Grain Refinement in Near-Eutectic Al-Si Alloy with Master Alloys Fabricated by Slip Casting Using Nano-Nb2O5 as a Precursor

Abstract

Aluminum-silicon (Al-Si) alloys are staple materials in the automotive and aerospace industries. However, effective grain refinement of alloys containing > 4 wt.% Si remains challenging with traditional Al-Ti-B master alloys. This difficulty arises from the silicon poisoning effect, in which Si reacts with titanium to form undesirable silicides, thereby reducing efficiency. Consequently, Al-Nb-B master alloys have been widely studied as effective alternatives. The present work proposes an innovative processing route for Al-Cu-Nb master alloys, using Nb2O5 powder as a precursor to replace costly metallic niobium. A slip-casting technique was employed to uniformly coat micrometric Al-Cu powders with nanometric Nb2O5 particles. Sintering these coated powders enabled the aluminothermic reduction of Nb2O5 and the subsequent in situ formation of potent, well-dispersed NbAl3 nucleant particles. Inoculation of SAE 305 alloy (Al ≈ 12 wt.% Si) with these master alloys (0.3 wt.% Nb) achieved pronounced grain refinement, yielding a 14-fold reduction in average grain size (from 3.13 mm to ≈ 221 µm). Consequently, the macroscopic hardness increased significantly from 68.8 to 87.2 HB. This performance is comparable to reports in the literature. A holding time of only 15 min proved sufficient to achieve maximum refinement, indicating this route is an effective and potentially viable strategy for producing high-performance Al-Si alloys.