Simulation Analysis And Optimal Control Of Typical Forming Defects In The Preparation Of Complex Aluminum Profiles For Vehicle Body

The aluminum space frame bodies are widely used in automobile industry for their low weight,high specific strength,stiffness and energy absorption.The high precision extrusion and stretch bending of aluminum profiles are the two key technologies for manufacturing aluminum space frame body.The present study is mainly focused on the research of the key technologies in the extrusion and stretch bending of automotive aluminum profiles.An method for accurately controlling the uniformity of cross-section velocity at the die exit was proposed.The effects of section structure,material and process parameters on cross-section distortion and springback of profiles in stretch bending were systematically studied.The integrated multi-objective optimization design method of structure,material,process and performance for automotive aluminum profiles was established.An accurate 3D-FE model for simulating the porthole die extrusion process of profiles was established based on the ALE formulation.To quantify the friction at the workpiece/bearing interface,the ball-on-disc tests were used to simulate the real condition in the extrusion interfaces.Compared with the friction coefficient,the shear friction stress in the wear process is relatively stable,and not affected by the change of the contact area,which makes it more suitable to reflect the real contact between aluminum and die bearing.The formation mechanism of extrusion defects was revealed based on the evaluation of material flow velocity standard deviation(SDV)at the die exit and welding pressure.To solve the problem of the non-uniform velocity distribution,three steps of the die optimization were proposed,including optimization of porthole and drainage channel,introduction of baffle plate and optimization of die bearing.After optimization,the SDV value in the cross-section of extrudate is decreased from 23.75 to 1.63 mm/s.At the same time,the temperature uniformity in the cross-section and seam weld quality are improved significantly.The elasto-plastic finite element bending model and unloading springback model of the process were established based on ANSA and LS-DYNA software platform.The stress state for stretch bending is uniaxial with only a circumferential stress.The tangential stress of the outer material along the length of profile is not uniform,and the maximum value is not located in the middle postion of profile,but at the position of L/L0=±0.18 from the middle section.The distribution of cross-section distortion along the length of profile is similar to that of the tangential stress.After springback,the cross-section distortion decreases obviously and the maximum error is 13%.Better bending quality can be obtained when the pre-tension and post-tension are about Aσs.The cross-section distortion is reduced by 83%by the stretch bending with filler rubber pad,while the springback is increased by 192.2%.Increasing friction coefficient can increase springback,while the cross-section distortion reduce slightly.With the increase of clamp deflection angle,the cross-section distortion increases obviously and springback increases slightly.Increasing elastic modulus,hardening exponent or decreasing yield strength can reduce significantly cross-section distortion and springback.The cross-section distortion and springback increase with increasing strength coefficient.With the increase of the height-width ratio of section,the number of internal stiffener and profile thickness,the cross-section distortion decreases gradually.The larger the position of stiffener deviated from the center of section,the greater the cross-section distortion,and springback increases slightly.The significant factors of section structure,material and process parameters on the cross-section distortion are the cavity with or without filling,height-width ratio of section,the position of stiffener and pre-tension.The significant factors on the springback are the cavity with or without filling,pre-tension,material and friction coefficient.The work hardening and variation of thickness in stretch bending have significant influences on the impact performance of pendulum.In order to accurately simulate the impact response of automobile bumper,the bending forming history of profile should be introduced.The prediction accuracy of three kinds of metamodels including quadratic polynomial,Kriging and radial basis function(RBF)for five different performance indexes were comparatively studied.The quadratic polynomial metamodel is suitable for the mass and profile displacement,while the RBF metamodel is more reasonable for the cross-section distortion,springback and max thinning rate.During optimization process,the sampling points of metamodels are iteratively added and updated by the sequential sampling strategy.Taking the minimum mass and cross-section distortion as the objective function,and springback,max thinning rate and pendulum displacement as the restraints,the integrated multi-objective optimization problem of structure,material,process and performance for automotive aluminum profiles was solved through NSGA-Ⅱ algorithm.The result of solution set for Pareto front is obtained,which can provide engineers with a wide range of possible options.After optimization,the weight of automobile bumper is reduced 0.63kg,the cross-section distortion is reduced from 2.72mm to 0.81mm,andthe other performance indexes are nearly unchanged.To further accurately compensate springback,with minimizing the mean squared residual error ε between computed curve and target curve,an adaptive optimization model for die-face parameters based on RBF neural network model and simulated annealing algorithm was established.The method avoids the defects of low compensation efficiency and low precision by artificial experience or virtual trial.The bending experimental results verify the validity and feasibility of the optimization method.