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Optical in-process height measurement system for process control of laser metal-wire deposition
Journal Pre-proof Optical in-process height measurement system for process control of laser metal-wire deposition Shigeru Takushima, Daiji Morita, Nobuhiro Shinohara, Hiroyuki Kawano, Yasuhiro Mizutani, Yasuhiro Takaya PII:
S0141-6359(19)30770-6
DOI:
https://doi.org/10.1016/j.precisioneng.2019.11.007
Reference:
PRE 7058
To appear in:
Precision Engineering
Received Date: 25 October 2019 Accepted Date: 5 November 2019
Please cite this article as: Takushima S, Morita D, Shinohara N, Kawano H, Mizutani Y, Takaya Y, Optical in-process height measurement system for process control of laser metal-wire deposition, Precision Engineering (2019), doi: https://doi.org/10.1016/j.precisioneng.2019.11.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Optical in-process height measurement system for process control of laser metal-wire deposition Shigeru Takushimaa*, Daiji Moritaa, Nobuhiro Shinoharaa, Hiroyuki Kawanoa, Yasuhiro Mizutanib, Yasuhiro Takayab a Advanced Technology R&D Center, Mitsubishi Electric Corp. 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b Department of Mechanical Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka, 565-0871 Japan We propose an in-process height measurement system for a weld bead and feedback control system for wire-feeding speed for high-quality laser deposition. Metal additive manufacturing, especially laser metal-wire deposition, is effective for complex shape fabrication and repair processing. However, we must control the gap between a weld bead and a feed wire in an optimal range for high-quality deposition. Conventionally, the Z-stage pitch for multi-layer deposition must be precisely adjusted by each deposition shape. In this paper, we design an in-process height measurement system that is integrated in a laser processing head, which measures the weld bead height by a line section method. We decreased the influence of the intense thermal radiation generated from a melt pool by inserting the band-pass filter of the line beam’s wavelength in the imaging system and optimizing its line laser power. Consequently, our system can measure the weld bead height near the melt pool, which is 4 mm in front of it. Next we show that our proposed system can measure the weld bead height during wire-laser metal deposition with 50µm accuracy by comparing its value to the true value. Finally, we achieved a cylinder shape deposition of 50-mm height, regardless of the Z-stage pitch and the cylinder diameter of the multi-layer deposition, by controlling the wire-feeding speed based on the measured weld bead height. Keywords: Height Measurement; Optical Sensing; Process Monitoring; Additive Manufacturing; Laser Metal Deposition; Light Section Method 1. Introduction Additive manufacturing (AM) technology is innovative because it can fabricate complex shaped parts quickly and inexpensively. In particular, it is effective in complex metal processing where fabrication is complicated by conventional cutting machining. In powder bed fusion (PBF) methods, lead metal powders are melted by lasers or electron beams and molded into layers [1,2]. Although PBF can fabricate precise shapes, long fabrication times are required and fabricating large parts is difficult. On the other hand, in the directed energy deposition (DED) method, metal wires or powders are fed to the added part and welded by lasers, arc plasma, or electron beams [3-5]. Owing to partial fabrication, the DED method is useful for repairing incomplete parts and the high-speed fabrication of large complex parts. In laser metal deposition (LMD), the metal powder method has been researched [3] because feeding metal material and process control are easy. However, the metal wire method’s [5] fabricating speed is faster and it has higher fabricated density [6]. Furthermore, a large variety of metal materials can be used easily with the wire materials of a welding machine. However, only the powder method in LMD is currently available on the market because the process control of the wire method is more difficult. A process monitoring technique is also critical for high-quality fabrication with laser metal-wire deposition (LMwD) [7], including melt pool monitoring by a camera [5,8], the thermal measurement of a melt pool [4,9], and the height measurement of a weld bead [5,8,10,11]. High-quality laser deposition is especially difficult without controlling the gap between a weld bead and a feed wire to an optimal range. The deposition result based on the gap between a weld bead and a feed wire is shown in Table 1. When the feed wire is too high above a weld bead, its surface becomes uneven due to droplets since the metal wire that was melted by the processing laser is not sufficiently attached to the weld bead surface. In contrast, when the feed wire is too low below a weld bead, stubbing occurs because the wire was not adequately melted by the processing laser since the feed wire was stuck to the weld bead. Consequently, we have to control the gap between a weld bead and a feed wire within the optimal range for high-quality laser deposition. In addition, the bead height of one layer becomes lower than the height of the first layer as the deposition progresses due to thermal storage during multi-layer deposition. The Z-stage pitch of the multi-layer deposition conventionally needs to be adjusted precisely based on each deposition shape and all of the deposition parameters (wire-feeding speed, laser power, scanning speed, and so on) for highquality deposition. Therefore, the gap between a weld bead and a feed wire must be controlled within an optimal range regardless of the Z-stage pitch and the cylinder diameter by the feedback control of a
deposition parameter based on the measured weld bead height at each layer [8,10,12]. As a result of our experiment, the target measurement accuracy of the bead height for continuous deposition without droplets and stubbing was ±150 µm. The light section method, which was previously proposed for measuring the weld bead height of the LMwD [8,10], measures an object’s height by calculating the centroid of the projected line beam that is imaged by a camera set obliquely to the line beam’s projection direction [13]. However, the setup of a previous method [8] was too large because the imaging and illuminating systems faced each other with a processing laser head between them. When there is a gas nozzle with a processing head and a wire that is fed from the side, measuring the projected position of the line beam is difficult. Furthermore, a method that images from an oblique direction to the deposition plane (X-Y plane) is also difficult for accurately measuring the bead height because the calculation accuracy of the line beam’s centroid was decreased by aberrations and defocusing due to the difference of the imaging distance in the camera’s field of view. An in-process height measurement is desirable for LMwD. Although separating the measurement process from the deposition process is easy, the manufacturing time becomes longer. Hence, it is effective to control the deposition parameters by the in-process measurement result of the previous layer’s bead height near the melt pool. However, intense thermal radiation with a broad spectrum is generated because a high power processing laser is irradiated at a melt pool during the deposition processing. The bead height must be measured with high accuracy without the influence of intense thermal radiation. However, if we measure the beads’ height at far position from the melt pool to reduce the influence of the intense thermal radiation, LMwD that fabricate complex shape parts cannot measure onto the bead because the measurement positon become off the processing route. Therefore, we must measure the bead’s height at near position from the melt pool in possible. We propose an optical in-process height measurement system for the feedback control of wire-feeding speed for LMwD and expect to launch it on the market by establishing an in-process height measurement and a feedback control technique. Our proposed measurement system consists of an imaging system coaxially integrated in a laser processing head and an illuminating system that obliquely projects a line beam, which measures the weld bead height by a line section method. We designed an imaging system that shares an objective lens for the laser deposition and the height measurement as well as the projection angle of the line beam to measure the weld bead height with high accuracy. We decreased the influence of the intense thermal radiation generated from a melt pool by inserting the band-pass filter of the line beam’s wavelength in the imaging system and optimizing its line laser power. Therefore, our system can measure the weld bead height near the melt pool, which is 4 mm in front of it. Finally, it can achieve high-quality deposition by controlling the wire-feeding speed based on the measured weld bead height and retaining the gap between the weld bead and the feed wire at an optimal value. In this paper, we show the optical design of the imaging and illuminating systems and quantitatively demonstrate the accuracy evaluation result of our height measurement system on an LMwD by comparing it to a highly accurate height measurement instrument. We also argue that the high-quality deposition of cylinder objects is possible regardless of the Z-stage pitch and the cylinder diameter by controlling the wire-feeding speed based on the measured weld bead height. 2. System We show the construction of our proposed height measurement system for LMwD in Fig. 1. Because it has a 5-axis stage, it fabricates freeform shapes by moving a workpiece relative to a processing head while feeding a metal wire to the deposition position and irradiating a high power processing laser. A processing laser light is reflected to the workpiece by a beam splitter. The reflected light is focused by an objective lens and irradiated to the deposition position through a shielding gas nozzle to prevent metal oxidation during deposition. The metal wire is fed to the deposition position and melted by the irradiated processing laser. We propose a height measurement system with a coaxial setup of the laser processing axis by integrating the imaging system with a processing head. We attached a line beam to the side of a processing head (Fig. 1) and projected it in front of the melt pool to measure through a gas nozzle. The line beam reflected from a weld bead is captured by an objective lens used for deposition processing. An imaging system images the reflected light that passed through a beam splitter onto a camera integrated with a processing head. Consequently, we can coaxially measure the line beam position through a gas nozzle by projecting it from the opposite direction of the feeding wire. Since our system measure the line beam through a gas nozzle, we can achieve both the bead’s height measurement and monitoring the melt pool by only one camera.
