time vibration monitoring in additive manufacturing. 
         [0002]    Additive manufacturing is a process in which material is built up layer-by-layer to form a component. 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]    One common type of additive manufacturing machine includes a blade-like recoater to apply powdered material over a build surface prior to laser fusing. 
         [0004]    A common cause of build crashes in this kind of machine is contact between the recoater blade and the workpiece being built as internal stresses build up causing the workpiece to warp or lift from a supporting build plate. One problem with such machines it that there is currently no simple means of monitoring this stress build up or the severity of contact between the workpiece and the recoater during the build process. 
         [0005]    Another problem is that 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 requires undesirable extra time and cost and eliminates the possibility of taking corrective actions during the build. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    At least one of these problems is addressed by a method of monitoring vibration during an additive manufacturing build process. 
         [0007]    According to one aspect of the technology described herein, a method is provided for monitoring an additive manufacturing process in which a layer of powdered material is deposited in a recoating process so as to define a build surface, and a directed energy source is used to selectively fuse the powdered material to form a workpiece. The method includes: measuring a vibration signal profile generated by the recoating process; and controlling at least one aspect of the additive manufacturing process in response to the measured vibration signal profile. 
         [0008]    According to one aspect of the technology described herein, a method of making a workpiece includes: using a moving recoater to deposit a powdered material so as to define a build surface; directing a focused energy beam from a directed energy source to selectively fuse the powdered material in a pattern corresponding to a cross-sectional layer of the workpiece; measuring a vibration signal profile generated by the recoater; and controlling at least one aspect of making the workpiece in response to the measured vibration signal profile. 
     
    
     
       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 an enlarged view of a warped workpiece being built by the apparatus of  FIG. 1 ; 
           [0013]      FIG. 4  is an enlarged view of a warped workpiece being built by the apparatus of  FIG. 1 ; 
           [0014]      FIG. 5  is a baseline vibration signal profile of a “known good” workpiece build process; 
           [0015]      FIG. 6  is a vibration signal profile showing a small or incipient defect in workpiece during a build process; and 
           [0016]      FIG. 7  is a vibration signal profile showing a large defect in a workpiece during a build process. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    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. 
         [0018]    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 . 
         [0019]    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. 
         [0020]    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 . It should be appreciated that other suitable forms and/or types of powder supply  14  may be used. For example, the powder supply  14  may be positioned above the worksurface  30  and include a powder container that drops powder onto the worksurface  30  at a pre-determined flow rate. 
         [0021]    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”. 
         [0022]    The overflow container  18  underlies and communicates with the overflow opening  36 , and serves as a repository for excess powder P. 
         [0023]    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. 
         [0024]    The beam steering apparatus  26  may include one or more electromagnets, mirrors, prisms, 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”. 
         [0025]    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. 
         [0026]    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 0.002 in.). 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  (known as the “recoating process”). 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 ). 
         [0027]    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 melt, flow, and 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. 
         [0028]    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 fuse both within the top layer and with the lower, previously-solidified layer. 
         [0029]    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. 
         [0030]      FIG. 3  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 raised portion and/or protrusion caused by a high stress area). Non-limiting examples of types of defects that can be detected using this method include warping of the workpiece W, lifting of the workpiece W from the build platform  20 , and uneven melting by the directed energy source  24 . This particular defect  66  is caused by warping (high stress area) which causes a portion of the workpiece W to be located above the build surface  54 . 
         [0031]    A real-time vibration monitoring process may be incorporated into the build process described above. Generally stated, the vibration monitoring process includes using a vibration sensor  68  (such as a transducer or microphone) to generate vibration signals during the recoating process and monitoring those signals for irregularities indicative of a defect in the workpiece W. The vibration sensor  68  may be mounted on and/or in the recoater  16  ( FIG. 3 ) and/or table  12  ( FIG. 4 ), or a chassis of the apparatus  10 , to measure and record vibration signals during the recoating process. 
