Abstract: Based on grain formation principle and kinetics characteristics of dendrite growth,two-dimensional mathematical and physical model of nucleation,dendrite growth,redistribution and diffusion of the solute were established to simulate the columnar- to-equiaxed transition in rapid solidification process of welding molten pool and the influence of different cooling rate on this transition process was also studied.The results show that, the solute redistribution and diffusion are evident in the process of rapid cooling.When the columnar grains transform into equiaxed grains,the discharged solute makes concentration sharply increased at the tip of columnar grain,which inhibits the growth of columnar crystals.Moreover,the change will be more likely to occur at higher cooling rate,and the required time will be also shorter.