Patent Description:
Control schemes for such processes have limitations. A typical control scheme permits a user limited control over process parameters, such as power, speed, and path of the energy beam (e.g., a laser or electron beam). The user controls the process via preset "themes" from which the user can select a theme for a given geometry of a component. For instance, a geometry that overhangs the powder bed (a "downskin") may have a corresponding theme with its own presets for a single power, single speed, and path parameters for stripe width and overlap. Outside of selecting that theme versus another theme that has different presets, there is no ability to vary these parameters once the theme begins.

Patent document <CIT> relates to an energy gun additive manufacturing system comprising monitoring means for detecting surface anomalies after solidification.

Non-patent document "<NPL>, relates to monitoring devices which can be implemented on selective laser melting systems in order to provide an adaptive feed-back control of the system.

Patent document <CIT> relates to the monitoring of a powder bed additive manufacturing process continually monitored unsing multiple eddy current sensor arrays.

The present application relates to a powder processing machine, a computer program and a method for use in a powder processing machine as disclosed in the appended set of claims.

In a powder bed fusion process, aside from selecting one theme versus another theme, there is no ability to vary process parameters once the theme begins. Moreover, if parameters such as power, speed, and path are to be adjusted during the process, there is still the matter of how to vary such parameters to enhance melting and fusion and thus improve the quality of the components built. In this regard, the present disclosure sets forth a model-based approach for implementation of a dynamic control scheme that is capable of adjusting parameters during the process to facilitate the production of high quality parts with fewer defects, such as key-holing, balling, and unmelt porosity defects commonly found in powder bed manufacturing.

The approach involves, inter alia, modeling of the melt pool, prediction of aspects such as the shape of the melt pool, energy density, and porosity. For instance, one output of the model may be a dynamic process map of energy beam power versus speed, although other modeling input parameters could be assessed by such process maps. Through the modelling, all input parameters may be included in the analysis and in the <NUM>-parameter process maps. The models are used to establish regions in the process map where defect conditions are predicted to exist. Thus, a control scheme can plot an instant point in the build on the process map, and adjust the power or speed to be outside of the region of the defect conditions, thereby facilitating the production of a higher quality, lower defect component.

<FIG> illustrates a powder processing machine <NUM>. The machine <NUM> may be used in an additive manufacturing process (e.g., powder bed fusion) to fabricate components (e.g., depicted as "build parts"). Although not limited, the components may be gas turbine engine components, such as airfoils, seals, tubes, brackets, fuel nozzles, heat shields, liners, or panels. Additionally, the components may be fabricated from a wide range of materials, including but not limited to, metal alloys.

The machine <NUM> includes a work bed <NUM>, a powder deposition device <NUM> that is operable to deposit powder (e.g., a metal powder) in the work bed <NUM>, an energy beam device <NUM> that is operable to emit an energy beam <NUM> with a variable beam power and direct the energy beam onto the work bed <NUM> with a variable beam scan rate to melt and fuse regions of the powder, and a controller <NUM> that is in communication with at least the energy beam device <NUM>. All but the controller <NUM> may be enclosed in an environmental chamber <NUM>. As will be appreciated, although not shown, the machine <NUM> may include additional components, such as but not limited to, a vacuum pump, process gas sources, and related valves.

In this example, the work bed <NUM> includes a build plate 22a upon which the powder is deposited and the component is built. The build plate may be actuated using a piston or the like to lower the build plate 22a during the process. The powder deposition device <NUM> may include a powder supply bed 24a supported on a bed plate 24b, and a recoater arm 24b. The bed plate 24b may be actuated using a piston or the like to raise the bed plate 24b during the process. The recoater arm 24b is operable to move across the supply bed 24a and work bed <NUM>, to deposit layers of powder in the work bed <NUM>. The operation of the work bed <NUM> and powder deposition device <NUM> may be controlled via the controller <NUM>.

In this example, the energy beam device <NUM> includes a laser 26a, one or more lenses 26b, and a mirror 26c. The mirror 26c may be actuated (at the command of the controller <NUM>) to control the direction of the energy beam <NUM> onto the work bed <NUM>. The laser 26a and one or more lenses 26b may be modulated (at the command of the controller <NUM>) to control the power of the energy beam <NUM>. For example, the energy beam <NUM> can be operated with varied energy levels from no power (off) to the highest power setting as required to maintain processing parameters within a safe zone to mitigate defect formation. Although shown with the laser 26a, it will be appreciated that the energy beam device <NUM> may alternatively utilize an electron beam gun, multiple electron beam guns, or multiple lasers, and the laser or lasers may be continuous or intermittent (pulsing).

