Patent Description:
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" (DMI,M), and "<NUM>-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. A prior art method of process monitoring is disclosed in <NPL>.

This problem is addressed by a method of imaging a melt pool during a manufacturing process and extracting a geometric length of the melt pool.

Claim <NUM> defines a method of controlling an additive manufacturing process. In the following, apparatus and/or methods referred to as embodiments that nevertheless do not fall within the scope of the claims should be understood as examples useful for understanding the invention.

The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:.

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, <FIG> illustrates schematically an additive manufacturing machine <NUM> suitable for carrying out an additive manufacturing method. The machine <NUM> and its operation are as representative example of a "powder bed machine".

It will be understood that the machine <NUM> 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 <NUM> include a table <NUM>, a powder supply <NUM>, a recoater <NUM>, an overflow container <NUM>, a build platform <NUM> surrounded by a build chamber <NUM>, a directed energy source <NUM>, and a beam steering apparatus <NUM>, all surrounded by a housing <NUM>. Each of these components will be described in more detail below.

The table <NUM> is a rigid structure defining a planar worksurface <NUM>. The worksurface <NUM> is coplanar with and defines a virtual workplane. In the illustrated example it includes a build opening <NUM> communicating with the build chamber <NUM> and exposing the build platform <NUM>, a supply opening <NUM> communicating with the powder supply <NUM>, and an overflow opening <NUM> communicating with the overflow container <NUM>.

The recoater <NUM> is a rigid, laterally-elongated structure that lies on the worksurface <NUM>. It is connected to an actuator <NUM> operable to selectively move the recoater <NUM> along the worksurface <NUM>. The actuator <NUM> is depicted schematically in <FIG>, 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 <NUM> comprises a supply container <NUM> underlying and communicating with the supply opening <NUM>, and an elevator <NUM>. The elevator <NUM> is a plate-like structure that is vertically slidable within the supply container <NUM>. It is connected to an actuator <NUM> operable to selectively move the elevator <NUM> up or down. The actuator <NUM> is depicted schematically in <FIG>, 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 <NUM> 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 <NUM>. When the elevator <NUM> is raised, it exposes the powder P above the worksurface <NUM>. Other types of powder supplies may be used; for example, powder may be dropped into the build chamber <NUM> by an overhead device (not shown).

The build platform <NUM> is a plate-like structure that is vertically slidable below the build opening <NUM>. It is connected to an actuator <NUM> operable to selectively move the build platform <NUM> up or down. The actuator <NUM> is depicted schematically in <FIG>, 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 <NUM> is lowered into the build chamber <NUM> during a build process, the build chamber <NUM> and the build platform <NUM> 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 <NUM> underlies and communicates with the overflow opening <NUM>, and serves as a repository for excess powder P.

The directed energy source <NUM> 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 <NUM> may be a laser. Other directed-energy sources such as electron beam guns are suitable alternatives to a laser.

The beam steering apparatus <NUM> 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 <NUM> can be focused to a desired spot size and steered to a desired position in plane coincident with the worksurface <NUM>. 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 <NUM> serves to isolate and protect the other components of the machine <NUM>. During the build process described above, the housing <NUM> 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 <NUM> may be coupled to a gas flow apparatus <NUM>, seen in <FIG>. The exemplary gas flow apparatus <NUM> includes, in serial fluid flow communication, a variable-speed fan <NUM>, a filter <NUM>, an inlet duct <NUM> communicating with the housing <NUM>, and a return duct <NUM> communicating with the housing <NUM>. All of the components of the gas flow apparatus <NUM> are interconnected with suitable ducting and define a gas flow circuit in combination with the housing <NUM>.

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 <NUM> may be connected to the gas flow apparatus <NUM>. Alternatively, the gas could be supplied using one or more pressurized cylinders <NUM>.

An exemplary basic build process for a workpiece W using the apparatus described above is as follows. The build platform <NUM> is moved to an initial high position. The build platform <NUM> is lowered below the worksurface <NUM> 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 <NUM> to <NUM> micrometers (<NUM>. <NUM> to <NUM> in. Powder "P" is then deposited over the build platform <NUM> for example, the elevator <NUM> of the supply container <NUM> may be raised to push powder through the supply opening <NUM>, exposing it above the worksurface <NUM>. The recoater <NUM> is moved across the worksurface to spread the raised powder P horizontally over the build platform <NUM>. Any excess powder P drops through the overflow opening <NUM> into the overflow container <NUM> as the recoater <NUM> passes from left to right. Subsequently, the recoater <NUM> 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 <NUM> is used to melt a two-dimensional cross-section or layer of the workpiece W being built. The directed energy source <NUM> emits a beam "B" and the beam steering apparatus <NUM> 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" <NUM> 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 <NUM> may be on the order of <NUM> micrometers (<NUM> in. This step may be referred to as fusing the powder P.

