Patent Publication Number: US-11020907-B2

Title: Method for melt pool monitoring using fractal dimensions

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to additive manufacturing and related processes, and more particularly to apparatus and methods for melt pool monitoring and process control in additive manufacturing. 
     Additive manufacturing is a process in which material is built up layer-by-layer to form a component. Additive manufacturing is limited primarily 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. 
     One type of additive manufacturing machine is referred to as a “powder bed” machine and includes a build chamber that encloses a mass of powder which is selectively fused by a laser to form a workpiece. 
     One problem with prior art additive manufacturing machines is that they operate in an open loop environment and cannot report back to an operator the stability of the process being applied. The systems in place to determine health of the process occur in quality steps after the build has finished. When issues are caught there can be work in progress that is scrapped due to machine issues that were undetected till the ex post facto quality system could catch them. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This problem is addressed by a method of imaging a melt pool during a manufacturing process and extracting a fractal dimension of the melt pool. 
     According to one aspect of the technology described herein, a method of controlling an additive manufacturing process is provided in which a directed energy source is used to selectively melt material to form a workpiece, forming a melt pool in the process of melting. The method includes: using an imaging apparatus to generate an image of the melt pool comprising an array of individual image elements, the image including a measurement of at least one physical property for each of the individual image elements; from the measurements, mapping a melt pool boundary of the melt pool; computing a fractal dimension of the melt pool; and controlling at least one aspect of the additive manufacturing process with reference to the fractal dimension. 
     According to another aspect of the technology described herein, a method of making a workpiece includes: depositing a material in a build chamber; directing a build beam from a directed energy source to selectively fuse the material in a pattern corresponding to a cross-sectional layer of the workpiece, wherein a melt pool is formed by the directed energy source; using an imaging apparatus to generate an image of the melt pool comprising an array of individual image elements, the image including a measurement of at least one physical property for each of the individual image elements; from the measurements, mapping a melt pool boundary of the melt pool; computing a fractal dimension of the melt pool; and controlling at least one aspect of making the workpiece with reference to the fractal dimension. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a schematic, partially-sectioned front elevation view of an exemplary additive manufacturing machine; 
         FIG. 2  is a schematic, partially-sectioned side elevation view of the machine of  FIG. 1 ; 
         FIG. 3  is a schematic top plan view graphic of an image of a powder bed including a melt pool; 
         FIG. 4  is an enlarged view of a portion of a melt pool image; 
         FIG. 5  is a schematic perspective view of a 3-D melt pool image; and 
         FIG. 6  is a schematic top plan view of a melt pool template 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates schematically an additive manufacturing machine  10  suitable for carrying out an additive manufacturing method. The machine  10  and its operation are as representative example of a “powder bed machine”. 
     It will be understood that the machine  10  is merely used as an example to provide context for describing the principles of the present invention. The principles described herein are applicable to other configurations of powder bed machines, as well as to other types of additive manufacturing machines and related processes. More generally, the principles described herein would be applicable to any manufacturing process in which a melt pool is generated. Nonlimiting examples of such processes include electron-beam melting (“EBM”), directed energy deposition (“DED”), and laser welding. The term “manufacturing process” could also encompass repair processes where components are built up or joined together using a technique that generates a melt pool. 
     Basic components of the machine  10  include a table  12 , a powder supply  14 , a 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 a housing  28 . Each of these components will be described in more detail below. 
