Flow structure optimization and the impact on the solidification structure

The adjustment of fine grain morphologies has been approved to be a crucial issue for improving characteristics and properties of cast and wrought aluminium alloys. Several methods are known to achieve grain refinement in solidification processes: add-on of grain refiners, rapid cooling conditions, mechanical or electromagnetic stirring, or ultrasonic treatment.
AC magnetic fields provide a contactless method to control the flow inside a liquid metal and the grain size of the solidified ingot. Many studies have shown that beneficial effects like a distinct grain refinement or the promotion of a transition from a columnar to an equiaxed dendritic growth (CET) can be obtained. However, electromagnetically-driven melt convection may also produce segregation freckles on the macroscale. The achievement of superior casting structures needs a well-aimed control of melt convection during solidification, which in turn requires a detailed knowledge of the flow structures and a profound understanding of the complex interaction between melt flow, temperature and concentration field.
Previous investigations considered the use of time-modulated AC magnetic fields to control the heat and mass transfer at the solidification front [1, 2]. It has been shown recently under laboratory conditions, that an accurate tuning of the magnetic field parameters can avoid segregation effects [3] and homogenize the mechanical properties [4].
This present study examines the directional solidification of commercial cast and wrought aluminium alloys from a water-cooled copper chill. Rotating magnetic fields were used to agitate the melt. The application of different stirring strategies, e.g. time-modulated magnetic fields, reveals the impact of diverse flow conditions on the resulting macro and micro structure. The solidified structure was reviewed in comparison to an unaffected solidified ingot. Our results demonstrate the potential of magnetic fields to control the grain size and the formation of segregation freckle. In particular, time–modulated rotating fields show their capability to homogenize both the grain size distribution and phase distribution.