Effect of natural aging on the properties of heat-treated A356 aluminum alloy

During the heat treatment of aluminum alloys, there is usually a delay time between quenching and the final artificial aging. This delay is called natural aging or preaging at room temperature. This research was conducted in order to study the effect of various natural aging times (0, 6, 12 and 20 hours) on the properties of unmodified and strontium modified A356 aluminum alloys solution heat-treated 4 hours at 540oC, water quenched and artificially aged 6 hours at 155oC and 170oC. The samples were tested for electrical conductivity, microhardness, and tensile properties. In the case of the samples artificially aged at 155oC from the results it can be seen a decrease in the microhardness and yield strength with natural aging. Regarding the samples aged at 170oC it is noticed that natural aging at 12 hours will result in the lowest electrical conductivity, yield strength and microhardness. However, there is evidence of recovery of those properties at 20 hours of natural aging. Regarding the elongation, natural aging seems to have a positive effect when artificial aging is carried out at 155oC, but at 170oC an optimum elongation is obtained only at 12 hours. (author)