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Remove warp from aluminium plate?

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aneevuser:
I have a sheet of 1050 grade, 6 mm aluminium, dimensions 250 mm x 175 mm.

After drilling, it seems to have acquired a warp. I've attached images. The line at which the deviation from flat starts is marked in black on the plan view image.

When the sheet is clamped at one edge, the maximum deflection at the other is about 2 mm.

So the obvious question: is there any way that I can remove this warp without specialist tools? - I don't want to have to mill it, for example.

T3sl4co1l:
Turn it over and slap it gently with a hammer until it lies flat against your platen.  Flip and repeat until acceptably flat.

Mind that heavy hits deform the material, pushing it out from the struck location -- you want just enough force to bend out the flex, not so much that the material is displaced.  Continued heavy hammering causes stretching, which is great for making 3D shapes, and, obviously, what you need to avoid here. :D  Example: hammering in the middle causes the material to stretch there, making a domed shape.  Hammering the edges causes it to stretch there, making a kinked / saddle shape.  Hammering evenly over the entire surface, balances these out of course, stretching and thinning the whole sheet.

If you're actually in such a situation, i.e. it doesn't lay flat after correcting the bend, it seems to kink itself into shapes, then it may need to be stretched in just the right places to counter that.  This is a bit nontrivial and best developed as a skill, so, not really in scope here.

And at which point, you're maybe better off heating it to annealing temp (400C or so), to relieve the stress, and then just tap it flat.  But that's probably also the last thing you want to do on already extremely soft 1050 aluminum... it'll be little better than metallic putty after annealing(!).

Tim

aneevuser:

--- Quote from: T3sl4co1l on May 23, 2022, 07:02:28 pm ---Turn it over and slap it gently with a hammer until it lies flat against your platen.  Flip and repeat until acceptably flat.

--- End quote ---
Yes, that's the obvious approach but I didn't want to try it without good reason, given the relative softness of the plate - seems a bit too easy to go horribly wrong.

I have tried bending it against the warp with G clamps, but that does nothing, AFAICS - simply rebounds after pressure is removed.

--- Quote ---And at which point, you're maybe better off heating it to annealing temp (400C or so), to relieve the stress, and then just tap it flat.  But that's probably also the last thing you want to do on already extremely soft 1050 aluminum... it'll be little better than metallic putty after annealing(!).

--- End quote ---
I don't want to try annealing it, not least as I can find no entirely authoratitive info. on how to un-anneal the stuff, or if it is even genuinely possible without specialist ovens (lots of waffle about "time hardening" or something, and work hardening by hammering, neither of which appeal).

Would it help if I heat it to sub-anneal temperatures while bashing it with a hammer, or is that pointless?

Ian.M:

--- Quote from: aneevuser on May 23, 2022, 07:43:08 pm ---Would it help if I heat it to sub-anneal temperatures while bashing it with a hammer, or is that pointless?

--- End quote ---
Pointless - aluminum is 'hot short' so working it hot vastly increases the risk of cracking.  Also holding many aluminum alloys at elevated temperature can result in grain growth, with increased intergranular precipitates, also increasing the risk of cracking

T3sl4co1l:
Yeah exactly, you can't harden it again, it's not a hardening alloy -- you could perhaps do that with 6061 or what have you, where you could anneal, quench from annealing temp, then age in the 200C ballpark for some hours (exact time/temp depends on alloy and condition) for a, T5 I think temper.  But 1xxx, 3003, etc. don't have the alloying elements that allow this kind of hardening cycle to work -- your only option is to hammer/roll/etc. until work hardening gets it there.  Which presumably, your plate already is, so it's as good as it's going to get.

That it doesn't take a set from being clamped flat, is encouraging -- it's not dead soft.  Having strength, means it has to be bent past flat, to get into the plastic deformation range, before it will take a set.  It's... not so useful of a property to you right now, :D but it's also a necessary property in general.

Which is kinda where hammering comes in, because hitting it tends to over-bend it (material curves up from the anvil).  So, hit it a few times against the curve, and at some point it should come flat.  Start with light taps, and get harder until it's taking a set.  Do it evenly over the curve, get it flatter; turn it over, correct the parts you over-corrected, etc.  Also, use a smooth clean hammer, don't want to mar the surface too much.  Can use plastic, cardboard, leather, etc. to soften and spread out the blow too.  Soft-blow hammer, rubber mallet, etc.

Also, the fact that it's work hardened, means there's internal stress in it, too -- this is probably not what caused the bend in the first place, not from just drilling anyway; maybe you dropped it, or cranked down on it too hard while drilling, or something... well, whatever, it is how it is now.  But this internal property is important when, for example, milling into the surface: relieving some of the stress from one side (by literally removing the material there) causes the other side to take over, and it warps towards or away from the cut.  Again, likely not what's happened here, but important to know when doing stuff like that.

Tim

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