Patent Grant
11440130
Various additive manufacturing technologies have been developed. One such technology is Electron Beam Freeform Fabrication (EBF3). The EBF3 process can be utilized to build complex, near-net-shape parts requiring substantially less raw material and finish machining than traditional manufacturing methods. It is envisioned that EBF3 may be utilized to manufacture a wide range of components such as metallic aerostructures and spare parts, tools, or structural elements. These parts span a wide range of scale and material choices ranging from small, detailed aluminum parts to large, near-net-shape superalloy components for rocket motors.
EBF3 devices and processes are described in U.S. Pat. Nos. 8,452,073 and 8,344,281, and U.S. Patent Publication Nos. 2015/0258626, 2010/0270274, and 2010/0260410. However, known additive manufacturing processes may suffer from various drawbacks.
One aspect of the present disclosure is a method of controlling operation of an electron beam gun and wire feeder during a deposition process in which pools of molten matter (e.g. metal) are deposited onto a substrate. Beads upon solidification of the molten matter. The method includes providing a substrate and a wire source. A molten pool of liquid phase metal is formed on the substrate by melting wire utilizing an electron beam generated by an electron beam gun. The liquid metal solidifies into a solid phase. A sensor is utilized to generate data related to at least one of a thermal transient, one or more alloy physical properties and/or melting range, a width and/or shape of the molten pool, tracking of the wire and/or the molten pool and flaws in the solidified metal. A process parameter is adjusted based, at least in part, on the data generated by the sensor. The sensor data may be generated at speeds of about 1 Hz or more or about 5 Hz or more and the process may be adjusted at speeds of about 10 Hz or more or about 20 Hz or more.
The method may include changing a raster pattern of the wire source and/or electron beam to deflect ahead, back, or to the side to spread heat. The raster pattern may be adjusted at speeds of about 10 Hz or more or about 20 Hz or more.
The method may also include detecting/tracking molten pool size and shape to detect increases and/or decreases in molten pool size, wherein the molten pool size comprises one or more of pool width, length, aspect ratio, area, centroid and reducing power of the electron beam gun to reduce molten pool size, or to detect decreases in molten pool size, and increasing power of the electron beam gun to increase molten pool size.
The process may include detecting changing thermal patterns that result in different bead geometries and/or microstructures and increasing or reducing power of the electron beam gun to increase or reduce molten pool size to correct bead geometries and/or microstructures.
The method may also include detecting a melting temperature of the molten matter (e.g. metal), and providing sufficient power, travel speed, and/or feed rate to melt the wire.
The method may include detecting dealloying in molten matter (e.g. metal), and reducing a power of the electron beam and/or increasing travel speed of the electron beam and/or changing beam focus to reduce the temperature of the molten matter (e.g. metal).
The method may include detecting viscosity of the molten matter (e.g. metal) and changing beam angle relative to the molten pool and/or wire entry angle relative to the molten pool.
The method may also include detecting a surface tension of the molten pool, and changing beam angle relative to the molten pool and/or wire entry angle relative to the molten pool to reduce surface tension.
The method may also include detecting excessive or deficient height and/or width of a molten pool at a start, and adjusting timing of one or more of a starting beam power, a travel speed, and a wire feed rate to reduce or increase height and/or width of the molten pool.
The method may also include detecting narrowing or widening of a solidified deposit, and adjusting one or more of a beam power, a beam raster, a travel speed, and a wire feed rate to increase or decrease the width of the solidified deposit.
The process may also include detecting variations in one or more of a deposit height, a width, and a shape at a stop location, and adjusting one more of a beam power, a travel speed and a wire feed rate to provide uniform height and/or width and/or shape.
The method may also include detecting variations in one or more of solidified deposit width and height due to abrupt changes in build direction, and adjusting one or more of a beam power, a beam raster, a travel speed, and a wire feed rate to reduce variations in at least one of a deposit width and a height.
The process may also include detecting incomplete wire melting, and providing beam focus and deflection up the wire to preheat the wire and/or preheating the wire via a separate heater, and/or reducing a wire feed rate and/or switching to a smaller diameter wire.
