Age hardening behaviour of semi-solid squeeze cast A319 aluminium alloy

The influence of age hardening treatment on the hardness and microstructure of semi-solid squeeze cast A319 alloy is investigated. At 175 degrees C, the age-hardening curve of the alloy shows two peaks between which there was an apparent valley. Analyses by transmission electron microscopy, selected area electron diffraction and energy-dispersive X-ray detection suggest that the first peak is caused by formation of G.P.2 zones while the second peak can be related to formation of the metastable theta '- and Q '-phases. Compared with gravity die cast A319 the semi-solid squeeze cast A319 shows a larger increase and a higher peak in hardness during the age hardening process. Additionally, the latter requires another 32 h to reach the first hardness peak while it reaches the second hardness peak about 24 h earlier than the former.