Microstructural features associated with the effect of temperature on the dimensional stability of an automotive Al-A319 alloy

In this work an automotive Al-A319 was given a solid solution heat treatment (T4) at 753 K (480 °C) for 4.5 hours and an ageing treatment (T7) at 513 K (240 °C) for various times up to 3.0 h. The alloy in the T4 condition was dilatometrically tested at various temperatures in order to measure its relative dimensional changes. It was found that the dimensional changes are due to both, alloy thermal expansion and nucleation and growth of second phases. In addition, in the T7 condition the alloy strength and ductility were determined as a function of ageing times. Ageing promoted alloy strength but at the expenses of a rather poor alloy ductility (down to 1%). Apparently, Cu rich intermetallic phases and regions provided a brittle path for fracturing. In particular, microstructural characterization using high resolution transmission electron microscopy indicated that not all the Cu in the matrix was dissolved during the T4 treatment. Hence, after ageing (T7) these Cu-rich regions seemed to coarsen into spherical particles.