Hence, there is a possibility that our system can control process parameter based on monitoring result of the melt pool during the bead’s height measurement in the future. In the line section method, the height measurement’s sensitivity is determined by the projection angle of the line beam and the transverse magnification of the imaging system. The relation between the measurement height and the projected position’s shift of the line beam is shown in Fig. 2. Displacement ∆Z by the projected position’s shift of line beam ∆X is shown in Eq. (1), where θ is the projection angle of the line beam: ∆Z = ∆X ∙ tan . (1) The image of the line beam on an image sensor is shown in Fig. 2 (b). The shift of line beam’s centroid by displacement ∆Z is ∆X’ which is equal to M･∆X, where M is the transverse magnification of the imaging system. The displacement by the camera’s 1-pixel ∆H is shown in Eq. (2), where the camera’s pixel size is P : H = ∙ tan . (2) We calculated the height of a weld bead by the line beam’s centroid on a camera image. The centroid in the X direction of each Y-direction’s pixel in a captured image was calculated, and the cross-section height distribution in the Y direction was estimated by Eq. (2). Next we propose a control system of the deposition parameters with our height measurement system. In the deposition parameters including laser power and stage scanning speed, the wire-feeding speed is the best parameter to control the bead height without influencing the bead width [8]. The configuration of our wire-feeding speed control system with an in-process height measurement system is shown in Fig. 3. Although the Z-stage pitch that was deposited every layer was conventionally constant during the multilayer deposition, the deposition height of each layer was not equal due to the thermal storage. Therefore, we controlled the wire-feeding speed by numerical control (NC) so that the deposition height reached the target height. The controlling value of the wire-feeding speed was calculated based on the gap between the weld bead and the feed wire, which was estimated by the measuring bead height of previous layer in front of the melt pool using our height measurement system. The control value of the wire-feeding speed was estimated experimentally from the relationship with the wire-feeding speed and the bead height in advance. The value of the wire-feeding speed was controlled linearly based on the measured bead height in our system. 3. Experiment setup We experimented with our prototype height measurement system attached to the LMwD. In this setup, the processing head has X-, Y-, and Z-stages and the workpiece is on a C-axis stage. The imaging system is set on the processing head, and the illuminating system is integrally attached to its side. We designed the sensitivity of the height measurement by Eq. (2). The line beam’s projection angle θ should be vertical because the weld bead surface has mostly specular reflection. We designed the projection angle of line beam θ, the transverse magnification of imaging system M, and the camera’s pixel size P so that the displacement by the camera’s 1-pixel ∆H is 30 µm. The line beam’s projection position should be as near the deposition position as possible for accurate height measurement. The projected line beam is not vertical to the deposition direction of the weld bead if the line beam’s projection position is far from the deposition position because LMwD fabricates complex shapes like curves. We designed the line beam’s projection position to be 4 mm from the laser processing axis’s center because the melt pool’s maximum diameter is φ6 mm and the gas nozzle’s diameter is φ10 mm. Since the line beam’s projection position at focus position was set 4 mm from the laser processing axis’s center, we can calculate ±1-mm centroid shift on a camera image. Hence, the height measurement range of our system is ±2-mm by Eq. (1). However, it is more difficult to optically measure the bead height due to the influence of the intense thermal radiation generated from a melt pool if the measurement position is close to the melt pool. Therefore, we inserted a 520-nm band-pass filter of the line beam’s wavelength into the imaging system to reduce the influence of the thermal radiation that has a broad spectrum whose center wavelength is infrared. We also measured the thermal radiation’s luminance based on the distance from the melt pool to estimate the required line beam power. The luminance result of a 520-nm line beam wavelength is shown in Fig. 4. The φ3-mm luminance from the melt pool’s center is very intense. Furthermore, the generated luminance of the thermal radiation is more intense when a wire was fed for the deposition. Consequently, we used a line laser whose power is 50 mW so that the line beam’s luminance at 4 mm from the melt pool is adequately high for the thermal radiation. When we measured the luminance of the 50-mW line beam (Fig. 4), the luminance at 4 mm from the melt pool is
more than twice the thermal radiation. Consequently, our system can measure the bead’ height at 4 mm in front of the melt pool although melt pool’ radius is 3mm. This shows that our system can measure the bead’ height at the physically nearest position from the melt pool. 4. Result and discussion 4.1. Accuracy evaluation The displacement measurement result between a weld bead and a processing head by the Z-stage shift is shown in Fig. 5(a). In this paper, the metal-wire material we used was Inconel. We measured a 1-layer weld bead when the X- and Y-stages are stopped without deposition processing. 0-mm displacement means that the Z-stage is the focus position of the processing laser. We measured from -3 to +3 mm by a 250-µm pitch. The estimated pitch from the measured displacement at the weld bead position (Y = -0.6 mm) and the flat workpiece position (Y = -3.5 mm and 2.5 mm) is shown in Fig. 5(b). The pitch of each Y position averages ±50 µm (11 pixel) in the Y direction. Our proposed system can measure the displacement both of a weld bead and a flat workpiece at ±50-µm accuracy. Next we evaluated whether our proposed system can measure the height at 4 mm in front of a melt pool during laser deposition processing like without laser processing. The images captured during and without laser processing are shown in Fig. 6. Our proposed system captured the line beam’s image at 4 mm in front of a melt pool as in the image without deposition because we inserted a band-pass filter in the imaging system and optimally designed the line beam’s power. The line beam’s image during laser deposition was not influenced by the intense thermal radiation. We evaluated the height measurement accuracy of our measurement system by comparing its result to a highly accurate height measurement instrument. We deposited 1- and 3-layer striped beads on a plate workpiece. The heights on the weld bead and the flat plate were measured by an optical non-contact height measurement instrument (NH-3N from Mitaka Kohki), where the measurement position of the flat plate was 2 mm from the bead’s center. We set the workpiece on the LMwD again and measured the weld bead height by our measurement system both during laser deposition when the thermal radiation is generated and without deposition. The weld bead height by the measurement position is shown in Fig. 7, where the true value is the result of the height measurement instrument. We calculated the weld bead height by the difference between the flat workpiece surface and the top of the weld bead, and the flat workpiece surface has an average result value at