THE REMOVAL OF COPPER-PHASE SEGREGATION AND THE SUBSEQUENT IMPROVEMENT IN MECHANICAL-PROPERTIES OF CAST 319-ALUMINUM-ALLOYS BY A 2-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 there by optimum mechanical properties are often not effective because the p recipitation strengthening process, which involves the precipitation o f 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 treatmen t thus interfering with the desired precipitation on aging. Thermal an alysis 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 degrees C, thus limiting single-stage (i.e., direct heating to temperature) solution treatment to temperatures of 4 95 degrees C or below. To overcome these problems, a two-stage solutio n treatment has been developed for the 319 alloy where the conventiona l solution treatment is followed by a second solution treatment at a t emperature above 495 degrees C. Optical metallography showed that the two-stage solution treatment significantly reduces the amount of (as-c ast) copper-rich phase in the alloy and gave rise to better homogeniza tion prior to aging. Mechanical property measurements of cast alloys s ubjected to either the conventional or the two-stage solution treatmen ts demonstrate the microstructural improvements generated by the two-s tage process give rise to improve mechanical properties.