The process may also include detecting dripping of melted wire, and increasing a wire feed rate and/or decreasing a beam power and/or increasing a travel speed of the wire source and/or changing (e.g. decreasing) a wire feed height distance.
The process may also include detecting stabbing and/or wire dragging, and decreasing a wire feed rate and/or increasing a beam power and/or decreasing a travel speed of the wire feeder and/or changing (e.g. increasing) a wire feed height distance.
The process may include detecting one or more of residual stress, curvature, or twist in the wire as it is fed into the molten pool, and providing beam deflection up the wire to redirect the wire into the molten pool.
The process may also include detecting if the wire does not enter the molten pool at a desired (correct) location, and adjusting a position of the wire feeder relative to the molten pool and/or halting the process if wire is detected outside the molten pool.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in
A first EBF3 machine 1A is shown in
The operational concept of the EBF3 process is to build a near-net-shape metal part directly from a CAD file in a layer-additive manner without the need for molds or tooling dies. In this process, an electron gun 5A, 5B, 5C generates an electron beam 6A, 6B, 6C, respectively that is used as a heat source to create a small molten pool on a substrate (e.g. baseplates 8A, 8B, 8C, respectively) into which wire 10A, 10B, 10C from a wire feeder 12A, 12B, 12C, respectively is fed. The substrate (baseplate) can be composed of identical or different material than the deposited metal depending upon the application. The electron beam 6A, 6B, 6C and wire feed assembly 12A, 12B, 12C, respectively are translated with respect to the baseplate 8A, 8B, 8C, respectively to follow a predetermined tool path, similar to conventional computer-aided machining practice. The deposited material solidifies immediately after the electron beam 6A, 6B, 6C has passed, having sufficient structural strength to support itself. This process is repeated in a layer-wise fashion, resulting in a near-net-shape part requiring minimal finish machining. Careful attention to process control minimizes transient thermal effects, such as distortion, until the part cools and the part can be removed from the vacuum chamber 4A, 4B, or 4C.
EBF3 offers several unique attributes among the competing metallic additive manufacturing approaches. First, it is scalable in size and deposition rate, allowing fabrication of parts measuring a few cubic centimeters up to parts 1 cubic meter or larger, restricted only by the size of the vacuum chamber. High deposition rates with larger bead sizes enable near-net-shape construction of large structures using established aerospace alloys such as aluminum 2219 and 6061, stainless steel 304 and 316, Inconel® 625 and 718, and titanium Ti-6A1-4V. Second, EBF3 uses wire feedstock as opposed to metal powder for high feedstock usage efficiencies and safe operability in reduced gravity environments such as low-Earth orbit. Third, the electron beam used to melt the metal wire requires that the process be conducted in a vacuum chamber; vacuum enables processing of traditional aerospace alloys, most of which cannot be melted in air without oxidation degrading their metallurgical properties. Finally, EBF3 offers the promise of fabrication of structures with functionally graded metallurgy. By selectively introducing wires of different compositions during the deposition process, the chemistry and resulting physical properties of the metal structure can be varied continuously throughout the part and tailored to its exact service requirements.
While the basic operational concept of EBF3 is simple, a closer look reveals dozens of variables, each affecting the outcome and many of which are interdependent. As discussed in more detail below, there are several major challenges to the acquisition and use of real-time sensor data to monitor and control the process. One aspect of the present disclosure is the use of imaging techniques and data analyses to detect anomalies and refine the EBF3 process through real-time monitoring and control to mitigate these challenges.