Y= ±2 mm from the weld bead. This evaluation acquired the measurement position of each image captured by the camera for the height measurement from the LMwD’s stage position by sending a trigger signal when the camera captured the image. This result shows that our system can identically measure the height of both the 1- and 3-layer beads during and without laser deposition. This height value is the same as the true value that was measured by the height measurement instrument. The start and end points of the beads were also measured accurately. The height measurement error of the 1- and 3-layer beads is shown in Fig. 8, calculated by the difference between the during-deposition and the true value results in Fig. 7. This evaluation shows the result at a flat position (X=5-55 mm), except for the start and end positions of a bead whose slope is sharp. The 1-layer bead result shows that our system’s accuracy is PP=64.6 µm (σ=13.2 µm), and the 3-layer bead result shows that its accuracy is PP=87.6 µm (σ=11.5 µm). We achieved ±50-µm accuracy for the height measurement during laser deposition, which was more accurate than the target accuracy ±150 µm. 4.2. Process control evaluation The cylinder deposition comparison between our wire-feeding speed control system and the conventional system without any process control is shown in Table 2. In gap measurement result, the negative value of the gap means that the previous layer’s bead height is higher than the wire height in Fig. 3. In contrast, the positive value of the gap means that the previous layer’s bead height is lower than the wire height. Therefore, if the gap is negative value, the bead height of current deposition layer must be lower by decreasing the wire-feeding speed. If he gap is positive value, the bead height of current deposition layer must be higher by increasing the wire-feeding speed. We estimated the gap between the weld bead and the feed wire in the gap measurement result at 180 degrees from the deposition start position. The deposition height in the horizontal axis was calculated from the number of deposition layers and the Z-stage pitch. For the conventional method without any wire-feeding speed control, the maximum continuous deposition height was 44 mm when the Z-stage pitch was 0.25 mm, the cylinder’s diameter was φ70 mm, and the wire-feeding speed was set to an optimal value. Here, the optimal value means the wire-feeding speed that could continuously highest deposition estimated experimentally in advance.
However, the gap between the weld bead and the feed wire in Table 2 was not constant during the deposition. The wire was near the bead until the deposition height was 15 mm, and after that it moved away from the bead. The deposition could not be continued due to the droplets when the deposition height was around 44 mm. On the other hand, when the Z-stage pitch was reduced to 0.2 mm, continuous deposition was stopped by stubbing at an 18-mm deposition height. When the deposition height was around 5 mm, height measurements were impossible because the bead surface was shaved by the stubbed wire. When the Z-stage pitch was increased to 0.3 mm, continuous deposition was stopped by droplets at a 10-mm deposition height. In this experiment, continuous deposition was difficult in the conventional method without-process control if the Z-stage pitch was not optimally adjusted at less than ±0.05 mm. When the deposition shape changed, for example, the cylinder diameter became φ50 mm, the continuous deposition height was reduced to 25 mm, even if the Z-stage pitch was 0.25 mm and the wire-feeding speed was set to an optimal value, which is same as the φ70-mm cylinder. This means that the optimal Zstage pitch must be adjusted for each deposition shape and the parameters. In contrast, we achieved continuous deposition of 50-mm height with our proposed control system of wire-feeding speed regardless of the deposition parameter and shape, such as the Z-stage pitch and the cylinder diameter. We controlled the gap between the weld bead and the feed wire at ±0.1-mm accuracy. Although we controlled the wire-feeding speed by a constant value based on the measured gap, we should dynamically modify the controlling value of the wire-feeding speed by the deposition temperature or the measured bead height for more accurate control. The control result of the wire-feeding speed by the deposition height is shown in Fig. 9. The wire-feeding speed was normalized by an optimal value when the Z-stage pitch was 0.25 mm in the conventional method. When the Z-stage pitch was 0.25 mm, the wire-feeding speed was reduced until the deposition height was around 15 mm; at this range, the gap was smaller than the optimal value. This means that Moreover, the wire-feeding speed was faster after the deposition height was around 15 mm; at this range, the gap was larger than the optimal value. This result shows that we controlled the 1-layer deposition height that was changed by the thermal storage to equal the Z-stage pitch. We also controlled the 1-layer deposition height to equal the Z-stage pitch even when it was different from 0.25 mm. The wire-feeding speed was reduced when the Z-stage pitch was 0.2 mm, and the wire-feeding speed was faster when the Z-stage pitch was 0.3 mm. The deposition height result of cylinder objects measured with a height gauge (HD-30AX from Mitutoyo) is shown in Fig. 10. We measured the cylinder deposition objects fabricated by controlling wire-feeding speed when Z-stage pitch is 0.2 mm, 0.25 mm and 0.3mm. We measured the deposition height from the base plate at each rotation position of a cylinder by a 45-degree pitch. The deposition height in the vertical axis is the difference from the average value in one rotation result. The deposition height accuracy in one rotation of our system was achieved ±0.1 mm by controlling the wire-feeding speed. This result is more accurate than the 0.4-mm accuracy of the previous method [8]. Consequently, our system could deposite the flat surface by controlling the wire-feeding speed based on the measured weld bead height. 5. Conclusion We proposed an in-process height measurement system of a weld bead by a line section method and a feedback control system of wire-feeding speed for high-quality laser deposition. Our proposed measurement system consisted of an imaging system integrated in a laser processing head and an obliquely illuminating system that projected a line beam. We designed an imaging system to share an objective lens for laser deposition and height measurement. Moreover, a projection angle of the line beam was also designed to measure the weld bead height with high accuracy. In addition, we achieved a height measurement of 4 mm in front of the melt pool by inserting the band-pass filter of the line beam’s wavelength into the imaging system and optimizing its line laser power to decrease the influence of the intense thermal radiation generated from a melt pool. Our measurement system measured the weld bead height during laser deposition with 50-µm accuracy compared to the true value measured by a highly accurate height measurement instrument. We controlled the gap between the weld bead and the feed wire at ±0.1-mm accuracy to an optimal value using the developed control system of the wire-feeding speed based on the measured weld bead height. Although we had to precisely adjust the Z-stage pitch for each deposition shape and the parameters in the conventional method without any process control, we achieved high-quality continuous deposition over 50-mm height by our proposed system regardless of the Z-stage pitch and the cylinder diameter. Our proposed in-process height measurement and feedback control
