Aluminum Aging

All precipitation hardening materials continue to precipitate with time. heat will accelerate it. the "aging" heat treatment is intended to "hurry it up". eventually they will overage and the material properties will steadily get worse. There was an article i read a while back about a museum that had aquired Buckminster Fuller's "Dymaxion house" which was essentially a prefab aluminum tent for lack of a better word made primarily from 6061-T6 but it had been outdoors in the sun for years and much of it's skin had overaged and was in danger of disintegrating under the loads required to remain standing. the museum had to replace much of the original skin with modern aluminum 6061-t6 in order to display it.

HI, I have welding job on 6802 T6 extrusions, do you think there will be any benefit in PWHT the welds or will the normal ageing be adequate? The welding will be completed 6 months before going into service. Thanks

Welding on heat treated aluminum is always a sticky situation because the weld itself will be in the O condition while the HAZ will be overaged. A (thermal) weld on a heat treated part will NEVER be as strong as the base metal. The CORRECT way to do this is to get the extrusions in the O condition, weld them in the O condition then send the whole assembly out for aging. Another less desirable plan is to send the whole assembly out to a heat treater and have it solution annealed and then re-aged after welding, but this causes the grains to grow large. There IS a relatively new welding process in which you can weld heat treated aluminum, and even "non-weldable"aluminum alloys without compromising strength and that is friction stir welding, but it requires some rather extensinve and expensive equipment and there are some joint geometry it simply can't handle.

Hi Rorschach, Thanks for the great answer. I went to a TWI seminar recently on FSW but the critical joints we do are not suitable for the process. FSW is a fantastic process if you get the chance to attend the seminar you should go the sample welds were amazing. Regards David

Rorschach,

My question is about a part of engine ( flywhell housing ) that was treated wit T6

If I have a hardness after the treatment according to the specification, what will be

the hardness of this part with the time, working in a ciclic process of heating ( from

engine ) and cooling ( room temperature )..? I will have a residual hardness bigger

( natural aging ) or smaller ( annealing from the ciclic process of motor )

tks

JB

The mechanical properties you obtain from precipitation follow a bell curve to a large extent. Once you have achieved the maximum hardness/strength, further aging will begin to degrade those properties and will continue to do so. but the change will probably take years under most circumstances. But it is all downhill from there. In many respects aging is not unlike concrete hardening, it will continue to cure forever but at some point the crystal lattice bonds will be stretched so tight that they will start to spontaneously break, but getting to that point can take a very long time.