The first major challenge to sensor integration into the EBF3 process is the deposition environment itself. Efficient generation and transmission of the electron beam requires high vacuum (10−5 Torr or lower), meaning that sensor electronics in the chamber must (preferably) be vacuum-rated. This implies little or no permissible outgassing from electronics or packaging and the ability to run uncooled or with conductive and radiative cooling only. Unlike vacuum processes such as electron microscopy, EBF3 is a relatively “dirty” process that results in metal vapor contamination of line-of-sight surfaces inside the vacuum chamber. Unprotected optical surfaces such as lenses or mirrors will become coated with metal vapor that will periodically need to be removed. Translation of the part and/or electron beam gun during part fabrication results in a moving molten pool that is difficult to track unless sensors are incorporated into the electron beam gun carriage. The size of the electron beam gun and the gun-to-work distance also place practical limits on the size and placement of cameras or other sensors.
Second, EBF3 is an inherently thermally transient process. Several factors contribute to this, beginning with its layer-additive nature. The continuously moving molten pool over a relatively cold baseplate or previous layer encounters a different cooling path and thermal mass with each successive layer. Variations in the geometry of the part such as section thickness changes, ends or corners also contribute to these changes. Thermal transients need to be better managed to enable repeatability necessary for certification of additive manufacturing processes for use in many applications. Therefore, sensors capable of measuring thermal distributions are important for tracking changes in the molten pool (by detecting the thermal gradients between liquid and solid phases) over time.
Third, the physical properties of each metal alloy (e.g., specific heat, melting temperature, thermal conductivity, vapor pressure) necessitate different process parameters such as beam power, beam raster, wire feed rate and translation speed. Factors of 2×-10× difference depending upon the material and part geometry are not uncommon, often requiring equally wideband sensors. Less obvious are process accommodations arising from material properties such as molten pool surface tension and liquid metal viscosity, which affect the shape and behavior of the molten pool during deposition. Differing vapor pressures of various alloying additions can result in selective de-alloying (constituent loss) in the deposited material. Controlling the temperature of the EBF3 process has been shown to impact alloy compositional losses. The temperature-dependent variation of emissivity also affects thermal imaging, requiring unique sensor calibrations and camera settings to optimize the imaging. While emissivity is well documented for many pure metals and common engineering alloys, it is not generally known for functionally graded materials deposited with EBF3. Since it is of interest to use EBF3 to deposit materials with widely varying physical properties, wideband sensors capable of detecting the thermal environment for low melting temperature alloys like aluminum up to high melting temperature alloys like nickel are more versatile.
The fourth challenge involves geometric variations in the deposit that result from imperfect coordination of heat and mass flow (electron beam power and wire feed) during starts, stops and abrupt changes in build direction. These variations include bulbs (build-up of excess material at starts), necking (narrowing of the deposit) or tailing-off (deficit in deposit height approaching a stop). Although bead widths are affected with each of these variations, the more significant impact is on the height of the deposited material. Since the EBF3 process is a layer-additive process, even minute perturbations in height accumulate and cause large errors over time as the build progresses. Even parts measuring as small as 10 cm in height can represent over 100 layers. Therefore, the sensor type and its location in the build chamber preferably provide precise measurement of the bead height.
The closely related fifth challenge involves the relative direction of wire in-feed and deposit direction to the geometric variations in the deposit. Changes in the wire orientation with respect to the translation direction (e.g., wire lagging or pushing the molten pool, or entering from the side) and wire elevation (e.g., entry angle relative to the horizontal) can subtly change the geometry of the deposit. Equally important, fabrication of some parts may require multiple side-by-side beads to develop the required section width, and the shape of the molten pool depends upon the presence and relative location of adjacent beads. For example, for multi-bead deposits it is often easier to “push into” adjacent beads than to reach over them but this is not always possible due to other constraints. If the width of the bead and the influence of the wire on the molten pool can be measured in real-time, minor adjustments to processing parameters such as wire feed rate and translation speed can be implemented to improve part precision.
