| Thin-walled pipes have been widely used in aviation,aerospace,energy,chemical and other engineering fields.The service environment involves complex working conditions such as high temperature,high pressure,corrosion,fatigue,etc.,which puts forward high requirements for structural safety.As an important manufacturing process for thin-walled pipes,welding stresses and deformation not only affects the dimensional accuracy and stability,but also seriously threatens the reliability and service life of the welded structure.In this paper,the research method combining experiments and welding computational mechanics was used to study the welding mechanics and control strategies of thin-walled pipes.The research contents mainly include analyzing the welding deformation mode and residual stress distribution characteristics in thin-walled pipes with seam weld and girth weld,discussing the influence of various factors including pipe dimensions and welding parameters,clarifying the mechanism,and then proposing control strategies.To investigate the characteristics of welding residual stresses and deformation of thin-walled seam welded pipes with a thickness of 2 mm and a diameters from 60 mm to 500 mm,the welding model was developed based on the solid-shell hybrid element modeling technology,which significantly improved calculation efficiency and showed good accuracy and applicability.The results showed that the seam welded pipes had a composite welding deformation mode,and the maximum radial deformation was generated at the end.Axial stress was mainly the membrane stress with little difference between inner and outer walls.And the hoop stress was typical bending stress,and the peak stress was located at the end.Peak stress in hoop and axial directions was both up to the yield strength at room temperature,namely,250 MPa to 300 MPa.Furthermore,this paper clarified the characteristics of plastic deformation of thin-walled seam welded pipes.Among them,the axial and hoop shrinkage were both compressive,but due to the hoop shrinkage on inner wall was greater than that of the outer wall,and hoop bending direction was opposite to the direction of angular deformation of the butt welded plate.Moreover,the formulas of axial shrinkage,hoop shrinkage and hoop bending related to dimensional parameters and welding parameters were developed.The formulas revealed the relationship between plastic deformation and pipe diameter and welding heat input,and could be used to estimate the welding deformation efficiently combined with inherent strain method.Thin-walled pipes were often seam welded with external constraints such as fixtures in actual production,so there was a significant difference in welding mechanical process with the free state.In this paper,considering the heat dissipation of the fixture and the mechanical restraint,the welding numerical model with external constrains was developed,and the influence on the welding residual stress and deformation in seam welded pipe was analyzed taking the seam welded pipe with a wall thickness of 2 mm and a diameter of 180 mm as the research object.The results showed that the external constraint effectively reduces the peak value of residual stress and the width of tensile stress zone,but does not change the distribution characteristics.The peak stresses in axial and hoop directions were reduced by 30% and 50%,respectively.Under external constraint,the convex deformation in hoop direction become the concave one,which was superimposed with axial bending,so that the maximum deformation occurred in the middle section,and the pipe ends obtained a good shape accuracy due to the canceling effect of the two.The mechanism of the external constraint on plastic deformation was analyzed,and it is believed that the mechanical constraint mainly acted by generating more tensile plastic deformation during the cooling process,but has no obvious effect on the heating process.The parametric calculation of the external constraint strategies showed that reducing the clamping distance is the most effective way.With a clamping distance of 5 mm,the maximum error of straightness and roundness controlled within 1 mm.The welding process of girth welding was significantly different from that of seam welding,and the welding stress and deformation problems were more complicated.The axis bending,radial deformation and the welding mechanics in welding start/end location were both required further research.Firstly,the axis bending in girth welded pipes with a thickness of 3 mm and a diameter from 20 mm to 100 mm was studied.The results showed that a one-way continuous welding always tended to generate axis bending to the opposite side of the welding starting point,and the bending value decreased with the increase in diameter,indicating that such deformation was more prominent in slender pipes.It was believed that axis bending was generated by the uneven distribution of axial shrinkage on the circumference,while the latter was caused by the difference in temperature field on the circumference and the interaction of the successive welding.The influence of the segmented welding schemes on the bending deformation was discussed,and it was clarified that the bending produced by two semi-circles welding in same direction could cancel each other out and effectively mitigate the axis bending.Furthermore,based on the deformation symmetry,an ectopic synchronous dual-torch welding scheme was proposed,which showed a good applicability to girth welded pipes with different sizes and process parameters.For radial deformation in girth welded pipes,it was found that the deformation direction not only had concave type in traditional cognition,but also might produce convex one according to our study on the pipes with a thickness of 1 mm and 2mm and a diameter from 60 mm to 254 mm.It is believed that the radial deformation was mainly controlled by hoop plastic strain,while less related to the axial bending.Mechanism analysis showed that the radial deformation was mainly controlled by the effect of axial expansion on the hoop strain cycles,and heated zone width was the main influencing factor.When the heated zone width was large enough under the axial restraint,the weld would deformed to the outward,causing that the hoop total strain was larger than hoop thermal strain,so that the hoop tensile plastic deformation was generated,and the radial convex deformation was produced after cooling.The role of welding parameters on the regulation of radial deformation was clarified,the results showed that changing heat input at a specific welding speed had a limited effect,while changing welding speed at a specific heat input worked better.Moreover,a control strategy was proposed to mitigate the radial concave and convex deformation,which were successfully achieved using the proposed auxiliary heat source method and auxiliary cooling method,respectively,reducing the deformation by 70%.The welding mechanics in the start/end position of girth welded pipes was studied based on the girth welded pipes with a thickness of 2 mm and a diameter of180 mm.And the distribution of plastic deformation on the circumference was clarified,and the axial and hoop plastic strains were divided into three typical regions.Based on this,it is believed that the stress release and stress re-balance at the starting area caused by the end welding determined the distribution of the residual stress in the starting/end position,and the distribution of plastic strain at the starting/end location caused a larger radial indentation.For example,a 40%increase in deformation was observed compared to the steady-state zone in a D500 girth welded pipe.The indentation could be mitigated by reducing the welding heat input in this region.In addition,the effect of lap welding process on residual stresses and deformation was also discussed. |
