Microstructure Simulation Of Low-carbon Steel In The Solidification Process Of Weld Pool

The performance of welded components is directly affected by microstructure of welded joints.During the welding process,solidification is the main way to form the weld.Depth analysis mechanism of the weld pool solidification is conducive to control the weld and predict the performance of welded joints.However,the weld pool is small,the temperature distribution is extremely uneven,and the presence time of liquid metal is very short.Those features make real-time monitoring solidification process of weld pool very difficult.In recent decades,with the rapid development of computer technology and the solidification theory,numerical simulation method to study the process of weld pool solidification becoming one of the hottest issues in the field of welding.In this paper,the process of dendrite growth and evolution on low-carbon steel in the solidification of weld pool is simulated by cellular automata and finite difference method based on the existing simulation study.Studies shows that simulated solute concentration rule in the process of dendritic growth is consistent with the basic theory of solidification.The larger th,e initial undercooling degree is,the faster the dendrite growth rate will be.The number of secondary dendrite and tertiary dendrite grow with the initial undercooling degree and its topography is more complex and diverse.When columnar grains transform to equiaxed grains,the solute released by equiaxed growth makes solute concentratrion of the columnar dendrite tip increases sharplly,which impedes continued growth of columnar grains.The cooling rate have greater impact on columnar dendrites to equiaxed dendrite transition in the solidification of the weld pool.The larger the cooling rate is,the more easily the transition will be,and the shorter required time will be.The temperature field and dendrite growth are coupled together to reproduces the solidification process of weld pool on Fe-0.04%C binary alloy by two measures of macro and micro.The temperature gradient is decreasing along the direction of thermal boundary to the insulating boundary,but the curvature of the isotherm is increasing.Temperature gradient increases with the increasing of boundary diffusion rate in simulation area.With the increase of number of nucleation substrate,the number of columnar crystals is relatively stable,while the number of equiaxed grain increased sharply,the distribution range is gradually expanded,and the size of equiaxed grain is reduced.In addition,solute concentration control model of phase change in which features of binary Fe-C phase diagram are considered is established.The improved model of low-carbon steel(Fe-0.04%C)are used to simulate the weld.Moreover,the influence of the initial solute concentration,cooling rate and the number of nucleation substrate on microstructure of weld metal are discussed.