Influence of process parameters and mold geometry on the shell zone of AW 6082 alloy billets produced by direct-chill casting

• Increasing casting speed significantly reduces the shell zone, providing to be the most influential parameter among those tested. • A less abrupt mold transitions from inlet to outlet halves the shell zone under identical process parameters. • A modified mold geometry alters shell zone thickness, also affecting the morphology and composition of intermetallic compounds. • Mold geometry likely impacts shell zone thickness through liquid shear conditions, needing further experimental and CFD validation. The direct-chill casting (DCC) process employing hot-top molds is a prevalent technique for producing Al alloys. Recent emphasis on enhancing process yield within the Al alloy market has directed equipment manufacturers towards minimizing the shell zone that forms during solidification. The shell zone is characterized by a fine microstructure, segregation of alloying elements, and a high risk of gas inclusions and micro-shrinkage formation. This work systematically investigates the influence of key parameters on shell zone characteristics, including water cooling system design, casting speed, liquid–solid transition zone position, and mold geometry. The objective of the present investigation was to identify the optimal parameter combination for shell zone thickness minimization, contributing to the development of innovative hot-top molds. Casting trials were conducted at an industrial production facility using the commercially relevant AW 6082 alloy. Microstructural characterization, employing light and electron microscopy with a focus on alloying element distribution, revealed that increasing casting speed significantly reduces shell zone thickness. Furthermore, variations in mold geometry under consistent process parameters also had a notable impact. This latter finding highlights the importance of thermo-fluid dynamic simulations in future investigations related to this project.