ADAPTIVE GENERATIVE MANUFACTURING PROCESS USING ON-SITE LASER ULTRASOUND TESTING

Abstract

Additive manufacturing process comprising: selectively heating a layer of powder (18) using a laser heating process to form a solid deposited layer (10) comprising a solid deposit (28), wherein the solid deposited layer (10) forms a part (24) of a component; propagating ultrasonic energy waves (50, 60) through the solid deposit (28) prior to completion of the component using a wave-generating laser (40) located remotely from a surface (44) of the solid deposit to direct a wave-generating laser beam (42) onto the surface, wherein the propagation of the ultrasonic energy waves (50, 60) through the solid deposit (28) is carried out when the material is near its melting temperature; detecting the propagated ultrasonic energy waves (62) using a wave-detector laser (70);Evaluating the detected propagated ultrasonic energy waves (50, 60) with respect to information about a residual stress of the solid deposit (28), wherein parametric data are used to predict a mapping between the detected residual stress and a residual stress in the solid deposit (28) after further cooling, and wherein the residual stress is compared with a threshold value;and forming another solid deposited layer (80) in response to the information about the residual stress of the solid deposit (28), wherein, if the residual stress does not exceed the threshold, it is determined how many more solid deposited layers (10) can be formed before another laser ultrasonic detection process is carried out, and wherein, if the residual stress exceeds the threshold, either the further solid deposited layer (80) is formed using other parameters for the laser heating process or a residual stress reduction process is carried out on the solid deposited layer (10).

Description

AREA OF INVENTION The present invention relates to the on-site laser ultrasonic testing of a component that takes place between the formation of layers in an additive manufacturing process. BACKGROUND OF THE INVENTION Additive manufacturing often begins by slicing a three-dimensional representation of an object to be manufactured into very thin layers, thereby creating a two-dimensional image of each layer. To form each layer, popular laser additive manufacturing techniques, such as selective laser melting (SLM) and selective laser sintering (SLS), involve the mechanical pre-positioning of a thin layer of metal powder of a precise thickness on a horizontal plane. Such pre-positioning is achieved using a mechanical stripper to apply or remove a uniform layer of powder, after which an energy beam, such as a laser, is advanced across the powder layer according to the two-dimensional pattern of the solid material for that layer.After the switching operation for the respective layer is completed, the horizontal plane of the applied material is lowered, and the process is repeated until the three-dimensional part is completed. The physical properties of a finished part of significance include both defects (flaws, cracks, etc.) and the amount of residual stress, partly because residual stress can cause warping and premature cracking. The amount of residual stress in the solid part of the component can be determined using known techniques, such as the borehole method. However, this requires material removal and is therefore at least semi-destructive. X-ray and neutron diffraction techniques are non-destructive, but they are expensive and cannot be performed on-site. Additionally, these techniques require the component to be removed for evaluation. Magnetic testing is also non-destructive, but it relies on the interaction between magnetization and elastic strain in a ferromagnetic material.Consequently, magnetic testing is necessarily limited to ferromagnetic materials. Laser-ultrasonic detection of physical properties is known in the field of welding and joining, but is hardly known in the field of additive manufacturing, where it is not performed simultaneously with the formation of the component and / or directly on the component being formed. Publication US 2007 / 0176312A1 describes an additive manufacturing process in which a layer of powder 3 is selectively heated via a laser heating process to form a solid layer 33 comprising a solid deposit. The solid layer 33 forms part of a component 36. Ultrasonic energy waves are directed onto a surface of the solid deposit using a wave-generating laser, and the propagated ultrasonic energy waves are detected and evaluated with respect to a physical property of the solid deposit. The publication Sanderson, RM et al.: Measurement of residual stress using laser-generated ultrasound, International Journal of Pressure Vessels and Piping 87, 2010, p. 762-765, describes a laser-based ultrasound method for measuring the stress behavior in a steel block. Variations in the velocity of the ultrasound wave can be correlated with the stress state, i.e., a stress amplitude. Finite element modeling is used to determine the sensitivity of the measurement method. Document US 6 925 346 B1 describes a method and system of a laser-based metal deposition process for manufacturing an object, taking into account the stress behavior of the object. Document US 2015 / 0041025A1 describes a method and apparatus for manufacturing a metallic component using an additive layer