Patent ID: 12208465

DETAILED DESCRIPTION

Techniques disclosed herein relate generally to systems that receive and/or process optical signals. More specifically, techniques disclosed herein relate to the use of a dual polarization optical filter to improve the signal to noise ratio (SNR) of optical signals. In some instances, embodiments of the disclosure are particularly well suited for use with additive manufacturing systems because of the relatively high noise level and the stochastic nature of the optical signals, as described in more detail below.

For example, in some embodiments an additive manufacturing system employs a laser energy source to melt and fuse a work region of metallic powder. An optical sensor receives light emitted from the melt pool and laser plume. The received light can be stochastic in nature and may include both spurious signals as well as mixed polarizations. The received light is passed through a first partially transmissive polarization filter having a first polarization axis and through a second partially transmissive polarization having a second polarization axis, where the second axis is orthogonal to the first polarization axis.

In order to better appreciate the features and aspects of the present disclosure, further context for the disclosure is provided in the following section by discussing one particular implementation of an additive manufacturing system that includes a dual polarization filter according to embodiments of the disclosure. These embodiments are for explanatory purposes only and such filters can be employed in other systems and configurations. One of skill in the art with the benefit of this disclosure will appreciate that the dual polarizing filter is not limited to laser-based additive manufacturing processes. Other optical systems can employ similar techniques to improve the SNR of optical sensors including, but not limited to, optical communications systems, electron beam-based systems and UV curing-based systems.

FIG.1shows an example embodiment of an additive manufacturing system100, according to embodiments of the disclosure. As shown inFIG.1, additive manufacturing system100includes an energy source105, which in this example is a laser, although energy source105could alternately take the form of an electron beam or other energy source. Beam110emitted by energy source105passes through a partially reflective optic115. Partially reflective optic115is designed to be transmissive at the specific wavelength that energy source105operates, and reflective at other optical wavelengths. In some embodiments the laser wavelength is infrared or near-infrared and in various embodiments is typically wavelengths of 1000 nanometers or greater.

System100can include a scanning head120that consists of x and y positioning galvanometers as well as a focus lens, such as an f-theta lens to manipulate beam110. Beam110can be focused and strikes a work region125of a build plane130that is covered with a layer of metallic powder135. Beam110imparts thermal energy to work region125creating a molten pool of metal that is fused to an underlying part137. As beam110is moved across build plane130by scanning head120, selective portions of build plane130are fused to underlying portions of part137to create a layer of the part. Once each layer has been fused to the underlying part137, a new layer of metal powder135is deposited on build plane130and beam110fuses the new layer to the underlying part in a sequential fashion until the part is completed.

As further shown inFIG.1, the molten pool creates a luminous plume140that emits optical radiation145. Optical radiation145can be stochastic in nature and may include both spurious signals150reflected off adjacent surfaces (e.g., metal powder135and features of system100) as well as light having mixed polarizations155. The presence of the elemental constituents of metal powder135can emit unique spectroscopic signatures from luminous plume140that issue from the heated and vaporized metal powder135. A portion of optical radiation145enters a receiving aperture160in scanning head103and is reflected by partially reflective optic115through optical filter165and into an optical detector170.

In some embodiments optical filter165includes a dual polarization filter that includes a first partially transmissive filter175having a first polarization axis and a second partially transmissive filter180having a second polarization axis wherein the first polarization axis is rotationally offset from the second polarization axis by 90 degrees. In some embodiments optical filter165improves a SNR of the optical signal received by optical detector170to improve an accuracy of the optical detector, as described in more detail below.

FIG.2is an exploded view of optical filter165and optical detector170of additive manufacturing system100illustrated inFIG.1. As shown inFIG.2, reflected optical radiation145is passed through optical filter165before being received by optical detector170. Optical filter165includes first partially transmissive polarized filter175having a first polarization axis210that is oriented at 90 degrees as shown by coordinate system215. As described herein, a polarization axis is the direction along which a filter passes an electric field of an incident electromagnetic wave. Thus, light signals having an electric field aligned with first polarization axis210will pass through with a high transmittance and light signals having an electric field not aligned with the first polarization axis will be attenuated to some degree. In some embodiments first partially transmissive filter175allows light signals having an electric field aligned with first polarization axis210to pass through with at least 90 percent transmittance (e.g., with less than 10 percent attenuation) and in some embodiments with at least 95 percent transmittance and in one embodiment with at least 99 percent transmittance.

