Precipitation strengthening of the aluminum alloy AA6111

Abstract

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 ␤ Љ and QЈ phases co-exist throughout the aging cycle with the relative amount of QЈ 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.

An Extended Age-Hardening Model for Al-Mg-Si Alloys Incorporating the Room-Temperature Storage and Cold Deformation Process Stages

“…[31][32][33]37,38] 1. Problem description With a few exceptions, there seems to be general agreement that prolonged storage at RT and up to about 343 K (70°C) will reduce the yield strength of the alloys following artificial aging.…”

“…[37,38] This is because the clusters, which initially form at low temperatures, tend to survive for a certain period of time during artificial aging and therefore temporarily tie-up solute that otherwise would have been consumed in b¢¢ formation. [11,[31][32][33] At higher storage temperatures, the situation is the opposite due to GP zone formation, since GP zones may convert directly into strengthening b¢¢ particles on subsequent artificial aging. [37,38] However, this type of aging response is not an issue here.…”

“…This type of thermomechanical processing is not dealt with in Version 1, although cold deformation operations like sheet forming and stretch bending in combination with prolonged room-temperature storage are known to strongly alter the subsequent artificial aging response of Al-Mg-Si alloys. [31][32][33] Therefore, incorporation of these two new stages in NaMo will largely increase the versatility of the model by allowing simulations of integrated production processes as well of the kind shown in Figure 2. To enable this, the important reactions contributing to the nanostructure evolution at each stage must first be identified (based on what is already known in the scientific literature) and then implemented, either in the existing NaMo sub-models, or in new ones.…”

The effect of a paint bake treatment on joint performance in friction stir spot welding AA6111-T4 sheet using a pinless tool

“…A number of studies have shown that prolonged natural ageing delays the artificial ageing kinetics by inhibiting the formation of ", the main strengthening phase [25,26]. In addition, it has also been shown that prolonged natural ageing can lead to the earlier formation of the Q' phase [25].…”

“…A number of studies have shown that prolonged natural ageing delays the artificial ageing kinetics by inhibiting the formation of ", the main strengthening phase [25,26]. In addition, it has also been shown that prolonged natural ageing can lead to the earlier formation of the Q' phase [25]. This has an important impact on the yield strength after a paint bake cycle, because the paint bake treatment is quite short relative to the full T6 temper and leaves the material in an under-aged condition, where its hardness is most sensitive to any delay in the ageing response [27].…”