Influence of Welding Parameters on the Mechanical Properties of a Laser-Welded Joint
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Abstract:
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.Keywords:
Fusion welding
Tensile testing
The welding technology of TC2 titanium alloy sheets with thickness of 4 mm was studied with high-power CO 2 laser welding equipment in this paper.Correct welding parameters were determined.Effect of welding parameters on the welding quality was analyzed and the rationality of the process test was proved by stretch test for laser welding butt joint of titanium.In laser welding process,the position of focus affected greatly on the molten depth from the test results,and the right position of focus was the necessary condition for penetration.The penetration was increased with welding heat input,and TC2 titanium alloy sheets with thickness of 4 mm were penetrated when the welding heat input was 2.4 kJ/cm.
Butt welding
Titanium alloy
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The experiments of vertical and flat position laser welding for 4 mm-thick 5A06 aluminum alloys were implemented, and the characteristics of weld dimension and porosity in the vertical position laser welding for aluminum alloys were investigated. The results show that the concave value and excessive penetration value of vertical welding is less than those of flat welding. Further, with the increase of heat input, the difference of vertical and flat welding becomes obvious. The weld appearance and dimension of the vertical welding and flat welding were slightly different. When the heat input is increased to a great extent, the weld depth of vertical welding is more than that of flat welding. However, the weld width of vertical welding is less than that of flat welding. The porosity of vertical position laser welding for aluminum alloys is composed of the large and irregular porosity or hole. It is not obviously different during vertical welding and flat welding, and a great deal porosity concentrates in the upper and middle part of weld section, which can be indicatied from the distributing position and shape of porosity. The number of porosity in vertical welding was slightly less than that of flat welding for the same welding parametors.
Horizontal position representation
Weld pool
Cold welding
Electrogas welding
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The experiments of CO 2 laser TIG paraxial hybrid welding with 4 mm thick AISI 321 stainless steel sheet have been performed. The arc images and welding characteristics have been investigated with different energy ratios between laser and arc. The experimental results indicate that the hybrid welding is similar to laser welding and has also two welding mechanisms: deep penetration welding and heat conduction welding. Because of the effect of keyhole induced by laser, the arc root can be stabilised and compressed, and the current density and the penetration depth are all increased significantly, which show the characteristics of deep penetration welding. However, when the current is increased to a critical value, the laser induced keyhole disappears and the arc expands obviously, which decreases the penetration depth, so that the welding mechanism has been changed from deep penetration welding to heat conduction welding. Furthermore, the effects of distance between laser beam and electrode, pulsed laser and hybrid manners on hybrid welding characteristics have also been studied.
Cold welding
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4 mm thickness plates of AZ31B alloy were welded by CO2 laser.Three welding modes of penetration welding,heat conduction welding and unstable mode welding were found,and laser power was found to be the main factor for welding mode.The ratio of topside width and penetration of deep and narrow segment was proposed to characterize the weld shape,and the influencing factors of weld shape were investigated.Heat input was found to be the most important factor,while the laser power efficiency affected by welding velocity and the size of plasma-flume-affected zone affected by laser power could also influence the weld shape.When welds are fully penetrated,low laser power,high welding velocity and parameter combination of low laser power and welding velocity is favorable for weld of and narrow shape.
Cold welding
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For CO2 laser welding of large output, when a deep penetration welding in single pass is done in the ambient atmosphere, it is known that blowholes may occur because of the in-keyhole gas being entrapped in molten metal. Keeping this problem in mind and changing the parameters, we conducted welding tests with full-penetration bead-on-plate welding, and checked by radiographic test for welding defects.The experimental results demonstrated that the larger is the specimen thickness, the more frequently occur welding defects, and that generation of welding defects depends upon the amount of welding heat input.Welding defects such as blowholes remain in metal, because the gas once entrapped into keyhole floats up in molten metal, and it is enclosed in the course of solidification. From this, we can verify the theory that a larger welding heat input, in the case of the full-penetration bead-on-plate welding, may be favorable for preventing welding blowholes. It can be assumed, therefore, that a larger heat input may hinder cooling of molten metal, and need a longer time for metal solidification; in this longer span of time, in-molten metal gas may escape while the metal is sufficiently heated.
Keyhole
Cold welding
Penetration (warfare)
Plastic welding
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A laser beam welding and an electric arc welding were combined, and the positive points of each welding method are drawn such as high speed, low thermal load, deep penetration, and high productivity. The fiber laser-MIG conjugated welding. namely the hybrid welding has been studied mainly for the automation industry of a pipeline welding. In this study, the MIG welding was combined with a fiber laser welding to make up the hybrid welding. The weld shapes, microstructures and mechanical properties for weld zones after the hybrid welding or only fiber laser welding were investigated on the 700 ㎫ grade Ultra Fine Grained(UFG) high strength steel. The amount of acicular ferrite in weld metals and HAZ(heat affected zone) was observed larger after hybrid welding compared with after only laser welding. The Vickers hardness of the top area of the fusion zone after fiber laser welding was higher compared with after hybrid welding.
Acicular ferrite
Cold welding
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A laser beam welding and an electric arc welding were combined, and the positive points of each welding method are drawn such as high speed, low thermal load, deep penetration, and high productivity. The fiber laser-MIG conjugated welding. namely the hybrid welding has been studied mainly for the automation industry of a pipeline welding. In this study, the MIG welding was combined with a fiber laser welding to make up the hybrid welding. The weld shapes, microstructures and mechanical properties for weld zones after the hybrid welding or only fiber laser welding were investigated on the 700 MPa grade Ultra Fine Grained(UFG) high strength steel. The amount of acicular ferrite in weld metals and HAZ(heat affected zone) was observed larger after hybrid welding compared with after only laser welding. The Vickers hardness of the top area of the fusion zone after fiber laser welding was higher compared with after hybrid welding.
Acicular ferrite
Cold welding
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Inert gas
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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.
Keyhole
Weld pool
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For CO2 laser welding of large output, when a deep penetration welding in single pass is done in the ambient atmosphere, it is known that blowholes may occur because of the in-keyhole gas being entrapped in molten metal. Keeping this problem in mind and changing the parameters, we conducted welding tests with full-penetration bead-on-plate welding, and checked by radiographic test for welding defects. The experimental results demonstrated that the larger the specimen thickness, the more frequently welding defects occur, and that generation of welding defects depends upon the amount of welding heat input. Welding defects such as blowholes remain in metal, because the gas once entrapped into keyhole floats up in molten metal, and it is enclosed in the course of solidification. From this, we can verify the theory that a larger welding heat input, in the case of the good appearance full-penetration bead-on-plate welding, may be favorable for preventing welding blowholes. It can be assumed, therefore, that a larger heat input may hinder cooling of molten metal, and need a longer time for metal solidification; in this longer span of time, in-molten metal gas may escape while the metal is sufficiently heated.
Keyhole
Cold welding
Penetration (warfare)
Plastic welding
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