         [0032]    During a typical recoating process, the recoater  16  slides across the worksurface  30  of the table  12  pushing powder over the build platform  20  to form a layer increment of powder to be melted by the directed energy source  24 . If the workpiece W is properly formed, then the recoating process is smooth and very little variation occurs in the amount of vibration being induced by the recoating process. However, if the workpiece W experiences warpage, portions of the workpiece W may protrude upwardly, thereby causing a change in the amount of vibration being induced by the recoating process. 
         [0033]    As illustrated in  FIG. 3 , in the most severe case, the workpiece W warps to a degree that a portion of the workpiece W protrudes in the Z direction above the build surface  54 . As the recoater  16  spreads powder across the build platform  20 , the recoater  16  makes contact with the workpiece W. In severe cases, such contact may cause damage to the workpiece W, recoater  16 , and/or apparatus  10 . More subtle cases may occur when the workpiece W has a smaller degree of warpage or when a directed energy source  24  provides for an uneven melting of the powder. In these cases, as the recoater  16  spreads powder across the build platform  20 , the undulations and/or projections in an upper surface of the workpiece W can cause a change in vibration induced by the recoating process. This is true even when the upper surface of the workpiece W lies below a layer of powder, due to lubrication flow. By monitoring these vibrations, a user can determine if the workpiece W is not being properly formed. 
         [0034]    The vibration monitoring process uses a “known good” workpiece vibration signal profile as a comparison to provide vibration signal analysis. For each specific build (different sized and shaped workpieces) a new signal profile can be developed for comparison. Thus, a baseline signal profile may be developed for each increment layer of the build. This is done so that variations in vibrations due to a specific profile of a workpiece, as opposed to a defect, is taken into account. A non-limiting example of a known good signal profile, showing vibration amplitude versus time, is shown in  FIG. 5 . 
         [0035]    As the build process for a workpiece W is conducted, the vibration sensor  68  measures vibration signals to be recorded for each increment layer of the build. These vibration signals are then compared to the known good signal profile for each build layer to determine if a defect such as warpage of the workpiece W is propagating (it should be appreciated that the vibration signal profile may be recorded at any suitable pre-determined interval of layer increments). The comparison may be conducted after the build process is complete, as a means of inspection and quality control. Alternatively, the comparison may take place in real time. 
         [0036]    If the defect is in its beginning stages or is subtle, an example signal profile like that shown in  FIG. 6  may occur. As shown, a small spike or peak  70  occurs indicating that a small defect is changing the vibration profile. Alternatively, if severe defect occurs (for example, causing contact with the recoater  16 ), an example signal profile like that shown in  FIG. 7  may occur, showing a significant spike or peak  72 . 
         [0037]    By monitoring the vibration signal profile for each build process and comparing the signal profile to the known good signal profile, a defect in the build process can be detected at an early stage of the build process. This enables the possibility of corrective actions such as: (1) stopping the build process before damage to the workpiece W, the recoater  16 , and/or the apparatus  10  occurs due to contact between the workpiece W and the recoater  16 ; (2) allowing an operator to perform quality control during the build by observing the signal spikes and determining if the spikes are below a pre-determined vibration threshold; and (3) using the apparatus  10  to repair the defect, by directing the build beam B to the workpiece W over the defect, creating a weld pool which re-melts the material and permits it to flow into and fill the defect. 
         [0038]    In addition to or as an alternative to the discrete actions described above, the vibration monitoring process 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 directed energy source power level, beam scan velocity, beam scan pattern, beam pulse length, or beam pulse frequency, may be changed to restore performance to nominal or eliminate the source of defects. 
         [0039]    The vibration monitoring process described above may also be used as part of a plan of statistical process control. In particular, the inspection process which 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. 
         [0040]    The foregoing has described a real-time vibration monitoring process for 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. 
         [0041]    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. 
         [0042]    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.