Research On Hydroforming Behavior Of Thin-walled Welded Tubes With Irregular Sections

Tube hydroforming is an advanced technology for the fabrication of lightweight hollow components with irregular cross-sections.The welded tubes have been widely used in the hydroforming of automobile parts due to their thickness uniformity and cost advantage.However,nonuniform deformation and fracture instability easily occur on the tubes during hydroforming process because of the existing of welding joint.In this paper,the theoretical analysis,the numerical simulation and the process experiment are used to investigate the hydroforming behavior of thin-walled welded tubes with irregular sections.The bi-axial mechanical properties and forming limit of the thin-walled welded tubes are investigated.Then the hydro-pressing process with concave cross-section was proposed.On this basis,the full-scale welded tube hydroforming with irregular sections was conducted using the multi-step simulation and experiment.All of these results could provide theoretical guidance for the welded tube hydroforming technology.In order to obtain bi-axial mechanical properties and forming limit of the thin-walled welded tubes,the hydraulic bulging tests were conducted and the deformation behavior of low carbon steel welded with different length/diameter ratios was investigated.Based on this,the change rule of the circumferential thickness and non-uniform deformation mechanism of the welded tubes are revealed.The theoretical model for the calculation of equivalent stress and strain of welded tubes was established using the plane stress anisotropic yield criterion and tangent circular geometry model in deformation zone.The bi-axial mechanical properties and forming limit diagram of the welded tubes were tested directly from whole tube hydraulic bulging experiment.It is found that fracture pressure and bulging height are slightly reduced with the increasing of length/diameter ratio,and the maximum thickness thinning is located at about ±30° on both sides of the welding line.The mechanism for this nonuniform deformation can be summarized as follows: a “waistnipper” phenomenon was found around the welding line during bulging process,which cause a bigger curvature radius for the base matel with a certain angle near the welding line than any other areas.This will make the ±30° position near welding line bearing the biggest stress,so plastic deformation is preferred at this position.Based on the analysis of effect of preforming billet's shape on the stress state on welding line during the forming process of rectangular cross-section,the hydro-pressing method of concave preforming billet was developed for the welded tubes,and the effect of supporting pressure on the preforming billet's shape and its thickness distribution was revealed.Moreover,the preforming billet's shape was optimized using the sensitivity analysis and combinatorial optimization strategy.It is found that the tensile stress on the welding line can be reduced using the concave preforming billet.As a result,the fracture possibility of the welding line and heat affect zone can be lowered efficiently.Furthermore,the undercut defect,which is frequented in the mechanical pressing process,can be completely avoided in the hydro-pressing process.In brief,the required concave preforming billet can be manufactured precisely by the hydro-pressing method.Based on this,the reasonable preforming billet can be supplied to solve the fracture problem of welding line during the hydroforming of welded tubes with complicated cross-sections.Effect of welding joint position,thickness and strength of the thin-walled welded tubes on their forming behavior has been revealed through the establishment of numerical simulation model of welded tube hydroforming with the consideration of weld properties.It is found that the forming limit can be improved and the welding failure can be avoided during the tube bending process when the welding line was placed at the top or inside of the part,where the welding line receive a compressive stress.Moreover,when the thickness ratio between welding line and parent metal increased from 0.95 to 1.05,the equivalent strain of the welding line can be reduced about 20 %.When the strength ratio between welding line and parent metal increased from 0.8 to 1.2,the equivalent strain of the welding line has an 18.5 % reduction.From the above we can know that increasing of welding joint thickness or strength could reduce its deformation amount,which is beneficial to reduce the cracking risk of welding line.The integrated hydroforming and hydro-piercing equipment has been designed and manufactured for the forming of welded tubular parts with complicated cross-sections used as car subframe.During the hydroforming process,the stress and strain of the hydroforming die can be effectively reduced by using the loading method of variable clamping force.The influencing rule of welding line position and supporting pressure has been revealed by the numerical simulation and experimental investigation of preforming billet hydro-pressing of bent welded tubes.The result show that the concave preforming billet with adequate shape precision can be hydro-pressed under the internal pressure of 6 MPa.On this basis,the experimental investigation of welded tube hydroforming and multihole synchronous hydro-piercing was conducted to manufacture the full-scale welded tubular parts with complicated cross-sections,and the effect of preforming billet shape and welding line position on welded tube hydroforming defects was revealed.A full set of welded tube hydroforming technology devoted to full-scale welded tubular parts with complicated cross-sections have been developed,including the CNC bending,preforming and hydroforming forming processes.Using this whole process,the subframe parts used in Mazda M8 car have been trial-produced on a large scale.In the end,the dimensional accuracy and mechanical properties of the subframe parts were measured.It is found that the maximum size deviation and the maximum relative error of subframe parts are 0.25 mm and 0.5 %,respectively,which are far less than the design tolerance.Moreover,the yield strength of the subframe parts is improved about 12 %,but their tensile strength almost remains unchanged and elongation has a slight decrease.All of these results show that both of the dimensional accuracy and mechanical properties of the hydroformed subframe parts meet the design requirement.