Enhanced Dendrite Coarsening and Microsegregation in Al–Cu Alloy under a Steady Magnetic Field

Abstract

The final mechanical properties of alloys are significantly influenced by the secondary dendrite arm spacing (SDAS). The application of a steady magnetic field (SMF) during solidification is a novel method to control the SDAS, however, the nature of the change in SDAS under an SMF is still an open question. In this work, dendrite coarsening in the Al–4.5 mass%Cu alloy in an SMF and its effect on microsegregation were investigated experimentally by the quenching technique. The coarsening experiments showed that the SDAS increased in an SMF, which was mainly attributed to the thermoelectric magnetic convection (TEMC) while the change in solid/liquid interfacial tension in the SMF played an adverse role. Further, the variation of the microsegregation level in the SMF was examined by composition measurements. It was shown that the segregation ratio increased in the SMF, which could be ascribed to the reduction of diffusivity in the solid phase and the enlargement of SDAS in the SMF. Using a modified analytical model developed by Voller, the microsegregation levels with and without an SMF were predicted, which was in agreement with the experimental results.

Fig. 3 Longitudinal microstructures of the Al–4.5 mass%Cu alloy quenched at different temperatures with and without an SMF, to which the samples were cooled at the rate of 0.3°C min⁻¹, (a) 0 T, 580°C; (b) 0 T, 570°C; (c) 0 T, 560°C; (d) 0 T, 550°C; (e) 5 T, 580°C; (f) 5 T, 570°C; (g) 5 T, 560°C; (h) 5 T, 550°C.