The controller <NUM> may include hardware (e.g., one or more microprocessors, memory, etc.), software, or combinations thereof that are programmed to perform any or all of the functions described herein. The controller <NUM> is operable to dynamically control at least one of the beam power (Watts) or the beam scan rate (meters per second) to change how the powder melts and fuses in the work bed <NUM>. The control of power and scan rate may also extend to "resting time" of the energy beam deice <NUM>, at which power is equal to zero and scan rate is equal to zero. For instance, the "resting time" parameter may be used when the powder bed is being recoated, and time can be added to start the process (which may also depend on the number of parts being built in the work bed <NUM> because the energy beam <NUM> "jumps" from one part to another). The term "dynamically control" refers to the ability of the controller <NUM> to change at least one of the power or the scan rate as the energy beam <NUM> scans across the powder to melt and fuse the powder during an additive manufacturing process. In this regard, the controller <NUM> is configured to determine whether an instant set of process parameters (variables) falls within a defect condition or a non-defect condition and adjust at least one of the beam power or the beam scan rate responsive to the defect condition such that the instant set of process parameters falls within the non-defect condition. For instance, the list below contains an example set of process parameter variables, which will be used in the subsequently described development of the models upon which the dynamic control is based. In some instances, example values are listed for the variables, but it is to be understood that the values are variable based on the composition of the metal, energy beam, bed design, etc..

In addition to or in place of powder bed local density, powder layer thickness and compaction degree may be used. For example, piston drop may be used (e.g., piston drop may be <NUM> micrometers, while the actual layer thickness becomes <NUM> micrometers after the initial layers because of material shrinkage; the piston drops <NUM> micrometers after the first layer, but spreads <NUM> micrometers because the first layer shrunk by <NUM>%).

The defect condition(s) correspond to one or more specific types of defects often found in additive manufacturing, such as (unstsable) key-holing, balling, and unmelt porosity. Key-holing, balling, and unmelt porosity defects are depicted, respectively, in <FIG>. In general, key-holing results from excessive evaporation of the melt due to high energy beam power, slow energy beam scan rate, or both; balling results from unstable elongated melt pools that break into discrete balls or islands; and unmelt porosity results from pockets of unmelted powder due to low energy beam power, fast scan rate, or both.

Each of these types of defects was modeled, as discussed further below, based on the process parameter variables. The modeling, in turn, enabled each type of defect to be mapped on a plot of energy beam power versus scan rate. Thus, for a given set of process parameter variables, there may be one or more regions on the plot where defect conditions are predicted to occur. In turn, if the power and scan rate at an instant location in the work bed <NUM> during a process lies within a region of a defect condition, the power, scan rate, or both, can be dynamically adjusted during the process such that the plot of the adjusted power and scan rate fall outside of the region of the defect condition (in a region where non-defect conditions exist). In this manner, as the energy beam <NUM> scans a path across the powder to melt and fuse the powder, the controller <NUM> may dynamically adjust power, scan rate, or both location-by-location along the path (e.g., voxel-by-voxel) to ensure that at each location the power and scan rate correspond to a plot location on the map with a non-defect condition.

The methodology herein may also provide a technique for rapid component qualification, wherein the output of additive manufacturing machine sensors compared with target parameters will help assess quality level and conformance of the built components. Further, the methodology herein may be used to simulate a build path for entire components and then use the simulated path to build the actual component. The control system could, for example, be of two types: i) follow explicitly the parameter sets pre-defined for all specific locations, but monitor the parameter separately as well to enable comparison and validation that the process was run to plan or ii) run the controller from the start with a pre-defined path, but the parameters for time, speed and power are determined in-process based on sensor readings and the established models. In addition to power and scan rate, the path of the energy beam could also be adjusted either before building via the modelling (e.g., to a less complex scan path with power and speed being the driving factors), or the path could be adjusted during a build (e.g., if the melt pool width varied outside its bounds, the scan spacing may be adjusted).

The following examples illustrate the mathematical modeling of defects for use in a dynamic control scheme.

Referring to <FIG>, a model of the cross-sectional area of the melt pool generated during melting of the powder was developed. The cross-sectional area is calculated from an energy balance.

