THE CHANGE IN THE STRUCTURE AND MECHANICAL PROPERTIES OF THE AM5 CASTING ALLOY STRENGTHENED WITH Zr

Abstract

This paper investigates how modifying the AM5 casting alloy with Zr by introducing it into the melt as part of a zirconium tetrafluoride-based flux composition affects the alloy's structure and mechanical properties. This study is relevant because there is a need to create high-quality Al-Cu alloys with increased strength and a stable microstructure for the aviation and automotive industries applications. The article analyzes the current state of research on alloying and modifying Al-Cu alloys, particularly the use of transition group elements, and demonstrates the potential effectiveness of using Zr. The research methodology involved melting the alloy and adding a complex flux, a mixture of zirconium tetrafluoride and lithium fluoride (ZrF₄ and LiF compounds at a ratio of 1:1.2), at temperatures of 750 °С and 900 °С for 15 minutes. To evaluate the mechanical properties, uniaxial tensile tests were performed on standard samples in the cast state and in the T5 condition after heat-treatment. The chemical composition and microstructure were analyzed using energy-dispersive X-ray spectroscopy, X-ray fluorescence analysis, and scanning electron microscopy. A full factorial experiment of type 22 and regression analysis were used for statistical data processing. These methods allow one to predict the influence of melting temperature and ZrF₄ concentration on strength and plasticity indicators based on the experimental results obtained in this study. The experimental data clearly show that introducing Zr into the melt causes the formation of the Al₃Zr intermetallic compound, which acts as a crystallization center and inhibits grain growth, ensuring a refined structure. T5 heat treatment disperses the alloy, strengthening it due to the formation of fine CuAl₂ phase inclusions and increasing the homogeneity of the structure. The best mechanical properties are achieved by adding 2% ZrF₄ to the melt at 750 °С, followed by T5 treatment. The resulting temporary tensile strength is ~365 MPa, exceeding the values of the unmodified alloy by 10–20%. At the same time, the relative elongation remains at ~5%; thus, strengthening the alloy does not result in a loss of plasticity. These results confirm the effectiveness of using complex fluxes containing Zr to modify aluminum alloys because they simultaneously increase strength while maintaining sufficient plasticity. This makes them suitable for critical structural elements in aviation and automotive equipment.