Effect of Minor Cu Addition on the Precipitation Sequence of an As-Cast Al-Mg-Si 6005 Alloy

The whole precipitation sequences of two as-cast Al-Mg-Si 6005 alloys (containing no Cu or 0.1 wt.% Cu) at 150◦C were investigated using X-ray diffraction, transmission electron microscopy and hardness examinations. The precipitation sequence of the Cu-free alloy can be expressed as: super-saturated solid solution (SSSS) → spherical G.P. zones → pre-β′′ → β′′ → U2 + Si +βx → Si +βx , while that of the Cu-containing alloy can be expressed as: SSSS → spherical G.P. zones + platelet-like G.P. zones → pre-β′′ → β′′ → Q′ + β+ Si → Q +β+ Si. A new type of β precipitate, namely βx here, has been discovered in the Cu-free alloy. The addition of minor Cu is found to accelerate the microstructural evolution by inducing the formation of a new type of platelet-like G.P. zone along {111} Al planes, and improve the hardening response at the over-aged stage by forming Q′ and Q precipitates.