Could residual stresses in a 6061-T6 stamping result in fracture during forming?
The process is that a blank is formed from sheet stock using a turret press, then two mirrored parts are made from the blank. Bends are formed at the same place but in opposite direction. Anecdotally, I've been told that one part tends to fracture more than the other.
Experimentally, I've found that fracture seems to coincide with the break-out side being on the outside of of the bend and parts that don't fracture have the break-out side on the inside of the bend. Hence my theory, that the edge opposite the break-out side leaves residual compressive stresses... For that matter, it could be the residual tensile stresses on the breakout side coupled with the outside bend tensile stresses.
Anyone confirm this theory
Answer:
I doubt it is "residual stresses" as much as it is microscopic cracks already existing on the break-out side.
I agree with the fist answerer that T6 may well be too hard for the forming being done, if possible, increasing the bend radius should help, but if the design can't be changed, then using an O temper and heat treating after forming would probably be the best solution, trying T4 might possibly be an option if it is strong enough. Shaving the part after blanking to remove the fractured material might help a little, but I'm not sure that is really practical on a turret press and probably wouldn't help enough to be worth the trouble.
I don't completely understand the geometry of what you're talking about, but it sounds like residual stress might be an issue. If you need -T6 for strength, you might consider getting un-heat-aged 6061, stamping then treating it to -T6. (I don't know if the un-treated 6061 is more malleable or not since I don't have my handbook with me now. This is something you'll want to check.) If strength is not an issue and you're not locked into 6061 for some reason, you might try a more workable alloy.
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