7075-T6 Vs 7075-T7x

[fpm]

Hi all,

I'm currently researching a suitable material for a component I'm designing.

I've pretty much settled on 7075 aluminium, but I'm now seemingly faced with the choice between T6 or T73/T76. From what I can see, the T73/76 offers better resistance to SCC than the T6, but at a price of a reduction of around 10% in the proof and UTS.

Is this about right...or are there other things to consider between the two? I'm not a materials scientist by any manner of means, but I'm curious as to why the T7x temper seems to lose around 10% of its max Proof and UTS?

Any thoughts on any of the above would be very gratefully received.
Thanks
FP

 

[TVP]

T6 is the peak aged condition, so maximum strength is developed when processed in this manner. T7 is the overaged condition, which means that the precipitates have coarsened beyond the size of maximum strengthening. As you have learned, this imparts greater resistance to SCC. If you can provide some additional details on the expected environment and service conditions, we can provide you with some recommendations for T6 vs. T7x.

 

[fpm]

The part will be a long bar, roughly rectangular, in a cyclic bending mode. Bending will be of the region of 50 microns between each load point, each point 100mm apart. There will be no sharp cuts or notches in the profile, other than a series of tapped holes to allow loading rods be fixed to the bar.

Bar will not be exposed to extremes of temperature, maybe highest at 50-60 deg C.

At first I was pretty sure T6 would be OK, as there are no notches or cuts in the bar to help start crack propogation. However, since it is in continuous operation, I'm maybe thinking it is worth sacrificing the extra strength for improved resistance to SCC. Mind you, I could do with as much strength as I can get, to keep a 'max stress to yield stress' ratio of 2:1, which is what I'm aiming for.

Why is engineering never simple....!
FP

 

[CoryPad]

You haven't really discussed the second 'C' - CORROSION. If this part is in a vacuum, use T6. If it is used in a coastal environment (salt spray), use T7x. If it used in between those, you will need to consider the corrosivity of the environment and the subsequent crack growth rate.

Regards,

Cory

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