Method for emissions plume monitoring in additive manufacturing

A method of monitoring an additive manufacturing process in which one or more energy beams are used to selectively fuse a powder to form a workpiece, in the presence of one or more plumes generated by interaction of the one or more energy beams with the powder. The method includes using at least one sensor to generate at least one signal representative of a trajectory of one or more of the plumes.

BACKGROUND OF THE INVENTION

This invention relates generally to additive manufacturing, and more particularly to apparatus and methods for monitoring an emissions plume in additive manufacturing.

Additive manufacturing is a process in which material is built up layer-by-layer to form a component. 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 radiant energy beam to form a workpiece. The build chamber is enclosed in a housing that typically includes provisions for a flow of shielding gas therein. The shielding gas is used to transfer heat away from the surface of the powder bed, to prevent vaporized powder from condensing on the surface of the workpiece, and to control undesired chemical reactions, such as oxidation.

In operation, the interaction of the radiant energy beam with the powder causes vaporization of the powder, generating a plume which originates in the vicinity of the melt pool and travels downstream, entrained in the shielding gas flow. In the immediate vicinity of a melt pool generated by the energy beam, the composition of the plume is mostly vaporized powder. At downstream locations, the vapor cools and condenses so that the plume comprises a mixture of gas and metallic particles (condensate).

The presence of the condensate can have detrimental effects on the build process, for example blockage of the energy beam, or a reduction in beam intensity. This effect can prevent rapid beam scanning or the use of multiple beams.

A problem with prior art additive manufacturing machines and processes is that the trajectory of the emissions plume is not tracked or known.

BRIEF DESCRIPTION OF THE INVENTION

this problem is addressed by a method of using one or more sensors to provide a real-time understanding of the emissions plume trajectories in an additive manufacturing process.

According to one aspect of the technology described herein, a method is provided of monitoring an additive manufacturing process in which one or more energy beams are used to selectively fuse a powder to form a workpiece, in the presence of one or more plumes generated by interaction of the one or more energy beams with the powder. The method includes using at least one sensor to generate at least one signal representative of a trajectory of one or more of the plumes.

According to another aspect of the technology described herein, a method of making a workpiece includes: depositing a powdered material in a build chamber disposed in a housing, while using a gas flow apparatus coupled in fluid communication with the housing to provide a gas flow over the powder; in the presence of the gas flow, directing one or more energy beams to selectively fuse the powdered material in a pattern corresponding to a cross-sectional layer of the workpiece, wherein interaction of the one or more energy beams with the powdered material generates one or more plumes entrained in the gas flow; using at least one sensor to generate at least one signal representative of a trajectory of one or more of the plumes; and controlling at least one aspect of the additive manufacturing process in response to the at least one signal.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,FIG. 1illustrates schematically an additive manufacturing machine10suitable for carrying out an additive manufacturing method. Basic components of the machine10include a table12, a powder supply14, a recoater16, an overflow container18, a build platform20surrounded by a build chamber22, and at least one beam generator24, all surrounded by a housing26. Each of these components will be described in more detail below.

The table12is a rigid structure defining a planar worksurface28. The worksurface28is coplanar with and defines a virtual workplane. In the illustrated example it includes a build opening30communicating with the build chamber22and exposing the build platform20, a supply opening32communicating with the powder supply14, and an overflow opening34communicating with the overflow container18.

The recoater16is a rigid, laterally-elongated structure that lies on the worksurface28. It is connected to an actuator36operable to selectively move the recoater16along the worksurface28. The actuator36is depicted schematically inFIG. 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 supply14comprises a supply container38underlying and communicating with the supply opening32, and an elevator40. The elevator40is a plate-like structure that is vertically slidable within the supply container38. It is connected to an actuator42operable to selectively move the elevator40up or down. The actuator42is depicted schematically inFIG. 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 elevator40is lowered, a supply of powder44of a desired composition (for example, metallic, polymeric, ceramic, and/or organic powder) may be loaded into the supply container38. When the elevator40is raised, it exposes the powder44above the worksurface28. Other types of powder supplies may be used; for example powder may be dropped into the build chamber22by an overhead device (not shown).

