Research On Hydraulic Push-pull Bending Process And Wrinkling Instability Of Thin-walled Metal Tube

Due to their special hollow section structure and excellent product performance,bent tubes are more and more used in many high-tech industries,such as aviation,aerospace,automotive,shipbuilding,petrochemical industries,medical devices and so on.As the thin-walled tubes(t/D<0.05)are prone to inner thickening and wrinkling,outer thinning and cracking,cross-section distortion,springback and other defects in the bending process,which seriously limits the application of bent components in the field of high precision ends.Some traditional tube bending processes have been unable to meet the increasingly high requirements of industrial production on the appearance,performance,bending accuracy and shape accuracy of bent tubes.Therefore,a hydraulic push-pull bending process for precision bending and forming of thin-walled tubes was proposed in this paper,which has the advantages of various bending methods such as push bending,pull bending and liquid-filled bending.In this paper,an in-depth and systematic study of this process were conducted through theoretical analysis,finite element numerical simulation and experimental research.In order to obtain the mechanical property parameters of metal tubes,the true stress-strain curves of DC06,08 Al,304 stainless steel and 316 stainless steel are obtained through unidirectional tensile test.And the appropriate material constitutive relationship models are selected,which lay a foundation for the theoretical analysis of the stress state of tube in pure bending condition.The stress-strain variation law of the inner and outer sides of the bent tube was studied by establishing the mechanical equilibrium differential equation.The calculation formula of the position of the strain neutral layer and the calculation formula of the variation of the wall thickness on the concave and convex sides of the elbow are deduced.The formulas for calculating the location of the strain-neutral layer and the variation of wall thickness change on the inner and outer side of bent tubes are derived.Based on ABAQUS finite element platform,the advantages and disadvantages of different buckling analysis simulation methods were compared and analyzed.A DYNAMIC/EXPLICIT finite element model with initial defects was proposed.This paper takes the example of the AA6061 diameter-reduced tubes by solid granule medium forming technology,a dynamic explicit finite element model with shape defects and thickness defects is established to accurately simulate the ripple morphology and number after tubes wrinkling and instability.The reliability of the simulation method is verified by comparing the force displacement curve between the simulation and the experiment.The finite element simulation model of hydraulic push-pull bending of thin-walled tubes with initial defects was constructed.Through the analysis of the finite element simulation results,the effects of material properties,relative bending radius,relative wall thickness,internal pressure,friction and clearance between the outer wall of tube and the inner cavity of die,internal pressure loading path,thrust and tension loading paths and other factors on the forming quality of the bent tubes were studied.It provided data support for the establishment of wrinkling limit diagram and wrinkling prediction model based on BP neural network,and provided an important reference for process experiments.The strain loading path at the critical wrinkling moment of the buckling point was extracted,and the critical wrinkling strain line based on the main strain was established to accurately predict the wrinkling instability region of the bent tube under specific working conditions.In order to improve the generality of the wrinkling determination line in predicting the wrinkling instability region under different bending process parameters,a unified critical wrinkling judgment line characterized by stress ratio and strain ratio was established by introducing two-dimensional principal stresses on the basis of two-dimensional principal strains.The wrinkling area of the bent tube is below the judgment line and the non-wrinkling area is above the judgment line,which proves that the unified critical wrinkling judgment line can identify the wrinkling instability area of bent tubes.Taking relative bending radius,relative wall thickness,internal pressure and die clearance as input parameters,a BP neural network wrinkling instability prediction model was established through the training and testing of a large number of simulation data,which can predict the maximum wrinkle height Δh,the number of wrinkle corrugations N and the maximum wall thickness thickening rate η.According to the results of finite element numerical simulation,an experimental study of the hydraulic push-pull bending process for thin-walled tubes was conducted.The effects of process parameters such as material parameters,internal pressure and lubrication conditions on the forming quality of bent tubes were obtained.The experimental results show that the hydraulic push-pull bending process is generally applicable to the precision bending and forming of thin-walled tubes.The experiment has accumulated experience for the popularization and practical application of this process.