Effect of time–modulated magnetic fields on the solidification structure and extrusion properties of wrought aluminium

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.
Previous investigations considered the use of time-modulated AC magnetic fields to control the heat and mass transfer at the solidification front. It has been shown recently under laboratory conditions that an accurate tuning of the magnetic field parameters can avoid segregation effects and homogenize the mechanical properties.
This present study examines the directional solidification of commercial wrought aluminium alloys from a water-cooled copper chill. Rotating time-modulated magnetic fields were used to agitate the melt. The impact of flow on the resulting macro and micro structure are investigated. The solidified structure was reviewed in comparison to an unaffected solidified ingot and ingot prepared with chemical grain refiner. In addition results from extrusion process experiments are introduce. Our results demonstrate the potential of time–modulated magnetic fields to control the grain size, the formation of intermetallic phases and the morphology and distribution of pores.