Sixth, random process errors result from variability in the wire feed. The wire is primarily at room temperature until it crosses into the electron beam path, at which point it is subjected to an abrupt thermal gradient, melting over a very short distance as it enters the molten pool. Larger diameter wire, therefore, retains substantial stiffness as it enters the molten pool, potentially resulting in several errors. In the event of excess heat, insufficient wire feed, or wire feeding above the electron beam/substrate intersection point, dripping may occur. Conversely, insufficient heat, excess wire feed, or wire feeding below the electron beam/substrate intersection point may result in “wire stabbing” which can cause the wire to oscillate back and forth in the molten pool; skip, causing fluctuations in the deposit height; or deflect off the bottom of the molten pool and divert out of the beam altogether. Improper wire location or poor timing of the start/stop sequence will often result in wire sticking through the welds. Some errors of this nature have also been observed due to cast (residual curvature or twist in the wire not removed by the wire straightener) or simple mechanical misalignment of the wire feed apparatus that was not immediately apparent. To measure wire feed anomalies, it may be beneficial to track wire position relative to the molten pool. This may require a sensor with a wider field of view and the ability to measure geometric features that have widely different temperatures.
A seventh major challenge is encapsulation of flaws within the EBF3 deposits. Voids and porosity have been observed due to contamination (cleanliness) of the wire, wire variability, or programming errors in the steps between side-by-side beads and layers. Inclusions can occur when the molten pool picks up a contaminant from contaminated wire or from metal vapor condensate flaking off and falling into the deposit. Lack of fusion between layers can occur due to insufficient heating before or during the deposition steps, improper programming of the layer height, or oxidized surfaces when the part has been exposed to air prior to initiating or resuming interrupted EBF3 deposition. Similar to welding operations, hot cracking or quench cracking can occur due to high thermal gradients, insufficient preheat or microsegregation that occurs with rapid cooling. Material property variations can also occur due to microstructural or chemical variations. For example some alloys will form strong textural orientation due to preferential crystallographic freezing that follows the cooling path. Finally, large thermal gradients and thermal contraction can result in residual stresses that will manifest as distortion, or in the extreme case, cracking. These defects and the mechanisms to control and avoid their formation are somewhat understood.
The ability to detect, quantify and repair flaws that occur during the EBF3 deposition process is beneficial. Evaluating thermal diffusivity using thermal imaging cameras is one way to detect embedded flaws during deposition.
The solution to these challenges resides in integration of imaging systems into the EBF3 systems to monitor the process. Image analyses to identify the anomalies and control logic to implement corrective action are then required to fully close the processing control loop.
Internal Mount CMOS Camera
Initial NIR imaging approaches to monitoring the molten pool during the EBF3 process were investigated from 2003 to 2005. The first camera selected was from Silicon Imaging, model SI-1280. This is a 12-bit, digital Complementary Metal-Oxide Semiconductor (CMOS) camera, which has particular advantages for the EBF3 process environment. Data is digitized in the camera reducing the susceptibility of signals to noise in transmission lines. CMOS cameras directly measure the current in the detector so there is no charge build-up in the pixel wells as in a Charged Coupled Device (CCD). Therefore CMOS cameras will not bloom or streak when overexposed. For example, if the electron beam energy density is too high, the resulting plasma would “wash out” a normal CCD camera, preventing viewing of the molten pool. A CMOS sensor will simply “peg out” a pixel at its maximum value, leaving adjacent pixels unaffected. The exposure was controlled through a LabView Camera Link interface. This camera has a 1.28 megapixel chip, but can be operated with a smaller region of interest at very high speeds. At 100×100 pixels, frame rates of 3000 fps can be achieved.
The CMOS camera consumes very low power, producing less heat and reducing the need for cooling in the vacuum environment. A water-cooled coldplate was installed against the camera body for precautionary measures, but the camera also operated for short periods of time without overheating in the vacuum environment without the coldplate. The compact design of this camera (45×52×50 mm) enabled installation on a bracket directly on the electron beam gun. Pinhole and secondary optics were installed with a flow controller to meter argon into the camera/pinhole system to protect the optics from being occluded by metal vapor. This optical camera was bandpass filtered down to the NIR range and calibrated to provide thermal information.
