| As a kind of high-strength hull structural steel,DH36 steel has the characteristics of high strength,good comprehensive mechanical properties,excellent processing and welding performance,and is widely used in hull manufacturing.As a large-line energy welding method,VEGW has the advantages of simple welding,excellent welding effect,high welding efficiency and low cost compared with traditional welding methods,and is favored by shipbuilding,heavy machinery and other manufacturing industries.During the welding process,the temperature field distribution and the internal fluid behavior of the molten pool are closely related to the weld forming and joint microstructure properties.In addition,the residual stress after welding is inevitable,which seriously affects the performance and service life of the workpiece.For VEGW of large thick plates,the dynamic behavior of the molten pool under high heat input and inter-plate constraints is more complex,and residual stress is more likely to be caused.In order to ensure the excellent weld forming and welding quality of DH36 steel in VEGW,it is necessary to conduct a comprehensive and accurate analysis of the dynamic characteristics of the workpiece melt pool and the residual stress after welding.In this study,a reasonable VEGW process is formulated according to the welding performance of DH36 steel,and a three-dimensional numerical analysis model of temperature field,stress field,heat field and flow field is established by comprehensively considering the physical properties of materials,geometric characteristics of components and characteristics of welding heat source.The dynamic behavior of the internal fluid of the molten pool under different process parameters was compared and analyzed.Based on the thermoelastic-plastic theory,the three-dimensional distribution characteristics of the residual stress field are calculated and analyzed by indirect coupling method.The thermal cycle curve and stress distribution curve of the heat affected zone temperature were measured by thermocouple method and blind hole method,respectively,and the accuracy of the model was verified.Under different process parameters,the residual stress value and distribution after VEGW were further explored.This has important guiding significance and application value for optimizing welding process and improving industrial production efficiency.The results show that for I-shaped VEGW,the sidewall fusion degree is small when there is no swing,and the bottom of the molten pool is inverted cone-shaped;when the peak flow rate of the molten pool occurs at 0.55 s,about 0.417m/s.When the swing frequency is10 Hz,with the welding,the bottom of the molten pool changes from bimodal to unipeak,and the convection area inside the molten pool increases significantly,which promotes the flow of liquid metal,so the fusion degree of the sidewall increases compared with that without oscillation;the peak flow rate of the molten pool occurred in the convection region of the molten pool surface at 0.69 s,about 0.541 m/s.When the swing frequency is 20 Hz,with the increase of the swing frequency,the convection area in the molten pool further increases,and the molten width size increases relatively;the peak flow rate occurred in the eddy current region inside the molten pool at 3.45 s,about 0.547 m/s.The weld morphology,thermal cycle curve of temperature in the heat-affected zone and residual stress distribution characteristics obtained by the VEGW simulation of DH36 steel are in good agreement with the experimental results,which proves the accuracy of the three-dimensional finite element numerical analysis model.For the straight reciprocating path in the I-shaped groove,when the welding speed is increased from 10mm/s to 15mm/s,the peak compressive stress of the transverse residual compressive stress of the water-cooled side weld increased from150 MPa to 182 MPa,and the longitudinal residual compressive stress increased from 20 MPa to 70 MPa,and the stress range of the weld area also increased;along the thickness direction,the transverse residual stress values of the weld and the heat-affected zone were significantly reduced.For the straight reciprocating path with a welding speed of 15mm/s,when V-shaped groove was considered compared with I-shaped groove,in the water-cooled side weld and near seam area,the peak transverse residual compressive stress increased from 182 MPa to 225 MPa,and the peak longitudinal compressive stress increased from70 MPa to 250 MPa,and the horizontal and longitudinal stress ranges were significantly larger;in the ceramic side weld and near seam area,the peak transverse compressive stress increased from 125 MPa to 160 MPa,the range of transverse tensile stress also became significantly larger,and the overall value of longitudinal tensile stress decreased,the peak value decreasing from 270 MPa to 250 MPa.For the straight reciprocating path in V-shaped groove,the residual stress in all directions of the weld and the near seam area is not greatly affected by the welding speed.For the I-shaped groove with a welding speed of 15mm/s,when the swing frequency changed from 0Hz to 20 Hz,on the water-cooled side,the peak transverse compressive stress of the weld was reduced from 182 MPa to 140 MPa,while the peak value of longitudinal tensile stress reducing from 70 MPa to 30MPa;on the ceramic side,the peak compressive stress of the weld was reduced from 125 MPa to 90MPa;along the plate thickness direction,the values of the transverse and longitudinal residual stresses in the heat-affected zone increased significantly with the increase of oscillation frequency. |
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