manufacturing process. A heat source is used to melt a surface of the component, creating a molten pool. Adding powder and moving the heat source then creates a new metallic layer. Accordingly, there remains room in technology for an improved, non-destructive process for detecting a physical property, such as residual stress or defects. This problem is solved by the features of the independent claims. Further advantageous embodiments of the invention are the subject of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained in the following description with reference to the drawings, which show the following: Fig. 1 represents an additive laser manufacturing process. Fig. 2 represents a laser-ultrasonic detection process for physical properties. Fig. 3 represents an option for forming a solid deposited layer after the laser-ultrasonic detection process for physical properties by deviating from the parameters used during the additive manufacturing process. Fig. 4 represents an option for performing a residual stress relief process after the laser-ultrasonic detection process for physical properties. Fig. 5 shows the additive laser manufacturing process and the laser-ultrasonic detection process for physical properties performed on a solid deposit.Figure 6 is a flowchart that presents an exemplary embodiment of an additive manufacturing process using a laser-ultrasonic detection process for physical properties. DETAILED DESCRIPTION OF THE INVENTION As with many manufacturing processes, selective laser heating processes (e.g., SLM, SLS) lead to physical properties such as defects and / or the build-up of residual stress. The residual stress level can be high and can affect the structural integrity of the component. Consequently, it is advantageous to know both the magnitude of the existing residual stress and any other defects. The inventors recognized that residual stress can occur within each layer and can build up with the formation of additional layers, and that it is advantageous to identify these physical properties during the additive manufacturing process. Previous techniques for controlling residual stress, for example during the formation of a blade tip, involve alternating the application of the laser beam from side to side to balance the residual stresses. These parts can then be heat-treated to further reduce the residual stresses. However, these processes do not necessarily measure the residual stress during component formation; instead, they predict its presence as a predetermined value and then compensate for / reduce the assumed residual stress. It is known that the properties of a melt pool used to form a layer in an additive manufacturing process can be evaluated using a camera to capture an image of the melt pool.While this technique provides information about the melt pool, it does not provide information about physical properties that may be present after the melt pool has solidified, nor about the layers beneath the melt pool. The inventors of the present invention have developed an additive manufacturing process that monitors physical properties within a component as the component is formed and adapts the additive manufacturing process in response to the information obtained about these physical properties. The physical properties (e.g., residual stress) are monitored using a laser-ultrasonic physical property detection process. This process employs a laser positioned remotely from the component to direct a wave-generating laser beam onto the surface of a recently formed solid deposited layer. Non-contact laser-ultrasonic physical property detection processes are known in the art, such as those developed by Daniel Levesque et al.“Defect Detection and Residual Stress Measurement in Friction Stir Welds using Laser Ultrasonics”, 1st International Symposium on Laser Ultrasonics: Science, Technology and Applications, July 16-18, 2008, Montreal, Canada. Laser ultrasonic detection of residual stress is described, for example, by Karabutov, Alexander, et al., Laser Ultrasonic Diagnostics of Residual Stress, Ultrasonics, 48, 631-635, (2008). In this process, the wave-generating laser beam produces sound waves that propagate through both the most recently formed solid deposited layer and any underlying solid deposited layers. The ultrasonic energy waves are reflected within the component, and these reflected waves can be detected by a wave-detection laser beam using established techniques. The ultrasonic energy waves are analyzed to determine the residual stress and / or defects in the most recently formed solid deposited layer and / or any underlying solid deposited layers. If desired, the additive manufacturing process can be adjusted as necessary to compensate for and / or reduce the residual stress.The settings can include changing the way in which a subsequently formed solid applied layer is created, and / or performing a residual stress relief process on the component before forming another solid applied layer. Fig. 1 shows an exemplary embodiment of an additive laser manufacturing process in which a solid deposited layer 10 is formed on top of previously formed solid deposited layers 12. During the additive manufacturing process, a heating laser 14 selectively directs a laser beam 16 to the powder 18 to heat the powder 18 to form the solid deposited layer 10. The laser beam 16 can