The build platform20is a plate-like structure that is vertically slidable below the build opening30. It is connected to an actuator46operable to selectively move the build platform20up or down. The actuator46is depicted schematically inFIG. 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 platform20is lowered into the build chamber22during a build process, the build chamber22and the build platform20collectively surround and support a mass of powder44along 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 container18underlies and communicates with the overflow opening34, and serves as a repository for excess powder44.

The apparatus10incorporates at least one beam generator24operable to generate an energy beam and direct it as desired. As will be explained in more detail below, multiple beam generators24may be provided and used simultaneously in order to increase this production speed of the apparatus10. In the illustrated example, two beam generators24are shown.

Each beam generator24includes a directed energy source48and a beam steering apparatus50. The directed energy source48may comprise any device operable to generate a beam of suitable power and other operating characteristics to melt and fuse the powder44during the build process, described in more detail below. For example, the directed energy source48may be a laser. Other directed-energy sources such as electron beam guns are suitable alternatives to a laser.

The beam steering apparatus50may include one or more mirrors, prisms, and/or lenses and provided with suitable actuators, and arranged so that a beam from the directed energy source48can be focused to a desired spot size and steered to a desired position in plane coincident with the worksurface28. 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 may be referred to herein as a “build beam”.

In the illustrated example, one of the beam generators24is operable to generate a first build beam54, and the other of the beam generators24is operable to generate a second build beam56.

An exemplary basic build process for a workpiece25using the apparatus described above is as follows. The build platform20is moved to an initial high position. The build platform20is lowered below the worksurface28by a selected layer increment. The layer increment affects the speed of the additive manufacturing process and the resolution of the workpiece25. As an example, the layer increment may be about 10 to 50 micrometers (0.0003 to 0.002 in.). Powder44is then deposited over the build platform20for example, the elevator40of the supply container38may be raised to push powder through the supply opening32, exposing it above the worksurface28. The recoater16is moved across the worksurface to spread the raised powder44horizontally over the build platform20. Any excess powder44drops through the overflow opening34into the overflow container18as the recoater16passes from left to right. Subsequently, the recoater16may be moved back to a starting position. The leveled powder44may be referred to as a “build layer” and the exposed upper surface thereof may be referred to as a “build surface”, designated45.

One or more of the beam generators24are used to melt a two-dimensional cross-section or layer of the workpiece25being built. Within the beam generator24, the directed energy source48emits a beam and the beam steering apparatus50is used to steer a focal spot of the build beam over the exposed powder surface in an appropriate pattern. A small portion of exposed layer of the powder44surrounding the focal spot, referred to herein as a “melt pool” is heated by the build beam to a temperature allowing it to sinter or melt, flow, and consolidate. This step may be referred to as “fusing” the powder44. As an example, the melt pool may be on the order of 100 micrometers (0.004 in.) wide. In the illustrated example using two beam generators24, the first build beam54generates a first melt pool58and the second build beam56generates a second melt pool60.

The build platform20is moved vertically downward by the layer increment, and another layer of powder44is applied in a similar thickness. The beam generators24again emit build beams54,56and the beam steering apparatus50is used to steer the focal spots of the build beams54,56over the exposed powder surface in an appropriate pattern. The exposed layer of the powder44is heated by the build beams54,56to a temperature allowing it to fuse as described above, and consolidate both within the top layer and with the lower, previously-solidified layer.

This cycle of moving the build platform20, applying powder44, and then directed energy fusing the powder44is repeated until the entire workpiece25is complete.

The machine10and its operation are as representative example of a “powder bed machine”. It will be understood that the principles described here are applicable to other configurations of powder bed machines.

