Effect of cooling rate on microstructure and mechanical properties of an Al-5.0Mg-3.0Zn-1.0Cu cast alloy

Abstract

This study examines the effect of cooling rate (specifically at 2.3, 3.4, 9.8 and 24.1 K/s as) on the microstructure and mechanical properties of an Al-5.0Mg-3.0Zn-1.0Cu (wt.%) alloy casted through a step-like iron mold. This study was performed in as-cast and T6-treated (470 °C × 24 h + 150 °C × 16 h) conditions. An examination of the as-cast microstructure shows that when the cooling rate increases from 2.3 to 24.1 K/s, the average secondary dendrite arm spacing decreases from 43.0 to 19.0 μm, and that the average width of the T-Mg32(AlZnCu)49 phase also decreases from 6.7 to 4.0 μm. The solution treatment has an impact on dissolving most of the T phases and aging treatment results in the formation of fine precipitates in matrix. The examinations of precipitates under different cooling rates show that a higher cooling rate promotes the transformation from the GP II zone to η′ phase and also keeps a higher concentration of Mg and Zn suspended in the alloy matrix. The results of tensile tests show that the strength and elongation of as-cast alloys are improved slightly by applying a higher cooling rate, and that of T6-treated alloys improved more significantly, especially for elongation (from 6.1% to 10.6%) and yield strength (from 402.5 to 441.9 MPa). The refinements of α-Al and Al3Fe phases are attributed to the significant increase of elongation for T6-treated alloys. Moreover, cooling rate regulates the yield strength by mainly affecting precipitation strengthening and the relative mechanisms are discussed. Overall, these results show that Al-5.0Mg-3.0Zn-1.0Cu cast alloy can exhibit a strength of 450 MPa, a high ductility of 10%, and can become one of high performance cast aluminum alloys.