Referring to <FIG>, balling is calculated from a modified Rayleigh condition of instability for the melt pool aspect ratio.

<FIG> illustrates an example of a map of the balling condition region (shaded) on a plot of power versus scan rate.

<FIG> depicts unmelt porosity without balling, and <FIG> illustrate the assumed melt pool geometry. The model of unmelt porosity is based on a regular array of pores predicted at deterministic unmelt conditions.

Referring to <FIG>, <FIG>, the model of key-hole porosity is based on a force and heat balance in a Marangoni vortex to determine flow velocity and maximal temperature under the energy beam.

Keyhole/porosity formation criterion: <MAT> Typical value Vf∼ <NUM>/s Keyhole forms when recoil vapor pressure ~ capillary pressure of the pool <MAT> Keyhole formation criterion: <MAT> <MAT>.

As depicted in <FIG>, Marangoni flow accelerates effective thermo-conductivity by approximately 75X and dramatically shifts criterion of keyhole formation.

Referring also to <FIG>, a keyhole instability criterion is also modeled.

<FIG> illustrates an example of a map of the keyhole condition region (shaded) on a plot of power versus scan rate. In this example, there is a region where no keyhole occurs, a region where a stable keyhole occurs (which may be tolerable or acceptable), and a region where an unstable keyhole occurs (defect region).

<FIG> shows a schematic representation of a process map for an instant location in the work bed <NUM> during a process. The process map is a composite of process maps for balling, unmelt porosity, and key-holing. That is, there is an "ideal" region on the map for which combinations of power and scan rate, at that location for a given starting temperature at that location and set of process parameters, do not result in balling, unmelt porosity, or key-holing (i.e., non-defect conditions). Thus, if the power and scan rate at that location for the given starting temperature at that location lie outside of the "ideal" region, the controller <NUM> adjusts the power, the scan rate, or both such that the adjusted power and scan rate fall within the "ideal" region for non-defect conditions.

<FIG> illustrates a more detailed example of such a process map and listing of process parameters upon which the process map is based. As shown, there is a region where balling and unmelt porosity will occur, a region where unmelt porosity will occur, a region where unstable keyhole porosity will occur, and a defect free region where no defects occur. For the given set of process parameters, if the power and scan rate at a location for the given starting temperature at that location lie outside of the defect free region, the controller <NUM> adjusts the power, the scan rate, or both such that the adjusted power and scan rate fall within the defect free region (non-defect conditions). Similarly, <FIG> illustrates another example process map in which at least some of the defect regions are further classified according to a predicted defect concentration.

The instant or starting temperature at an energy beam location in the work bed <NUM> is modeled and may serve as the basis of the modeling examples above. For example, the instant temperature is determined based on at least one of the temperature of the build plate surface 22a in the work bed <NUM>, the temperature change due to previous energy beam passes in a current stripe, the temperature change due to a previous stripe in the same layer, the temperature change due to previous powder layers, or an edge factor that represents the instant location of the energy beam in the work bed <NUM> relative to an edge of the component being formed from the powder (collectively, "temperature factors"). As represented by the equation below, the instant temperature may be determined based on all of these temperature factors, although it will be appreciated that the temperature factors may be used individually or in combinations or two or more factors.

Calculation of the temperature at energy beam location, T<NUM> : <MAT>.

With reference to <FIG>, the stripe is the path in the y-direction. The energy beam passes back and forth in the x-direction along the stripe. At each pass, the energy beam is adjacent the prior pass, which adds heat to locations along the current pass. As shown in <FIG>, this causes a large temperature increase immediately after the energy beam turns into a new pass because the adjacent prior pass has not had time to cool. As the energy beam continues in the x-direction though, the immediately adjacent pass has had more time to cool and thus adds less heat.

Referring to <FIG>, the effect of the prior pass may also be influenced by proximity of the instant energy beam location to an edge or perimeter of the component being built. At an edge, there is lower heat transport and heat thus accumulates to further increase temperature. This effect is modeled and represented in the determination of the temperature at the instant energy beam location as an edge factor, f. The edge factor is a function of the distance from the instant location of the energy beam to the edge.

As shown in the example of <FIG>, a distribution of temperature can be mapped on the energy beam path. In this example, the path stripes are angled to the right in the figure. Each point in the map represents the temperature, i.e., the temperature T<NUM> in the point (x,y) at a time moment, when the energy beam passes that point.

