Influence of the cooling rate on the structure formation of the AM4.5kd (VAL10) alloy

Abstract

High-strength cast aluminum alloy AM4.5Kd (VAL10) belongs to the Al-Cu system and due to the combination of a high level of physical, mechanical and operational properties, is widely used in high-tech industries and technology: aviation, space, shipbuilding, transport. Products from the alloy AM4.5Kd (VAL10) are obtained by all known methods of casting (in sand molds, in a chill mold, under pressure), differing in cooling rates. This has a significant effect on the structure and properties of the alloy, not only in the as-cast, but also in the heat-treated state, which determines the relevance of scientific work in this direction.
The article presents the results of a study of the microstructure of the AM4.5Kd (VAL10) alloy with a change in its cooling rate during the curing process (V_cool.) from 0.4 °C/s to ≥ 10⁵ °C/s. The alloy was melted, refined and, at a temperature of 750°C, poured into molds with different heat sinks. It is shown that an increase in the cooling rate during the hardening process leads to a decrease in the size of structural components, in particular, aluminum (Alα) solid solution crystals and to an increase in their microhardness. At a cooling rate of 0.4 °C/s, corresponding to solidification in a sandy form, primary Alα crystals are formed in the form of coarsened dendrites with an average size slightly larger than 800 μm, along the boundaries of which a fine network of particles of Al₃Ti, Al₁₂Mn₂Cu phases and Al_α+СuАl₂ eutectics is formed. With an increase in the cooling rate, the branching of the dendrites and the volume fraction of the finely differentiated eutectic increase, the cooperative growth of phases in which is maintained throughout the entire range of cooling rates studied. The value of the dendritic parameter of the solid solution of aluminum regularly decreases with a practically unchanged shape factor of its crystals, which is almost up to V_cool. ≈ 10⁵ °C/s is from 1.45 to 3.15. A similar dependence of the change in the macrograin size on the cooling rate was not found. Its anomalous growth was recorded at a cooling rate of  ≥ 120 °C/s, at which the macrograin size is commensurate with the alloy cooled at a rate of  0.4 °C/s. In the work, such a discrepancy is explained from the standpoint of the theory of the nucleus and growth of crystals.