     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 . 
     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. 
     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 . Other types of powder supplies may be used; for example, powder may be dropped into the build chamber  22  by an overhead device (not shown). 
     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”. 
     The overflow container  18  underlies and communicates with the overflow opening  36 , and serves as a repository for excess powder P. 
     The directed energy source  24  may comprise any device operable to generate a beam of suitable power and other operating characteristics to melt and fuse the powder P 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. 
     The beam steering apparatus  26  may include one or more 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 a 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”. 
     The housing  28  serves to isolate and protect the other components of the machine  10 . During the build process described above, the housing  28  is provided with a flow of an appropriate shielding gas which, among other functions, excludes oxygen from the build environment. To provide this flow the machine  10  may be coupled to a gas flow apparatus  54 , seen in  FIG. 2 . The exemplary gas flow apparatus  54  includes, in serial fluid flow communication, a variable-speed fan  56 , a filter  58 , an inlet duct  60  communicating with the housing  28 , and a return duct  64  communicating with the housing  28 . All of the components of the gas flow apparatus  54  are interconnected with suitable ducting and define a gas flow circuit in combination with the housing  28 . 
     The composition of the gas used may similar to that used as shielding gas for conventional welding operations. For example, gases such as nitrogen, argon, or mixtures thereof may be used. Any convenient source of gas may be used. For example, if the gas is nitrogen, a conventional nitrogen generator  66  may be connected to the gas flow apparatus  54 . Alternatively, the gas could be supplied using one or more pressurized cylinders  68 . 
     An exemplary 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 . 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” and the exposed upper surface thereof may be referred to as a “build surface”. 
     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 a focal spot 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, referred to herein as a “melt pool”  52  is heated by the build beam B to a temperature allowing it to sinter or melt, flow, and consolidate. As an example, the melt pool  52  may be on the order of 100 micrometers (0.004 in.) wide. This step may be referred to as fusing the powder P. 
     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 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 sinter or melt, flow, and consolidate both within the top layer and with the lower, previously-solidified layer. 
     This cycle of moving the build platform  20 , applying powder P, and then directed energy fusing the powder P is repeated until the entire workpiece W is complete. 
     The additive manufacturing machine  10  is provided with an imaging apparatus  70  which is operable to produce a digital image of the melt pool  52  comprising an array of individual image elements, i.e., pixels for a 2-D array or voxels for a 3-D array. An example of such an image  72  is shown in  FIG. 3 , with image elements  74 . The imaging apparatus  70  is operable to produce, for each image element, a measurement of at least one physical property. The measurement may include at least one scalar value such as brightness, intensity, frequency, temperature, or Z-height. Alternatively, the imaging apparatus  70  may produce a signal representative of multiple factors, for example RGB color values. The imaging apparatus  70  is also operable to produce relative or absolute positional information for each imaging element. For example, the output of the imaging apparatus  70  for a particular image element  74  may be in the format X, Y, T where X equals X-position, Y equals Y-position, and T equals temperature. 
     Nonlimiting examples of suitable imaging apparatus  70  include photodiode arrays, photomultiplier tube (“PMT”) arrays, digital cameras (e.g. CMOS or CCD), or optical coherence tomography (“OCT”) apparatus. Optical coherence tomography (“OCT”) is a known technique which is capable of providing Z-axis information as well as X and Y information (e.g., “3-D information”). In the illustrated example, the imaging apparatus  70  is depicted as a digital camera placed so that its field-of-view encompasses the melt pool  52 . 