The build platform <NUM> is moved vertically downward by the layer increment, and another layer of powder P is applied in a similar thickness. The directed energy source <NUM> again emits a build beam B and the beam steering apparatus <NUM> 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 <NUM>, applying powder P, and then directed energy fusing the powder P is repeated until the entire workpiece W is complete.

The additive manufacturing machine <NUM> is provided with an imaging apparatus <NUM> which is operable to produce a digital image of the melt pool <NUM> comprising an array of individual image elements, i.e., pixels for a <NUM>-D array or voxels for a <NUM>-D array. An example of such an image <NUM> is shown in <FIG>, with image elements <NUM>. The imaging apparatus <NUM> 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 <NUM> may produce a signal representative of multiple factors, for example RGB color values. The imaging apparatus <NUM> is also operable to produce relative or absolute positional information for each imaging element. For example, the output of the imaging apparatus <NUM> for a particular image element <NUM> 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 <NUM> 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., "<NUM>-D information"). In the illustrated example, the imaging apparatus <NUM> is depicted as a digital camera placed so that its field-of-view encompasses the melt pool <NUM>. This particular example shows an "on-axis" device which shares the same optical path as the build beam B.

Alternatively, the imaging apparatus <NUM> could be mounted "off-axis", i.e., outside the optical axis of the build beam B. The imaging apparatus <NUM> 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 <NUM>.

The imaging apparatus <NUM> may be configured to acquire a series of static images at regular intervals, or it may operate continuously.

The melt pool images <NUM> create a base dataset on which analysis may be performed. The next step is to determine a boundary of the melt pool <NUM> based on the image <NUM> produced by the imaging apparatus <NUM>.

The process of determining a boundary of the melt pool <NUM> 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 <NUM>, or it may be incorporated into the machine <NUM>, for example the software may be run by the controller described below.

Continuing to refer to <FIG>, a portion of the powder bed P is shown with a melt pool <NUM> superimposed thereon. The melt pool <NUM> is shown having a peripheral melt pool boundary <NUM> or closed perimeter, identified by the shaded image elements <NUM>, which is a demarcation between the interior of the melt pool <NUM> and the exterior of the melt pool <NUM>.

Various criteria may be used for determining the location of the melt pool boundary <NUM>. In one example, a threshold value may be established, and the melt pool boundary <NUM> may include any image elements which are equal to the threshold value. For example, <FIG> shows a simplified representation of a portion of the melt pool <NUM> in which each image element <NUM> is assigned a scalar value corresponding to sensed data. For example, temperature data might be represented on a <NUM>-<NUM> scale. In this example, the threshold value is "<NUM>" (this is an arbitrary value used as an example). Accordingly, each image element <NUM> returning the value <NUM> is declared or defined to constitute a portion of the melt pool boundary <NUM>. Any image element <NUM> returning a value greater than <NUM> is declared or defined to be inside the melt pool <NUM>, and any image element <NUM> returning a value less than <NUM> is declared or defined to be outside the melt pool <NUM>.

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 <NUM>-<NUM> scale, the threshold value representing inclusion in the melt pool boundary <NUM> might be any value greater than <NUM> and less than <NUM>.

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 <NUM> 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 <NUM> may be based on a simple scalar value in a <NUM>-D image as described in the above example, for example temperature, image element intensity, etc..

Alternatively, another <NUM>-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 <NUM>. 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 as an example, a certain material with a given melting point can be characterized by the sensing system to calibrate the appropriate values to consider melted versus un-melted material. The boundary is then tied to correlated material properties, namely the liquid versus solid phases.

Alternatively, a <NUM>-D property or combination of properties may be used as a criteria for determining location of the melt pool boundary <NUM>. 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. A further example would use the presence of the meniscus in the <NUM>-D data by means of a topographic transition in the data to segregate the data into melted and unmelted regions.

