NbB₂ Modified Al–Cu Alloys Fabricated by Freeze-Ablation Casting under High Cooling Rate Solidification

At present, improving the properties of aluminum alloys is generally achieved by increasing the cooling rate of the melt and adding micro-nano particles. Controlling the cooling rate of the melt to improve the refining effect of grain refiner is still a difficult problem in the aluminum alloy casting industry. An innovative and environmentally friendly casting process, known as freeze ablation, was introduced during the preparation of an Al–NbB₂ intermediate alloy. This process significantly enhanced the cooling rate of the melt. The results indicated that the Al–NbB₂ intermediate alloy produced under high cooling rates had a noticeable refining effect on Al–Cu alloys, with smaller NbB₂ particles demonstrating superior refining performance. The average grain size of the refined Al–Cu alloy decreased from 154 to 69 μm, the tensile strength increased by 12%, the fluidity increased by 18.4%, and the hot tearing index decreased from 144 to 12. The matching degree between NbB₂ and α-Al was calculated using high-resolution transmission electron microscopy and the edge-to-edge model. It was found that the atomic interplanar spacing and the interatomic spacing mismatch between NbB₂’s <11 \({\overline{\text{2}}}\) $\overline{\text{2}}$ 0> plane and Al were both less than 10%, which further proved that NbB₂ could serve as an effective nucleation site for α-Al grains to achieve grain refinement.