Control And Optimization Of Loading Path In Hydroforming Of Tube

Hydro forming is a modern plastic processing technology with near net shape and almost no chipping,which is gradually developed on the basis of the traditional combined stamping and welding process.Compared with the traditional process,it has energy saving and weight reduction,improving product quality and precision,reducing the number of subsequent machining and assembling welding work,and improving the strength and rigidity of the formed parts,especially increasing the fatigue strength.The final forming quality of metal tubes is affected by many factors such as loading path(including axial feeding,internal pressure,back-displacement),friction,mold structure and material properties.During the hydroforming process for the tube,the loading path plays a crucial role in the final forming quality for the tube.Defects such as bursting,wrinkling and instability during the forming process are caused by unreasonable loading paths,and there are mutual constraints and connections between the various factors in the loading path.If the axial feeding increases too fast,while internal pressure cannot be timely followed,it will cause the material accumulation on the symmetrical side and the fillet corner of branch tube.On the contrary,if the axial feeding increases slowly,but the internal pressure increases too fast,the tube is prone to excessive thinning or even rupture,and it is difficult to form an ideal branch height.Only when the parameters between the loading paths are properly matched,the qualified forming parts can be obtained,so the control and optimization of loading path is very necessary for tube hydroforming.In this paper,X-type tube and T-type tube were taken as research objects.The main research contents were as follows:(1)The 3D drawing software UG was used to establish the geometric modeling of X-type tube and T-type tube for hydroforming process.Based on Dynaform,the combination of numerical simulation and hydroforming experiment was used to study the loading rules of three stages of linear loading path(internal pressure,axial feed,back displacement).It was found that the loading rate is the fastest in the initial stage of formation(filling and sealing stage and deformation stage),and the loading rate is the slowest in the late forming stage(shaping stage).(2)The influence of different friction coefficients on the thickness distribution of X-type tube and T-type tube was analyzed.With the increase of friction coefficient,the metal flow resistance was increased,the thinning rate was increased,and the thickness distribution was more uneven.From the perspective of stress and strain,the reason for the expansion of the top of the branch tube during the hydroforming process of the X-type tube and the T-type tube was revealed,because the top of the branch tube was in the state of biaxial tensile stress.The reason why the center position of the main axis was prone to wrinkle that because it was subjected to the maximum tensile strain and compressive strain,so the wrinkle defect was occurred easily due to the severe deformation.(3)The internal pressure,axial feed,back-displacement and friction coefficient were selected as the test factors.Based on Box-Behnken Design and response surface method,the response surface model was created with the minimum thickness,branch height and ultimate fillet radius as targets.The effect of various factors on the X-type tube and T-type tube by response surface method was studied,and the optimal parameter combination of the model was determined through analysis of variance and regression equation.Then the hydroforming experiment was carried out according to the same loading path.It was also found that the experimental results were consistent with the simulation results.The accuracy of the response surface model and the feasibility of the method are verified.In view of its own flexibility and intelligence,the response surface method can be applied to the hydroforming experiments for other shape tube,thereby the popularization and application of the technology were promoted in industrial actual production.