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Today numerous age hardenable Al-Mg-Si alloys are used as structural materials in cast, wrought, rolled or extruded form in the automotive, shipbuilding, architectural, and aviation industry. Most commercial Al-Mg-Si alloys show a distinct negative effect of natural pre-aging on artificial aging at conventional, low temperatures (150 - 180 °C). Compared to ideal direct aging, this effect retards hardening kinetics at artificial aging and reduces the achievable strength. As the negative effect appears within minutes after quenching from solution heat treatment, its origin has been linked to the clustering processes at room temperature. Recently a concept has been presented1, which retards the formation of clusters for > 2 weeks by adding trace amounts of Sn to the alloy AA6061. While suppression of natural aging prevails, artificial aging kinetics at 170 °C is significantly enhanced, compared to the natural pre-aged case. This study investigates the effect of Sn on artificial aging at unconventional high temperatures (> 200 °C) compared to natural pre-aging and direct aging: besides rapid aging kinetics, superior peak hardness can be reached which is ex-plained by strong Sn-vacancy binding. Nucleation processes occur shortly after the beginning of artificial aging, thus the concentration of vacancies available at this stage is critical. In the direct aging case, simulations of the characteristic time scale of excess-vacancy annihilation reveal decreasing excess vacancy-availability with in-creasing temperature for pure aluminum. Natural pre-aging clusters are supposed to trap quenched-in excess vacancies which are essential for the nucleation of age-hardening precipitates. Aging tem-peratures above 200 °C positively enhance the dissolution kinetics of these clusters, resulting in a faster nucleation rate of the major hardening phase β´´. Further, the decreasing cluster density results in a continuous vacancy supply which supports nucleation over a longer period of time than for direct aging. This leads to higher precipitate number density and peak hardness than for direct aging. Based on the knowledge from direct aging and natural pre-aging treatments of commercial Sn-free AA6061, suggestions for understanding the Sn-added case are discussed: (i) Sn forms heterogene-ous nucleation sites, (ii) more vacancies can be quenched-in, (iii) Sn-vacancy pairs contribute to diffusion and (iv) Sn retards annihilation of quenched-in vacancies. 1 S. Pogatscher et al., Phys. Rev. Lett. 112, 225701–225705 (2014).
|Publication status||Published - 15 May 2015|
|Event||European Metallurgical Conference 2015 - Kongresszentrum, Düsseldorf, Germany|
Duration: 14 Jun 2015 → 17 Jun 2015
|Conference||European Metallurgical Conference 2015|
|Abbreviated title||EMC 2015|
|Period||14/06/15 → 17/06/15|