The Effects of Low Cu Additions and Predeformation on the Precipitation in a 6060 Al-Mg-Si Alloy

Abstract

Effects of low Cu additions (≤0.10 wt%) and 10% pre-deformation before aging on precipitate microstructures and types in a 6060 Al-Mg-Si alloy have been investigated using transmission electron microscopy (TEM). It was found that pre-deformation enhances precipitation kinetics and leads to formation of heterogeneous precipitate distributions along dislocation lines. These precipitates were often disordered. Cu additions caused finer microstructures, which resulted in highest materials hardness, in both the un-deformed and the pre-deformed conditions. The introduced pre-deformation led to microstructure coarsening. This effect was less pronounced in the presence of Cu. The precipitate structure was studied in detail by high resolution TEM (HRTEM) and high angle annular dark field scanning TEM (HAADF-STEM). The Cu additions did not alter the respective precipitation sequence in either the un-deformed or the pre-deformed conditions, but caused a large fraction of β'' precipitates to be partially disordered in the undeformed conditions. Cu atomic columns were found in all the investigated precipitates, except for perfect β''. Although no unit cell was observed in the disordered precipitates, the presence of a periodicity having hexagonal symmetry along the precipitate length was inferred from the fast Fourier transforms (FFT) of HRTEM images, and sometimes directly observed in filtered HAADF-STEM images.

The effects of quench rate and pre-deformation on precipitation hardening in Al–Mg–Si alloys with different Cu amounts

“…However a Cu content of ~0.1 wt% significantly affects the hardness in all the thermo-mechanical treatments. The addition of ~0.1 wt% Cu leads to higher hardness corresponding to higher number densities of shorter precipitates, which is in good agreement with our previous study [11]. Interestingly, these observations were made not only for the quenched condition (the condition (a)) but also for the slow cooling, the 1% pre-deformation and combination of these conditions (the conditions (b), (c) and (d)).…”

“…This suggests that a 0.1 wt% Cu addition significantly increases the hardness, which is in good agreement with previous results [11]. This strength increase is more pronounced when peak hardness is reached as compared to the early stages of precipitation.…”

“…Precipitates are present in form of needleshaped with the main growth direction along <001>Al. The precipitates were identified to be mostly β" in the LC1 alloy and to be in coexistence with β" with a partially disordered structure in the same precipitate needle in the LC3 alloy at peak hardness condition [11]. The corresponding precipitate statistics (needle lengths, number densities, cross sections and volume fractions) are summarized in Figures 4 and 5.…”

“…It was previously demonstrated [11] that a low Cu addition (~0.1 wt%) does not alter the precipitation sequence in the Al-Mg-Si alloy system, although this level of Cu leads to higher strength by means of forming a higher number density of shorter precipitates having a partially disordered structure [11,12]. Aluminum recycling is increasing.…”

“…It has been reported [27][28][29][30][31] that 10%-15% pre-deformation leads to formation of heterogeneous nucleation along dislocation lines. After artificial aging, different types of precipitates (except for β") form: a continuous decoration of the dislocation lines ("string-like" precipitates) together with discrete precipitation consisting mainly of B' [27][28][29][30], and together with (often disordered) Q' [11,12,31,32] if Cu is present in the Al-Mg-Si alloys. In the case of 0.5%-5% pre-deformation, β" is dominant at peak hardness condition, and this level of pre-deformation accelerates formation of β" due to decreased activation energy [33][34][35][36].…”