Laser Welding Head Research Articles

All-fiber 3 kW LP02 laser output based on long-period fiber grating for precise welding

An innovative all-fiber mode converter specifically designed for high-power fiber lasers is introduced. The mode converter relies on the coupling theory of long-period fiber gratings (LPFGs) and involves the fabrication of a LPFG, where the performance is meticulously studied by examining various fabrication parameters. The LPFG is integrated into a high-power fiber laser system to achieve efficient conversion from the LP01 mode to the LP02 mode with a conversion ratio of 90 % and a low transmission loss of only 0.088 dB. Under a 3.03 kW laser output, the LP02 fiber laser exhibits a remarkable laser-to-raman peak intensity ratio, which is 15.98 dB higher than that of the signal-mode fiber laser. Furthermore, during the welding process of a thick aluminum plate, the LP02 fiber laser demonstrates a significant increase of 1.2 times in the depth of the weld pool morphology compared to the signal-mode fiber laser, particularly evident when operating at a laser welding head speed of 100 mm/s. The mode converter with LPFG exhibits lower insertion loss, enhanced power-handling capacity, making it ideal for diverse high-power fiber laser applications with promising practical prospects.

High-power polarization-maintaining LP11-mode fiber laser based on long-period fiber grating for precise welding.

An LP11-mode output all-fiber laser was presented, utilizing long-period fiber gratings (LPFGs) and polarization-maintaining optical fiber (PMF). The LPFG was designed and fabricated, achieving a 90.56% efficiency in LP01 to LP11 mode conversion. Furthermore, the transmission stability of LP11-mode in the PMF was also explored, with the spatial mode overlap ratio exceeding 0.95. Ultimately, the high-power polarization-maintaining (PM) fiber laser, capable of the LP11 mode output, was constructed, with the output power of 600 W and the beam quality M2 of 2.84. During the process of welding a thick Al-plate, the LP11 fiber laser exhibits a notable 1.88 times greater depth of fusion compared to the commercial single-mode fiber laser, when operating at the laser welding head speed of 100 mm/s. For applications demanding non-circular symmetric high-order modes, this research holds substantial potential for widespread adoption within the field of industrial processing.

Process monitoring by deep neural networks in directed energy deposition: CNN-based detection, segmentation, and statistical analysis of melt pools

The complex interaction between laser and material in Laser Wire Direct Energy Deposition (LW-DED) Additive Manufacturing (AM) benefits from process monitoring methods to ensure process stability and final part quality. Understanding the relationship between process parameters and melt pool geometrical characteristics can be used to effectively monitor and in-process control the process, as the melt pool geometrical characteristics serve as crucial indicators of process stability and quality.This study presents a novel in-situ monitoring approach for LW-DED, utilizing process images for melt pool segmentation, melt pool geometrical characteristics estimation, process stability assessment, and bead geometry prediction. The segmentation of melt pool objects was successfully accomplished using Convolutional Neural Networks (CNN)-based models, enabling the prediction of essential parameters such as melt pool area, height, width, center of area, and the center point of the bounding box enclosing the melt pool. Multiple models were compared regarding the accuracy and processing speed using a controlled central composite design and random experiments. We used an Inconel alloy 625 wire and two distinct substrate materials for deposition, a coaxial laser welding head with a 3 kW fiber laser, and an off-axis welding camera for monitoring.Among the CNN architectures evaluated, YOLOv8l demonstrated superior accuracy with mean Average Precision (mAP) values of 0.925 and 0.853 for Stainless Steel (SS) and low carbon steel (S355) substrates, respectively. Additionally, YOLOv8s exhibited a notable processing speed of over 114 frames per second, which indicates its suitability for real-time process control. Furthermore, the results indicate a significant correlation between process parameters and melt pool geometry variables. Notably, a clear correlation was established between melt pool characteristics and bead geometries obtained through microscopic examinations, including penetration depth and heat-affected zone. The analysis revealed a significant correlation for the bead area and width parameters. In relation to the bead height, while the correlation exhibited a lower magnitude compared to bead area and width, it remained responsive. In addition, the tensor masks derived from the developed models have proven to be highly effective in accurately predicting bead geometries.The results demonstrate the effectiveness of YOLO-based algorithms for detecting and segmenting the melt pool. Statistical analysis confirms the significance of stabilized process data and the accuracy of melt pool geometric models. We demonstrate that integrating advanced monitoring and control techniques using artificial intelligence methods like CNN can facilitate process stability and quality control.