Light signals having an electric field not aligned with first polarization axis210will pass through with a relatively lower transmittance (e.g., a greater attenuation) as compared to those that are aligned with the first polarization axis. In some embodiments first partially transmissive polarized filter175allows light signals having an electric field not aligned with first polarization axis210to pass through with between 20 percent to 80 percent transmittance (e.g., with between 80 percent to 20 percent attenuation, respectively) and in some embodiments with between at between 40 percent to 70 percent transmittance and in one embodiment with between 45 percent to 55 percent transmittance. Thus, first partially transmissive filter175has a higher transmittance for on-axis light than it does for off-axis light.

In one embodiment a transmittance ratio for each of first and second partially transmissive polarized filters175,180, respectively can be defined herein as a ratio of on-axis transmittance to off-axis transmittance of incident light. In one embodiment each of first and second partially transmissive polarized filters175,180, respectively, have a transmittance ratio between 1.5 and 2.5 and in various embodiments the transmittance ratio is between 1.8 and 2.2 while in one embodiment the transmittance ratio is between 1.9 and 2.1. In some embodiments the transmittance ratio is substantially 2, meaning that twice the intensity of incident light that is aligned with the polarization axis is allowed to pass through (e.g., 99.9 percent) as compared to the intensity of the incident light that is not aligned with the polarization axis and is allowed to pass through (e.g., 50 percent).

Optical filter165further includes second partially transmissive polarized filter180having a second polarization axis220that is oriented at 0 degrees as shown by coordinate system215. Thus, second polarization axis220of second partially transmissive polarized filter180is rotated approximately 90 degrees in comparison to first polarization axis210of first partially transmissive polarized filter175. In some embodiments second polarization axis220is rotated between 80 degrees and 110 degrees relative to first polarization axis210. In further embodiments second polarization axis220is rotated between 85 degrees and 95 degrees relative to first polarization axis210.

Second partially transmissive polarized filter180can have similar transmittance characteristics as first partially transmissive polarized filter175. More specifically, in some embodiments second partially transmissive filter180allows light signals having an electric field aligned with second polarization axis220to pass through with at least 90 percent transmittance (e.g., with less than 10 percent attenuation) and in some embodiments with at least 95 percent transmittance and in one embodiment with at least 99 percent transmittance. In some embodiments, second partially transmissive polarized filter180allows light signals having an electric field not aligned with second polarization axis220to pass through with between 20 percent to 80 percent transmittance (e.g., with between 80 percent to 20 percent attenuation, respectively) and in some embodiments with between at between 40 percent to 70 percent transmittance and in one embodiment with between 45 percent to 55 percent transmittance. Thus, second partially transmissive filter180has a higher transmittance for on-axis light than it does for off-axis light.

In some embodiments optical filter165can also include one or more bandpass filters225and or focusing lenses230that can be positioned at any location within optical filter165. In some embodiments optical detector170can be a pyrometer, photodiode, spectrometer, high or low speed camera operating in visible, ultraviolet, or IR spectral ranges, etc.

In further embodiments optical filter165can be made from a single polarized optical filter. More specifically, one filter element can have both a first polarization axis and a second polarization axis that is oriented approximately 90 degrees relative to the first polarization axis. The filter element can have similar ranges of transmissivity described above for on-axis and off axis light signals, thus a single filter element can perform the same dual polarization filtering functions describe above and as performed by first and second partially transmissive polarized filters175,180, respectively.

FIG.3illustrates an example image300from optical detector170(seeFIGS.1,2) without first and second partially transmissive polarized filters175,180, respectively. As shown inFIG.3image300has a high degree of variation including an interference pattern indicating that signals of differing phase are collected by optical detector170. Thus, the SNR ofFIG.3is relatively low (e.g., a low signal strength compared to the noise floor) and it can be difficult to accurately ascertain the conditions at work region125(seeFIG.1). In contrast,FIG.4illustrates example image400from optical detector170with first and second partially transmissive polarized filters175,180, respectively. As shown inFIG.4image400is relatively uniform and the interference pattern is not apparent. Thus the SNR ofFIG.4is significantly improved (e.g., a high signal strength compared to the noise floor) and the conditions at work region125(seeFIG.1) can be more accurately ascertained. In some embodiments employing optical filter165improves the SNR by a factor of 100:1 through the elimination of undesirable interference patterns and the attenuation of optical noise. The improved SNR enables the detection of micron-size defects in each affected layer of a build.