technique greatly improved LMwD for practical use. Our future work will improve the centroid calculation algorithm to decrease the influence of the diffuse reflection by the local roughness on the bead surface. Furthermore, we will apply this system to other complex shape depositions. 6. References [1] Simchi A, Pohl H. Direct laser sintering of iron–graphite powder mixture. Materials Science and Engineering 2004;383:191-200. [2] Simchi A. Direct laser sintering of metal powders: Mechanism, kinetics and microstructural features. Materials Science and Engineering 2006;428:148-158. [3] Milewski O. J, Lewis K.G, Thoma J.D, Keel I. G, Nemec B. R, Reinert A. R. Directed light fabrication of a solid metal hemisphere using 5-axis powder deposition. Journal of Materials Processing Technology 1998;75:165-172. [4] Zalameda N. J, Burke R. E, Hafley A. R, Taminger M. K, Domack S. C, et al. Thermal Imaging for Assessment of Electron-Beam Freeform Fabrication (EBF3) Additive Manufacturing Deposits. SPIE Proceedings 2013;8705:1-8. [5] Liu S, Liu W, Harooni M, Ma J, Kovacevic R. Real-time monitoring of laser hot-wire cladding of Inconel 625. Optics & Laser Technology 2014;62:124-134. [6] Nurminen J, Riihimäki J, Näkki J, Vuoristo P. Comparison of laser cladding with powder and hot and cold wire techniques. Proceedings of international congress on application of lasers & electro-optics 2006:634-637. [7] Everton K. S, Hirsch M, Stravroulakis P, Leach K. R, Clare T. A. Review of in-situ process monitoring and in-situ metrology for metal additive manufacturing. Materials and Design 2016;98:431-445. [8] Heralic A, Christiansson A, Ottosson M, Lennartson B. Increased stability in laser metal wire deposition through feedback from optical measurements. Optics and Lasers in Engineering 2010;48:478-485. [9] Yan Z, Liu W, Tang Z, Liu X, Zhang N, Li M, Zhang H. Review on thermal analysis in laser-based additive manufacturing. Optics and Laser Technology 2018;106:427-441. [10] Heralic A, Christiansson A, Lennartson B. Height control of laser metal-wire deposition based on iterative learning control and 3D scanning. Optics and Lasers in Engineering 2012;50:1230-1241. [11] Donadello S, Motta M, Demir G. A, Previtali B. Monitoring of laser metal deposition height by means of coaxial laser triangulation. Optics and Lasers in Engineering 2019;112:136-144. [12] Hagqvist P, Heralic A, Christiansson A, Lennartson B. Resistance based iterative learning control of additive manufacturing with wire. Mechatronics 2015;31:116-123. [13] Gao H, Gu Q, Takaki T, Ishii I. A self-projected light-section method for fast three-dimensional shape inspection. International Journal of Optomechatronics 2012;6:289-303. Fig. 1 Construction of height measurement system for laser metal-wire deposition: (a) height measurement system and (b) enlarged view of deposition position. Fig. 2 Bead height by projection position shift of line beam. M is the transverse magnification of the imaging system: (a) illustration of line beam and a weld bead and (b) image of line beam. Fig. 3 Configuration of wire-feeding speed control system using in-process height measurement system. Fig. 4 Luminance result by distance from a melt pool. Fig. 5 Displacement measurement result on a 1-layer bead. Z = 0 mm denotes focus position of a processing head: (a) displacement measurement result by Y position and (b) Z-pitch measurement result by Z-stage displacement, where each Y position’s result is calculated by ±50 µm range. Fig. 6 Line beam’s images captured by height measurement system: (a) during deposition and (b) without deposition.
Fig. 7 Comparison of height measurement results during and without deposition to true value: (a) 1-layer bead and (b) 3-layer bead. Fig. 8 Height measurement error of 1- and 3-layer beads. Error was estimated from differences between measurement result and true value. Fig. 9 Wire-feeding speed by deposition height. Fig. 10 Height measurement result of cylinder deposition object with a height gauge. Table 1. Deposition result by gap between weld bead and feed wire. Table 2. Cylinder deposition comparison result between our wire-feeding speed control system and conventional system without any process control. Gap between weld bead and feed wire in gap measurement result is at 180-degree position from deposition start position.
(a)
Camera
Collimate lens
Imaging system Band-pass filter Laser processing head
High power laser Gas nozzle Metal wire
Beam splitter Laser light Objective lens
(b) Gas nozzle
Line laser Line beam
Stage
Z
Bead
Y X Workpiece
Expansion Wire Bead Workpiece moves relative to a processing head by a stage
Fig. 1 Construction of height measurement system for laser metal-wire deposition: (a) height measurement system and (b) enlarged view of deposition position.
(a)
(b) Line beam
Wire
DX’=M・DX
Bead
q DZ DX
Fig. 2 Bead height by projection position shift of line beam: (a) illustration of line beam and a weld bead and (b) image of line beam.
Gap
NC machine
Height measurement System
Control Measurement Wire-feeding speed Z-stage pitch
Target height Gap
Line beam
Fig. 3 Configuration of wire-feeding speed control system using in-process height measurement system.
Luminance [kcd/m2]
60
50 40 No wire supply Wire supply Line laser
30 20
10 0
0
1
2 3 4 5 6 7 8 9 Distance from melt pool [mm]
10
Fig. 4 Luminance result by distance from a melt pool.
(a)
Z displacement-3～3 mm (Pitch = 0.25 mm)
Displacement [mm]
5 4 3 2 1 0 -1 -2 -3 -4 -5 -5
-4
-3
(b) Measured pitch [mm]
350
-2
-1 0 1 2 Y position [mm]
3
4
5
Y=-3.5 mm Y=-0.6 mm Y=2.5 mm
300 250
200 150 -3
-2
-1 0 1 Z displacement [mm]
2
3
Fig. 5 Displacement measurement result on a 1-layer bead. Z = 0 mm denotes focus position of a processing head: (a) displacement measurement result by Y position and (b) Z-pitch measurement result by Z-stage displacement, where each Y position’s result is calculated by ±50 mm range.
Line beam Melt pool
4 mm
Y X
(a)
(b)
Fig. 6 Line beam’s images captured by height measurement system: (a) during deposition and (b) without deposition.
2.5
During deposition Without deposition True value
0.9
Bead height [mm]
Bead height [mm]
1
0.8 0.7 0.6
During deposition Without deposition True value
2.3 2.1 1.9 1.7 1.5
0.5
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Measurement position [mm]
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Measurement position [mm]
Expansion During deposition Without deposition True value
0.8
0.75
0.7
0.65
20
25 30 35 40 45 Measurement position [mm]
(a)
50
2.1 2.05 2 1.95 1.9 1.85 1.8 1.75 1.7
During deposition Without deposition True value
Bead height [mm]
Bead height [mm]
0.85
Expansion
20
25 30 35 40 45 Measurement position [mm]
50
(b)
Fig. 7 Comparison of height measurement results during and without deposition to true value: (a) 1-layer bead and (b) 3-layer bead.
Measurement error [μm]
100 80 60 40 20 0 -20 -40 -60 -80 -100
1-layer bead 3-layer bead
5
15 25 35 45 Measurement position [mm]
55
Fig. 8 Height measurement error of 1- and 3-layer beads. Error was estimated from differences between measurement result and true value.
Wire-feeding speed [a.u.]
1.6
Z=0.20 mm Z=0.25 mm Z=0.30 mm
1.4 1.2
1 0.8 0.6
0
10
20 30 40 Deposition height [mm]
50
Fig. 9 Wire-feeding speed by deposition height.
Deposition height [mm]
0.3
Z=0.20 mm Z=0.25 mm Z=0.30 mm
0.2
0.1 0 -0.1
-0.2 -0.3
0
45 90 135 180 225 270 315 Rotation position of a cylinder [deg]
Fig. 10 Height measurement result of cylinder deposition object with a height gauge.