Design of a laser welding plugging system for the steam generators of high-temperature gas-cooled reactor

As fourth-generation nuclear power equipment, high-temperature gas-cooled reactor (HTGR) is attracting more and more attention both in industry and academia. The core outlet temperature of the steam generator (SG) in HTGR is much higher than that of pressurized water reactor steam generators, which is more likely to cause damage. Till now, welding plugging is still one of the important means to prevent the leakage of damaged heat transfer tubes. However, how to improve the automation of welding plugging to reduce the short-range operation time in a radioactive environment is still difficult. This work proposes the design of a laser welding plugging system for the HTGR. Firstly, the design requirements are analyzed and the overall design scheme is given. The whole system consists of a laser generator, a multi-axis robot, and supporting devices. Secondly, the key supporting devices including a laser welding head, a pneumatic clamping jaw, and a single-axis motion platform are explained in detail. The structure simulation based on SolidWorks is adopted, which provides the basis for the design of the single-axis motion platform. Thirdly, the control system which takes the PLC control system as the main control unit and the industrial computer as the monitoring data processing unit is described. Finally, an experimental system is built and the laser welding experiment is carried out to test the feasibility of the designed laser welding plugging system.

Seam detection method design and automatic calibration based on an active light coaxial visual sensor

Seam feature detection is one of the critical premises of automatic welding. However, the current seam detection method is vulnerable to complex laser environments, such as illumination interference and material defects, thereby limiting the application of active light coaxial visual sensors in seam tracking. This paper proposes a seam detection and calibration method based on an active light coaxial visual sensor in a laser welding digital twin environment. First, a vision sensor with an 808 nm auxiliary illumination and a customized lens are mounted on the laser welding head to filter out noises and capture the welding image, including the seam and melt pool. Next, a seam information extraction approach based on image processing algorithms is designed to extract seam position and seam width. In addition, an artificial neural network based Kalman filter is proposed to eliminate measurement errors and reduce measurement noise. The calibration of the seam detection parameters is a tedious process. Therefore, the calibration problem is transformed into an optimization problem. A genetic algorithm-based calibration method is proposed to automatically determine the optimum parameters of the seam detection algorithm. Finally, seam detection experiments on laser welding are conducted in both the digital twin environment and the laser welding system. The experimental results show that the proposed method is more efficient than the Hough transfer-based seam detection method. The calibration method can ensure the accuracy of the seam detection method, especially in complex and changeable welding conditions.

Laser welded lip seal for UHV class fusion vessels–A methodical study

Lip seals are considered, the most promising options for the vacuum boundary joints in large size vacuum vessels (like rectangular neutral beam vessel with an opening size of ∼30 m) of fusion machines. Lip seals are designed with the corner welding of 2 mm thickness sheets of SS304L with the welding penetration requirement of 3 mm. With the first phase of development, laser welding parameter optimization and the sensitivity analysis was carried out to understand the effect of manufacturing tolerances (like air gap between the plates, laser beam misalignment and plate misalignment) on the weld quality. To ascertain the defined tolerances, a specialized clamping tool was configured, designed, manufactured and tested by integrating that with the assembly of laser welding head and robotic arm. For the prototype welding demonstration, the Lip seal welding of a rectangular configuration and an elliptical configuration of size 3 m × 1.5 m (∼1/3rd size of Diagnostic Neutral Beam (DNB) vessel lip seal); and 1 m length with 1.2 m diameter (exact size of DNB high voltage bushing lip seal) are considered. Welding qualification was done with the Non-Destructive and Destructive examinations. Further, the camera based feedback system for the seam tracking was incorporated to the system, which enables the remote detection and correction of the weld path. This development also extended to enable the repair of welding operation.

Design of the integrated laser welding head with a continuously adjustable focus rotation radius

In this paper, the integrated laser welding head with a continuously adjustable focus rotation radius was achieved by using the output shaft (hollow shaft) of the high-frequency motor as the laser transmission tube for laser transmission, laser deflection and laser focus rotation. The continuous adjustment of the focus rotation radius was achieved by changing the relative phase angle between the two wedge prisms and using a focusing lens. And the function relationship between the focal rotation radius and the relative phase angle of two wedge prisms is verified and modified based on the measured results. The energy loss rate of the 5 kW laser through the inner diameter of the hollow shaft is less than 5% and it has little effect on weld depth. The temperature of the hollow shaft can rise to 57.4 °C only at the entrance of the hollow shaft under the long action time (120 s) of high-power (6 kW) laser. Both the weld forming and porosity defects of 5083 aluminiumalloy can be effectively regulated and controlled by using the integrated laser welding head.

Hybrid Laser-GMA Welding of High-Strength Steel Grades

Abstract The results of investigations on hybrid laser + gas metal arc (GMA) welding of butt joints of different fine-grained and thermomechanically rolled steel plates 5.0 mm thick are presented in the article. The hybrid welding tests were carried out on steel grade Domex 700MC, Domex 960, and Domex 1100. The experimental stand for hybrid welding consisted of the laser welding head and also the arc GMA torch. Butt joints of Domex 700MC plates were hybrid welded applying a metal cored wire Stein Megafil 742B (Charpy V: 80 J at −20°C). In turn, the butt joints of Domex 960, and Domex 1100 steel plates were hybrid welded applying a metal cored wire Stein Megafil 1100M (Charpy V: 55 J at −20°C). The quality of the test joints was analyzed by means of visual inspection and cross-section observation. In turn, the mechanical properties were evaluated by means of a static tensile test of the welded joints and also impact toughness of the weld metal. The results showed that the hybrid welding can provide a proper shape of weld even at altering gap between the butt surfaces of the groove and also at distinct shift of the electric arc to the laser beam spot. Results of tensile tests showed that the hybrid welded joints of Domex 960 and 1100 have the strength at similar levels of approximately 1,140 MPa, which is significantly higher compared to the joints of Domex 700MC at approximately 820 MPa. In turn, the weld metal of the Domex 700MC steel exhibited the highest impact toughness at a mean value of 139 J/cm2, whereas the test joints of Domex 960 showed lower impact toughness at approximately 122 J/cm2. Surprisingly, the lowest impact toughness was determined for the joints of Domex 1100, despite the same wire being applied for the Domex 960.

