Remote Laser Welding of Multi-Alloy Aluminum at Close-Edge Position
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6000 series (magnesium and silicon based) aluminum alloys are highly susceptible to hot cracking during laser beam welding. The occurrence of hot cracks depends on thermo-mechanical strain at the solidifying stage of the melt and in the micro-scale on the solidification structure. A new metallurgical approach to avoid these cracks by affecting the solidification process is using a multi-alloy aluminum with a silicon-rich layer. Within this paper infrared videos were used to investigate and compare the behavior of this new material with the 6xxx monolithic alloy at hot crack sensitive close-edge positions.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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Laser welding is a well-known and relatively widely-used joining process in the engineering industry. Laser welding is also a proven suitable welding method for the joining of modern, high-strength structural steels. Welding of these steels can be challenging when using traditional manual fusion welding, since limits are set for the minimum and maximum welding energy and the cooling time, to retain the original properties of the base material. In the laser welding process, constant welding parameters are used and the movement is performed mechanically, to achieve a high and even processing speed, so that the welding values set can be fulfilled. In the study, the mechanical properties of laser-welded joints were researched in respect of the welding energy used and the cooling time resulting from different combinations of laser power and travelling speed. Several welds with a variable laser power and travelling speed were joined. The thicknesses of the test materials were 3, 4 and 6 mm and the welding energies used for each thickness were 0.05, 0.07 and 0.15 kJ/mm, respectively. The test material was thermo-mechanically rolled structural steel with 500 MPa of yield strength. The joint configuration used was bead-on-plate. Various destructive testing was performed for welded joints. For example, the transversal tensile strength results only showed just minor differences between the values, whereas the hardness values showed clearer differences between the joints.
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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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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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Following phenomena were experienced on the welding with plate thickness 100 mm to 200 mm.(1) The weld cracking increases with an increases of the plate thickness (number of welded layer).(2) Weld cracking occurs several days after welding.In order to assure above phenomena and study the preventive method of the weld cracking, the weld cracking test was carried out and the weld cracking phenomena were observed.The main results obtained are as follows.(1) It is clarified that weld cracking increases with an in crease of the thickness of welded plate, and the initiating position of weld cracking lies just under the surface bead.(2) Incubation period of weld cracking is 4 days after welding.(3) Weld cracking is prevented with lower temperature postheating for 0.5 hr at 300°C in case of local gas heating for a 2 1/4 Cr-lMo steel.(4) It is clarified that above phenomena are based on the distribution of hydrogen in thick weldment which is reported by authors in the previous reportt.
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Various welding positions need be used in laser welding of structures with complex configurations. Therefore, it is necessary to gain knowledge of how the welding positions can influence the keyhole and weld pool behavior in order to better control the laser weld quality. In the present study, a computational fluid mechanics (CFD) model was constructed to simulate the laser-welding process of the titanium alloy Ti6Al4V, with which the keyhole stability and the fluid flow characteristics in weld pool were studied for four welding positions, i.e., flat welding, horizontal welding, vertical-up welding, and vertical-down welding. Results showed that the stability of the keyhole was the best in flat welding, the worst in horizontal welding, and moderate in vertical welding positions. Increasing heat input (the ratio of laser power to welding speed) could increase the keyhole stability. When the small heat input was used, the dimensions and flow patterns of weld pools were similar for different welding positions. When the heat input was increased, the weld pool size was increased, and the fluid flow in the weld pool became turbulent. The influences of gravity became significant when a large heat input was used, especially for laser welding with vertical positions. Too high a heat input in vertical-up laser welding would lead to oscillation and separation of molten metal around the keyhole, and in turn result in burn-through holes in the laser weld. Based on the present study, moderate heat input was suggested in positional laser welding to generate a stable keyhole and, meanwhile, to guarantee good weld quality.
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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.
Cold welding
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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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In our first paper, we have proposed the monitoring method using reflected Ar+ laser beam in YAG laser welding. It has been shown that this method was effective to monitor the changes of the welding conditions in YAG laser welding. It is also very important to detect the welding defects in YAG laser welding. In this paper, signals of the light and acoustic emission in addition to that of reflected Ar+ laser were measured at the same time. We investigated changes of these monitoring signals due to welding defects, such as lack of penetration, underfill by a wide gap and misalignment in butt joints. Main results are as follows; 1) The signal of the light emission was suitable to detect the penetration was full or partial. 2) The reflected Ar+ laser signal was suitable to detect the underfill due to a wide gap and the misalignment. 3) Only one signal was changed when these welding defects were occurred. While various signals were altered at same time when the welding conditions were varied. 4) The signal of the acoustic emission was not changed by these welding defects.
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Butt welding
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