The development of nucleation controlled microstructures during laser treatment of Al-Si alloys

Solidification of laser-remelted Al-26 wt% Si alloys has revealed a transition from constrained growth to nucleation control of microstructural 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 by fibrous eutectic. An analysis of the transition from growth-controlled to nucleation-controlled microstructure development has been formulated based upon the limiting solidification interface temperature at the transition and a heterogeneous volume nucleation model. With a nucleant distribution and potency that is continuous with increasing undercooling, the model analysis provides a good account of the variation in silicon particle density with scan velocity. The reliability of the model is explored by a sensitivity analysis to establish bounds on unmeasured parameter values and to determine the processing domain of nucleation controlled microstructural evolution. Copyright (C) 1996 Acta Metallurgica Inc.