On the Modelling of Macrosegregation during Twin-Roll Casting

Abstract

Solidification during twin-roll casting happens by cooling of the melt between two counter-rotating rolls, where the molten alloy is constantly fed in. For an inoculated Al-melt, grain growth leads to a gradual increase of solid fraction, so that a coherent solid network forms. Depending on the process condition, this solid network might be subjected to compression within the gap between the two rolls. By using a two-phase volume average model that accounts for (i) transport and growth of spherical grains within a flowing melt, (ii) formation of a coherent solid network above a specific solid fraction and (iii) viscoplastic flow of the solid network saturated with interstitial melt during casting and compression, the process is numerically analysed. It is found that an optimum process with minimum macrosegregation can be achieved for conditions where the kissing point of the two viscoplastic semi-solid shells nearly coincides with the roll nip. It is demonstrated how casting speed, cooling intensity and strand thickness must be related to hit the optimum process window.