Analyzing multispectral emission and synchrotron data to evaluate the quality of laser welds on copper
Jan BrüggenjürgenChristoph SpurkMarc HummelChristoph FranzAndré HäuslerAlexander OlowinskyFelix BeckmannJulian Moosmann
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The validation of laser welding of metallic materials is challenging due to its highly dynamic processes and limited accessibility to the weld. The measurement of process emissions and the processing laser beam are one way to record highly dynamic process phenomena. However, these recordings always take place via the surface of the weld. Phenomena on the inside are only implicitly recognizable in the data and require further processing. To increase the validity of the diagnostic process, the multispectral emission data are synchronized with synchrotron data consisting of in situ high-speed images based on phase contrast videography. The welding process is transilluminated by synchrotron radiation and recorded during execution, providing clear contrasts between solid, liquid, and gaseous material phases. Thus, dynamics of the vapor capillary and the formation of defects such as pores can be recorded with high spatial and temporal resolution of <5 μm and >5 kHz. In this paper, laser welding of copper Cu-ETP and CuSn6 is investigated at the Deutsches Elektronen-Synchrotron (DESY). The synchronization is achieved by leveraging a three-stage deep learning approach. A preprocessing Mask-R-CNN, dimensionality reduction PCA/Autoencoders, and a final LSTM/Transformer stage provide end-to-end defect detection capabilities. Integrated gradients allow for the extraction of correlations between defects and emission data. The novel approach of correlating image and sensor data increases the informative value of the sensor data. It aims to characterize welds based on the sensor data not only according to IO/NIO but also to provide a quantitative description of the defects in the weld.ABSTRACT Fundamentals of high-power laser welding are reviewed and unique features relative to other welding processes are noted. A brief description is given of the preferred characteristics of laser, focusing and ancillary equipment suitable for high-power production applications. Specific welding performance is noted for a range of steel compositions and thicknesses and process limitations are identified. INTRODUCTION The potential of high-power laser systems for production welding applications has long been recognized. Seam-welding procedures employing pulsed laser systems were initially developed shortly after the first operation of a laser in 1960. Although such procedures were well-suited for precision welding of delicate and/or complex assemblies, however, it was found that welding speeds and penetration capabilities for pulsed systems were inadequate for large-scale production applications. Development of high-power, industrially-suited, continuously-operating, C02M laser systems in the late 1960's significantly enhanced the laser's capability for welding. Within the past fifteen years, the pace of laser welding development has quickened at an increasing rate. Welding performance has been demonstrated in stainless, low-carbon and alloy steels, titanium alloys, nickel-base alloys and in some aluminum alloys. A maximum single-pass weld penetration of 50 mm has been achieved in alloy steel and welding speeds to approximately 1000 mm/sec have been demonstrated in 0.2 mm thick material. The influence of process parameters on welding performance has been identified and the range of current applicability of laser welding has been delineated. Within the past five years, the most significant advances in laser welding have come in the area of reduction to routine production practice. An ever increasing number of multi kilowatt, carbon-dioxide laser systems are demonstrating their capability for reliable operation under severe production conditions. In the following, current laser welding technology is identified with specific emphasis on laser welding performance in steels. PROCESS FUNDAMENTALS From a welding viewpoint, the laser may be considered simply as a radiant energy source. The individual photons which comprise the laser beam exhibit an energy corresponding to the laser transition. For the carbon-dioxide laser, which is currently the only system suitable for multikilowatt production use, the photon energy is 0.12 eV and the corresponding wavelength is 10.6 micron. Since this wavelength in the infrared portion of the electromagnetic spectrum does not transmit through ordinary optical materials (glass, quartz, etc.), special materials such as zinc selenide must be employed. Extensive use of front surface reflective optics also characterizes carbon-dioxide laser applications. Because the energy in a laser beam is highly ordered, the beam can be focused to provide extremely high power densities. From basic principles, it is known that the minimum spot diameter to which the beam can be focused is of the order of the beam wavelength. This provides the capability for attaining power densities of the order of 106 W/cm2 with relatively modest power levels of a few kilowatts.