Table 1. Deposition result by gap between weld bead and feed wire. Droplets
Smooth
Stubbing
High
Optimal value
Low
Wire height
Laser Too high above a weld bead
Configuration
Droplet
Wire Bead
Bead
Stub of wire
Too low below a weld bead
Table 2. Cylinder deposition comparison result between our wire-feeding speed control system and conventional system without any process control. Gap between weld bead and feed wire in gap measurement result is at 180-degree position from deposition start position. 70
70
50
Z stage pitch [mm]
0.20
0.25
0.30
0.25
Gap measurement result
1.5 1 0.5 0 -0.5 -1 -1.5
Gap = -0.1 mm
0
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Highlights An in–process height measurement system for laser metal-wire deposition is proposed Weld bead height near melt pool is measured by light section method during deposition Measurement system is designed to be integrated in a laser processing head Wire–feeding speed is controlled based on bead height for high–quality deposition
S0141-6359(19)30770-6
DOI:
https://doi.org/10.1016/j.precisioneng.2019.11.007
Reference:
PRE 7058
To appear in:
Precision Engineering
Received Date: 25 October 2019 Accepted Date: 5 November 2019
Please cite this article as: Takushima S, Morita D, Shinohara N, Kawano H, Mizutani Y, Takaya Y, Optical in-process height measurement system for process control of laser metal-wire deposition, Precision Engineering (2019), doi: https://doi.org/10.1016/j.precisioneng.2019.11.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Optical in-process height measurement system for process control of laser metal-wire deposition Shigeru Takushimaa*, Daiji Moritaa, Nobuhiro Shinoharaa, Hiroyuki Kawanoa, Yasuhiro Mizutanib, Yasuhiro Takayab a Advanced Technology R&D Center, Mitsubishi Electric Corp. 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b Department of Mechanical Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka, 565-0871 Japan We propose an in-process height measurement system for a weld bead and feedback control system for wire-feeding speed for high-quality laser deposition. Metal additive manufacturing, especially laser metal-wire deposition, is effective for complex shape fabrication and repair processing. However, we must control the gap between a weld bead and a feed wire in an optimal range for high-quality deposition. Conventionally, the Z-stage pitch for multi-layer deposition must be precisely adjusted by each deposition shape. In this paper, we design an in-process height measurement system that is integrated in a laser processing head, which measures the weld bead height by a line section method. We decreased the influence of the intense thermal radiation generated from a melt pool by inserting the band-pass filter of the line beam’s wavelength in the imaging system and optimizing its line laser power. Consequently, our system can measure the weld bead height near the melt pool, which is 4 mm in front of it. Next we show that our proposed system can measure the weld bead height during wire-laser metal deposition with 50µm accuracy by comparing its value to the true value. Finally, we achieved a cylinder shape deposition of 50-mm height, regardless of the Z-stage pitch and the cylinder diameter of the multi-layer deposition, by controlling the wire-feeding speed based on the measured weld bead height. Keywords: Height Measurement; Optical Sensing; Process Monitoring; Additive Manufacturing; Laser Metal Deposition; Light Section Method 1. Introduction Additive manufacturing (AM) technology is innovative because it can fabricate complex shaped parts quickly and inexpensively. In particular, it is effective in complex metal processing where fabrication is complicated by conventional cutting machining. In powder bed fusion (PBF) methods, lead metal powders are melted by lasers or electron beams and molded into layers [1,2]. Although PBF can fabricate precise shapes, long fabrication times are required and fabricating large parts is difficult. On the other hand, in the directed energy deposition (DED) method, metal wires or powders are fed to the added part and welded by lasers, arc plasma, or electron beams [3-5]. Owing to partial fabrication, the DED method is useful for repairing incomplete parts and the high-speed fabrication of large complex parts. In laser metal deposition (LMD), the metal powder method has been researched [3] because feeding metal material and process control are easy. However, the metal wire method’s [5] fabricating speed is faster and it has higher fabricated density [6]. Furthermore, a large variety of metal materials can be used easily with the wire materials of a welding machine. However, only the powder method in LMD is currently available on the market because the process control of the wire method is more difficult. A process monitoring technique is also critical for high-quality fabrication with laser metal-wire deposition (LMwD) [7], including melt pool monitoring by a camera [5,8], the thermal measurement of a melt pool [4,9], and the height measurement of a weld bead [5,8,10,11]. High-quality laser deposition is especially difficult without controlling the gap between a weld bead and a feed wire to an optimal range. The deposition result based on the gap between a weld bead and a feed wire is shown in Table 1. When the feed wire is too high above a weld bead, its surface becomes uneven due to droplets since the metal wire that was melted by the processing laser is not sufficiently attached to the weld bead surface. In contrast, when the feed wire is too low below a weld bead, stubbing occurs because the wire was not adequately melted by the processing laser since the feed wire was stuck to the weld bead. Consequently, we have to control the gap between a weld bead and a feed wire within the optimal range for high-quality laser deposition. In addition, the bead height of one layer becomes lower than the height of the first layer as the deposition progresses due to thermal storage during multi-layer deposition. The Z-stage pitch of the multi-layer deposition conventionally needs to be adjusted precisely based on each deposition shape and all of the deposition parameters (wire-feeding speed, laser power, scanning speed, and so on) for highquality deposition. Therefore, the gap between a weld bead and a feed wire must be controlled within an optimal range regardless of the Z-stage pitch and the cylinder diameter by the feedback control of a
deposition parameter based on the measured weld bead height at each layer [8,10,12]. As a result of our experiment, the target measurement accuracy of the bead height for continuous deposition without droplets and stubbing was ±150 µm. The light section method, which was previously proposed for measuring the weld bead height of the LMwD [8,10], measures an object’s height by calculating the centroid of the projected line beam that is imaged by a camera set obliquely to the line beam’s projection direction [13]. However, the setup of a previous method [8] was too large because the imaging and illuminating systems faced each other with a processing laser head between them. When there is a gas nozzle with a processing head and a wire that is fed from the side, measuring the projected position of the line beam is difficult. Furthermore, a method that images from an oblique direction to the deposition plane (X-Y plane) is also difficult for accurately measuring the bead height because the calculation accuracy of the line beam’s centroid was decreased by aberrations and defocusing due to the difference of the imaging distance in the camera’s field of view. An in-process height measurement is desirable for LMwD. Although separating the measurement process from the deposition process is easy, the manufacturing time becomes longer. Hence, it is effective to control the deposition parameters by the in-process measurement result of the previous layer’s bead height near the melt pool. However, intense thermal radiation with a broad spectrum is generated because a high power processing laser is irradiated at a melt pool during the deposition processing. The bead height must be measured with high accuracy without the influence of intense thermal radiation. However, if we measure the beads’ height at far position from the melt pool to reduce the influence of the intense thermal radiation, LMwD that fabricate complex shape parts cannot measure onto the bead because the measurement positon become off the processing route. Therefore, we must measure the bead’s height at near position from the melt pool in possible. We propose an optical in-process height measurement system for the feedback control of wire-feeding speed for LMwD and expect to launch it on the market by establishing an in-process height measurement and a feedback control technique. Our proposed measurement system consists of an imaging system coaxially integrated in a laser processing head and an illuminating system that obliquely projects a line beam, which measures the weld bead height by a line section method. We designed an imaging system that shares an objective lens for the laser deposition and the height measurement as well as the projection angle of the line beam to measure the weld bead height with high accuracy. We decreased the influence of the intense thermal radiation generated from a melt pool by inserting the band-pass filter of the line beam’s wavelength in the imaging system and optimizing its line laser power. Therefore, our system can measure the weld bead height near the melt pool, which is 4 mm in front of it. Finally, it can achieve high-quality deposition by controlling the wire-feeding speed based on the measured weld bead height and retaining the gap between the weld bead and the feed wire at an optimal value. In this paper, we show the optical design of the imaging and illuminating systems and quantitatively demonstrate the accuracy evaluation result of our height measurement system on an LMwD by comparing it to a highly accurate height measurement instrument. We also argue that the high-quality deposition of cylinder objects is possible regardless of the Z-stage pitch and the cylinder diameter by controlling the wire-feeding speed based on the measured weld bead height. 2. System We show the construction of our proposed height measurement system for LMwD in Fig. 1. Because it has a 5-axis stage, it fabricates freeform shapes by moving a workpiece relative to a processing head while feeding a metal wire to the deposition position and irradiating a high power processing laser. A processing laser light is reflected to the workpiece by a beam splitter. The reflected light is focused by an objective lens and irradiated to the deposition position through a shielding gas nozzle to prevent metal oxidation during deposition. The metal wire is fed to the deposition position and melted by the irradiated processing laser. We propose a height measurement system with a coaxial setup of the laser processing axis by integrating the imaging system with a processing head. We attached a line beam to the side of a processing head (Fig. 1) and projected it in front of the melt pool to measure through a gas nozzle. The line beam reflected from a weld bead is captured by an objective lens used for deposition processing. An imaging system images the reflected light that passed through a beam splitter onto a camera integrated with a processing head. Consequently, we can coaxially measure the line beam position through a gas nozzle by projecting it from the opposite direction of the feeding wire. Since our system measure the line beam through a gas nozzle, we can achieve both the bead’s height measurement and monitoring the melt pool by only one camera.
