Precipitation strengthening of the aluminum alloy AA6111

There is an increasing use of aluminum alloy sheet in automotive applications due to the desire to decrease vehicle weight. The current study provides a detailed quantitative study on precipitation strengthening in AA6111, which is the alloy of choice in North America for exposed body panels. Transmission electron microscopy was used to characterize the average size, the size distribution, the volume fraction, and the crystal structure of the hardening precipitates for aging at 180 °C (a) directly after solution treatment and (b) following 2 weeks of natural aging. The results indicate that both β″ and Q′ phases co-exist throughout the aging cycle with the relative amount of Q′ being increased both with increased aging time at 180 °C and when natural aging precedes aging at 180 °C. A strengthening model was developed which uses the size distribution and the volume fraction of precipitates as the primary inputs to predict the yield stress. An important feature of this model was that only one fitting parameter was necessary to give very good agreement with the experimental results.