Would it be possible for the aluminium alloy AA6082-T651 to increase in strength/hardness over time (years)? Or are the mechanical properties stable over such long periods at room temperature storage?
As per recent research (https://www.researchgate.net/publication/228058487_Continuous_Cooling_Precipitation_Diagrams_Depending_on_the_Composition_of_Aluminum-Magnesium-Silicon_Alloys) it is evident that AA 6082 has to be quenched to attain a critical rate. The values shown in this publication are about 100 and 800 K/min for two grades of AA6082 (Fig.5) For optimal artificial aging specified by 651 condition, the quench rate, aging temperature and aging time should be as per established specifications. If this is adhered to, the AA6082 T651 should have stable period over a long period of time. Departure from any of the three may result in non-optimal aging. If the component was in underaged condition when put into service, then there will be a good probability of natural aging occurring while in service resulting in slow, but gradual increase in strength and hardness.
Conference Paper Continuous Cooling Precipitation Diagrams Depending on the C...
Hi, the values were cited slightly wrong in the above answer: the critical cooling rate of a low conentrated batch of 6082 was determined to be about 1000 K/min (i.e. ca. 17 K/s), while the critical cooling rate of an high concentrated version was 8000 K/min (130 K/s). I.e. there is a large dependence on the batch composition.
please read/cite the full paper instead the cited conference paper:
Continuous cooling precipitation diagrams of Al-Mg-Si alloys
Article in Materials Science and Engineering A 550:87-96 (2012), which can be found in my publication list.
However, the question is on the potential of further natural ageing after prior artificial ageing. I have not observed this for 6082 but already for a AA7150, showing a further increase in hardness after additional 9 month ageing (after initial quenching + 120 °C 24 h, which is a common procedure) - so I would say, natural ageing might be possible after artifical ageing.
Regards
benjamin
My mistake! I should have indicated 1000K/min for low concentrated batch and 8000K/min for the high concentrated batch. From the energetic view point attainment of equilibrium is essential for stable structure and properties. Underaged alloys would attain equilibrium state after some lapse of time assuming constant service temperature.
Dear Erik Carton,
Some remarks:
1. T651 denotes: Solution heat treated, stress relieved by stretching then artificially aged.
2. It is assumed that AA6082-T651 refers to a condition prescribed by the corresponding heat treatment recipes.
3. It is further assumed that natural ageing refers to prolonged maintaining the alloy at an ambient temperature beneath the recovery temperature.
4. It is also assumed that stretching has been performed at room temperature.
5. The foregoing hints at the possibility of residual cold deformation not fully recovered during artificially ageing, thereby enhancing kinetics during natural ageing.
6. It is noted that prolonged ageing is on principle possible as long as the alloy has not attained thermodynamical equilibrium; whether or not this prolonged ageing is going to occur depends on the kinetical factors, like the presence of areas with residual cold deformation.
7. In this particular case natural ageing might yield strength increase; however, natural ageing may eventually lead to overageing, resulting in a strength decrease.
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