Aluminum 7075 - T6

[M. BR]

Hello guys,

I'm doing a failure analisys on an aluminum 7075 T6. I think the failure mode is brittle. Attached the SEM image.

The supplier send the "recipe" for the T6. What do you think about this recipe?

Solution Heat treatment: 470°C(880F) - 2h - water cooling
Precipitation: 120°C(250F)2h + 180°C(360F)
Recristalization: 260°C(500F) - 3h - air cooling

Some times they do the T6 twice. Do you think this is a problem?

 

[rb1957]

I'd be surprised if 7075T6 is "brittle" ... sure it may be an overload failure but Aluminium has some ductility.

You can find out about 7075T6 for online sources, like Mil Handbook 5J or AR-MMPDS-01 (or -0x, there are several versions, some behind pay walls).

"T6" should be thin sheet can be thick plate (but more often T651). some specs ... AMS 4044, AMS 4045, AMS QQ-A-250/12

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[M. BR]

I was also suprised analysing it..... but on SEM images/analysis there was no signs of deformation, voids... nothing...

 

[rb1957]

is it thin sheet ? plate ?? machining ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[mfgenggear]

OP
As RB said not enough info.
What it is and what are the stress loads. Shear or tensile

 

[3DDave]

In the latest pictures it looks like beach marks, possibly low cycle fatigue from stress exceeding the material properties.

 

[M. BR]

Its not fatigue, the part was just machined and assembled.

 

[3DDave]

It spontaneously snapped sitting by itself?

 

[M. BR]

no, after assembly, obnline testing

 

[3DDave]

What testing was done? Is there a photo of the item in the test fixture?

 

[mfgenggear]

Was there any straightening involved.
It may be from forging. And how it was heat treated.
Take the whole part and have it zyglo penetrant inspected. Now that is scrap. And not repairable. Cut it up and do met lab samples.
Spectral analysis
Sem
Pull a couple of tensile
Do a charpy test. If it's brittle that will tell you. So will the tensile.
Your on a mission to what it is not.

 

[rb1957]

not that it's a big deal but heat treating to T6 after the original melt makes T62, which has all the properties of T6.

There look to be some awful stress concentrations in those sharp corners ?

yes, cut coupons from the scrap and verify properties (just to be sure).



"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[drawoh]

I'd be surprised if 7075T6 is "brittle"...

I tried to move a tent that had poles made from Aluminium 7005-T6. There was no discernable ductility. The first sign of trouble was when they snapped. Now that is not 7075 alloy, but the yield and ultimate stresses are awfully close together.

--
JHG

 

[3DDave]

One other thought - check to see if that really is 7075. I've had suppliers substitute other alloys before.

 

[mfgenggear]

Spectral analysis

 

[BrianE22]

Its not fatigue, the part was just machined and assembled.

Did you watch the assembly or do it yourself? Perhaps something was off and a lot of stress induced during the assembly.