The removal of copper-phase segregation and the subsequent improvement in mechanical properties of cast 319 aluminium alloys by a two-stage solution heat treatment

The commercial heat treatment schedules that are utilized with cast AlSiCu alloys in order to obtain the optimum microstructure, and thereby optimum mechanical properties are often not effective because the precipitation strengthening process, which involves the precipitation of CuAl2 or its precursors, does not occur in the desired manner since copper-rich phases, which form in the as-cast structure, are not fully dissolved in the aluminum solid solution during the solution treatment thus interfering with the desired precipitation on aging. Thermal analysis experiments for a 319 alloy show that the copper-rich phase in the as-cast alloy causes localized melting once the solution treatment temperature reaches 495°C, thus limiting single-stage (i.e., direct heating to temperature) solution treatment to temperatures of 495°C or below. To overcome these problems, a two-stage solution treatment has been developed for the 319 alloy where the conventional solution treatment is followed by a second solution treatment at a temperature above 495°C. Optical metallography showed that the two-stage solution treatment significantly reduces the amount of (as-cast) copper-rich phase in the alloy and gave rise to better homogenization prior to aging. Mechanical property measurements of cast alloys subjected to either the conventional or the two-stage solution treatments demonstrate the microstructural improvements generated by the two-stage process give rise to improve mechanical properties.