Structure formation and properties of cast billets under the influence of vibration during melt crystallization

Abstract

The authors investigated the formation of the structure and properties of cast billets made from a transparent organic camphene-based alloy and an aluminum alloy AD31, depending on the conditions of their crystallization, including under external physical influences. It was shown that the structural formation of all billets occurs according to the same patterns, regardless of the alloy type and crystallization conditions. It was established that in the control billets (i.e., without external influence), crystallization mainly proceeds according to the scheme of sequential solidification, as clearly evidenced by their macrostructures, which consist only of columnar crystal zones. During the formation of experimental billets (i.e., under vibrational influence), significant changes in the crystallization conditions occur, leading to a mixed solidification scheme. Visual observations of the formation of experimental camphene alloy billets and macrostructure analysis of AD31 aluminum alloy billets showed that, at the initial stage, the nucleation and growth of only columnar crystals take place, meaning that solidification occurs sequentially. As the columnar crystals reach a certain size (length), they undergo partial destruction under the influence of bending stresses of elastic waves, and the crystal fragments, due to vibrational mixing, are evenly distributed throughout the cross-section of the solidifying melt. At the same time, the appearance of supercooled micro-volumes of the melt around each crystal fragment causes accelerated cooling of the remaining liquid core of the billet, thereby ensuring its subsequent volumetric solidification. As a result of such vibrational effects during billet formation, the structural heterogeneity decreases and the dispersion of the cast grain significantly increases. A natural consequence of these structural changes in the experimental billets is the improvement of the physicomechanical properties of the cast metal, accompanied by a decrease in their anisotropy across different zones.