Hence, there is a possibility that our system can control process parameter based on monitoring result of the melt pool during the bead’s height measurement in the future. In the line section method, the height measurement’s sensitivity is determined by the projection angle of the line beam and the transverse magnification of the imaging system. The relation between the measurement height and the projected position’s shift of the line beam is shown in Fig. 2. Displacement ∆Z by the projected position’s shift of line beam ∆X is shown in Eq. (1), where θ is the projection angle of the line beam: ∆Z = ∆X ∙ tan . (1) The image of the line beam on an image sensor is shown in Fig. 2 (b). The shift of line beam’s centroid by displacement ∆Z is ∆X’ which is equal to M･∆X, where M is the transverse magnification of the imaging system. The displacement by the camera’s 1-pixel ∆H is shown in Eq. (2), where the camera’s pixel size is P : H = ∙ tan . (2) We calculated the height of a weld bead by the line beam’s centroid on a camera image. The centroid in the X direction of each Y-direction’s pixel in a captured image was calculated, and the cross-section height distribution in the Y direction was estimated by Eq. (2). Next we propose a control system of the deposition parameters with our height measurement system. In the deposition parameters including laser power and stage scanning speed, the wire-feeding speed is the best parameter to control the bead height without influencing the bead width [8]. The configuration of our wire-feeding speed control system with an in-process height measurement system is shown in Fig. 3. Although the Z-stage pitch that was deposited every layer was conventionally constant during the multilayer deposition, the deposition height of each layer was not equal due to the thermal storage. Therefore, we controlled the wire-feeding speed by numerical control (NC) so that the deposition height reached the target height. The controlling value of the wire-feeding speed was calculated based on the gap between the weld bead and the feed wire, which was estimated by the measuring bead height of previous layer in front of the melt pool using our height measurement system. The control value of the wire-feeding speed was estimated experimentally from the relationship with the wire-feeding speed and the bead height in advance. The value of the wire-feeding speed was controlled linearly based on the measured bead height in our system. 3. Experiment setup We experimented with our prototype height measurement system attached to the LMwD. In this setup, the processing head has X-, Y-, and Z-stages and the workpiece is on a C-axis stage. The imaging system is set on the processing head, and the illuminating system is integrally attached to its side. We designed the sensitivity of the height measurement by Eq. (2). The line beam’s projection angle θ should be vertical because the weld bead surface has mostly specular reflection. We designed the projection angle of line beam θ, the transverse magnification of imaging system M, and the camera’s pixel size P so that the displacement by the camera’s 1-pixel ∆H is 30 µm. The line beam’s projection position should be as near the deposition position as possible for accurate height measurement. The projected line beam is not vertical to the deposition direction of the weld bead if the line beam’s projection position is far from the deposition position because LMwD fabricates complex shapes like curves. We designed the line beam’s projection position to be 4 mm from the laser processing axis’s center because the melt pool’s maximum diameter is φ6 mm and the gas nozzle’s diameter is φ10 mm. Since the line beam’s projection position at focus position was set 4 mm from the laser processing axis’s center, we can calculate ±1-mm centroid shift on a camera image. Hence, the height measurement range of our system is ±2-mm by Eq. (1). However, it is more difficult to optically measure the bead height due to the influence of the intense thermal radiation generated from a melt pool if the measurement position is close to the melt pool. Therefore, we inserted a 520-nm band-pass filter of the line beam’s wavelength into the imaging system to reduce the influence of the thermal radiation that has a broad spectrum whose center wavelength is infrared. We also measured the thermal radiation’s luminance based on the distance from the melt pool to estimate the required line beam power. The luminance result of a 520-nm line beam wavelength is shown in Fig. 4. The φ3-mm luminance from the melt pool’s center is very intense. Furthermore, the generated luminance of the thermal radiation is more intense when a wire was fed for the deposition. Consequently, we used a line laser whose power is 50 mW so that the line beam’s luminance at 4 mm from the melt pool is adequately high for the thermal radiation. When we measured the luminance of the 50-mW line beam (Fig. 4), the luminance at 4 mm from the melt pool is
more than twice the thermal radiation. Consequently, our system can measure the bead’ height at 4 mm in front of the melt pool although melt pool’ radius is 3mm. This shows that our system can measure the bead’ height at the physically nearest position from the melt pool. 4. Result and discussion 4.1. Accuracy evaluation The displacement measurement result between a weld bead and a processing head by the Z-stage shift is shown in Fig. 5(a). In this paper, the metal-wire material we used was Inconel. We measured a 1-layer weld bead when the X- and Y-stages are stopped without deposition processing. 0-mm displacement means that the Z-stage is the focus position of the processing laser. We measured from -3 to +3 mm by a 250-µm pitch. The estimated pitch from the measured displacement at the weld bead position (Y = -0.6 mm) and the flat workpiece position (Y = -3.5 mm and 2.5 mm) is shown in Fig. 5(b). The pitch of each Y position averages ±50 µm (11 pixel) in the Y direction. Our proposed system can measure the displacement both of a weld bead and a flat workpiece at ±50-µm accuracy. Next we evaluated whether our proposed system can measure the height at 4 mm in front of a melt pool during laser deposition processing like without laser processing. The images captured during and without laser processing are shown in Fig. 6. Our proposed system captured the line beam’s image at 4 mm in front of a melt pool as in the image without deposition because we inserted a band-pass filter in the imaging system and optimally designed the line beam’s power. The line beam’s image during laser deposition was not influenced by the intense thermal radiation. We evaluated the height measurement accuracy of our measurement system by comparing its result to a highly accurate height measurement instrument. We deposited 1- and 3-layer striped beads on a plate workpiece. The heights on the weld bead and the flat plate were measured by an optical non-contact height measurement instrument (NH-3N from Mitaka Kohki), where the measurement position of the flat plate was 2 mm from the bead’s center. We set the workpiece on the LMwD again and measured the weld bead height by our measurement system both during laser deposition when the thermal radiation is generated and without deposition. The weld bead height by the measurement position is shown in Fig. 7, where the true value is the result of the height measurement instrument. We calculated the weld bead height by the difference between the flat workpiece