Fiber Optic+External Mount CCD Camera
Although the CMOS camera has high speed data acquisition, good compatibility with the vacuum environment, and resistance to blooming of oversaturated pixels to adjacent pixels, the resolution and low light capabilities were inferior to CCD cameras in the 2003 to 2005 time period. For comparison, the second camera evaluated at that time was a Hamamatsu C8484-05 12-bit CCD 14 (
The Hamamatsu camera 14 was too large to mount on the electron beam gun and was not well adapted to the vacuum environment. With reference to
Backscattered Electron (BSE) Detector
From mid-2008 to mid-2009 an effort was made to develop an electron imaging system that could be used to actively monitor and measure the EBF3 deposit height and wire position relative to the beam. Additional objectives of this work were to explore the capabilities of the electron beam gun system to raster the beam at relatively high frequency in specific patterns to preferentially direct energy to the wire or the molten pool, and to detect the onset of process errors due to wire feed errors or misalignment.
Normal operation of the EBF3 electron beam generates large quantities of primary electrons, secondary electrons, x-rays, neutral particles, ions and backscattered electrons, all with a wide range of energies. Of these possible signal sources, backscattered electrons (BSEs) were chosen because of their directionality and high retained energy fraction. A custom-built BSE detector 34 (
Several different mounting arrangements for the detector 34 were tried with varying degrees of success for producing an unobstructed view of the EBF3 process zone without exposing the sensor to metal vapor. The best viewing angle was ultimately obtained with the detector mounted directly in line with the wire feed nozzle 32 (
Some high quality images were obtained with the BSE detector 34; however, these images were obtained with very low beam current (1-5 mA) and a raster scan pattern specifically designed for imaging, not deposition. In other words, images of this type could not be obtained using the beam parameters in real time during an actual build; imaging scans would have to be conducted separate from the build process. Also, during the build process the beam path is confined to the molten pool and not rastered outside a very small target area. It was possible to infer, but not directly measure, the deposit height with a single BSE detector. The efforts to monitor wire position and preferentially direct beam energy into the wire or the molten pool with the BSE detector were quite successful, and revealed that some information as to geometric error conditions (dripping, wire feeding off-center to the molten pool) could easily be detected.
Despite these promising results, BSE imaging was ultimately abandoned as a means to provide information to the closed-loop control system. This was mainly due to its relatively high cost and the bulkiness of the detector system. In addition, the BSE detector required modification of the electron beam rastering and focusing coils to coordinate the beam location with the sensors in the detector. As a research and development tool, the BSE detector was useful, but challenges with obtaining images during beam power and raster patterns typical for deposition, and the ability to image but not measure temperatures led to the decision to explore other sensors.
Internal Mount CCD Camera
It was learned from the experiences described above that camera location may be important for observation of the molten pool sufficient to provide data usable for closed-loop control of the process.
With reference to
The NIR band was selected to image metals of higher melting temperatures such as 316 stainless steel, Ti-6A1-4V, and Inconel® 625. The Prosilica GC1380H camera 42 is a non-cooled CCD camera with a frame rate of 30 Hz at full resolution through a GigE interface. A water-cooled coldplate 44 was installed against the camera body 36A to provide cooling in the vacuum environment of vacuum chamber 4D. The camera pixel array size is 1360×1024 (pixel pitch size of 6.45 μm×6.45 μm). The dynamic range of camera 42 is 12 bits. The imaging spectral band may be set at 0.875 to 1.05 μm using a long pass filter to allow imaging temperatures from 700 to approximately 2,200° C. An NIR neutral density filter may be used to reduce the transmission by a factor of 10. The C-mount optical package contains a 150-mm relay lens pair, a protective window (B270 material), and an extension tube with a gas fitting 46 (
A radiometric calibration may be utilized to convert the pixel values (intensity counts) to temperature, define the temperature imaging limits of the system, determine the optimal threshold values for closed loop control metrics (based on the wire melting temperature), correct for material dependent emissivity, and selection of the optimal integration time. A radiometric characterization was performed on the NIR camera/optic 42 using a calibrated blackbody radiation source set at various temperatures. The process involves the calibration of the radiance counts to actual temperature values. The temperature values used were 700, 800, 900, 1,000 and 1,100° C. at specified sensor integration times ranging from 10 to 50,000 μs. Using a given emissivity value, the radiance is then converted to temperature using a linear interpolation of the system's radiance response.