As shown in <FIG>, a distribution of defects can also be mapped. In this example, the distribution of defects is mapped in a layer located at height h from the build plate 22a. As shown in <FIG>, rather than a singular height, a distribution of defects can also be mapped in a three-dimensional volumetric space of the component. In this example, the distribution reveals a non-optimal build in which there are repeating point defects and repeating linear defects.

<FIG> shows a micrograph of a downskin perimeter region of a component with porosity defects. The occurrence of such defects is modeled and provides opportunity to employ a control scheme to reduce occurrence of such defects. A "downskin" is a section of a component that overhangs the powder in the work bed <NUM> (versus a section that is on top of the component).

<FIG> illustrates a representative model of a downskin section and the path of the energy beam in a stripe at the edge. The downskin accumulates heat because of low thermal conductivity of the powder. The heat, in turn, causes defects such as porosity and excessive surface roughness. As shown in <FIG>, the angle (ϕ) is the compliment to the angle (ψ) of the downskin section. The angle (ϕ) has a strong influence on the accumulation of heat. Thus, for higher angles (ϕ), which correspond to lower angles (ψ), there is greater heat accumulation and correspondingly higher temperature increase. This indicates that for defects that occur due to excessive temperature in the downskin region, decreasing the angle (ϕ) will reduce temperature in the downskin region.

Temperature profile in downskin region: <MAT> <MAT>.

Equations for downskin control: <MAT> <MAT> wherein, <MAT> <MAT> <MAT>.

<FIG> illustrates the claimed powder processing machine <NUM> that is similar to the machine <NUM> but additionally includes one or more sensors <NUM>. In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate modified elements that are understood to incorporate the same features and benefits of the corresponding elements. The controller <NUM> may be operable to carry our one or more functions and methods as described above but may additionally or alternatively be operable to carry out functions as will be described below.

The sensor or sensors <NUM> may be located in or around the work bed <NUM>, depending on the type of sensor. The sensor <NUM> is operable to detect process characteristics in the work bed <NUM> by location in the work bed <NUM> during scanning of the energy beam <NUM> and produce signals representative of the process characteristics and locations. In this regard, the sensor <NUM> is connected to communicate with the controller <NUM>. The controller <NUM> is in communication with the sensor <NUM> to receive the signals. The controller <NUM> is configured to identify one or more anomalies in the process characteristics and responsively adjust at least one of the beam power or the beam scan rate at the locations of the anomalies.

With continued reference to <FIG>, as shown in <FIG>, the locations in the work bed <NUM> may be represented in a Cartesian coordinate system, wherein the x-y plane is typically in the plane of the powder that is spread in the work bed <NUM>. Thus, any point in the work bed <NUM> can be mapped by its x-y coordinates. If needed, a third coordinate of the height (h) above the build plate 22a may also be used. As will be appreciated, location in the work bed <NUM> may be expressed in other forms and is not limited to a Cartesian coordinate system.

The process characteristics include temperature (of the powder) in the work bed <NUM> by location, and optionally topography of the powder in the work bed <NUM>. In particular, one phenomenon in powder bed fusion that potentially leads to defects is expulsion of material from local points in the work bed <NUM>. The phenomenon is depicted in <FIG>.

With continued reference to <FIG>, as shown in <FIG>, the energy beam <NUM> is traveling from left-to-right and has thus fused the metal to the left of the arrowhead while the metal to the right of the arrowhead is still powder. Due to porosity in the powder, contamination in the powder, adsorbed moisture, or other disorder in the process, there can be a "spark" of material that is ejected from the melt pool. Here, as shown at the location indicated at <NUM>, the expulsion of material has resulted in a crater 134a in the fused bead of metal.

With continued reference to <FIG>, as shown in <FIG>, the recoater arm 24c has already spread a second, subsequent layer of powder over the first layer of powder that was fused in <FIG>. This layer of powder is of uniform thickness (t1), except that the crater 134a becomes filled-in with powder. Thus, at the location <NUM> of the crater 134a there is a greater thickness (t2) of powder than the thickness (t1) adjacent the crater 134a.

