THE DEVELOPMENT OF NUCLEATION CONTROLLED MICROSTRUCTURES DURING LASERTREATMENT OF AL-SI ALLOYS

Solidification of laser-remelted Al-26 wt% Si alloys has revealed a tr ansition from constrained growth to nucleation control of microstructu ral evolution. Laser scanning velocities between 5 and 8 mm/s resulted in a fibrous eutectic microstructure. For scanning velocities above 8 mm/s, the microstructure consisted of equiaxed, micron-sized silicon crystals surrounded by alpha-Al cells that, in turn, were surrounded b y fibrous eutectic. An analysis of the transition from growth-controll ed to nucleation-controlled microstructure development has been formul ated based upon the limiting solidification interface temperature at t he transition and a heterogeneous volume nucleation model. With a nucl eant distribution and potency that is continuous with increasing under cooling, the model analysis provides a good account of the variation i n silicon particle density with scan velocity. The reliability of the model is explored by a sensitivity analysis to establish bounds on unm easured parameter values and to determine the processing domain of nuc leation controlled microstructural evolution. Copyright (C) 1996 Acta Metallurgica Inc.