Internal Mount Short-Wave IR (SWIR) Camera
Longer wavelength cameras may be utilized for imaging lower melting temperature alloys (e.g. aluminum). With reference to
The SWIR band may be utilized to image metals of lower melting temperatures such as aluminum. The digital SWIR camera 50 may comprise an Allied Goldeye G-032. This camera requires temperature stabilization using an internal thermoelectric cooler. The camera pixel array size is 636×508 (pixel pitch size of 25 μm×25 μm). The camera's dynamic range is 14 bits with a maximum frame rate of 100 Hz at full resolution through a GigE interface. The spectral response of the camera's detector is approximately 0.950-1.7 μm and this allowed measurement of temperatures from about 300° C. to about a 1000° C. The camera 50 may be protected within an aluminum enclosure that is liquid cooled. The camera's optic is a C-mount 50 mm lens with 75% or better transmission between 0.700 μm to 1.9 μm. A protective window comprising a Mylar® polyester film 62 may be used to protect the optical system from metal vapor resulting from the electron beam deposition process. Camera resolution is approximately 0.015 cm/pixel. With a molten pool size of 0.2 cm to 0.6 cm, this camera resolution results in the regions of interest being captured by 10's of pixels across.
A radiometric characterization may be performed using a calibrated blackbody radiation source set at various temperatures for the SWIR camera 50. The process involves the calibration of the radiance counts to actual temperature values. The temperature values used were 300, 400, 500, 550, and 600° C. at specified sensor integration times ranging from 120 to 15,000 μs.
A SWIR temperature image and line plot collected during the deposition of aluminum 2219 are shown in
The vacuum, thermal, metal vapor environment and limited space in the EBF3 process pose challenges for sensor selection and integration into the system for real-time process monitoring and feedback. Since EBF3 is inherently a thermally transient process, thermal data are of interest to correlate with microstructural and non-destructive evaluations to identify metallurgical features and flaws that drive the mechanical properties of the deposited materials. Due to space constraints, multiple sensors may not be feasible, so sensors that can capture all of the suitable data are sought. Suitability of a particular imaging technology for use in the EBF3 process is also a concern. Suitability may be determined by consideration of the available frame rate, resolution, signal-to-noise ratio, spectral response, vacuum compatibility, ruggedness, size, and cost.
If these various defects are detected by sensors such as cameras 14 and/or 24, the wire feed rate, angle α1 and/or θ1 or θ2 (
During operation, the wire feeder moves in the direction X which is generally parallel to the base plate 8 at a height “HW” (
In general, defects caused by excessive heat can be corrected by reducing the energy of electron gun 5/electron beam 6 and/or by raising the rate of movement of wire feeder 12 in the direction X and/or by increasing the wire feed rate. Conversely, defects resulting from insufficient heat can be corrected by reducing the power of electron gun 5/electron beam 6 and/or reducing the movement of the wire feeder 12 in the direction X and/or reducing the wire feed rate.
Table 1 (below) summarizes qualitatively how well the five sensors capture data to address the EBF3 process challenges.
Various issues associated with the deposition environment can be corrected as shown in Table 2.
The Process Challenges and Corrective Actions of Table 2 are believed to be straightforward, such that a detailed description is not necessary.
With reference to Table 3, various issues related to thermal transients can be detected/measured and corrected as shown.
In an Electron Beam Freeform Fabrication (EBF3) process according to one aspect of the present disclosure, an electron beam 6 (
EBF3 is an inherently thermally transient process. Several factors contribute to this, beginning with its layer-additive nature. The continuously moving molten pool 64 over a relatively cold baseplate 8 or previous layer encounters a different cooling path and thermal mass with each successive layer. Variations in the geometry of the part such as section thickness changes, ends or corners also contribute to these changes. Thermal transients can be taken into account by controller 72 to enable repeatability necessary for certification of additive manufacturing processes for use in many applications. Sensors (e.g. cameras 14, 24) capable of detecting/measuring thermal distributions may be utilized to detect/track changes in the molten pool 64 by detecting the thermal gradients between liquid and solid phases over time.
