UDC 669.715:669-154 EFFECT OF HEATING TEMPERATURE AND MODIFICATION OF Al – 27% Cu – 6% Si MELT ON THE STRUCTURE AND PHASE COMPOSITION OF CRYSTALLIZED SPECIMENS

Alloy Al – 27% Cu – 6% Si has a composition close to an eutectic one [1, 2] and is widely used as a solder (A34) for soldering aluminum and its alloys [3]. The main problem in fabrication of cast solder A34 is the formation of a ternary ( + CuAl 2 + Si) eutectic and primary silicon crystals in its structure, which lowers substantially the process and operating properties of the alloys. It is known [4, 5] that superheating of melts of the Al – Si system to a temperature T * specific for each composition transfers the alloy irreversibly into a more homogeneous state (i.e., homogenizes the metal), which causes substantial changes in the structure and phase composition of the solid alloy in subsequent cooling and crystallization even at a moderate rate (on the order of 1 – 10 K sec). According to [4, 5], the temperature T * (the temperature of homogenization of liquid metal) corresponds to the start of the coinciding high-temperature region of the heating and cooling polytherms for some structurally sensitive property of the melt (for example, the viscosity). In an earlier study [6] we analyzed temperature dependences of the kinematic viscosity of a melt obtained in heating and subsequent cooling of a specimen and determined the temperature regime of homogenization of the melt considered, i.e., T * = 950 – 1000°C. In the present work heating of a melt to a temperature above T * = 950°C is treated as an independent method of modifying of the microstructure of a cast metal. The aim of the present work was to study the effect of the temperature of heating and modifying of a melt of Al – 27% Cu – 6% Si on the structure and phase composition of crystallized specimens with the aim to optimize the process of fabrication of ingots of solder A34.