A 3-D transient modelling based on the numerical resolution of the fluid flow ant the heat transfer equations is developed for butt welding using the cladding process. The physical mechanisms included in our model concern the matter melting, and the re-solidification. The implementation of developed procedures called User Defined Functions (UDFs) working interactively with Fluent CFD code, and a dynamic mesh method, allows to treat the problem with its specific and complex boundary conditions. The fusion, resolidification, and the temperature dependence of the physical properties are taken into account. As results, clad formation and information on the temperature field are obtained.