Formation of Internal Crack in High-Speed Twin-Roll Cast 6022 Aluminum Alloy Strip

Abstract

An Al-Mg-Si based 6022 aluminum alloy, which represents a wide freezing temperature range in solidification, was cast through high-speed twin-roll casting (HSTRC). Under a roll rotation speed of 60 m/min and an initial roll separating force of 14.1 kN, the cast strip showed a large central crack, which deteriorated a quality of the cast strip. The strip exhibited an equiaxed grain structure at the near-surface region and a band of fine globular grains at the mid-central region. The large central crack was mostly observed at a boundary between the central band and dendritic solidified shell. It was considered that decrease of cooling rate near the minimum roll gap caused the cracking due to residual liquid. In order to find the condition for defect-free strip casting, HSTRC was carried out under various casting conditions. Large-scale central cracking was prevented when a sufficient roll separating force was applied by increasing a solidification time, or increasing an initial roll separating force. From a direct temperature measurement during the HSTRC using a sensitive thermocouple, it was confirmed that a cooling rate increased with increasing the roll separating force.