With continued reference to <FIG>, as shown in <FIG>, the second layer has been fused by the energy beam. However, the theme used for the process in this case was based on an assumption that the powder always has the uniform powder layer thickness (t1). Under that assumption, although powder in the second layer may be consolidated, there may not be enough energy in the energy beam at location <NUM> to melt and fuse all of the powder in the greater powder thickness (t2). Consequently, the powder over the crater 134a fuses but the powder in the crater 134a is not fused. As a result, the unfused powder in the crater 134a, now sealed over by fused powder, becomes a powder-filled pore <NUM> left in the component.

In contrast, as shown in <FIG> (with continued reference to <FIG>), the second layer is fused using adaptive control. For instance, the sensor <NUM> detects the expulsion of material at the stage in <FIG> and communicates the signal of the location <NUM> to the controller <NUM>. When the energy beam fuses the second layer and reaches the location <NUM> of the greater thickness (t2), the controller <NUM> adjusts the beam power, the beam scan rate, or both to account for the greater thickness (t2). For instance, the controller <NUM> increases beam power, decreases scan rate, or both. Consequently, as shown in <FIG>, there is higher energy input and the powder in the crater 134a melts and fuses, thereby reducing or eliminating the powder-filled pore <NUM> that would otherwise have been produced without adaptive control. Thus, adaptive control refers to the ability in the process to adjust beam power or scan rate during scanning of the energy beam in response to a detected discontinuity of the work bed <NUM>.

Defects, such as but not limited to expulsion of material, may be detected by one or more anomalies in one or more of the process characteristics. For instance, the sensor <NUM> may include a topography sensor that is operable to detect topographical process characteristics of the powder in the work bed <NUM> by location in the work bed <NUM>. The controller <NUM> may identify the crater 134a as an anomaly from the topographical process characteristics and responsively adjust the beam power, scan rate, or both. As an example, the controller <NUM> may be programmed with one or more threshold crater sizes above which the controller <NUM> executes the adjustment. As will be appreciated, the crater 134a is the subject topographic feature in the examples above; however, the topographic feature is not limited to craters and other topographic anomalies may be detected and trigger the responsive adjustment of the beam power, scan rate, or both. As examples, the topographic anomalies may be positive or negative features in the fused powder or in the powder before it is fused. Example topographic anomalies in the powder may include streaks (e.g., a trench in the unfused powder from the recoater arm dragging a larger particle across the work bed), pocks or indentations, elevated powder agglomerates or protuberances, or the like. Similarly, the sensor <NUM> may include one or more optical sensors to detect expulsion or topographic anomalies.

The sensor <NUM> includes a temperature sensor that is operable to detect temperature process characteristics of the powder in the work bed <NUM> by location in the work bed <NUM> and to detect ejected material by temperature excursions coming off of the powder. The controller <NUM> may identify a temperature anomaly that violates a temperature threshold and responsively adjust the beam power, scan rate, or both.

Claim 1:
A powder processing machine comprising:
a work bed (<NUM>);
a powder deposition device (<NUM>) operable to deposit layers of a powder in the work bed;
at least one energy beam device (<NUM>) operable to emit an energy beam (<NUM>) with a variable beam power and scan the energy beam (<NUM>) in a path across the powder in the work bed (<NUM>) with a variable beam scan rate to melt and fuse regions of the powder to produce fused powder;
at least one sensor comprising a temperature sensor (<NUM>) operable to detect temperature in the work bed (<NUM>) by location in the work bed (<NUM>) during scanning of the energy beam (<NUM>) and produce signals representative of the temperature and locations; and
a controller (<NUM>) in communication with the temperature sensor (<NUM>) to receive the signals, the controller (<NUM>) configured to identify one or more temperature excursions that are associated with defects in which ejected material coming off of the powder leaves one or more craters in the fused powder and responsively adjust at least one of the beam power or the beam scan rate at the locations corresponding to the craters,
characterised in that the controller is adapted to detect the temperature excursions during scanning of a first layer of the powder by detecting a temperature anomaly that violates a temperature threshold, thereby indicating the presence of craters in the fused powder produced from the first layer of the powder, wherein on the fused powder produced from a first layer of the powder there is subsequently deposited a second layer of the powder that is of uniform thickness except that there is a greater thickness of the second layer of the powder in the craters, and the controller (<NUM>) is adapted to increase energy input of the energy beam into the second layer of the powder at the locations corresponding to the craters in order to fuse the greater thickness of the second layer of the powder in the crater by increasing beam power of the energy beam, decreasing the scan rate of the energy beam, or both, in order to fuse the greater thickness of the second layer of the powder in the craters.