Abstract
Over the past several decades, it was generally believed that the strength of the industrially widely used cast Al–Si–Mg–Cu alloys enhanced monotonously with increasing Cu content. However, in this study using cast Al9Si0.5MgxCu (x = 0,0.2,0.4,0.6,0.85,1.0,1.25, in wt.%) alloys under T6 heat-treated condition, it was found that the hardness and yield strength of the heat-treated alloys showed a platform in the composition range from 0.4 wt.% to 0.85 wt.% Cu, while still increased with increasing Cu content before and after the platform. With increasing Cu content, the β-Mg2Si intermetallic phase decreased and disappeared at 0.85 wt.% Cu, while the Q–Al5Cu2Mg8Si6 and θ–Al2Cu intermetallic phases increased in the as-cast alloys. After heat treatment, the micron-scale intermetallic phases were dissolved into the Al matrix and precipitated as the nanoscale β″, Q′ and θ′ strengthening phases. With increasing Cu content, the β″ precipitate decreased and vanished at 0.85 wt.% Cu, while the Q′ and θ′ precipitates increased in the heat-treated alloys. The trade-off of the phases induces the platform in the strength of the heat-treated alloys, and further increase of the Cu content in this undetected trapped platform range is not favorited industrially as it only decreases ductility.
Highlights
Over the past several decades, it was generally believed that the strength of the industrially widely used cast Al–Si–Mg–Cu alloys enhanced monotonously with increasing Cu content
The cast Al–Si–Mg alloys can be significantly strengthened after T6 heat treatment including solid solution and artificial ageing, due to the precipitation of the nanoscale β′′–Mg2Si strengthening phase, and the alloys are mainly used after T6 heat treatment2–4
The hardness of the 0 wt.% and 0.4 wt.% Cu alloys reached the peak at an ageing time of 4 hour, while the hardness of the 0.85 wt.% and 1.25 wt.% Cu alloys reached the peak at an ageing time of 8 hour
Summary
Over the past several decades, it was generally believed that the strength of the industrially widely used cast Al–Si–Mg–Cu alloys enhanced monotonously with increasing Cu content. Over the past several decades, based on the insufficient investigating Cu content with a coarse interval of at least 0.5 wt.%, it was generally believed that the strength of the T6 heat-treated cast Al–Si–Mg–Cu alloys increased monotonously with the increase of Cu content. Different phases including the Cu-free β′′ precipitate and the Cu-containing Q′–Al5Cu2Mg8Si6 and θ′–Al2Cu precipitates were reported for the strengthening of T6 heat-treated cast Al–Si–Mg–Cu alloys under sporadic Cu contents, while the effect of Cu in the refined composition range on the strengthening of the alloys is still scientifically blind. The trade-off of the Cu-free and Cu-containing strengthening phases with increasing Cu content and their possible balancing effect to strength in specific composition range were hardly noticed, and the blind addition of Cu might fall into the trap of the deterioration of ductility without any enhancement in strength
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