This type of aluminum has natural aging? In this case, in what time change hardness significantly and in which quantity?
The -T6 temper is stable. You won't see any changes in properties, unless you expose the material to elevated temperatures.
Elevated Temperature means that is above recrystaline temperature range (around 60% of melting temp of the alloy)
This alloy hardens only artifially. Solution treatment app. 525-535 oC 6-8 hrs/water,
hardening 150-170 oC 6-8 hrs
It consists of a solution (510 to 530C,6-8 hrs/cooling in water) and artificial aging (180-200C/8-10hrs) that increase (to ~ 50%) the mechanical properties
(tensile strength, hardness) due to strengthening precipitation of alloy.
Extruded Rod, bar, shapes, tube, drawn tube: 520 C (time depends on thickness, for thin sessions a few hours are enough, i.e. for 1 mm you need just 30 min, for 6.35-12.7 mm, 65min, then add +30 min for each additional 12.7mm - source: ASM Handbook, Aluminum Alloys), water quench, 175 C for 8h.
AA6063 is a typical precipitation (or age) hardenable Al alloy. Its state of artificial aging is denoted by the the temper designations (T4, T6, T7 etc.). T6 stands for the peak hardened state by artificial aging. (age hardening consists of three stages: (i) solution treatment (~520-530oC) for 1-2 h (time depends on thickness) to get single phase solid solution, (ii) water quenching to get supersaturated solid solution at ambient temp, and (iii) finally artificial aging at ~180-190oC for suitable time period to get fine precipitation of strengthening phase.)
The alloy once artificially aged at a higher temperature (say 180oC), it becomes significantly stable at ambient temperature, hence there will be no further natural aging. (natural aging will occur if the alloy is kept without artificial aging after solution treatment and water quenching).
However, the T6 treated material will overage if exposed to its peak aging temperature or higher. As a result, it will loose its hardness due to coarsening of the precipitates.
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