The output of the boundary determination process described above is a digital map of the melt pool boundary <NUM>. The process of determining the location of the melt pool boundary <NUM> may be referred to as "mapping the melt pool boundary".

Once the melt pool boundary <NUM> is established, one or more geometric analytical constructs may be used to analyze the boundary and determine the quality of the melt pool <NUM>.

One analytical construct for evaluating the melt pool boundary <NUM> is to consider its geometric length. Geometric length may also be referred to as arc length; the two terms are used interchangeably herein.

Numerous techniques are available for computing geometric length of the melt pool boundary <NUM>. For the purposes of the present invention, any known method of computing the geometric length for a given melt pool boundary <NUM> would be acceptable.

For example, because the melt pool image <NUM> is digitized, one simple method involves counting the image elements <NUM> that define the melt pool boundary <NUM>. Another exemplary method would involve breaking down the melt pool boundary <NUM> into a plurality of curves that can be defined as a function y = f(x) in a Cartesian space. An example is shown in <FIG>, with the individual curves labeled <NUM>. The arc length of the individual curves <NUM> can then be integrated using analytical or numerical methods, and the individual arc lengths can be summed. Yet another exemplary method would involve breaking down the melt pool boundary <NUM> into a plurality of straight line segments and summing the lengths of the line segments. <FIG> shows an example of a plurality of line segments <NUM> intersecting at vertices <NUM>, which are shown by enlarged circles for ease of visibility.

The geometric length, once computed, provides a parameter that can be used to evaluate the melt pool boundary <NUM>.

As an alternative to using only the geometric length, a ratio of the melt pool intensity to the geometric length may be used as a figure of merit.

According to the invention, a limit value is established for the geometric length or the ratio described above as a basis for concluding that the melt pool <NUM> is acceptable or not, corresponding to the additive manufacturing process being "unfaulted" or "faulted".

For example, <FIG> depicts a model of an idealized melt pool template <NUM>. The melt pool template <NUM> is made up of image elements <NUM> and includes a predetermined template boundary <NUM> which is representative of a known good process (i.e., unfaulted). While no specific shape is necessarily required for an acceptable process, it is generally true that the more complex shape of <FIG>, having a higher geometric length, is more likely to be unacceptable and indicative of a process problem, than a less complex shape.

In one example, software may be used to compute the geometric length based on the measured data. If the geometric length exceeds the limit value, the melt pool boundary <NUM> of the melt pool <NUM> may be declared to be unacceptable (i.e., process faulted).

The melt pool quality determination may be repeated for each individual melt pool image <NUM> as they are acquired.

Other methods may be used beyond a simple comparison of the geometric length to a threshold value. In one example, the computed geometric length for each melt pool image <NUM> 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 geometric length 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 geometric length in multivariate SPC methodologies such as partial least squares ("PLS"). In PLS, one could use geometric length as an input for doing process control on another variable. An example would be that geometric length, 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, geometric length could be used as a dependent variable in PLS and then the methodology could be used to predict the geometric length, which would serve as a basis for process control.

The process may include creating populations of unfaulted and faulted process states based on the geometric length. Specifically, each melt pool image <NUM> would be assigned to either the unfaulted or faulted population as its geometric length is computed.

Various manual or automated methods may be used to assign the populations. In one example, the geometric length 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 <NUM> described above to acquire melt pool images <NUM>, evaluating the melt pool <NUM> using one or more of the geometric length 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 <NUM>.

The monitoring process may include taking a discrete action in response to the geometric length 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 geometric length 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 <NUM>. 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 <NUM> and imaging apparatus <NUM> 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> illustrates schematically a controller <NUM> which includes one or more processors operable to control the machine <NUM>.

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.

Claim 1:
A method of controlling an additive manufacturing process in which a directed energy source is used to selectively melt material to form a workpiece, forming a melt pool (<NUM>) in the process of melting, the method comprising:
using an imaging apparatus to generate an image (<NUM>) of the melt pool (<NUM>) comprising an array of individual image elements (<NUM>), the image (<NUM>) including a measurement of at least one physical property for each of the individual image elements (<NUM>);
from the measurements, mapping a melt pool boundary (<NUM>) of the melt pool (<NUM>);
computing a geometric length of the melt pool boundary (<NUM>);
controlling at least one aspect of the additive manufacturing process with reference to the geometric length; and
evaluating the geometric length for indications of a process fault, wherein the geometric length exceeding a predetermined limit value indicates a process fault.