     The imaging apparatus  70  may be statically mounted or it may be mounted so that it can be driven by one or more actuators (not shown) in order to track the position of the melt pool  52 . 
     The imaging apparatus  70  may be configured to acquire a series of static images at regular intervals, or it may operate continuously. 
     The melt pool images  72  create a base dataset on which analysis may be performed. The next step is to determine a boundary of the melt pool  52  based on the image  72  produced by the imaging apparatus  70 . 
     The process of determining a boundary of the melt pool  52  may be carried out using appropriately-programmed software running on one or more processors embodied in a device such as a microcomputer (not shown). Such software may be implemented on a device separate from the machine  10 , or it may be incorporated into the machine  10 , for example the software may be run by the controller described below. 
     Continuing to refer to  FIG. 3 , a portion of the powder bed P is shown with a melt pool  52  superimposed thereon. The melt pool  52  is shown having a peripheral melt pool boundary  76  or closed perimeter, identified by the shaded image elements  74 , which is a demarcation between the interior of the melt pool  52  and the exterior of the melt pool  52 . 
     Various criteria may be used for determining the location of the melt pool boundary  76 . In one example, a threshold value may be established, and the melt pool boundary  76  may include any image elements which are equal to the threshold value. For example,  FIG. 4  shows a simplified representation of a portion of the melt pool  52  in which each image element  74  is assigned a scalar value corresponding to sensed data. For example, temperature data might be represented on a 0-10 scale. In this example, the threshold value is “4” (this is an arbitrary value used as an example). Accordingly, each image element  74  returning the value 4 is declared or defined to constitute a portion of the melt pool boundary  76 . Any image element  74  returning a value greater than 4 is declared or defined to be inside the melt pool  52 , and any image element  74  returning a value less than 4 is declared or defined to be outside the melt pool  52 . 
     The threshold value may include a range or band of values, for the purpose of resolving ambiguity. For example, in the above-described situation where temperature is represented on a 0-10 scale, the threshold value representing inclusion in the melt pool boundary  76  might be any value greater than 3.0 and less than 5.0. 
     In practice, the range of sensor values and the threshold value or threshold range may be stored in a calibration table which is then referenced by the software to evaluate the melt pool  52  during machine operation. The values for the calibration table may be determined analytically or empirically. 
     The criteria for determining the location of the melt pool boundary  76  may be based on a simple scalar value in a 2-D image as described in the above example, for example temperature, image element intensity, etc. 
     Alternatively, another 2-D property or combination of properties such as image element color, emission frequency, or image element sheen may be used as a criteria for determining the location of the melt pool boundary  76 . Such properties are combination of properties may be more directly indicative of the presence of a difference in phase (liquid versus solid), or of melting or incipient melting. 
     Alternatively, a 3-D property or combination of properties may be used as a criteria for determining location of the melt pool boundary  76 . For example, a height increase or decrease relative to the surrounding material may be indicative of the presence of the difference in phase (liquid versus solid), or of melting or incipient melting. 
     The output of the boundary determination process described above is a digital map of the melt pool boundary  76 . The process of determining the location of the melt pool boundary  76  may be referred to as “mapping the melt pool boundary”. 
     Once the melt pool boundary  76  is established, one or more geometric analytical constructs may be used to analyze the boundary and determine the quality of the melt pool  52 . 
     One analytical construct for evaluating the melt pool boundary  76  is to consider its fractal dimension. 
     Generally, the fractal dimension “D” of a shape is a parameter that quantifies its relative complexity as a ratio of change in detail to change in scale. The fractal dimension does not uniquely define a shape. 
     Numerous methods are known for computing fractal dimensions of a generalized shape. One known expression for computing fractal dimension D is as follows: 
     
