Numerical Analysis Of Narrow Gap Oscillating Laser Welding Process Of High-strength Steel

The narrow gap(NG)laser welding technology has many advantages in manufacturing hull thick plate structures in high-tech ship,which can effectively reduce the total welding heat input,the residual stress and residual deformation of the welded parts through decreasing the groove size and the filler metal under the condition that the welding production efficiency is not decreased.Conventional narrow gap laser welded joints are prone to defects of lack of side-wall fusion.The laser heat source in narrow gap oscillating laser welding process can effectively prevent lack of side-wall fusion due to side wall of groove heated by oscillating laser.However,there is still a lack of in-depth and quantitative studies of the mechanism of the influence of the oscillating laser on the side-wall fusion,evolution of the temperature field and the stress and strain fields.Based on oscillating laser welding process experiments,a thermo-elastic-plastic finite element mathematical model for the narrow gap oscillating laser welding process was established.The influences of the oscillating parameters on the side-wall temperature field and the penetration depth of the side-wall were analyzed,and the mechanism of the influence on the sidewall fusion of the groove was revealed.Based on the comprehensive consideration of the oscillating laser welding physical process and structural nonlinearity of 921A high-strength steel,the life-death element method was used to simulate the generation of the weld seam.The combined heat source for Gaussian surface heat source+attenuation Gaussian body heat source for laser welding and the node activation temperature+attenuation Gauss body heat source for laser wire welding were proposed respectively.By using the local coordinate system to realize the loading of the oscillating laser heat source,a finite element numerical analysis model for the narrow gap oscillating laser welding process was founded.In order to further improve the calculation efficiency,based on the law of conservation of energy,two equivalent heat source models of quasi-Gaussian surface heat source and quasi-attenuated Gaussian cylinder heat source are derived to simulate laser oscillation by analyzing the characteristics of laser oscillation.The thermal-mechanical sequential coupling method was used to calculate the temperature field and stress-strain field of the narrow gap oscillating laser welding.The accuracy of the model was verified by the thermal cycle,the fusion line criterion and the residual stress by the hole drilling method.The verification results show that the model is reliable and can be used to analyze the NG laser welding process.The influence of the oscillating laser on the single-pass narrow gap oscillating welding temperature field of 30 mm high-strength steel was analyzed by numerical simulation.The results show that the weld center and side wall are heated by multiple times,and a multi-peak thermal cycle curve appears,and the times that the peak temperature appears are equal to the laser oscillating frequency.In the range of oscillating amplitude of 0-1.5 mm,as the oscillating amplitude increases,the peak temperature at the sidewall increases from 1857? to 2115 ?,the bottom of the cross section gradually transition from the tip shape to the arc shape,the penetration depth of the side wall increases from 0.82 mm to 0.96 mm,which improves the cross-sectional morphology and can effectively prevent the side wall from lack of fusion.In the oscillating frequency range of 20 Hz-100 Hz,as the laser oscillating frequency increases,the peak temperature at the sidewall decreases from 2036? to 1936?,and the fusion depth in side wall varies from 0,9 mm to 0.94 mm.The temperature field and stress-strain field of 30 mm-thick narrow gap oscillating laser welding were numerically analyzed using an equivalent heat source.The results show that in the multi-pass oscillating laser welding process,the first pass is full penetration,the cross-sections of the other welds are similar,the inter-layer temperature will cause the isothermal surface of the weldment to gradually expand,and the overall temperature of the weldment will increase.The equivalent residual stress of the weldment is mainly distributed near the weld zone,and the stress concentration is mainly distributed in the middle and upper parts and roots.The peak value is 669 MPa.After unloading the rigid constraint,the stress concentration at the constraint positions disappears.The longitudinal residual stress and transverse residual stress in the direction of the weld are stable in the middle zone of the weld.The longitudinal residual stress is the tensile stress in the stable zone of the center path of the upper surface and the lower surface of the weld.The values are about 620MPa and 512MPa.The stable zone of residual stress under the center path of the upper surface of the weld is compressive stress with a value of about 100 MPa,and the stable zone along the center path of the lower surface of the weld is tensile stress of about 600 MPa.Band tensile stress layers exists at the root and the middle and upper parts of the weld.As the number of welding passes increases,the groove gap gradually decreases.The groove gap decreases from 4.82 mm before welding to 3.18 mm at the beginning of eleventh welding.