Thermal gradients from deposition onto a cold substrate 8 can cause rapid fluxuations in the molten pool 64, resulting in changing width W (
Table 4 is a summary of various issues related to thermal properties and associated corrective actions.
Differing vapor pressures of the alloying additions can result in selective de-alloying (constituent loss) in the deposited material (e.g. deposit 66,
Tables 5, 6, and 7 summarize issues associated with height/width measurement, wire entry, and wire anomalies, respectively.
As shown in Table 6, various issues associated with the wire entry direction can be corrected as shown.
Geometric variations in the deposit 66 (
Imaging may be accomplished with one or more near-infrared, infrared, or band-pass filtered optical cameras (14, 24) positioned at an angle with respect to the molten pool 64 to allow observation of bead height H. Most angles will work, as long as the camera (14, 24) is not oriented straight down on the molten pool 64 (looking along the axis of the electron beam 6). The geometry of the location of the camera is typically fixed, so the vertical and horizontal location of the molten pool 64 relative to the wire feeder 12 and substrate 8 can be determined with simple trigonometry. Identification of the location of the molten pool 64 may be determined by calculating the centroid and comparing it to the wire location. As the centroid moves closer to the wire location, the height H is increasing, and as the centroid moves farther away from the wire location, the height H is decreasing. This height measurement can then be used by controller 72 to increase or decrease the wire feed rate to raise (increased wire feed rate) or lower (decreased wire feed rate) the height H of the deposited bead 63. Furthermore, the wire feed rate can be synchronized with the translation speed, using the X and Y speeds from feedback on the X and Y translation stage motors, to automatically correct the wire feed rate for speed changes at starts, stops, and changes in translation direction. The measured height H would further increase or decrease the wire feed rate to maintain a constant height H across the entire layer on the part. If the wire feed rate cannot be synchronized to the translation speed, corrections based solely on height H will also work (i.e. the wire feed rate does not necessarily need to be synchronized with the translation speed).
Control of the EBF3 process also includes consideration of the relative direction of wire in-feed and deposit direction to the geometric variations in the deposit 66. Changes in the orientation of wire 13 with respect to the translation direction (e.g., wire lagging or pushing the molten pool 64, or entering from the side) and wire elevation (e.g., entry angle α, relative to the horizontal) can subtly change the geometry of the deposit. Also, parts may require multiple side-by-side beads 63 to develop the required section width, and the shape of the molten pool 64 depends upon the presence and relative location of adjacent beads 63. For example, for multi-bead deposits it is often easier to “push into” adjacent beads than to reach over them but this is not always possible due to other constraints. Use of cameras (such as optical, band-pass filtered optical, near-infrared, infrared) to measure the width W of the bead 63 and the direction of travel can be used to monitor the effects of relative direction of wire in-feed into the molten pool 64. The control system 72 is programmed to make minor adjustments based on the direction of the wire feed into the molten pool 64 as follows: If the wire 13 is feeding into the trailing edge of the molten pool 64 or into the side of the molten pool 64, the beam power may be increased slightly (1 milliamp or 1 kilovolt), the electron beam deflection may be changed to deflect the beam 6A onto the wire 13 to preheat the wire 13 before it enters the molten pool 64, the translation speed may be decreased by 1-2 inches per minute, the wire feed rate can be decreased by 5-10 inches per minute, or the wire feed height distance HW (
Random process errors result from variability in the wire feed. The wire 13 is generally at or near room temperature until it crosses into the electron beam path, at which point it is subjected to an abrupt thermal gradient, melting over a very short distance as it enters the molten pool 64. Larger diameter wire, therefore, retains substantial stiffness as it enters the molten pool 64, which may, result in various errors or defects. In the event of excess heat, insufficient wire feed, or wire feeding above the electron beam/substrate intersection point, dripping may occur. Conversely, insufficient heat, excess wire feed, or wire feeding below the electron beam/substrate intersection point may result in wire stabbing which can cause the wire 13 to oscillate back and forth in the molten pool 64; skip, causing fluctuations in the deposit height H; or deflect off the bottom of the molten pool 64 and divert out of the beam 6 altogether.
