Aluminum for Low Temperature 1

[DanStro]

I am trying to decide which Al alloy to use.

I have a cylinder approx 38mm long with very thin walls (<1mm along most of it). Aluminum with black anodize is really the only option due to weight and other factors so I need to determine if I should use 6061-T6 or 7075-T6. These are the only alloys that are allowed by the customer so I can't stray from those.

The temperature range is +71C down to -41C. At low temperature there is very little radial stress but the axial stress is fairly high (given the wall thickness) and I want to make sure that it will not break. I am worried about the brittleness of 7075-T6 when compared with 6061-T6.

Any suggestions?

Thanks,
Dan

 

[CoryPad]

Use fracture mechanics to decide what is the allowable flaw size and see if 7075 can be used. Can you use a different temper for the 7075? A T7-style overaging can increase toughness.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[DanStro]

Cory,
Thanks for that. I did that and they both look OK. I guess what I am really worried about is how the fracture toughness changes (if at all) when you get down to -41C.

I can't find any reference with anything but, I am assuming, at 20C and no mention of if that changes with temperature.

Dan

 

[CoryPad]

ASM Handbook Volume 19 Fatigue and Fracture has the following data:

[tt]Alloy K_Ic @ 24 [&deg;]C K_Ic @ -196 [&deg;]C
--------- ----------- ------------
6061-T651 29.1 MPa m^0.5 41.6 MPa m^0.5
7075-T651 22.5 MPa m^0.5 27.6 MPa m^0.5[/tt]

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[DanStro]

Cory,

Thats exactly what I needed, thanks.

Dan