Research On Constitutive Integrational Algorithms For Automotive Lightweight Sheet In Warm/Hot Forming

In nowadays, hot forming and warm forming as innovative techniques have played an important role in achieving energy conservation and emission reduction in automobile industry, which has attracted scholars’attention both at home and abroad. Hot forming as an innovative manufacturing process combine the cold forming and hot forging technology, including heating, forming and quenching process. Warm forming is suit for the production of parts for aluminum or magnesium alloys and the forming temperature is lower than the material’s recrystallization temperature. The high strength steel car structure parts produced by hot forming have some advantage of good formability, low springback and high strength, which can save weight and. improve crash safety.Comparing of traditional cold forming, the introduced temperature can strongly affect the material’s mechanical responding, which is a typical thermo-mechanical coupled problem. The plastic deformation of metal is dependent on its own micro-structures. In order to present a reasonable constitutive model and corresponding computational algorithm to describing the mechanical behavior physically, the effect of the micro-structures should be considered. In this thesis, several thermo-mechanical coupled integration algorithms based on the crystal plasticity and thermodynamic theory are presented. The effects of strain, strain rates and different heating/cooling rates in the hot forming are investigated, and the research work mainly covers the following aspects:(1) First, the research developments of warm/hot forming and crystal plasticity theory are introduced. This part focuses on the thermo-mechanical coupled models, the constitutive models for automotive lightweight sheet in warm/hot forming and the traditional crystal plasticity algorithms. Rate-dependent crystal plasticity model is introduced systematically. The methods to deal with highly non-linear effect during the computational process are introduced, which is caused by the expression of the slip shearing rate.(2) Thermo-mechanical coupled models of single crystal for finite deformation are presented. Based on the crystal plasticity theory, these models introduce the thermal part of deformation gradient into the multiplicative decomposition of the total deformation gradient, which means an intermediate thermal configuration representing the thermal history is introduced between the reference configuration and relaxed configuration. Considering the thermodynamics, the elastic deformation gradient and the plastic deformation gradient are chosen as the basic integration variable separately in two implicit algorithms. Newton-Raphson method is used in solving process in order to ensure the numerical stability. The effects of loading direction, anisotropic, different heating or cooling rates on effective stress-effective strain response for1100Al single crystal are computed. Based on the above theories, an explicit algorithm is developed using the Euler forward method.(3) Hot forming experiments of high strength steel22MnB5are performed in the isothermal and non-isothermal conditions. In isothermal tensile process, the blank is deformed in the temperature range between600-900℃, and at different strain rates. In non-isothermal tensile process, the22MnB5is cooled form900℃, and the tensile deformation of the blank is started at different temperatures during the cooling process. The material’s mechanical behavior in this thermo-mechanical coupled stage is investigated.(4) Based on the established computational models for single crystal, polycrystalline computational models are presented by using the homogenized method. The computational process, stress update and orientation of single crystal are introduced. In applications, the mechanical behavior of1100Al in warm forming and22MnB5boron steel in hot tensile deformation are computed using these models. The effects of temperature, cooling rate and strain rate during the deformation are investigated. Inorder to test the validity of these models, the predicted results have been compared with the experimental data.