Effects of ultrasound on the microstructure of Al-7Si-Sc alloy prepared via molten salt electrolysis

Al-Si alloys are widely used in modern industries, transportation, and other engineering applications. Processability and mechanical properties of Al-based alloys can be improved via the addition of scandium. Sc added to Al metal for fabricating Sc-containing Al alloys using molten salt electrolysis has been recently considered as promising technology. However, alloying elements, such as Sc and Si are often unevenly distributed in such Al-based alloys. In this study, Al-7Si-Sc ternary alloy was prepared via molten salt electrolysis aided with ultrasound to investigate the effects of ultrasound on the microstructure and distribution of the strengthening phase. Electrolysis was performed on molten salts of Na₃AlF₆-19%KF-29%AlF₃-2%CaF₂ at a temperature of 800℃ and current density of 1 A·cm², in which Sc₂O₃ (99.99% purity) and Al-7Si alloy served as the raw material and cathodic metal, respectively. Ultrasound (20 kHz, 200 W) was introduced into the cathode metal from the cell bottom. Sc contents in the as-prepared alloy samples were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). Microstructures of the alloy samples were characterized using optical microscope and scanning electron microscope coupled with an energy dispersive X-ray analyzer. Results reveal that ultrasound can increase Sc content in the ternary alloy prepared via molten salt electrolysis and refine the eutectic silicon clusters and the ternary AlSi₂Sc₂ phase. Compared with the alloys made without ultrasound aid, the silicon cluster size decreases from approximately 500 to 200 μm (~60%) and the refined ternary phase of AlSi₂Sc₂ uniformly distributes in the metal matrix. Results also indicate that ultrasound can considerably optimize the microstructure of Al-7Si-Sc alloy prepared via molten salt electrolysis. This process can prevent problems such as the segregation of alloying elements and uneven microstructures observed when using the traditional alloy-making process.