Fast reactive interdiffusion between solid brass and liquid aluminium

Abstract

The formation mechanisms of intermetallic layers and the development of the interface between solid brass and molten Al alloy were experimentally investigated using semi-infinite solid-liquid diffusion couples. Isothermal annealing of commercial CuZn37 – AA7075 diffusion couples was conducted for the selected temperatures of 650 °C, 660 °C, 680 °C and 700 °C, for three annealing times of 10 s, 20 s and 40 s, followed by quenching. Regardless of the temperature, the interface consisted of three intermetallic layers that are reaction products as defined in the Al–Cu–Zn phase diagram. In order to explain the growth behaviour and bring insights into the interface formation mechanisms, the thermodynamics of the Al–Cu–Zn was investigated by CALPHAD (calculation of phase diagrams). Based on these ex-situ results, three distinct phenomena were observed. First, intermetallic layers dissolve simultaneously into the molten Al alloy when the annealing time exceeds 40 s. Second, parts of the intermetallic layer that are in contact with the molten Al, detach from the interface region in the direction of molten Al alloy leading to a decrease in the layer thickness and deviation from planar-like geometry. Third, the formation of the three intermetallic layers is exclusively the result of solid-liquid interactions followed by quenching. Previous studies have shown a scattered interface character between solid brass and molten aluminium which results from interactions in the solid-solid state due to low cooling rates. As a result, the concept of primary intermetallic layers was introduced to describe the interface formation upon the contact of solid brass with molten aluminium alloy.