Study On The Lightweight Optimization Design For The Whole Developing Process Of Aluminum Alloy Body

With the popularization of new energy vehicles and t he stricter requirements for energy saving and emission reduction,aluminum alloy body has been paid more and more attention by OEMs due to its good lightweight effect and superior performance.However,the lightweight design research at different stages i n the whole developing process of aluminum alloy body is not systematic.This thesis takes aluminum body as the research object,and carries out a systematic study on the theory and method of lightweight optimization design in three different stages of early planning,conceptual design and detailed design in the developing process of aluminum alloy body:(1)In the early planning stage of aluminum alloy body,the quantitative relationship between the static stiffness,lightweight coefficient and various mod al parameters of the aluminum alloy body platform is derived according to the modal theory.Taking the finite element model of the aluminum alloy body platform as the carrier,the first 50 modal parameters were extracted and the approximate solutions of bending and torsional stiffness and lightweight coefficient of the aluminum alloy body were calculated.The results showed that the errors were only 4.32%,1.85% and1.78%.It is indicated that the static compliance of aluminum alloy body can be approximated by the sum of the contributions of each order of modal compliance,and the more order selected,the better the approximation effect.Meanwhile,it is found that the largest contribution to the first-order bending(torsional)stiffness is the corresponding first-order bending(torsional)mode.Finally,by comparing the bending stiffness,torsional stiffness and lightweight coefficient derived from finite element theory and modal theory with the experimental value,it is concluded that the accuracy of modal theory algorithm is higher than that of finite element theory algorithm.The error of bending stiffness,torsional stiffness and lightweight coefficient based on modal theory are 1.85% 1.82%,and 1.89% respectively.In this way,the validity of the above m ethod was verified.It provided theoretical foundation for the lightweight design and performance target setting of the aluminum alloy body in the early developing stage.(2)In the conceptual design stage of aluminum alloy body,the parametric conceptual model of aluminum body was established based on the detailed joint s and beam element,which meets the requirements of engineering development as the error between the parametric model and the detailed model is less than 15%.Then,8 key sections were selected as design variables through the relative sensitivity analysis of area A,moment of inertia Iz,Iy and torsion constant J of the aluminum body section.The EWL approximate model was proposed.The KRI,RSM,RBF,EI,EG and EWL approximate model of basic properties were established by Latin hypercube sampling.Based on boxplot analysis of evaluation indexes(the deterministic coefficient,the root mean square error,the relative maximum absolute error)of these approximate model,it is manifested that the EWL combined approximate model has the highest fitting accuracy among the six approximate models,and its stability has obvious advantages over EI and EG method.Thus,the EWL approximate model is suitable for the lightweight optimization in the conceptual design stage of aluminum body.Finally,combined optimization algorithm(global ASA annealing algorithm and NLPQL method)is used to optimize the EWL combined approximate model of parametric conceptual aluminum body model.It is concluded that the weight reduction of the aluminum body is 6.7kg,which accounts for 6.58% of the total weight of the vehicle.The above method has achieved a better lightweight effect and provides guidance for the lightweight design of aluminum body in the conceptual design stage.(3)In the detailed design stage of aluminum alloy body,CVOD sequential sampling method was proposed,which effectively improved the modeling efficiency of RBF approximate model.Considering the influence of uncertain factors such as design variables and noise factors on performance,the multi-objective reliability optimization for lightweight was carried out by using the moment-based dual-cycle reliability optimization strategy.Besides,the multi-objective quadratic optimization method was proposed to optimize the obtained Pareto solution set again,aiming to solve the local convergence problem existing in the NSGA-II genetic algorithm.At last,the reasonable optimal solution was selected by comparing the gray level correlation analysis in multi-criteria decision making based on TOPSIS method and lightweight coefficient method.As a result,the weight reduction is 3.87 kg,accounting for 3.7% of the weight of the whole vehicle,and the error with test results is less than 5%.It is proved that the method used in this thesis is effective and has high practical value for the lightweight design of aluminum bo dy in the detailed design stage.