Study On Thermal-Metallurgical-Mechanical Coupling Numerical Simulation In Laser Welding Of Martensitic Steel 1700MS

Advanced high strength steel(AHSS)has a very important role in automotive light weighting industry and promoting passive security,due to its good energy absorption.Meanwhile,laser welding with high efficiency and high flexibility,are very important for high quality and efficient manufacturing of thin-wall body components.However,the peak temperature and heating/cooling rate vary rapidly with changes in positions of welding joint during welding,leading to highly uneven distribution of microstructure,properties,and residual stresses.This put forward higher requirements for welding numerical simulation technology: 1)As the size of the pool in the laser welding process of AHSS thin plate is usually in the order of millimeter or even submillimeter,it is difficult to predict the transient temperature field accurately;2)The simulation accuracy of the existing welding solid-state phase transformation(SSPT)model under ultra-fast heating is low,and there is a serious problem of martensite tempering softening during welding.The relevant laser welding SSPT need to be improved urgently;3)Solid-state phase transformation is usually accompanied by a variety of phase products,each of which has a different flow stress behavior,resulting in the variability of the adaptive rules of stress to strain during welding.Based on the above problems,this paper took martensitic steel 1700 MS as the research object,and carried out a study on thermal-metallurgical-mechanical coupling numerical simulation in laser welding of martensitic steel 1700 MS.The main contents are as follows:(1)This part was performed to meet the establishment and verification requirements of the related models of the thermal-metallurgical-mechanical coupling numerical simulation.The experiments contained welding experiment,tempering simulation and tensile test,aiming to provide data support for the establishment and verification of heat source model,martensite tempering model and flow stress model.Meanwhile,the microhardness and residual stress of the welded samples were tested,aiming to verify the simulation results of microstructure and residual stress.In addition,martensite tempering was the main reason for the reduction in the mechanical properties of welded joints.Joints with middle soft heat-affected zone,and side hard zones(weld zone and base material)was formed.The hard zone accelerated the fracture failure of the softened zone during tensile process.(2)This part was performed aiming to the difficulty in predicting the transient temperature field of the welded joints with size in decimillimetre,and a laser welding temperature field simulation based on particle swarm optimization was carried out.A double cylinder heat source model with inclined body was established,which was closer to the real laser energy loading.Based on the upper/lower width and length of molten pool,a simulation error evaluation method considering the three-dimensional morphology of molten pool was proposed.Combined with swarm intelligence algorithm,a heat source coefficient optimization method based on variable gradient discrete particle swarm optimization was proposed.ANSYS and MATLAB software were used to develop the heat source coefficient optimization program,and the multi-process parameters adaptability of the heat source model was verified.The accurate prediction of the submillimeter molten pool morphology was realized.The minimum and average 3D morphology errors were 2.32% and 3.93%,respectively.(3)This part was performed due to the low simulation accuracy under the condition of ultra-fast heating and the lack of martensitic tempering model in the SSPT model,and a SSPT model considering martensite tempering for laser welding was constructed.The austenite phase transformation model was improved and the austenite grain growth model was modified based on the heating rate.The method for predicting the initial temperature of martensitic transformation based on austenite grains was introduced and used to predict martensitic transformation.The model of non-isothermal martensite tempering was derived,and the model parameters were optimized using the experiment results.After improving the existing SSPT model,the welding microstructure simulation was carried out according to the temperature simulation results.The accuracy of the improved SSPT model was verified by the hardness test results in Chapter 2,with an average hardness error of 4.84%.(4)This part was performed aiming at the complex and changeable flow stress behavior during SSPT,and a thermal-metallurgical-mechanical coupling constitutive model for laser welding of 1700 MS was developed.Based on the thermo-elastoplastic constitutive relation,the main factors of stress-strain relationship were analyzed.According to the real stress-strain curve of 1700 MS and temperature sensitivity coefficient,the flow stress model parameters were optimized.The model was improved aimed at the yield strength reduction caused by martensite tempering.Considering the strain caused by SSPT(volumetric strain,plastic strain),the effects of SSPT on strain increment were revealed.The UPFs module of ANSYS software was used to develop USERMAT subroutines.The stress field simulation based on the thermalmetallurgical-mechanical coupling was carried out,and the simulation results showed good consistency with the test results.The influence of SSPT on stress field was analyzed,and the evolution process of residual stress and plastic strain was studied based on the yield strength and thermal strain change curve with temperature during welding.