7075 aluminum issues

I'm making small parts with M4 (close to 8-32) threaded male ends. The metal ends are occasionally snapping off, usually near the shoulder.
Can suggest a way of increasing the strength of the part? They're made on a screw machine using a die, and are made from 7075 aluminum, T6. I can't change/increase the thread size.
I'm testing out re-artificially ageing them, at 250 deg. F for 24 hours in a small oven. I'm not sure how I'll test if they're stronger than before, but I'll figure out some creative way.
Is re-ageing them even worthwile? My thoughts are that the screw-machine is stressing them, and that this will strengthen them.
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What specifically are you starting with? Extruded rod? Drawn & rolled rod? It matters.
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I'll have to ask our supplier, but I'd guess it's extruded 7075 T6. I'll contact them tomorrow and will update the thread.
Harry Andreas wrote:

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It's extruded 7075 T6 aluminum. 0.250 rod.
Harry Andreas wrote:

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If I have this right from the initial post , the piece is breaking at the shank right where the threads start. Right?
Are you using a thread cutting die, or rolling the threads?
Either way, it's well known in the fastener industry that the transition area from the shank to the first thread is a high stress concentration. I don't recall why this is, but it's a fact.
Things to try: If you are cutting the thread, start rolling it. Change the fastener design to use a Pitch Diameter shank. These have the shank machined down to, obviously, the pitch diameter. This is a well known solution in the fastener industry to the stress concentration problem. Look up fasteners with a "PD" shank, or with PD in the name, such as SAE MA3286
Switch the initial condition of the 7075 from T6 to T73. While T6 is a little stronger, T73 is a little tougher.
My 2cents
good luck
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Yes, that's correct. Here are images of failed parts: http://jdcarbon.com/testing/testing.html

Cutting die at this time.

Yes, it seems they always break at the start of the first thread.

I'll investigate this with my machinist.

The shank is inserted into a tube, which is of fixed inside diameter. I can't reduce its diameter. Others in a machinist thread have suggested a radius at the end of the threads, and my machinist is experimenting with this on some stainless samples.

I'll see if I can get that in my next material order. How much stronger (%) would you imagine it to be?

Thanks, I appreciate your advice!
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Aluminum is notch sensitive enough as it is. When you're cutting the threads you're shearing right through the grain structure. Thread rolling deforms the grain around the thread without (hopefully) breaking it. Thus it's stronger.

T73511 extruded rod has a fracture toughness about 22% higher than T6510 extruded rod according to my source (MIL-HDBK-5)
As I said above however, the overall tensile strength of the T73 tempers is lower than T6 or T65. In the LT direction: T6 is 75 ksi T73 is 66 ksi
So you lose a little bit of UTS but gain fracture toughness.
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One thing to check would be the native ultimate tensile strength of the rod stock which you are using, that is, before machining. This would eliminate the possibility of the unknown defect being simply an improperly mixed or treated batch of aluminum. Find a reliable materials testing laboratory and submit some rod specimens for testing in their tensile testing machine. I would ask for yield strength data as well as UTS. ... Maybe they can provide some kind of plot or graph of the deformation data. After, you get the results, compare them to standard values, and hopefully, proceed with confidence to possible problems such as stress concentrations, dull thread dies, irregular threading procedures, or whatever else. ... If you don't do this, you might be missing something important. ...

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