The formation mechanism of non-dendritic primary α–Al phase in semi-solid AlSi₇Mg Alloy

In this paper, a process to make non-dendritic semi-solid AlSi7Mg alloy by electromagnetic stirring and the temperature field of the stirred melt cooled is continuously investigated. It is proposed that a new kinetic factor for primary α-Al nucleation is that a low thermal gradient exists in the electromagnetically stirred melt, for which the primary dendrite arms and secondary dendrite arms are refined. The results also show that the root remelting of the secondary dendrite arms is an important mechanism of the primary α-Al refinement. Strong electromagnetic stirring greatly reduces the composition supercooling in the melt and eliminates the preferred growth of primary dendrite arms, therefore, many rosettes or spherical primary α-Al phase particles form finally.