Study On The Effect Of Phase Transformation And Precipitation On Thermo-elastic-plastic Mechanism And Mechanical Properties In Laser Welding Of High Strength Steel

High-strength steel with its advantages of high strength,large toughness and good machining performance,has been widely used in marine engineering equipment.For example,the application of high strength steel in jack-up drilling platform has reached 55%?60%.At the same time,the laser welding technology,with its advantages of low heat input,small deformation,high depth-width ratio,has provides a new technical method for marine structures manufacturing.However,temperature and mechanics characteristics are influenced by high heating and cooling rate in laser welding processing:it is difficult to express the actual temperature field using current heat source model;large plastic behavior is induced by microstructure and its transformation;yield stress is also changed because across or shear mechanism behavior occurred between precipitation particles and dislocation.Therefore,mechanism of thermal elastic plastic behavior and mechanical properties of the weld in laser welding high strength steel were studied in this paper.There were four aspects can be concluded as follows.A peak increment-double cone heat source model considering the actual bead geometry was developed to improve computation accuracy of heat source model in full penetration.Assuming that the peak heat flux in the weld waist plane was a constant,while peak heat flux exponentially increased along the direction of weldment thickness,thus the peak index increment-double cone heat source model was deduced;Combining with the weld appearance size,the choosing method of geometric parameters and the adjusting parameters for the heat source model were designed;the equivalent specific heat model considering the latent heat of solid/liquid bond was analyzed,the finite element mathematical model was also established to compute the welding temperature field.To explain the nucleation and growth behaviors of microstructure in welding process,solid-state phase transformation model based on the theory of the phase transition thermodynamics was established.Austenitizing was a reversible process with diffusive transformation,but the nucleation and growth speed was different,with the help of Johnson Mehl Avrami Kolmogorov(JMAK)theory,the mathematical models of austenitic and diffusion transformation model were deduced;Bainite transformation presented a transformation from single-phase(gamma)to two phase(alpha and carbide),involving the carbon atom diffusion,and thus bainite transition model was established;According to the related research work of Koistinen and Marburger,the martensite phase transformation model was built;the explicit discretization algorithms for these four solid-state phase transformation models were deduced,and further the influence from some conditions(temperature,time,cooling rate and so forth)on the phase volume fraction was also studied.Considering the influence of solid-state phase transformation and precipitation strengthening on thermodynamics behavior of laser welding,the thermal elastic-plastic finite element model integrated with the solid state phase transition and precipitation strengthening was established.Analysis of thermal elastoplastic equilibrium process was executed,stress-strain mathematical relationship between stress and strain was derived;Material properties was solved based on composition phase and their phase volume fraction,and the mathematical model of strain increment was developed from the point of unit volume expansion,martensite shear strain and plastic strain induced by martensite;Based on Ashby-Orowan dispersion strengthening theory,the yield strength increment model was established;The thermal elastic-plastic finite element program considering solid-state phase transformation and precipitation strengthening was developed.Experiment of laser welding EH36 steel was completed,mechanical properties of the weld was investigated,and further simulation results of the finite element model was also verified.The influence of laser welding parameters(welding power,defocus length and welding speed)on the weld appearance and mechanical properties(hardness,tensile strength and residual stress)was studied,while data(welding morphology,phase composition distribution,stress)for verifying the correctness of the finite element model was extracted,the actual state of welding was thus reflected;It was the first time to couple solid-state phase transformation and precipitation strengthening behavior into thermal elastic-plastic finite element model by APDL and UPFs synergy method,the average error of temperature based bead profile was 11.17%,the biggest error of phase volume fraction was 4.51%,the residual stress in the simulation results kept a good consistency with that of the experiment results,and the influence of the solid-state phase transformation and precipitation strengthening behavior on residual stress and plastic strain was also analyzed.