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This article analyzed penetration of CO2 laser welding,and its three important mechanisms to realize its stability.These three mechanisms are as follows:how to stably transfer laser beam power to welding pool.After fused hole formed in weld part,how the welding pool metal around fused hole stably flow to form stable weld.How to control metal plasma plummes in fused hole caused by laser beam.It analyzed the effect of narrow weld and HAZ on extending laser welding,introduced the effect of gap between LBW and GMAW on droplet transfer stability,while adopting LBW+GMAW combined welding.Finally,it listed several examples of high power laser deep-penetration welding used in production,and pointed out the research trend of laser deep-penetration welding.
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Current high power laser welding technology is reviewed. The nature of the beam-material interaction, the influence of laser, process and material parameters on welding response and the unique characteristics of the process are discussed. Examples are given of representative laser welding performance in ferrous and nonferrous alloys and selected illustrations are presented of weld mechanical properties. A relationship is noted for the dependence of maximum single-pass laser weld penetration on power and a range of laser welding applicability is delineated. Laser welding efficiency is identified and process advantages and disadvantages are compared. The potential for future utilization of lasers in welding is discussed.
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The research of welding parameters, such as surface absorption rate, laser power, welding speed on laser welding process, weld depth and weld width are discussed by means of finite element simulation. In the article, it takes 5A06 aluminum alloy welding as the example to build the finite element model. Finite element model can forecast the weld shape under different welding parameters, and can realize choosing and optimizing laser welding parameters to show the advantage of aluminum alloy laser welding.
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Recently laser power is increased rapidly and the laser welding ability is advanced, then we can use it for thick plate welding in heavy industries. In such a background, we aimed to apply YAG laser welding to manufacture our products, and introduced 10kW and 7kW class YAG laser processing system. Then we have developed the optical fiber transmitting technology for high power beam, and welding technology which could make over 20mmt 1pass weld joint. To achieve such welding ability, we optimized welding parameters and controlled keyhole state, penetration shape and welding efficiency. Then we could get the high quality weld efficiently. After we confirmed the mechanical property of weld joint based on the examination standard for power plants, we applied the YAG laser welding to manufacture stainless vessels for reprocessing plants.
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Two types of material,including 5A06 aluminum alloy with 1.2mm thickness and 5A90 aluminum-lithium alloy with 3mm thickness,were joined by laser welding with ER5356 filler wire.The effects of welding parameters(the distance between the laser and the filler wire,weld feed speed,laser power and welding speed) on welding appearance were researched.The results show that the good welding appearance can be obtained when the distance between the laser and the filler wire is controlled within a range,and the range is affected obviously by the welding speed.Meanwhile,the higher wire feed speed and greater wire feed speed range tend to appear when the thinner aluminum alloy sheet is welded.The effect of welding speed on welding appearance is larger than that of laser power.As the welding speed increases,the laser power increases in order to realize the two welding parameters match each other.Besides,during laser welding with filler wire on the condition of good welding appearance and complete-penetration,the welding input should be kept as lower as possible.
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Fiber lasers have been receiving considerable attention because of their advantages of high power, high beam quality and high efficiency, and are expected as one of the desirable heat sources for high-speed and deep-penetration welding. In our researches, therefore, the effects of laser powers and their densities on the weld penetration and the formation of sound welds were investigated in welding of Type 304 austenitic stainless steel, A5052 aluminum alloy or high strength steel plates with four laser beams of about 0.12 to 1 mm in focused spot diameter, and their welding phenomena were observed with high-speed video cameras and X-ray transmission real-time imaging system. It was found that the laser power density exerted a remarkable effect on the increase in weld penetration at higher welding speeds, but on the other hand at low welding speeds deeper-penetration welds could be produced at higher power. Laser-induced plume behavior and its effect on weld penetration, and the mechanisms of spattering, underfilling, porosity and humping were elucidated, sound welds without welding defects could be produced under the improved welding conditions. In addition, importance of the development of focusing optics and the removal of a plume during remote welding will be emphasized in terms of the stable production of constant deep-penetration welds and the reduction in welding defects in high power laser welding.
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