Effect of the Cooling Rate on the Microstructure of a Quasi-Binary Al-Cu-Gd Alloy in the As-Cast and Homogenized States

Abstract

The paper studies the effect of the cooling rate during solidification on the microstructure of the quasi-binary Al-6Cu-3Gd alloy after casting and homogenization. Different cooling rates are implemented by laser surface melting (LSM), solidification in a cold or heated mold and with a furnace. It is shown that an increase in the cooling rate from 0.02 K/s to 10⁵–10⁷ K/s leads to a significant refinement of dendritic cells from 126 to 0.5 μm and intermetallic phases from 0.24 μm to 0.05–0.1 μm, which improves the hardness of ingots from 25 to 75 HV. The dependence of the dendritic cell size is accurately described by an empirical equation obtained for hypoeutectic silumin. The microstructure contains dispersed eutectic (Al) + Al₈Cu₄Gd (τ1) and individual inclusions of the (Al, Cu)₁₇Gd₂ (τ4) phase, which demonstrate high thermal stability during homogenization at 590 °C. The microstructure after LSM contains a network of larger particles about 1 μm in size, while the main proportion of 0.1–0.2 μm particles is uniformly distributed throughout the volume. In the alloys obtained at the intermediate cooling rates 1–15 K/s, which are close to industrial ones, the processes of fragmentation and spheroidization occur almost identically: the particle size changes from 0.1–0.2 μm in the as-cast state to 0.5–3 μm after 1–24 h of homogenization. In the alloy cooled at the minimum rate of 0.02 K/s, the particle morphology remains almost unchanged.