Microstructural evolution during remelting of laser surface alloyed hyper-monotectic Al–Bi alloy

The present investigation explores the possibility of synthesizing a two-phase microstructure consisting of a fine dispersion of bismuth particles in an aluminium matrix using the laser surface alloying technique. The possibility of controlling the size distribution of bismuth particles by subsequent remelting is also investigated. The microstructural analysis of the surface alloyed samples shows that the average size of the bismuth particles reduces with increase in laser scan speed. In order to understand the factors that determine the nature of the size distribution of the particles, a detailed model is developed. The model incorporates heat and fluid flow induced by the laser to arrive at the evolution of the temperature and velocity of the melt in three dimensions. Using these as inputs, a kinetic analysis of the nucleation, growth and coarsening induced by collision-controlled coalescence of the bismuth particles from the melt is carried out. Comparison with the experiments indicates that coalescence due to convection plays an important role in the evolution of the size distribution of bismuth particles.