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

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.