The housing26serves to isolate and protect the other components of the machine10. During the build process described above, the housing26is provided with a flow of an appropriate shielding gas which, among other functions, excludes oxygen from the build environment. To provide this flow the machine10may be coupled to a gas flow apparatus62, seen inFIG. 2. The exemplary gas flow apparatus62includes, in serial fluid flow communication, a variable-speed fan64, a filter66, an inlet duct68communicating with the housing26, and a return duct70communicating with the housing26. All of the components of the gas flow apparatus62are interconnected with suitable ducting and define a gas flow circuit in combination with the housing26.

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 generator72may be connected to the gas flow apparatus62. Alternatively, the gas could be supplied using one or more pressurized cylinders74.

Once the gas flow apparatus62and machine10are initially purged with gas, the fan64is used to recirculate the gas through the gas flow circuit in a substantially closed loop, so as to maintain the positive pressure described above, with additional added makeup gas added as needed. Increasing the fan speed increases the velocity and flow rate of gas in the gas flow circuit; conversely, decreasing the fan speed decreases the velocity and flow rate of gas in the gas flow circuit. As an alternative to recirculation, the gas flow apparatus62could operate in a total loss mode; for example instead of the gas flowing through the return duct70and back to the fan64, it could simply be vented to atmosphere after passing over the build chamber22. In the illustrated example, the thermal mass of the gas provides a heat transfer function, however an optional heat exchanger (not shown) could be incorporated into the gas flow apparatus62.

The inlet duct68is positioned near the bottom of the housing26. During operation it provides a stream or flow of gas (see arrow76). As seen inFIG. 1, the inlet duct68has an elongated shape (for example rectangular) and discharges gas across the width of the build chamber22. For reference purposes the width of the build chamber22may be considered parallel to the “X” direction. As shown inFIG. 3, the edge of the build chamber22closest to the upper inlet duct68is referred to as a “leading edge”78, and the opposite parallel edge is referred to as a “trailing edge”80. For reference purposes the length of the build chamber (i.e. distance from leading edge78to trailing edge80) may be considered parallel to the “Y” direction.

The gas flow76has two functions. First, it is used to effect heat transfer and carry heat away from the surface of the uppermost built layer within the build chamber22. Second, during the build process, some of the powder44is vaporized. This vapor can cool and condense on the surface of the workpiece25, in turn causing an undesirable surface roughness or “recast” layer. Part of the gas flow76is used to carry away the vapors and/or condensate.

In operation, the interaction of the build beams54,56with the powder44causes heating and vaporization of the powder44. As shown inFIG. 4, this generates first and second “plumes”82,84respectively which originate in the vicinity of the melt pools58,60and travel downstream, entrained in the gas flow76. In the immediate vicinity of the melt pools58,60the composition of the plumes82,84respectively is mostly vaporized powder. At downstream locations, the vapor can cool and condense so that the plumes82,84comprise a mixture of gas and metallic particles.

To enable the monitoring techniques described below, it is desirable to quantify the behavior of the plumes82,84. In particular, it is desirable to create a “plume map” describing the location and dimensions of each plume82,84in 3-D space for any given time, and the propagation of the plumes82,84over time. This process may also be described as determining the trajectory of the plumes82,84. For the purposes of convenient description, plume82will be used as an example with the understanding that the same methods may be used for plume84or for any additional plume, where multiple energy beams are used.

One possible method for creating a map of the plume82involves sensing the plume82. Any visualization technique capable of distinguishing the plume82from the gas flow76may be used for this purpose.

For example, an illumination source may be provided to illuminate the plume82in concert with one or more sensors to detect light scattered or reflected from the plume82. Nonlimiting examples of suitable illumination sources include: a laser operated at a low output wattage (such as the beam generators24); one or more additional dedicated low-power lasers or other energy beams (shown schematically at85inFIG. 5), a supplementary light-emitting diode (“LED”), or a chamber light in an appropriate wavelength (e.g. infrared or visible). Both backscatter and forward scatter sensing techniques may be used, and multiple images from multiple sensors may be combined to generate a 3-D plume map.