sinter the powder particles together as part of a selective laser sintering process. Alternatively, the laser beam 16 can melt the powder particles together in a melt pool 20, which then solidifies to form the solid deposited layer 10. The solid deposited layer 10 and the previously formed solid deposited layers 12 form a stack 22, which is part 24 of a component (not shown) being formed.During the formation of the solid applied layer 10, one or more solid deposits 28 are formed, which, when the layer is complete, constitute the solid applied layer 10. A single solid deposit 28 can be formed and grow continuously until the solid applied layer 10 is formed. Alternatively, several discrete solid deposits 28 can be formed in any pattern until they merge to form the solid applied layer 10. The selective laser heating process can be performed using a set of parameters. These process parameters include powder-related parameters, such as particle size and layer thickness. The powder particle size can be varied for the entire layer or locally within a layer. For example, finer powder particles require less energy to heat, while larger particles require more heat. The particle size can then be varied to meet the local heating requirements needed to relieve local residual stress. These process parameters can also include laser-related parameters, such as the direction 32 of the laser beam path, the laser beam energy, the laser beam diameter 34, and the laser beam path rate (through the powder). In the case of a pulsed laser, the laser properties can include pulse characteristics such as frequency and duration, etc. Additionally, the laser path taken when forming the solid deposited layer 10 can vary. Instead of following a path from one end to the other of the deposited powder 18 to form the solid deposited layer 10, the laser beam 16 can, for example, jump from one location to a more distant location within the deposited powder 18.In such a case, the laser beam 16 can first process a location or locations in the powder 18 in a manner that is effective in reducing the residual stress that has been detected, and then process a remainder of the powder 18 to complete the solid applied layer 10. Fig. 2 illustrates a laser-ultrasonic detection process for physical properties. The process can be implemented after the completion of a solid deposited layer 10, in which case a wave-generating laser 40 emits a wave-generating laser beam 42, which is directed to a surface 44 of a recently formed solid deposited layer 46. Alternatively or additionally, the process can be implemented during the formation of the solid deposited layer 10. In this exemplary embodiment, the wave-generating laser 40 emits the wave-generating laser beam 42 to the surface 44 of the solid deposit, which is the solid section of the partially formed solid deposited layer 10. The process is described here generally with respect to a solid deposited layer 10, but it is recognized that the principles can be applied to the solidified section (e.g.,the solid deposit) of a forming solid applied layer 10 are applicable. The wave-generating laser 40 can be located away from the surface (i.e., not in contact with the surface 44) during this process. When the wave-generating laser beam 42 comes into contact with the surface 44, ultrasonic energy waves 50 are generated. These ultrasonic energy waves 50 propagate through the most recently formed solid applied layer 46 and can be reflected by any number of features. These features include an interface 52, such as the interface 52 between the most recently formed solid applied layer 46 and an adjacent underlying applied layer 54, a bottom surface 56 of the stack 22, or a defect 58, such as a void or crack. Upon encountering these features, the ultrasonic energy waves 48 can be reflected, thereby generating reflected ultrasonic energy waves 60.The reflected ultrasonic energy waves 60 propagate through the stack 22 until they finally reach the surface 44. A wave detection laser 70 generates a wave detection laser beam 72, which is directed towards the surface 44 and reflected back to the wave detection laser 70, carrying information about the reflected ultrasonic energy waves 60. Alternatively, some of the ultrasonic energy waves 50 may propagate unimpeded through the last solid deposited layer 46 until they are detected by the wave detection laser 70. Consequently, the propagated energy waves 62 detected by the wave detection laser 70 may contain unimpeded ultrasonic energy waves 50 and / or reflected ultrasonic energy waves 60. In an exemplary embodiment, the heating laser 14, the wave-generating laser 40, and the wave-detection laser 70 can be separate lasers. Alternatively, a single laser can be one of any two or all three of the lasers 14, 40, or 70. For example, a single laser can be used to process the powder 18 and then ping the surface 44 to generate the ultrasonic energy waves 50. The same single laser can also be used to detect the propagated energy waves 62, or a separate laser can be used to detect the propagated energy waves 62. When the wave-detection laser 70 detects the propagated waves, it can be used, for example, in conjunction with an interferometer, as is known in the art. The physical properties of a material through which the energy waves pass can modify the properties of the energy