Improper wire location or poor timing of the start/stop sequence may result in wire sticks. Some errors of this nature may occur due to cast (residual curvature or twist in the wire 13 not removed by the wire straightener) or simple mechanical misalignment of the wire feed apparatus 12 that may not be immediately apparent. To measure wire feed anomalies, an important feature to track is the position of wire 13 relative to the molten pool 64. This may be accomplished with optical, near infrared, infrared, or band-pass filtered optical cameras (14, 24), where the camera is focused on the molten pool 64 and wire entry into the molten pool 64. Because the wire 13 is entering the electron beam 6 above the surface of the molten pool 64, it appears as a step feature in the molten pool images, when viewed from most angles (except for within about 15 degrees of straight down on the molten pool 64). The wire location (either the edges of the wire or the calculated centroid) may be tracked relative to the molten pool 64 (again, using either the edges of the molten pool 64 or the calculated centroid). The wire location preferably remains in a constant relationship to the molten pool 64 (i.e. they line up on the same centerline at a constant distance apart). Corrective actions may be implemented by controller 72 if the wire location changes with respect to the molten pool location by more than a preset threshold amount.
At higher wire feed rates, incomplete wire melting or “chopped” wire bits transmitted into molten pool 64 or bits of wire 70 (
As shown in Table 8 below, various issues associated with miscellaneous factors can be corrected as shown.
The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein.
All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.
All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. Each range disclosed herein constitutes a disclosure of any point or sub-range lying within the disclosed range.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. As also used herein, the term “combinations thereof” includes combinations having at least one of the associated listed items, wherein the combination can further include additional, like non-listed items. Further, the terms “first,” “second,” and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity).
Reference throughout the specification to “another embodiment”, “an embodiment”, “exemplary embodiments”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and can or cannot be present in other embodiments. In addition, it is to be understood that the described elements can be combined in any suitable manner in the various embodiments and are not limited to the specific combination in which they are discussed.
This patent application claims the benefit of and priority to U.S. Provisional Application No. 62/323,355, filed on Apr. 15, 2016, titled “APPARATUS AND METHOD FOR DETECTING FLAWS AND FEATURES FOR PROCESS CONTROL OF ELECTRON BEAM WIRE ADDITIVE MANUFACTURING,” the entire contents of which is hereby incorporated by reference in its entirety.
The invention described herein was made in the performance of work under a NASA contract and by employees of the United States Government and is subject to the provisions of the National Aeronautics and Space Act, Public Law 111-314, § 3 (124 Stat. 3330, 51 U.S.C. Chapter 201), and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6122564 | Koch | Sep 2000 | A |
| 6627850 | Koga | Sep 2003 | B1 |
| 8344281 | Taminger et al. | Jan 2013 | B2 |
| 8452073 | Taminger et al. | May 2013 | B2 |
| 20050173380 | Carbone | Aug 2005 | A1 |
| 20100260410 | Taminger | Oct 2010 | A1 |
| 20140124483 | Henn | May 2014 | A1 |
| 20160144452 | Liou | May 2016 | A1 |
| 20170259373 | Albert | Sep 2017 | A1 |
| Entry |
|---|
| Karen M. Taminger et al., In-Process Thermal imaging of the ElectronBeam Freeform Fabrication Process, Proceedings of SPIE, vol. 9861, 986102-11 (11 pages), 2016. |
| Number | Date | Country | |
|---|---|---|---|
| 20170297140 A1 | Oct 2017 | US |
| Number | Date | Country | |
|---|---|---|---|
| 62323355 | Apr 2016 | US |