FIG.5illustrates an example additive manufacturing system500, according to embodiments of the disclosure. As shown inFIG.5, additive manufacturing system500is similar to additive manufacturing system100shown inFIG.1, however additive manufacturing system500employs multiple on-axis and multiple off-axis sensors that can receive light signals through one or more dual polarization optical filters, as described in more detail below.

Laser550emits a laser beam501that passes through a partially reflective mirror502and enters a scanning and focusing system503which then projects the beam to a work region504on work platform505. In some embodiments, work platform505is a powder bed. Optical energy506is emitted from work region504due to the high material temperatures.

In some embodiments, scanning and focusing system503can be configured to collect some of optical energy506emitted from beam interaction region504. Partially reflective mirror502can reflect optical energy506as depicted by optical signal507. Optical signal507may be interrogated by multiple on-axis optical sensors509A-509C each receiving a portion of the optical signal through a series of additional partially reflective mirrors508. In some embodiments, additive manufacturing system may include only one on-axis optical sensor509A with a fully reflective mirror508A.

In some embodiments optical signal507may not have the same spectral content as optical energy506emitted from beam interaction region504because signal507has been attenuated and/or filtered by optical elements such as partially reflective mirror502, scanning and focusing system503, series of partially reflective mirrors508and dual polarization filter551. Each optical element may have its own transmission and absorption characteristics. More specifically, optical sensor509A may receive optical signal507filtered and/or attenuated by scanning and focusing system503and partially reflective mirror502. Similarly, optical sensor509B may receive optical signal507filtered and/or attenuated by scanning and focusing system503, partially reflective mirror502and dual polarization optical filter551. In some embodiments dual polarization optical filter551includes a first partially transmissive filter552having a first polarization axis and a second partially transmissive filter553having a second polarization axis wherein the second polarization axis is rotationally offset from the first polarization axis. In some embodiments dual polarization optical filter551improves a SNR of optical sensors509B. Similarly, in some embodiments dual optical polarization filter551can also improve a SNR of optical sensor509C.

Examples of on-axis optical sensors509A-509C include but are not limited to photo to electrical signal transducers (i.e. photodetectors) such as pyrometers and photodiodes. The optical sensors can also include spectrometers, and low or high speed cameras that operate in the visible, ultraviolet, or the infrared frequency spectrum. On-axis optical sensors509A-509C are in a frame of reference that moves with beam501, (i.e., they collect readings from all regions that are fused by the laser beam and are able to collect optical signals507from all regions of work platform505as the laser beam scans across the work platform). Because optical energy506collected by scanning and focusing system503travels a path that is near parallel to laser beam501, sensors509A-509C can be considered on-axis sensors.

In some embodiments, additive manufacturing system500can include off-axis sensors510A,510B that are in a stationary frame of reference with respect to laser beam501. Off-axis sensors510A,510B have a given field of view511that in some embodiments can be relatively narrow or in other embodiments the field of view could encompass entire work platform505. Examples of these sensors could include but are not limited to pyrometers, photodiodes, spectrometers, high or low speed cameras operating in visible, ultraviolet, or IR spectral ranges, etc. Off-axis sensors510A,510B, not aligned with the energy source, are considered off-axis sensors.

In some embodiments a dual polarization optical filter554can be employed to improve a SNR of one or more off-axis sensors510A,510B. In this particular embodiment dual polarization optical filter554can be positioned to filter incoming optical signal from field of view511that enters off-axis sensor510A. In some embodiments dual polarization optical filter554includes a first partially transmissive filter555having a first polarization axis and a second partially transmissive filter556having a second polarization axis wherein the second polarization axis is rotationally offset from the first polarization axis by 90 degrees. In some embodiments dual polarization optical filter554improves a SNR of off-axis optical sensor510A.

In some embodiments, off-axis sensors510A,510B could also be sensors that combine a series of physical measurement modalities such as a laser ultrasonic sensor which could actively excite or “ping” the deposit with one laser beam and then use a laser interferometer to measure the resultant ultrasonic waves or “ringing” of the structure in order to measure or predict mechanical properties or mechanical integrity of the deposit as it is being built. The laser ultrasonic sensor/interferometer system can be used to measure the elastic properties of the material, which can provide insight into, for example, the porosity of the material and other materials properties. Additionally, defect formation that results in material vibration can be measured using the laser ultrasonic/sensor interferometer system.

