Effect of Growth Rate on the Microstructure and Microhardness in a Directionally Solidified Al-Zn-Mg Alloy

The Al-5.5Zn-2.5Mg (wt pct) ternary alloy was prepared using a vacuum melting furnace and a casting furnace. Five samples were directionally solidified upwards at a constant temperature gradient (G = 5.5 K/mm) under different growth rates (V = 8.3–165 μm/s) in a Bridgman-type directional solidification furnace. The primary dendrite arm spacing, λ ₁, secondary dendrite arm spacing, λ ₂, and microhardness, HV, of the samples were measured. The effects of V on λ ₁, λ ₂ and HV properties of the Al-Zn-Mg alloy were studied by microstructure analysis and mechanical characterization. Microstructure characterization of the alloys was carried out using optical microscopy, scanning electron microscopy, wavelength-dispersive X-ray fluorescence spectrometry, and energy dispersive X-ray spectroscopy. From the experimental results, it is found that the λ ₁, λ ₂ values decrease, but HV values increase with the increase in V, and HV values decrease with the increase in λ ₁ and λ ₂. Dependencies of dendritic spacing and microhardness on the growth rate were determined using linear regression analysis. The growth rate, microstructure, and Hall–Petch-type relationships obtained in this work have been compared with the results of previous studies.