Abstract: In this paper,anew method called "rapid fixing" has been established for the study of the solute distribution at the solid-liquid interface during solidification of the welds.It was the first time to measure the concentration of Si solute in LD11R Al alloy at the frontier of the S-L interface and to show that the distribution of the concentration of Si solute was declinde in an exponential way.This work shows the transient distribution of the Si solute at different time during the crystal nucleation and growth.A relationship between the thickness of the Si solute build-up in the solid phase and that of the Si solute diffusion in the liquid phase at the S-L interface was found out.From the results,it can be concluded that the constitutional supercooling at the interface is a driving force for the crystal growth with variable rate.This work also shows that under overheating condition in the weld the assumption of homogeneous nucleation is not applicable.