Additionally, additive manufacturing system500can include contact sensors513on the mechanical device, recoater arm512, which spreads the metallic powder. These sensors could be accelerometers, vibration sensors, etc. Lastly, there could be other types of sensors514. These could include contact sensors such as thermocouples to measure macro thermal fields or could include acoustic emission sensors which could detect cracking and other metallurgical phenomena occurring in the deposit as it is being built. These contact sensors can be utilized during the powder addition process to characterize the operation of the recoater arm512. Data collected by the on-axis optical sensors509A-509C and the off-axis sensors510A,510B can be used to detect process parameters associated with recoater arm512. Accordingly, non-uniformities in the surface of the spread powder can be detected and addressed by the system. Rough surfaces resulting from variations in the powder spreading process can be characterized by contact sensors513in order to anticipate possible problem areas or non-uniformities in the resulting part.

In some embodiments, on-axis optical sensors509A-509C, off-axis sensors510A,510B, contact sensors513, and other sensors514can be configured to generate in-process raw sensor data. In other embodiments, on-axis optical sensors509A-509C, off-axis optical sensors510A,510B, contact sensors513, and other sensors514can be configured to process the data and generate reduced order sensor data.

In some embodiments, a computer516, including a processor518, computer readable medium520, and an I/O interface522, is provided and coupled to suitable system components of additive manufacturing system500in order to collect data from the various sensors. Data received by the computer516can include in-process raw sensor data and/or reduced order sensor data. The processor518can use in-process raw sensor data and/or reduced order sensor data to determine laser550power and control information, including coordinates in relation to the work platform505. In other embodiments, the computer516, including the processor518, computer readable medium520, and an I/O interface522, can provide for control of the various system components. The computer516can send, receive, and monitor control information associated with the laser550, the work platform505, and the recoater arm512in order to control and adjust the respective process parameters for each component.

The processor518can be used to perform calculations using the data collected by the various sensors to generate in process quality metrics. In some embodiments, data generated by on-axis optical sensors509, and/or the off-axis sensors510can be used to determine the thermal energy density during the build process. Control information associated with movement of the energy source across the build plane can be received by the processor. The processor can then use the control information to correlate data from on-axis optical sensor(s)509A-509C and/or off-axis optical sensor(s)510A,510B with a corresponding location. This correlated data can then be combined to calculate thermal energy density. In some embodiments, the thermal energy density and/or other metrics can be used by the processor518to generate control signals for process parameters, for example, laser power, laser speed, hatch spacing, and other process parameters in response to the thermal energy density or other metrics falling outside of desired ranges. In this way, a problem that might otherwise ruin a production part can be ameliorated. In embodiments where multiple parts are being generated at once, prompt corrections to the process parameters in response to metrics falling outside desired ranges can prevent adjacent parts from receiving too much or too little energy from the energy source.

In some embodiments, the I/O interface522can be configured to transmit data collected to a remote location. The I/O interface can be configured to receive data from a remote location. The data received can include baseline datasets, historical data, post-process inspection data, and classifier data. The remote computing system can calculate in-process quality metrics using the data transmitted by the additive manufacturing system. The remote computing system can transmit information to the I/O interface522in response to particular in-process quality metrics.

In the case of an electron beam system, an electron beam gun generates an electron beam that is focused by an electromagnetic focusing system and is then deflected by the electromagnetic deflection system resulting in a finely focused and targeted electron beam. The electron beam creates a hot beam-material interaction zone on the workpiece. Optical energy is radiated from the workpiece similar to the laser-based systems described above inFIGS.1-5. The optical energy can be collected by both on-axis and off-axis optical sensors as described above. One or more of the optical sensors can employ a dual polarization optical filter as described herein to improve the SNR of the respective optical sensor.

The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a computer readable medium for controlling manufacturing operations or as computer readable code on a computer readable medium for controlling a manufacturing line. The computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, HDDs, DVDs, magnetic tape, and optical data storage devices. The computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion.

In the foregoing specification, embodiments of the disclosure have been described with reference to numerous specific details that can vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The sole and exclusive indicator of the scope of the disclosure, and what is intended by the applicants to be the scope of the disclosure, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. The specific details of particular embodiments can be combined in any suitable manner without departing from the spirit and scope of embodiments of the disclosure.

Additionally, spatially relative terms, such as “bottom or “top” and the like can be used to describe an element and/or feature's relationship to another element(s) and/or feature(s) as, for example, illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use and/or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as a “bottom” surface can then be oriented “above” other elements or features. The device can be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.