wave. Consequently, the propagated energy waves 62 transmit information about the physical properties of the most recently formed solid applied layer 46 and / or the previously formed solid applied layers 12. An analysis of the properties of the propagated energy waves 62 allows for a determination regarding the physical properties, including both whether certain features (e.g., defects and / or cracks) are present and the magnitude of any residual stress that is present. The information can be obtained directly from the properties of the propagated energy waves 62. If, for example, a property (e.g., an amplitude, etc.) of the propagated energy waves falls to one side or the other of a threshold value, a predefined measure can be taken, such as a change in the additive manufacturing process to reduce or compensate for the residual stress. Alternatively or additionally, the properties of the propagated energy waves 62 can be evaluated, and the physical properties can be deduced from the evaluation. These physical properties can then be assessed for acceptability, whereby, if they are unacceptable, a measure can be taken, such as a change in the additive manufacturing process to reduce or compensate for the residual stress.In the event of a defect being found, the additive manufacturing process can be stopped to reformulate it and then either finish the part or to reject the part. The laser-ultrasonic detection process for physical properties is performed on the most recently formed solid deposited layer 46, while it is in a solid state. The laser-ultrasonic detection process can be performed, for example, after the entire most recently formed solid deposited layer 46 has been formed and has cooled to ambient temperature. The laser-ultrasonic detection process can be performed immediately after the powder 18 has been treated with the laser, in which case the material being processed is relatively warm. In the case of selective laser melting, the material may be near its melting temperature. Because the properties and the amount of residual stress change as the material cools, the residual stress detected in the latter case is not the same as the stress that is present once the component is finished and at ambient temperature.The parametric data can be used to establish a correlation between the detected residual stress at a relatively warm temperature and the residual stress after further cooling. Performing the laser-ultrasonic detection process shortly after laser treatment is complete can save considerable time compared to waiting for the part to cool and then performing the laser-ultrasonic detection. This can also allow for a less drastic corrective action to prevent the formation of residual stress, which is predicted to develop during cooling after the laser-ultrasonic detection process. The parametric data can be developed by actually measuring the residual stress in the components at varying temperatures and finishing conditions, etc., and applying these measurements to the observed data.Alternatively or additionally, the residual stress during cooling can be predicted using various modeling algorithms and the like. In one exemplary embodiment, the laser-ultrasonic detection process can occur as often as each time a solid deposited layer 10 is formed. Alternatively, the laser-ultrasonic detection process can occur at predetermined intervals, such as every second deposited solid layer 10, or every third, etc. Other factors can be incorporated into the process to determine when the laser-ultrasonic detection process should occur, including the geometry of the component and / or the solid deposited layer 10. For example, where a stress intensifier, such as a fillet, is formed, or any other geometry that is subject to high residual stress upon cooling, the laser-ultrasonic detection process can occur more frequently during the component's formation.Conversely, if the geometry is less susceptible to residual stress, the laser-ultrasonic detection process may occur less frequently during the formation of the component. The timing of the laser-ultrasonic detection process can be a predefined pattern built into the additive manufacturing process. However, the additive manufacturing process can modify the predefined pattern during the additive manufacturing process in response to residual stresses detected during the process. For example, if the predefined pattern is based on a specific level of predicted residual stress at a given point during the additive manufacturing process, and if the actual residual stress at that point is lower, the predefined pattern can be modified so that more solid, deposited layers can be formed before the next laser-ultrasonic detection process than would have been formed with the predefined pattern. For example, if...The laser-ultrasonic detection process would take place after the last solid applied layer 46 was formed and again after only three more solid applied layers 10 have been formed, and if the laser-ultrasonic detection process determines that the residual stress is lower than predicted when the last solid applied layer 46 is tested, the predetermined pattern can be changed to schedule the next laser-ultrasonic detection process after four or five or more solid applied layers 10 have been formed. Conversely, if the predicted residual stress is greater than expected and if