Research On The Graded Material Property Distribution Of Autobody Crashworthy Components Based On Hot Stamping

With the rapid development of the automobile industry,the shortage of energy and the pollution of the environment are becoming more and more important.The automobile lightweight is the most effective means to improve the fuel economy and reduce the energy consumption.As the main approach to realize the lightweight of automobile,the lightweight technology of car body has become an important research topic.High strength steels can realize the lightweight of the body while ensuring the safety.Hot stamping of high strength steels can realize the lightweight of the body while ensuring the safety.Hot stamping of high strength steels have been gradually used in the design and production of automotive crashworthiness components.The process can reduce the forming load and springback of the sheet metal,and improve the forming ability of the material.Due to the excellent hardenability,boron steel becomes one of the main materials for body safety components of hot stamping.Good crashworthiness of car body can be achieved by the design of part structures and material properties.The selective cooling and selective heating hot stamping processes can control the microstructure of materials by the selection of forming process parameters.It can realize the graded distribution of the material properties of a single part to ensure that the appropriate property designs are located in suitable locations,and provides a solution for property design.It has broad prospects and significance.In this paper,with the guidance of the collision service performance of body parts,a series of research on the materials modelling and simulation analysis for the hot stamping of boron steel was conducted,on the basis of experimental data and by means of the hot stamping technology of graded material property distribution.Furthermore,the effect of material damage during forming process on the service performance of the parts was studied.The material property design and collision service performance were guaranteed by reasonably controlling selective hot stamping processes.Firstly,the foundations of deformation,heat transfer,and phase transformation related to the selective cooling and selective heating hot stamping processes were studied.It mainly included: the deformation mechanism,strengthening mechanism,softening mechanism and damage mechanism of thermoplastic deformation,the analysis of heat transfer of hot stamping forming process,the foundations of thermodynamics and kinetics for metal solid phase transformation and the microstructure characteristics,and formation mechanism of boron steel during heating and cooling processes,etc.Secondly,the materials modelling and numerical simulation analysis of the selective cooling hot stamping process were studied.A viscoplastic-damage constitutive model at elevated temperatures and an austenite decomposition model were established respectively based on the experimental data.The hot stamping process simulation was conducted by invoking damage-phase transformation material model in LS-DYNA finite element software,and the validity of the results was verified based on the experimental data.Furthermore,a numerical simulation model of the selective cooling hot stamping process was established to investigate the effects of process parameters on material formability and material properties after phase transformation.Then,the simulation process of the selective heating hot stamping process was studied.An austenite formation model and a deformation constitutive model at elevated temperatures were established respectively based on the experimental data,obtained by Li et al.An austenite decomposition model was established using quenching test results of hot stamping.The integral and stepwise simulation schemes of the selective heating hot stamping process were proposed,and the advantages and disadvantages of the two schemes were compared.The process of the running subroutine for the stepwise simulation scheme was introduced in detail.Furthermore,the manufacturing process of a U shaped beam was simulated by the selective heating hot stamping process,and the part with graded properties was obtained.In addition,the effect of forming process for boron steel on the service performance for parts was studied.A viscoplastic-damage constitutive model at room temperature,only capable of describing the effects of phase composition and strain rate on the deformation behavior of the material was established,and the bending simulation analysis of the parts without considering forming effect was carried out.The forming and service tests of boron steel were conducted to study the effects of the cold and hot forming processes on the service performance respectively and the cause of the effects was explained by the microscopic test results.A service constitutive model at room temperature,capable of describing the effects of hot forming history,phase composition and strain rate on the deformation behavior of the material was established,and it could predict the material behaviors under a variety of complex service conditions.Then,the bending simulation analysis of the parts considering forming effect was carried out.Differences in bending characteristics of the parts when ignoring and considering forming effect were compared.The effect of different lengths of martensitic phase region on the parts with multiphase structure and the effect of different forming strain ratios on the failure form and bearing capacity of the parts were studied.Finally,a material performance design method of crashworthy components was proposed based on the side impact performance of automobile,and the application process of the method was demonstrated by taking the B pillar as an example.According to the method,a side impact simulation model consisting of a vehicle FE model and a moving deformation barrier FE model was established.Through the simulation results,the intrusion velocity and displacement at the measured points of the B pillar for the collision side were obtained and taken as the initial crashworthiness requirement.According to the forming damage and the design experience of crashworthy components,the safety margin was estimated.On the premise of ensuring the safety margin,the thickness and material partition of crashworthy components were optimized in side impact simulations based on the principle of the selective heating hot stamping,and an optimization program of thickness and material partition for crashworthy components was chosen.Then,the required forming process plan related to the optimization program was developed and carried out.The feasibility of the forming process plan was validated through side impact simulations,and the effect of the forming damage on the service performance for parts was validated by the comparison and analysis of side impact simulation results for both with and without forming damage of crashworthy components.