In the example shown inFIG. 5, an illumination source86(shown schematically) is provided at a fixed location within the housing26. Sensors88are provided within the housing26with a clear field of view of the build surface45. Each sensor88is sensitive to forward scattered light90or backward scattered light92. Alternatively or in addition to the illumination source86, a low-power energy beam85may be used to provide a source of light scattered from the plume82. As used herein, the term “low-power” refers to a beam intensity which is sufficient to produce a detectable scattered light signal but does not cause significant melting or fusing of the powder44. If a low-power energy beam85is used, it may be scanned across the build surface45in a fixed pattern, or it may be scanned in a method so as to track a predicted location of the plume82.

The sensors88are of a type and configured such that they can detect the forward scattered or backward scattered light90,92and in response produce a signal representative of the position of the plume82. For example, they may be imaging sensors, or a plurality of simpler sensors arranged in an X-Y array may be provided in order to provide positional reference. The pattern of signals from the sensors88is indicative of the location of the plume82.

The sensors88may be used to generate a plume map in real time as the build process proceeds. The information provided by the sensors88is useful in improving the performance and efficiency of the machine10.

For example, the plume map may be used to initially establish an adequate process capability for the machine10. Upon initial machine set up, a test build would be performed using a nominal set of operating parameters. The sensors88would be used to create a plume map as described above. The plume maps would then be analyzed, compared to predetermined performance limits, and/or compared to a specified trajectory, thereby confirming that the plume trajectory meets acceptable performance requirements and/or adheres to a predicted behavior. The plume maps generated during initial machine runs may be stored for future reference.

If the plume maps indicate unsatisfactory behavior, changes could be made to one or more process parameters to manipulate the plume behavior. The sensors88could be used again in a subsequent build to determine the effectiveness of the changes. A series of iterations may be performed until the plume performance meets operational requirements. Once this set of iterations is complete, subsequent builds could be performed in an open loop fashion, using the optimized set of operating parameters.

In establishing the initial process capability, it may be useful to characterize the plume82by modeling the plume82. This may be done for example, using a commercially available computational fluid dynamics (“CFD”) software package. The inputs to the software include, but are not limited to, the aerodynamic and thermal characteristics of the shielding gas flow76and the aerodynamic and thermal characteristics of the plume generation and propagation process. The inputs may take into consideration factors such as: air flow rates, energy beam wavelength, intensity, or focus, consolidated or unconsolidated powder material composition and physical characteristics, melt pool dimensions and thermal characteristics, the type of fusing process (e.g. heating, melting, or sintering), and the composition of the plume (e.g. gases/and/or metal alloys). The CFD software is then capable of producing as an output a plume map.

The plume map produced by the sensors88may be compared to the modeled plume map.

Once the initial machine capability is established as described above, the stored plume maps can serve as a baseline for a trend monitoring process. Generally stated, the monitoring process includes using the sensors88to monitor the plume82and then adjust one or more process parameters as necessary. As used herein, “process parameters” can refer to any controllable aspect of the machine10and/or the gas flow apparatus62.

The monitoring method may include establishing one or more predetermined limits for the plume behavior, such as a maximum permissible deviation from a baseline trajectory. These may be referred to as “plume trajectory limits”.

The monitoring method may include taking a discrete action in response to one or more plume trajectory limits being exceeded, such as providing a visual or audible alarm to a local or remote operator.

The monitoring method may include stopping the build process in response to one or more plume trajectory limits being exceeded. This is another example of a discrete action.

The monitoring method may include real-time control of one or more process parameters using 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 machine10and/or the gas flow apparatus62. Plume trajectory measurements may be measured and stored during several build cycles and compared between cycles. For example, changing trajectories could indicate clogging of filter66, or blockage in a portion of the inlet duct68. 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 machine10and gas flow apparatus62may 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.

The method described herein has several advantages over the prior art. In particular, it provides a detailed understanding of the emissions plumes in an additive manufacturing process. This detailed understanding will enable build strategies that avoid beam/plume interactions, optimize build speed, and/or monitor machine future behavior vs. documented baseline for trends and exceptions.

The foregoing has described an apparatus and method for emissions plume monitoring in 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.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to 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.