Alum 6061-T6 property changes 1

[vibman]

I have a strange observation and was wondering if any experts here would provide an explanation, if possible.

We have a product made from an aluminum 6061-T6 cylinder which has to be inductively heated via AC power coils. We try to maintain
constant current output by varying the output voltage.

Initially, we start outputting the power with a known current and voltage. Part is heated to approx. 500F for 30 minutes. Then it is cooled using forced convection (air). When we initiate the heating again, we notice that, given the fixed output current, the required voltage is lower than the original reading by about 10%. This indicates that the resistance of the ring has decreased.

From what's I've found online, the 6061 resistivity actually increases during its temperature rise. But, strangely, what I'm observing is that after the heating process, at room temp, the resistance seems to have dropped.

My questions are: What's the mechanism in the resistivity drop? Would heating the ring and cooling it back down repeatedly affect the overall strength of the alloy?

Thanks.

 

[EdStainless]

You are changing the properties a lot.
1/2 hr at 500F knocks the UTS down about 20% and the yield strength down about 30%. (at room temp afterwards)
I would expect elongation to be increased a few percent.
Repeated or extended exposure will continue these trends.
You are simply partially annealing the material, undoing the T6 aging effects.

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P.E. Metallurgy, consulting work welcomed

 

[vibman]

So with repeated temp cycling, the T6 temper will eventually become 0 temper?
That would explain why resistance is lower.
Thanks for clarifying!

 

[MagBen]

You basically dissolved precipitates (mainly Mg2Si) leading to a lower resistivity. Reported resistivity for T6 and O drops by 8.1% from 4.0E-8 to 3.7E-8 ohm.m. in the meanwhile the conductivity increases by 7.1%.

 

[vibman]

I confirm the 8.1% drop when looking up resistivity references for the different tempers. Thanks for explaining the mechanism behind this drop.