The formation of surface segregates during twin roll casting of aluminium alloys

When a certain productivity/casting speed is exceeded during twin roll casting, surface segregations form in a number of aluminium alloys with wide freezing ranges. Such segregations limit the productivity as mechanical properties and surface quality suffer during and after subsequent rolling and forming. Simulation results obtained by a 2-D thermo-mechanical model for twin roll casting, AlStrip, have been combined with studies in scanning electron microscope SEM/microprobe to assess the responsible mechanisms for the formation of segregates. The proposed mechanism consists of a sequence of events, where the initial source for the segregation is the progressive enrichment of alloying elements in the interdendritic liquid during solidification (microsegregation). Under certain conditions, i.e. when the critical productivity/casting speed is exceeded, low-pressure zones develop in the surface regions. Due to this pressure distribution, the enriched liquid flows through the coherent solid network to the surface, where solidification and the subsequent formation of segregations take place.