Effect of permanent magnetic stirring on the solidification microstructure and ingot quality of Al-Cu alloys

A permanent magnetic stirring (PMS) facility which was fabricated for investigating the influence of PMS on the solidification of Al-4Cu and 2024 Al alloys is described. It was found that PMS can effectively refine the grain structures of these alloys with the finest grain structure about 100 μm in grain diameter. PMS at 600 rpm has a similar grain refining effect to adding grain refiner. At the stirring rates over 600 rpm, too big gas holes are induced into the ingots. The cracks on the ingot side surface and elongated holes on the ingot cross section caused by unidirectional PMS can be avoided by applying a direction-reversing PMS. PMS can lead to a slightly more uniform eutectic phase distribution in 2024 Al alloy without changing eutectic phase types. A new nucleation mechanism based on the big-bang nucleation theory is proposed to explain the grain refining effect and limit of PMS.