Microstructural Evolution of Solid-solution-treated Zn–22Al in the Semisolid State

Abstract

The effect of solid-solution-treatment on the semisolid microstructure of Zn–22Al with developed dendrites was investigated. Forming Zn–22Al products by semisolid metal processing offers significant advantages, such as reductions in macro-segregation, porosity and forming costs. Thermal and microstructural analyses of the formed Zn–22Al alloy were performed by differential scanning calorimetry, scanning electron microscopy and optical microscopy. The changes in the microstructures and phase transformation in response to various solid-solution-treatments were analysed. In this study, as-cast samples were held isothermally at 330 °C for 0.5–5 h and then partially remelted at a semisolid temperature of 438 °C for 1 h to produce a solid-globular grain structure in a liquid matrix. A non-dendritic semisolid microstructure could not be obtained when the traditionally cast Zn–22Al alloy with developed dendrites was subjected directly to partial remelting. After solid-solution-treatment at 330 °C, the black interdendritic eutectics were dissolved, and the dendritic structures gradually transformed into uniform β structures when the treatment time was increased. The coarsened and merged dendrites were separated as a result of penetration by the liquid phase and melting of the residual eutectic at sites along the former grain boundaries. The microstructure of the solid-solution-treated sample transformed into a small globular structure; the best shape factor of 0.9, corresponding to a particle size of 40 ± 16 μm, is achieved when the sample was treated for 3 h followed by direct partial remelting into its semisolid zone.