Melting and coarsening of A356 during preheating for semisolid forming

Abstract

The melting and coarsening behaviour of A356 during preheating for semisolid forming has been investigated theoretically and experimentally. The preheating involves a relatively rapid heating and a partial melting, transforming the material into a mushy state, i.e.a mixture of solid particles and melt. The experiments reveal a change in melting behaviour over the temperature interval 580–590°C, where a re-solidification occurs during heating. During the initial phase of melting, the smallest solid particles melt completely and larger particles are split into smaller particles, making the shape of the particles more irregular. Particle fragmentation influences the occurrence of the enclosed eutectic, which temporarily maintains a higher solid fraction as a result of increased diffusion distances. Resolidification and coarsening contribute to a rapid particle size increase. The model derived fits the experimental series well and supports the coarsening and particle fragmentation observations.