Research On Damage Evolution Mechanical Behavior And Formability Prediction In Hot Stamping

Hot stamping of high strength steel is an advanced manufacturing technology which includes heating,forming and quenching of sheet metal.It is an important way to realize the lightweight of vehicle body and strengthen the vehicle crash safety.Hot stamping process involves complex thermo-mechanical coupling issues.The damage evolution,which is affected by temperature and strain rate,makes the accurate prediction of thermal formability of high strength steel more difficult.The unreasonable forming process under elevated temperature will result in the internal damage evolution of sheet metal,and eventually lead to the instability and fracture,which will seriously affect the quality of products and cause waste of resources.Therefore,it is of great practical significance to study the damage evolution behavior of high strength steel sheet,and then establish accurate formability prediction method for hot stamping process.In this paper,the high-temperature deformation mechanical behaviors and fracture mechanism of high strength boron steel 22MnB5 are systematically studied by means of thermal-mechanical tensile tests and forming limit experiments.Furthermore,based on the experimental results,the constitutive integration algorithm of mesoscopic damage model,and the forming limit prediction method considering meso-damage are developed for hot stamping.Finally,by using the calibrated damage characteristic parameters,the accurate prediction of 22MnB5 forming limit at elevated temperatures and fracture of sheet metal in the actual hot stamping process is realized.To this end,the research work focuses on the following aspects:(1)The effects of cooling rate,tensile speed,deformation temperature on the flow behavior and fracture mechanism of Al-Si coated high strength boron steel 22MnB5 are investigated by conducting systematic thermo-mechanical simulation tensile tests.With the help of non-contact optical measurement technology and infrared temperature measurement technology,the influences of gauge length on the calculation results of the stress-strain curve for high temperature deformation are also discussed.The experimental results show that the cooling rate,tensile speed and deformation temperature have significant effects on the mechanical behavior of the studied material.In the process of uniform tension,different positions of the specimen will show different deformation rates in different tensile stages,which will affect the mechanical properties of the material.Based on fracture surface observation and deformation characteristics analysis of the tensile specimens,the fracture mechanism of 22MnB5 sheet is analyzed and can be indicated as:microvoid nucleation,microvoid growth,void coalescence,micro-crack generation,macro-crack generation and final fracture.Meanwhile,it is found that both temperature and strain rate have significant effects on the damage evolution of the specimens.(2)According to the hot deformation characteristics of high strength steel,the constitutive integration algorithm of rate-dependent Gurson-Tvergaard-Needleman(GTN)meso-damage model is established based on implicit backward Euler stress return method,and the VUMAT user material subroutine is developed.Furthermore,the damage characteristic parameters of 22MnB5 at different deformation temperatures and strain rates are identified accurately by the parameter inverse method.The influence of local strain rate on material deformation behavior is studied by finite element numerical simulation:under the same tensile displacement,the uniform deformation degree of the specimen will increase significantly,compared with the simulation without considering the strain rate.Meanwhile,damage evolution during the tensile process and the influences of loading conditions on damage parameters are studied.Finally,the effects of damage characteristic parameters on the mechanical behavior of materials are also analyzed.(3)The forming limit tests of high strength boron steel have been carried out by a novel hot forming limit test equipment developed independently in China.Based on the Nakazima bulging test principle,the thermal forming limit diagrams(TFLD)of 22MnB5 under different bulging temperatures are established and the influences of temperature and friction on the formability of sheet metal are analyzed.With the help of non-contact optical measurement technology DIC,the deformation characteristics of the specimens in the bulging process are analyzed in detail.Based on the biaxial pre-straining theory,a reasonable explanation is given for the lowest forming limit point appearing in the right half of the forming limit diagram.(4)The forming limit prediction method of M-K model coupled with mesoscopic damage is established and the prediction process is deduced in detail.The critical void volume fraction and the failure void volume fraction are treated as instability criteria to predict the forming limits of 22MnB5 at ambient temperature and high temperature respectively,and the evolution laws of void damage under different loading paths are analyzed.Meanwhile,the finite element model of Nakazima bulging is established.By using the identified damage parameters and the VUMAT subroutine,the fracture position of the specimen can be predicted accurately.Furthermore,the forming limit diagrams of 22MnB5 at different temperatures and deformation speeds are established by finite element simulation.Finally,the forming limit stress diagram(FLSD)of 22MnB5 at different temperatures is established and analyzed.(5)The hot stamping process of a B-pillar is simulated by implementing the VUMAT subroutine,and verified by experiment.The temperature field and damage distribution of sheet metal during the forming process are analyzed.By predicting the force-displacement curve of punch and the fracture location of sheet metal,the validity of the established coupling damage constitutive integration algorithm and damage parameters is further verified,and it can be used to predict the formability of sheet metal during actual hot stamping process.Finally,the effects of the initial forming temperature and stamping speed on the thermal formability of sheet metal 22MnB5 are analyzed.