surface and the top of the weld bead, and the flat workpiece surface has an average result value at Y= ±2 mm from the weld bead. This evaluation acquired the measurement position of each image captured by the camera for the height measurement from the LMwD’s stage position by sending a trigger signal when the camera captured the image. This result shows that our system can identically measure the height of both the 1- and 3-layer beads during and without laser deposition. This height value is the same as the true value that was measured by the height measurement instrument. The start and end points of the beads were also measured accurately. The height measurement error of the 1- and 3-layer beads is shown in Fig. 8, calculated by the difference between the during-deposition and the true value results in Fig. 7. This evaluation shows the result at a flat position (X=5-55 mm), except for the start and end positions of a bead whose slope is sharp. The 1-layer bead result shows that our system’s accuracy is PP=64.6 µm (σ=13.2 µm), and the 3-layer bead result shows that its accuracy is PP=87.6 µm (σ=11.5 µm). We achieved ±50-µm accuracy for the height measurement during laser deposition, which was more accurate than the target accuracy ±150 µm. 4.2. Process control evaluation The cylinder deposition comparison between our wire-feeding speed control system and the conventional system without any process control is shown in Table 2. In gap measurement result, the negative value of the gap means that the previous layer’s bead height is higher than the wire height in Fig. 3. In contrast, the positive value of the gap means that the previous layer’s bead height is lower than the wire height. Therefore, if the gap is negative value, the bead height of current deposition layer must be lower by decreasing the wire-feeding speed. If he gap is positive value, the bead height of current deposition layer must be higher by increasing the wire-feeding speed. We estimated the gap between the weld bead and the feed wire in the gap measurement result at 180 degrees from the deposition start position. The deposition height in the horizontal axis was calculated from the number of deposition layers and the Z-stage pitch. For the conventional method without any wire-feeding speed control, the maximum continuous deposition height was 44 mm when the Z-stage pitch was 0.25 mm, the cylinder’s diameter was φ70 mm, and the wire-feeding speed was set to an optimal value. Here, the optimal value means the wire-feeding speed that could continuously highest deposition estimated experimentally in advance.
However, the gap between the weld bead and the feed wire in Table 2 was not constant during the deposition. The wire was near the bead until the deposition height was 15 mm, and after that it moved away from the bead. The deposition could not be continued due to the droplets when the deposition height was around 44 mm. On the other hand, when the Z-stage pitch was reduced to 0.2 mm, continuous deposition was stopped by stubbing at an 18-mm deposition height. When the deposition height was around 5 mm, height measurements were impossible because the bead surface was shaved by the stubbed wire. When the Z-stage pitch was increased to 0.3 mm, continuous deposition was stopped by droplets at a 10-mm deposition height. In this experiment, continuous deposition was difficult in the conventional method without-process control if the Z-stage pitch was not optimally adjusted at less than ±0.05 mm. When the deposition shape changed, for example, the cylinder diameter became φ50 mm, the continuous deposition height was reduced to 25 mm, even if the Z-stage pitch was 0.25 mm and the wire-feeding speed was set to an optimal value, which is same as the φ70-mm cylinder. This means that the optimal Zstage pitch must be adjusted for each deposition shape and the parameters. In contrast, we achieved continuous deposition of 50-mm height with our proposed control system of wire-feeding speed regardless of the deposition parameter and shape, such as the Z-stage pitch and the cylinder diameter. We controlled the gap between the weld bead and the feed wire at ±0.1-mm accuracy. Although we controlled the wire-feeding speed by a constant value based on the measured gap, we should dynamically modify the controlling value of the wire-feeding speed by the deposition temperature or the measured bead height for more accurate control. The control result of the wire-feeding speed by the deposition height is shown in Fig. 9. The wire-feeding speed was normalized by an optimal value when the Z-stage pitch was 0.25 mm in the conventional method. When the Z-stage pitch was 0.25 mm, the wire-feeding speed was reduced until the deposition height was around 15 mm; at this range, the gap was smaller than the optimal value. This means that Moreover, the wire-feeding speed was faster after the deposition height was around 15 mm; at this range, the gap was larger than the optimal value. This result shows that we controlled the 1-layer deposition height that was changed by the thermal storage to equal the Z-stage pitch. We also controlled the 1-layer deposition height to equal the Z-stage pitch even when it was different from 0.25 mm. The wire-feeding speed was reduced when the Z-stage pitch was 0.2 mm, and the wire-feeding speed was faster when the Z-stage pitch was 0.3 mm. The deposition height result of cylinder objects measured with a height gauge (HD-30AX from Mitutoyo) is shown in Fig. 10. We measured the cylinder deposition objects fabricated by controlling wire-feeding speed when Z-stage pitch is 0.2 mm, 0.25 mm and 0.3mm. We measured the deposition height from the base plate at each rotation position of a cylinder by a 45-degree pitch. The deposition height in the vertical axis is the difference from the average value in one rotation result. The deposition height accuracy in one rotation of our system was achieved ±0.1 mm by controlling the wire-feeding speed. This result is more accurate than the 0.4-mm accuracy of the previous method [8]. Consequently, our system could deposite the flat surface by controlling the wire-feeding speed based on the measured weld bead height. 5. Conclusion We proposed an in-process height measurement system of a weld bead by a line section method and a feedback control system of wire-feeding speed for high-quality laser deposition. Our proposed measurement system consisted of an imaging system integrated in a laser processing head and an obliquely illuminating system that projected a line beam. We designed an imaging system to share an objective lens for laser deposition and height measurement. Moreover, a projection angle of the line beam was also designed to measure the weld bead height with high accuracy. In addition, we achieved a height measurement of 4 mm in front of the melt pool by inserting the band-pass filter of the line beam’s wavelength into the imaging system and optimizing its line laser power to decrease the influence of the intense thermal radiation generated from a melt pool. Our measurement system measured the weld bead height during laser deposition with 50-µm accuracy compared to the true value measured by a highly accurate height measurement instrument. We controlled the gap between the weld bead and the feed wire at ±0.1-mm accuracy to an optimal value using the developed control system of the wire-feeding speed based on the measured weld bead height. Although we had to precisely adjust the Z-stage pitch for each deposition shape and the parameters in the conventional method without any process control, we achieved high-quality continuous deposition over 50-mm height by our proposed system regardless of the Z-stage pitch and the cylinder diameter. Our proposed in-process height measurement and feedback control
