Role of T7 Heat Treating on the Dimensional Stability of Automotive A319 Al Alloys

Alloy A319 is extensively used in the automotive industry. In the as-cast condition the alloy lacks the required strength. A solid solution heat treatment (T4) followed by room temperature quenching improves tensile strength. Accordingly, engine castings in T4 condition are used in the manufacture of automobiles. However, A319 alloy is not thermodynamically stable after solid solution. The matrix is supersaturated with Cu, and a precipitation sequence is expected to occur overtime, which leads to dimensional changes. Alternatively, an aging treatment (T7) can activate the precipitation reactions to achieve dimensional stability in castings. In this work, dilatometric measurements were made during a T7 heat treatment in order to establish the extent of dimensional changes, resulting from the precipitation reactions. In addition, high-resolution electron microscopy was used to follow up the precipitation reactions.