Synergistic Effects of Using Trace NbB2 and Ti Additions to Refine the Microstructure and Enhance the Mechanical Properties of PDC Al-Si Alloy

Grain refinement is a crucial technological strategy for achieving microstructural homogenization and enhancing the mechanical performance of aluminum alloys. This study examines the synergistic effects of trace additions of NbB2 and Ti on microstructural refinement and the enhancement of mechanical properties in high-pressure die-cast (HPDC) Al-Si series alloys. Through systematic investigations utilizing scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) analyses, the mechanisms by which trace additions of NbB2 and Ti contribute to the synergistic refinement and strengthening of mechanical properties in HPDC Al-Si alloys are elucidated. The incorporation of NbB2 effectively refines both the externally solidified crystals (ESCs) and secondary α-AlII grains. The combined addition of NbB2 and Ti further amplifies this effect, resulting in optimal refinement outcomes in the Al-Si series alloy, characterized by an ESC grain size of 19.1 μm and an area fraction of 27.5%, as well as an α-AlII grain size of 6 μm. TEM observations reveal the formation of a Ti-rich transition layer between NbB2 and Al with the synergistic addition of Ti and NbB2, which fills interface nucleation vacancies and enhances refinement performance.