       
         
           
             D 
             = 
             
               
                 log 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 n 
               
               
                 log 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   1 
                   / 
                   s 
                 
               
             
           
         
       
     
     Where n is the number of line segments used to bound the shape and s is a scaling factor. For the purposes of the present invention, any of the known methods of computing a fractal dimension for a given shape would be acceptable. 
     A similar technique may be applied to the melt pool  52  when considered as a 3-D surface rather than a closed line 2-D figure. For example,  FIG. 5  illustrates a simplified 3-D representation  172  of a portion of the melt pool  52  in which each image element  174  is assigned an elevation (Z-axis height) corresponding to sensed data, for example a scalar quantity such as pixel intensity. A fractal dimension may be computed for the surface  172  using known techniques. 
     The fractal dimension, once computed, provides a parameter that can be used to evaluate the melt pool boundary  76 . In example shown in  FIG. 3 , the fractal dimension D of the melt pool boundary  76 , shown bounded by line segments  77 , would be approximately 1.065. 
     As an alternative to using only the fractal dimension, a ratio of the melt pool intensity to the fractal dimension may be used as a figure of merit. 
     In one technique, a limit value may be established for the fractal dimension or ratio described above as a basis for concluding that the melt pool  52  is acceptable or not, corresponding to the additive manufacturing process being “unfaulted” or “faulted”. 
     For example,  FIG. 6  depicts a model of an idealized melt pool template  78 . The melt pool template  78  is made up of image elements  80  and includes a predetermined template boundary  82  which is representative of a known good process (i.e., unfaulted). In this example the template boundary  82  is approximately circular and would have a fractal dimension of 1. While no specific shape is necessarily required for an acceptable process, it is generally true that the more complex shape of  FIG. 3 , having a higher fractal dimension, is more likely to be unacceptable and indicative of a process problem, than a less complex shape. Accordingly, in this example, a limit value could be set somewhere in the range between 1 and 1.065. 
     In one example, software may be used to compute the fractal dimension based on the measured data. If the fractal dimension exceeds the limit value, the melt pool boundary  76  of the melt pool  52  may be declared to be unacceptable (i.e., process faulted). 
     The melt pool quality determination may be repeated for each individual melt pool image  72  as they are acquired. 
     Other methods may be used beyond a simple comparison of the fractal dimension to a threshold value. In one example, the computed fractal dimension for each melt pool image  72  may be used as an input into a single or multivariate statistical process control (“SPC”) process. Nonlimiting examples of known SPC methods include principal component analysis (“PCA”), independent component analysis (“ICA”), and kernel PCA. For example, the computed fractal dimension would be in input into one of the above-noted SPC methods along with other process parameters or extracted values from the process (such as melt pool intensity, melt pool area, etc.). The PCA could then be performed and process control could be implemented based on the reduced variables. Another example would be to use the computed fractal dimension in multivariate SPC methodologies such as partial least squares (“PLS”). In PLS, one could use fractal dimension as an input for doing process control on another variable. An example would be that fractal dimension, along with other possible independent variables (inputs or “Xs”) would be used in PLS for predicting other dependent variable (outputs or “Ys”), such as melt pool intensity. Alternately, fractal dimension could be used as a dependent variable in PLS and then the methodology could be used to predict the fractal dimension, which would serve as a basis for process control. 
     The process may include creating populations of unfaulted and faulted process states based on the fractal dimension. Specifically, each melt pool image  72  would be assigned to either the unfaulted or faulted population as its fractal dimension is computed. 
     Various manual or automated methods may be used to assign the populations. In one example, the fractal dimension of current process could be assigned to the populations of unfaulted and faulted process through a Multiple Model Hypothesis Test framework. 
     A melt pool monitoring process may be incorporated into the build process described above. Generally stated, the monitoring process includes using the imaging apparatus  70  described above to acquire melt pool images  72 , evaluating the melt pool  52  using one or more of the fractal dimension techniques described above, and then adjusting one or more process parameters as necessary. As used herein, “process parameters” can refer to any controllable aspect of the machine  10 . 
     The monitoring process may include taking a discrete action in response to the fractal dimension evaluation indicating a process fault, such as providing a visual or audible alarm to a local or remote operator. 
     The monitoring process may include stopping the build process in response to fractal dimension evaluation indicating a process fault. This is another example of a discrete action. 
     The monitoring process may include real-time control of one or more process parameters, such as directed energy source power level or beam scan velocity, using a method such as: statistical process control, feedforward control, feedback control using proportional, proportional-integral, or proportional-integral-derivative control logic, neural network control algorithms, or fuzzy logic control algorithms. 
     The monitoring method may include monitoring of the condition or “health” of the machine  10 . Melt pool measurements may be measured and stored during several build cycles and compared between cycles. For example, a change in melt pool consistency between cycles could indicate machine miscalibration or degradation. Corrective action could take the form of machine maintenance or repairs, or modification of process parameters in subsequent builds to compensate for machine degradation. 
     The operation of the apparatus described above including the machine  10  and imaging apparatus  70  may be controlled, for example, by software running on one or more processors embodied in one or more devices such as a programmable logic controller (“PLC”) or a microcomputer (not shown). Such processors may be coupled to the sensors and operating components, for example, through wired or wireless connections. The same processor or processors may be used to retrieve and analyze sensor data, for statistical analysis, and for feedback control.  FIG. 1  illustrates schematically a controller  84  which includes one or more processors operable to control the machine  10   
     The method described herein has several advantages over the prior art. In particular, direct analysis of the melt pool is a key indicator of the stability of the manufacturing process and ultimately of the conformity of workpieces to geometric, metallurgical and mechanical requirements. 
     Establishing melt pool stability using the method described herein can also reduce machine setup costs through validation of the process to a known good standard, reduce existing material development for additive, reduce application development and be an enabler for novel alloy for additive development. 
     The foregoing has described an apparatus and method for melt pool monitoring in a 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. 
     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. 
     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 claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.