| Low-alloy structural steel is one of the most productive carbon steels in the world,and medium-thick low-alloy structural steel not only has excellent mechanical properties such as strength,plasticity and toughness,but also great welding performance such as no need for heat treatment after welding,it is widely used in bridges,ships,rail transit,mining machinery and other engineering fields.Narrow gap laser welding(NGLW)is a technology that uses more concentrated laser beam and smaller groove for welding.Compared with traditional welding,it can significantly reduce the groove filling amount,heat input and welding deformation,and improve welding efficiency and quality of welded joints.It is widely used in industrial production.With the development of industrial technology,how to improve the defects such as coarse structure and poorly fused of side wall in the welding process of medium and thick plate become urgent problems to be solved.In order to solve the above problems,it usually uses wobble laser beam to optimize the welding process in industry.Narrow gap woble laser welding(NGWLW)can effectively increase the laser action area,refine the grain structure,and promote better fusion inside the narrow gap groove.However,aiming at the process optimization,side-wall fusion mechanism of NGWLW for low alloy structural steel,and the influence mechanism of wobble laser on welding temperature and stress field need to be further research.Based on these,the finite element model was established through process experiments such as high-speed photography of NGWLW under different process parameters in this thesis.The heat source model was calibrated and verified through experiments and simulations,the influence of the wobble parameters on the fusion of the inner side wall of the narrow gap groove is analyzed though the theory of thermoelasticity and plasticity.The temperature and stress fields of 22 mm thick low alloy structural steel in NGWLW are simulated,and the distribution law of temperature and stress fields was investigated.First of all,the narrow gap welding process and droplet transfer state is recorded and observed by high-speed photography device,and the influence of different welding power,welding speed,wobble mode,amplitude,and frequency on the droplet transfer form were researched.The results show that with the increase of welding power,the form of droplet transfer changes from spreading transition to liquid bridge transition,and with the increase of welding speed,the form of droplet transfer gradually changes from spreading transition to liquid bridge transition.The corresponding droplet transition forms are spread transition and liquid bridge transition when the wobble mode is Circle,and the corresponding droplet transition forms in Line and Infinity modes are spread transition.The laser beam is concentrated in the middle of the groove and agitated violently,causing the melt pool inside the groove to tilt when the wobble amplitude is 0.5mm.The droplet transfer is relatively stable and uniform and the surface weld morphology is aesthetically pleasing when the wobble amplitude is 1-1.5 mm and the wobble frequency is 60 Hz.The optimized welding process was used to weld 22 mm thick low-alloy structural steel,and the comprehensive mechanical properties measured by experiment is excellent.Furthermore,considering the complex physical and chemical changes in the welding process,the finite element models of NGLW self-melting welding and wire filling welding were established.Based on ABAQUS,Python and Fortran languages are used to establish a secondary development model of NGWLW,which realizes the simulation of different wobble processes.Using the life-death element technology,the law of distribution of the temperature field and stress field of 22 mm thick low-alloy structural steel 1+6(one self-melting and six filling wires)was research by sequential thermo-mechanical coupling.The accuracy and reliability of the heat source model and the secondary development model was verified by the thermal cycle curve and the fusion line rule.Finally,the influence of welding process on the temperature field of the narrow gap groove side wall and the distribution law of welding residual stress are analyzed.The results show that the peak temperature at the sidewall is 1534 ℃ when no wobble,which near the melting point of carbon steel,and the sidewall of the material cannot be fully melted.The peak temperature at the side wall exceeds 1800 ℃ when the wobble amplitude is 1-1.5 mm and the wobble frequency is 60 Hz,so the wobble laser effectively promotes the side wall fusion.In the Circle swing mode,the peak temperature of the side wall increased from 1546 ℃ to 1860 ℃ as the amplitude of wobble increased from 0.5 mm to 1.5 mm.In the Line wobble mode,the peak temperature of the side wall increased from 1568 ℃ to 1737 ℃ as the wobble frequency increased from 0.5 mm to 2 mm.The simulated thermal cycle curve fluctuates greatly when the wobble frequency is 20 Hz and 80 Hz,and the temperature curve is relatively stable when the wobble frequency is 60 Hz,which can provide stable energy input,Hoever,the wobble frequency has little influence on the peak temperature at the side wall.The equivalent residual stress is mainly distributed in the constrained position and the middle and upper part of the weldment,the maximum equivalent residual stress is distributed where the weldment is constrained,with a maximum value of 325 MPa.The upper part of the transverse residual stress is tensile stress and the lower part is compressive stress along the weld direction.The transverse residual stress from the bottom to the cover of the material has the same trend as the longitudinal residual stress,the maximum transverse residual tensile stress is 345 MPa,and the maximum longitudinal residual tensile stress is 447 MPa. |
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