that is the beauty of this Al–Zn–Mg–Cu alloy, micros change from a solid solution to GP zones, then to metastable precipitate η’, and finally to the equilibrium phase η. natural aging could form precipitates that increases hardness and strength in 5 minutes, so cold forming shall be done ASAP after quenching before losing ductility.
Thank you for your response. Hypothetically, if large, thick parts (about 5ft long, 1 ft. wide and tall, with sections several inches thick) of 7xxx in the AQ condition were to be stored at ambient temperature while forming and machining were to be performed over many weeks, would not only the parts be more susceptible to cracks, but could internal defects, stresses, cracks, or other detrimental issues be produced and go undetected/be undetectable?
Everything is detectable with the correct NDT.
Cracking will nearly always happen first.
It can be on the tension face of the bend or compression face.
The compression face is where the residual stress is the highest from the spring back.
And don't ever try to re-form an area that you already bent.
another effect of aging is the deformation bands due to diffusion of mobile solute atoms through the newly created dislocations at deforming. Heavy slab will have slower cool rate which could be "beneficial" to suppress RT aging.