the next laser-ultrasonic detection process is planned after only three more solid applied layers 10 have been formed, the specified pattern can be changed so that the laser-ultrasonic detection process takes place after each solid applied layer 10 has been formed.Figures 3 and 4 illustrate the available options if the detected residual stress exceeds a predetermined threshold and a modification to the additive manufacturing process, or the reduction and / or attenuation of the residual stress, is deemed necessary. Figure 3 depicts one option for compensating the residual stress by forming a solid deposited layer 10 after the laser-ultrasonic detection process and by deviating from the parameters used during the additive manufacturing process. For example, if residual stress is detected and it is determined that it can be compensated by forming the current solid deposited layer 10, then compensation can occur when the current solid deposited layer 10 is formed. Alternatively or additionally, compensation can occur when a subsequent solid deposited layer 80 is formed.Any, several, or all of the process parameters associated with the formation of the solid applied layer 10 can be adjusted, and the adjustment can take place in the current and / or the subsequent solid applied layer. These process parameters include the powder-related parameters and the laser-related parameters disclosed above, and any others that are known to the average person skilled in the art. In one exemplary embodiment, the formation of residual stress in the solid deposited layer 10 being processed can be detected before the residual stress reaches a threshold, and the process parameters can be adjusted to prevent further increases in the residual stress level. In another exemplary embodiment, residual stress can be intentionally generated in the most recently formed solid deposited layer 46 or its solid deposit 28 to counteract residual stress in one or more of the previously formed solid deposited layers 12. This localizes the residual stress, as opposed to potentially building it up. Accordingly, by adjusting the process parameters, the development of residual stress in a layer being processed can be stopped and / or a previously generated residual stress can be counteracted. Fig. 4 illustrates one option for performing a residual stress relief process following the laser ultrasonic stress detection process. These residual stress relief processes include those known in the art, such as shot peening (e.g., laser shot peening), laser reheating, and heat treatment (e.g., inductive heat treatment). Instead of modifying, or in addition to modifying, the parameters associated with the formation of the subsequent solid deposited layer 80 to reduce the residual stress, the stack 22 can be left in place or removed to perform the stress relief process. In an exemplary embodiment, the stack 22 is left in place for the stress relief process. A laser shot penning process is suitable for on-site stress relief because the laser used can be located in the same process chamber / environment and can be the same heating laser 14 that processes the powder. In laser shot penning, the laser beam 16 can be directed onto the surface 44 of the most recently formed solid deposited layer 46 or its solid deposit 28 to perform the shot penning process. Laser reheating can use the heating laser 14 to heat some or all of the most recently formed solid deposited layer 46 or its solid deposit 28, as necessary to reduce the residual stress. Induction heat treatment can be performed on-site if the heating coils are located in the same process chamber / environment.Induction heating can then be easily performed by activating the heating coils as needed. To reduce residual stress, the heating coils can also be used to control the cooling rate of the melt pool 20 and / or the solidified deposited layer 10. Any or all of these and other residual stress reduction processes can be used in combination. Furthermore, they can be used after the solidified deposited layer 10 has formed, or during the formation of the solidified deposited layer. Fig. 5 shows an alternative exemplary embodiment of the laser-ultrasonic process implemented during the formation of the solid deposited layer 10. It is shown that the two processes take place simultaneously. Alternatively or additionally, they can be carried out sequentially. In this exemplary embodiment, the wave-generating laser 40 emits the wave-generating laser beam 42 to the surface 44 of the solid deposit 28, which is the solid portion of the partially formed solid deposited layer 10. Consequently, the principles disclosed above are applicable to a solid deposit 28 of a partially formed solid deposited layer 10.The laser-ultrasonic process can be performed on the solid deposit 28 of a forming solid applied layer 10, wherein one or more of the on-site residual stress reduction processes can be performed on the solid deposit 28 or on any other part of the stack 24. Therefore, the laser heating process, the laser-ultrasonic process, and the residual stress reduction process can be performed on a solid applied layer 10, on a solid deposit 28 of a forming solid applied layer 10, and / or on several discrete solid deposits 28 of a solid applied layer 10 in any sequence and as often as necessary to adapt the additive manufacturing