Influence of Intermetallic Phases and Grain Refinement on Hot-Tearing Susceptibility in 6xxx-Series Aluminum Alloys

The hot-tearing susceptibility (HTS) of three experimental 6XXX-series Al alloys with controlled variations in Si, Mg, Fe, and Mn content was investigated using constrained rod casting (CRC) tests, and the effect of an Al–Ti–B grain-refining agent was also evaluated. CRC test results validated the hot-tearing indices predicted by Kou’s criterion, demonstrating strong agreement between theoretical predictions and experimental observations. Microstructural observations and fracture-surface analyses revealed that the presence of the Al 15 (Fe,Mn) 3 Si 2 phase with Chinese-script morphology contributes to the formation of a strong solid skeleton in the α -Al matrix, enhancing hot-tearing resistance. In contrast, the β -Al 5 FeSi phase, exhibiting a plate-like morphology, acts as a stress concentrator and disrupts interdendritic feeding, promoting hot-tear initiation. The influence of adding a Ti-based grain refiner was also evaluated, as grain refinement generally reduces hot-tear propagation along intergranular voids. However, because the alloys already exhibited a fine as-cast microstructure in the CRC castings, further grain refinement yielded only a marginal decrease in hot-tearing resistance. These implications provide important insights for optimizing alloy design and process parameters, improving the structural integrity of aluminum castings for industrial applications and supporting the use of recycled feedstocks in industrial applications.