What temperature affects annealing of 6061 T4 and/or T6

[RackleyJ]

I have a cast aluminum part that was made from 6061 Aluminum (it is either T4 or T6). The part was heated to 750°F in order to remove some press fitted bearings (I am unsure of the duration), and then was allowed to air cool. Is it possible that the heating may have compromised the material? In particular could the heat have affected the annealing or tempering properties?

Doing a quick Google search I found that temperatures of 400°F can affect the annealing properties of 6061-T6 aluminum. The only information I found regarding 6061-T4 is that in time it will repair itself (due to aging). Is this correct? How much time is required for natural aging? Can you please educate me on this topic? I would greatly appreciate any and all information you can provide.

Also, is this the correct forum for questions like this?

 

[TVP]

First, yes this is the correct forum for questions like this. Second, the artificial aging temperature for 6061-T6 (second step that strengthens the product) is ~ 320-350 F, so any amount of time spent at 750 F definitely caused softening by overaging the precipitates. You should not heat 6061-T6 above 350-400 F unless you can tolerate significantly lower strength.

With regards to natural aging, this occurs when the product has been properly solution treated (~ 985 F) and immediately quenched into water, then allowed to naturally age at ambient temperature. What you have described does not meet this description.

 

[tbuelna]

RackleyJ-

First, 6061 aluminum is not a casting alloy. Second, the T4 condition is naturally aged and the T6 condition is artificially aged. Depending upon how long your aluminum part was exposed to the 750degF temps (ie. did the entire part mass achieve that temperature?) and how it was air-cooled, it may or may not have been annealed.

It is always possible to re-heat-treat aluminum parts, but there is also the risk of creating distortions in the part when doing so.