process to compensate for the residual stress. After the stress reduction process has been carried out, a further laser-ultrasonic detection process can optionally be performed to assess its effectiveness. If it is satisfactory, the next solid deposited layer 80 can be formed, either using the same or different parameters as those used for the other solid deposited layers 10 and 12. If it is not satisfactory, another stress reduction process can be carried out. This process can be repeated as many times as necessary to achieve the desired residual stress level, and may include any combination of stress reduction processes and modifications to the next solid deposited layer 80, as required. Figure 6 is a flowchart illustrating an exemplary embodiment of an additive manufacturing process using a laser-ultrasonic detection process. In step 100, the solid deposited layer 10 is formed. In step 102, the laser-ultrasonic detection process is performed. In step 104, the residual stress is determined by the laser-ultrasonic detection process. In step 106, it is determined whether the residual stress is below a threshold, equal to a threshold, or exceeds a threshold. If the residual stress does not exceed the threshold (e.g., the stack 22 passes the test), then in step 108, it is determined how many more solid deposited layers 10 can be formed before another laser-ultrasonic detection process is performed.In step 110, the specified number of solid, applied layers 10 is formed, after which the process returns to step 102. If the residual stress exceeds the threshold (e.g., stack 22 fails the test), then either step 112 or step 114 is executed. In step 112, the next solid, deposited layer 80 can be formed using different parameters for the laser heating process. In step 114, a residual stress reduction process is performed on stack 22. Step 114 can be followed by either step 112 or step 116. In step 116, the next solid, deposited layer 80 can be formed using the same parameters that were used when one of the previously formed solid, deposited layers 12 was formed. Step 102 can follow steps 112 and 116. From the foregoing, it can be seen that the inventors have applied a new technique to an additive manufacturing process to allow non-destructive, on-site online testing of a component for physical defects and residual stress. The process enables the correction of certain conditions, thereby saving costs and avoiding the reduced service life associated with parts that would otherwise fail to meet the standards enabled by this process. Consequently, this represents an improvement in the technology. While various embodiments of the present invention have been shown and described here, it is obvious that such embodiments are provided only as examples. Numerous variations, modifications, and substitutions can be made without deviating from the invention presented here. Accordingly, it is intended that the invention is limited only by the inventive concept and the scope of protection of the appended claims.

Claims

Additive manufacturing process comprising: selectively heating a layer of powder (18) using a laser heating process to form a solid deposited layer (10) comprising a solid deposit (28), wherein the solid deposited layer (10) forms a part (24) of a component; propagating ultrasonic energy waves (50, 60) through the solid deposit (28) prior to completion of the component using a wave-generating laser (40) located remotely from a surface (44) of the solid deposit to direct a wave-generating laser beam (42) onto the surface, wherein the propagation of the ultrasonic energy waves (50, 60) through the solid deposit (28) is carried out when the material is near its melting temperature; detecting the propagated ultrasonic energy waves (62) using a wave-detector laser (70);Evaluating the detected propagated ultrasonic energy waves (50, 60) with respect to information about a residual stress of the solid deposit (28), wherein parametric data are used to predict a mapping between the detected residual stress and a residual stress in the solid deposit (28) after further cooling, and wherein the residual stress is compared with a threshold value;and forming another solid deposited layer (80) in response to the information about the residual stress of the solid deposit (28), wherein, if the residual stress does not exceed the threshold, it is determined how many more solid deposited layers (10) can be formed before another laser ultrasonic detection process is carried out, and wherein, if the residual stress exceeds the threshold, either the further solid deposited layer (80) is formed using other parameters for the laser heating process or a residual stress reduction process is carried out on the solid deposited layer (10). Additive manufacturing process according to claim 1, wherein the one solid applied layer (10) is formed on previously formed solid applied layers (12). Additive manufacturing process according to claim 1 or 2, wherein a single laser performs the selective laser heating process and generates the wave-generating laser beam. Additive manufacturing process according to one of claims 1 to 3, wherein the residual stress reduction procedure comprises laser shot penning and / or inductive heat treatment and / or laser reheating of the solid applied layer.

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