Floating Grain Characterization and Its Effects on Centerline Segregation of Direct-Chill Cast Al-Mg-Si Alloy Billets

Microstructure and microsegregation analysis were performed on a direct-chill (DC) cast Al-Mg-Si alloy billet, aiming to quantitatively investigate the microscopic features of floating grain and its contribution to the negative centerline segregation. Microstructure results show some irregular grains, whose grain size and secondary dendrite arm spacing (DAS) are two times larger than that of the regular fine grains, appear only in the billet central region. The duplex grain structure, a mixture of internal coarse and peripheral finer dendrites within one grain, was clearly shown by the EPMA results, exhibiting special microsegregation features. The coarse dendrites are more solute-depleted than the nearby finer-DAS structure, in which the average concentration of solute Mg nearly equals to the nominal content. Based on the microscale features and solidification information of the alloy, the irregular grains exhibiting duplex dendrite feature are confirmed to be floating grain, which internal coarse dendrites are believed to mainly contribute to the negative centerline segregation. And the resulting segregation degree is quantitatively evaluated to be similar to 0.041 for solute Mg considering the floating grains in the billet center as against with the assumed 100% fine-DAS microstructure.