Aluminum skins

[gutboy17]

I'm working for a small company and we just had some skin panels chemically milled and they came back deformed (6" deflection over about 43" span). They were .12" thk 7075-T6 flat panels originally that were milled to .050" thk in large pocket regions on only one side for weight reduction. From what I understand is that the material had internal residual stresses and during the milling process the unsymmetrical removal of material caused the internal stresses to warp the panels. My first question is the previous a correct assessment of the cause. Secondly, is there a cold working process (i.e. rolling, shot peen, etc.) that could flatten the panels, or can the panels be heat treated and made flat again.

 

[GBor]

Did they deflect away from the chem mill pocket or towards the pocket? Where did the panels originally come from and were they not stress relieved from what I presume to be a rolling process?

The statement is viable, although 6" seems like a lot, but I haven't really thought about this problem lately.

 

[berkshire]

gutboy17 (Aeronautics)
Your first question is a correct assessment of the cause.
GBors follow up question is to assess whether or not they were under rolled or over rolled, prior to Chem milling.
Depending on the size of the panel, hand straightening after re rolling may be your best bet, stretch straightening may be an option if the panel is not trimmed. The biggest question will be,how much, time, do you want to put into them?
Figuring how much curve to put into a panel that is going to get chem milled can be a real black art.
B.E.

 

[berkshire]

gutboy17
Apologies for not reading the complete question,
you are dealing with FLAT panels. Was the stock Chem Mill quality? Properly stress relieved before etching? If not then my prior comments apply.
B.E.

 

[gutboy17]

The panels are suppose to be flat. The curvature is towards the chem milled pockets. I don't know if it was stress relieved as raw material (I don't think so, it is 7075-T6 bare Aluminum, AMS-QQ-A-250/12). I don't think it was stress relieved during its manufacturing (cutting to outer dimensions and then chem milling lightening regions). It doesn't sound like it is financially feasiable to put much effort into these three panels. I should scrap these and start new with stress relieved material. Thanks for the info though.

 

[rb1957]

whilst this deformation isn't wanted, how much pressure needs to be applied to the milled panels to get them to be flat, ie to attach to the underlying structure ? probably not much. but then how wavy is the outer surface ?

maybe you need a stress relieve heat treatment ? shot peening would help flatten them too, but i think this is sort of "two wrongs not making a right".

could you machine in O condition ? then heat treat to T6.

 

[RPstress]

The usual way this sort of distortion is handled is by there being a rule about how much force is needed to flatten the parts out, often by hand pressure.

If it's not much, then it's ok to go ahead and rivet the skin to whatever it's attached to; this op will flatten the skin out with acceptable levels of prestress.

If you want to get more numerate about it then you can calculate the stress in the part as-assembled, but this is usually only done to start with to establish what an acceptable level of distortion is.

You of course have to make your assembly operation tolerant of a distorted skin.