The synergistic effects of copper content, cooling rate and nanoprecipitates on microstructure and tensile properties of Al–Si–Mg–Cu alloys

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The effect of copper addition and heat treatment on the as-cast microstructure, and nano-precipitates evolution of Al–Si–Mg–Cu alloys were investigated. The relationship between the microstructure and the tensile properties of the alloy was quantitatively evaluated by measuring the properties of the tenfold elongated tensile specimens. The results show that Cu content has less effect on grain size and secondary dendrite arm spacing, but reduces the number of primary Si phase, especially in low solidification cooling rate. The addition of copper makes Q-Al 4 Cu 2 Mg 8 Si 7 and θ-Al 2 Cu phases appeared in the as-cast microstructure, and the metastable nano-phases β''-Mg 5 Si 6 , Q′-Al 3 Cu 2 Mg 9 Si 7 and θ′-Al 2 Cu precipitated after heat treatment, which promoted remarkable strengthening. Based on the evaluation about the influences of the nano-precipitates to the strength of the alloys, the multiple nano precipitates of β'', Q′ and θ′ is yielded the strength values about 40 % higher than that of the single nano-phase β''. Comparing the addition of 0.52 wt%Cu with that of 1.2 wt%, it can be found that too much copper will cause performance degradation. Moreover, too long aging durations will also lead to coarse precipitated phase and uneven distribution, which will lead to a decline in mechanical properties.

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