激光焊接头焊缝自动检测及跟踪研究

激光焊接头焊缝自动检测及跟踪研究

Microstructure of Fiber Laser Deposited WC-Co Cemented Carbide and Carbon Steel

Using a fiber laser welding head, crack-free WC-Co/Steel weld depositions are obtained with optimized welding parameters. The microstructure, composition, phase, structure, and bend strength are analyzed using optical metallography, scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and bend test. The influence of the microstructure and WC/matrix interfaces in the vicinity of joint failure at the cemented carbide side is discussed. It is evident that the deposit consists of austenite dendrites and interdendritic eutectic carbides. The austenite further transforms to martensite on-cooling. The flexural bend strength and yield strength of the joints attained 970 MPa and 876 MPa, respectively. Bend fracture occurs at the HAZ in the cemented carbide side of the joint, characterized with cleavage fracture and quasi-cleavage fracture. TEM and HRTEM image of WC/Co interfaces verified the W2C and eta phase formation in the HAZ that contributed to the embrittlement.

Laser Welding Head Tailored to Tube-Sheet Joint Requirements for Heat Exchangers Manufacturing

Tube to tube-sheet joints in heat exchangers are currently welded by the orbital TIG process characterized by very high quality of the weld beads and good repeatability. However, due to high number of welds, a reduction in the welding cycle time would have an interesting impact on manufacturing costs and delays and laser welding technology is aimed to improve this factor. The main disadvantage is the positioning accuracy required by the laser welding process since beam deviations from real joint cause lack of penetration. It is expected that the Orbital laser welding head developed under the European project ORBITAL will avoid this drawback.

Weldability evaluation and tensile strength estimation model for aluminum alloy lap joint welding using hybrid system with laser and scanner head

Aluminum welding using a hybrid system with a laser and scanner welding head was performed under various welding conditions to verify the feasibility of applying an aluminum alloy to a car body. The experimental material was 5J32 aluminum alloy, and the laser power, welding speed, and laser incidence angle were used as the control variables. The weld bead shape and the tensile shear strength were evaluated in order to understand the aluminum lap joint weld characteristics. Analysis of variance (ANOVA) was conducted to identify the effect of the process variables on the tensile shear strength. Tensile strength estimation models using three different regression models were also suggested. The input variables were the laser power, welding speed, and laser incidence angle, and the output was the tensile shear strength. Among the models, the second-order polynomial estimation model had the best estimation performance, and the average error rate of this model was 0.058.

Infrared image recognition for seam tracking monitoring during fiber laser welding

Seam-tracking ability of a laser-welding system is important for welding process, and the accurate detection of deviations between the laser-beam focus and the weld seam position is prerequisite for seam-tracking control. Infrared image sensing and visual recognition techniques for real-time seam tracking monitoring during high-power fiber laser welding is researched to improve the accuracy of seam-tracking ability. Molten pool images are caught by an infrared sensitive high speed camera arranged off-axis orientation of a laser-welding head which is fixed to a robot. Through the image processing, the feature detection of a near-infrared image is used in visual tracking. The gray-value gradient of near-infrared image is calculated and the keyhole margin of a molten pool is also detected. Combining the gradient and keyhole margin of a molten pool image, the thermal gradient parameter based on the thermal distribution of a molten pool is extracted. As a visual feature in robot control system, this parameter can be used to determine the deviations between the laser-beam focus and the weld-seam center. In comparison with direct detection of the narrow gap position, this parameter can be measured easily and the delay error resulted from the forerun of the sensor can be eliminated. This provides a practical approach to detect the deviations and the possibility to adjust the laser-beam focus position in real time, which can sensibly promote seam tracking accuracy. The proposed algorithm is tested during a butt-joint laser welding of Type 304 austenitic stainless steel plates at a continuous wave fiber laser power of 6kW and 10kW. Its effectiveness is confirmed by the welding experiments.

Development of Remote Welding System Using Fiber Laser

In recent years, many of automotive companies are using laser welding in car body assembly shop. Even though laser welding is better than resistance spot welding in many points, its application has been limited in special manufacturing areas. This paper describes a new laser welding system named by Remote Welding System (RWS) developed at Hyundai Motor Company and comprised of fiber laser, industrial robot and remote welding head. In this technology, welding efficiency can be dramatically improved since its remote welding head where laser continuously finds next welding stitches in advance. Therefore, positioning time of laser is near to zero. It is a good solution for welding in mass production.