technique greatly improved LMwD for practical use. Our future work will improve the centroid calculation algorithm to decrease the influence of the diffuse reflection by the local roughness on the bead surface. Furthermore, we will apply this system to other complex shape depositions. 6. References [1] Simchi A, Pohl H. Direct laser sintering of iron–graphite powder mixture. Materials Science and Engineering 2004;383:191-200. [2] Simchi A. Direct laser sintering of metal powders: Mechanism, kinetics and microstructural features. Materials Science and Engineering 2006;428:148-158. [3] Milewski O. J, Lewis K.G, Thoma J.D, Keel I. G, Nemec B. R, Reinert A. R. Directed light fabrication of a solid metal hemisphere using 5-axis powder deposition. Journal of Materials Processing Technology 1998;75:165-172. [4] Zalameda N. J, Burke R. E, Hafley A. R, Taminger M. K, Domack S. C, et al. Thermal Imaging for Assessment of Electron-Beam Freeform Fabrication (EBF3) Additive Manufacturing Deposits. SPIE Proceedings 2013;8705:1-8. [5] Liu S, Liu W, Harooni M, Ma J, Kovacevic R. Real-time monitoring of laser hot-wire cladding of Inconel 625. Optics & Laser Technology 2014;62:124-134. [6] Nurminen J, Riihimäki J, Näkki J, Vuoristo P. Comparison of laser cladding with powder and hot and cold wire techniques. Proceedings of international congress on application of lasers & electro-optics 2006:634-637. [7] Everton K. S, Hirsch M, Stravroulakis P, Leach K. R, Clare T. A. Review of in-situ process monitoring and in-situ metrology for metal additive manufacturing. Materials and Design 2016;98:431-445. [8] Heralic A, Christiansson A, Ottosson M, Lennartson B. Increased stability in laser metal wire deposition through feedback from optical measurements. Optics and Lasers in Engineering 2010;48:478-485. [9] Yan Z, Liu W, Tang Z, Liu X, Zhang N, Li M, Zhang H. Review on thermal analysis in laser-based additive manufacturing. Optics and Laser Technology 2018;106:427-441. [10] Heralic A, Christiansson A, Lennartson B. Height control of laser metal-wire deposition based on iterative learning control and 3D scanning. Optics and Lasers in Engineering 2012;50:1230-1241. [11] Donadello S, Motta M, Demir G. A, Previtali B. Monitoring of laser metal deposition height by means of coaxial laser triangulation. Optics and Lasers in Engineering 2019;112:136-144. [12] Hagqvist P, Heralic A, Christiansson A, Lennartson B. Resistance based iterative learning control of additive manufacturing with wire. Mechatronics 2015;31:116-123. [13] Gao H, Gu Q, Takaki T, Ishii I. A self-projected light-section method for fast three-dimensional shape inspection. International Journal of Optomechatronics 2012;6:289-303. Fig. 1 Construction of height measurement system for laser metal-wire deposition: (a) height measurement system and (b) enlarged view of deposition position. Fig. 2 Bead height by projection position shift of line beam. M is the transverse magnification of the imaging system: (a) illustration of line beam and a weld bead and (b) image of line beam. Fig. 3 Configuration of wire-feeding speed control system using in-process height measurement system. Fig. 4 Luminance result by distance from a melt pool. Fig. 5 Displacement measurement result on a 1-layer bead. Z = 0 mm denotes focus position of a processing head: (a) displacement measurement result by Y position and (b) Z-pitch measurement result by Z-stage displacement, where each Y position’s result is calculated by ±50 µm range. Fig. 6 Line beam’s images captured by height measurement system: (a) during deposition and (b) without deposition.
Fig. 7 Comparison of height measurement results during and without deposition to true value: (a) 1-layer bead and (b) 3-layer bead. Fig. 8 Height measurement error of 1- and 3-layer beads. Error was estimated from differences between measurement result and true value. Fig. 9 Wire-feeding speed by deposition height. Fig. 10 Height measurement result of cylinder deposition object with a height gauge. Table 1. Deposition result by gap between weld bead and feed wire. Table 2. Cylinder deposition comparison result between our wire-feeding speed control system and conventional system without any process control. Gap between weld bead and feed wire in gap measurement result is at 180-degree position from deposition start position.
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Fig. 7 Comparison of height measurement results during and without deposition to true value: (a) 1-layer bead and (b) 3-layer bead.
Measurement error [μm]
100 80 60 40 20 0 -20 -40 -60 -80 -100
1-layer bead 3-layer bead
5
15 25 35 45 Measurement position [mm]
55
Fig. 8 Height measurement error of 1- and 3-layer beads. Error was estimated from differences between measurement result and true value.
Wire-feeding speed [a.u.]
1.6
Z=0.20 mm Z=0.25 mm Z=0.30 mm
1.4 1.2
1 0.8 0.6
0
10
20 30 40 Deposition height [mm]
50
Fig. 9 Wire-feeding speed by deposition height.
Deposition height [mm]
0.3
Z=0.20 mm Z=0.25 mm Z=0.30 mm
0.2
0.1 0 -0.1
-0.2 -0.3
0
45 90 135 180 225 270 315 Rotation position of a cylinder [deg]
Fig. 10 Height measurement result of cylinder deposition object with a height gauge.
Table 1. Deposition result by gap between weld bead and feed wire. Droplets
Smooth
Stubbing
High
Optimal value
Low
Wire height
Laser Too high above a weld bead
Configuration
Droplet
Wire Bead
Bead
Stub of wire
Too low below a weld bead
Table 2. Cylinder deposition comparison result between our wire-feeding speed control system and conventional system without any process control. Gap between weld bead and feed wire in gap measurement result is at 180-degree position from deposition start position. 70
70
50
Z stage pitch [mm]
0.20
0.25
0.30
0.25
Gap measurement result
1.5 1 0.5 0 -0.5 -1 -1.5
Gap = -0.1 mm
0
10 20 30 40 Deposition height [mm]
50
10 20 30 40 Deposition height [mm]
50
0
Deposition stopped at 44 mm height by droplets 1.5 1 0.5 0 -0.5 -1 -1.5
Gap = 0 mm
0
10 20 30 40 Deposition height [mm]
50
Gap [mm]
Gap [mm]
0
50
Deposition stopped at 18 mm height by stubbing
Gap [mm]
Picture of deposited cylinder
10 20 30 40 Deposition height [mm]
1.5 1 0.5 0 -0.5 -1 -1.5 10 20 30 40 Deposition height [mm]
Deposition stopped at 11 mm height by droplets 1.5 1 0.5 0 -0.5 -1 -1.5
Gap = +0.1 mm
0
10 20 30 40 Deposition height [mm]
1.5 1 0.5 0 -0.5 -1 -1.5
50
0
50
10 20 30 40 Deposition height [mm]
1.5 1 0.5 0 -0.5 -1 -1.5
Gap = +0.1 mm
0
10 20 30 40 Deposition height [mm]
Control Picture of deposited cylinder
Deposition height reaches 50 mm
Deposition height reaches 50 mm
Deposition height reaches 50 mm
50
Deposition stopped at 25 mm height by droplets
Gap [mm]
0
Gap [mm]
Without control
Target value
1.5 1 0.5 0 -0.5 -1 -1.5
Gap [mm]
Gap measurement result
1.5 1 0.5 0 -0.5 -1 -1.5
Gap [mm]
70
Gap [mm]
Cylinder diameter f [mm]
Deposition height reaches 50 mm
50
Highlights An in–process height measurement system for laser metal-wire deposition is proposed Weld bead height near melt pool is measured by light section method during deposition Measurement system is designed to be integrated in a laser processing head Wire–feeding speed is controlled based on bead height for high–quality deposition