stamping aluminum alloy sheet 1

[andrewjmorin]

We've got an old (30 years, actually) product that is an aluminum bridge that automobiles are driven over. It is 56" long, 22" wide and about 2" deep. It is stamped into a corrugated shape that coforms to a very specific gov't specification. The old material was 1/4" thick aluminum 5083(-m321? I'm not sure of the actual spec, but it involves heat & stress to harden the mat'l and let it retain some of it's ductility-->14% elongation-- for a relatively deep draw stamping).

Now, several years ago the working load specification was doubled+ from 1500 lbs to 3600 lbs, but the maximum mass of the product remains unchanged at 40 lbs. We've got very expensive legacy tooling, so major geometric changes are out. We've been trying 7075 aluminum, formed in the -t0 and treated to -t6. 'Of course' the parts warp nearly beyond recognition upon quenching. The parts (ingoring the deformities) do pass the proof load test (10,800 lbs) but we can't seem to get rid of the deformations.

Any suggestions (new material, with nearly similar yeild strength & weight; deviations from the -t6 spec that might help; painless methods of suicide?) would be appreciated.

-andy

 

[BillPSU]

The heat treating process will probably distort any other materials also. My thoughts would be die quenching the part to control distortion or part redesign.

 

[btrueblood]

Have you contacted Alcoa regarding the heat treatment? I'm not familiar with 7075 heat treating, but am surprised that a rapid quench is required? Unless you are re-solution treating after forming...but then the residual stresses should go away since you are above the stress relieving temperature.

The aging of 7075 is done at about 240 F, for 24 hours. This shouldn't require a rapid quench after 24 hours, just turn off the oven and wait for everything to cool down. If parts are still warping, consider fixtures to restrain it in the oven?

 

[TVP]

This is a very serious redesign, which may not be able to be fulfilled using an aluminum alloy. I certainly would not use 7075 in the T6 condition, even if you could manufacture it, due to problems with SCC.

 

[kenvlach]

Two comments:
Form the part after solutionizing + quenching, in temper 'W.' Then, artificially age to the 'T6' temper.

Al 7075 is much more corrosion prone than Al 5083. Any measures to prevent corrosion, eg., anodize + dichromate seal? Topcoat?

 

[andrewjmorin]

Thanks for all the input.

We have been in heavy contact with alcoa, they've not been particularly helpful, IMO. We've run trials at 2 different heat-treat facilities, using Kaiser, reynolds & alcoa materials. So far, we've Formed the sheets in the annealed (-t0, is that 'zero' or 'oh'? I've heard it both ways, anyhow...) condition, and it is after the quench (which IS specified to be rapid, 15 seconds maximum after removal from furnace) that the distortion becomes noticable. Is 870 degF material 'soft' enough that hydraulic pressure from the quench water could bring about this distortion? Or is this strictly some chaotic thermal phenomenon? Is there a non-water quench that we could try: perhaps a CO2 or liquid N2 spray, or would that not work?

As I've come to understand, the idea with 'W' temper is that if the material is kept below freezing (0degC, 32degF) after the quench, the aging/hardening process can be delayed for up to several days, and then the blanks can be formed before the onset of the strength increase. This presents us with a logistical nightmare, assuming we can get decent results in the do-over trial; the heat treaters around Chicago are not known for their bulletproof handling processes. Our last trial was intended to provide us with 'W' temper blanks for forming, but we had a SNAFU, and the parts were run too early and then naturally aged beyond the useable range; they cracked quite badly. Still, if it works it WILL be worth it.

What about warm-forming? We've discussed trying it, but using the plasma torch & IR thermometer (looking for 250-300 degF) method of heating, we split the part in the die worse than the snafu'ed W temper. Not to mention the risk of over-aging...

Does 7178 material have any significant features that might give us some new hope?

Actually, SCC could be our friend. With the 3:1 safety factor, our parts have a useful life expectancy well beyond 10 years. Some corrosion would provide for obsolescence; are there inspections that can predict the onset of problematic SCC? (These parts will never see a 'corrosive' environment, they are for use transloading automobiles to & from rail; salt-spray is kept to a minimum.)

I've done a good bit of work on mechanical fixturing to keep the deformations from manifesting, with mixed results. Is there any science to fixturing thin parts to control distortion?


Thanks again for your thoughtfull comments,

andy

 

[btrueblood]

Okay, to nail this down a bit firmer: you buy 7075 sheet, and have a local source solution heat treat and quench it to T0, then you form it and send it out for aging.

So the sheet is distorted in the flat (before you form it)? Is the T0 quench done by spray quenching with water now? The distortion may simply be due to the pattern of the spray jets and non-uniform cooling rates across the face of the sheet. But you should be able to stretch the sheet a bit in T0 condition to straighten without affecting properties too much. Alcoa doesn't talk to you about how to do it because they own(ed?) a patent on the process; you need to see if a.) you can locate that patent and use the information to help you in heat treating, and b.) talk to Alcoa about licensing their process for your application. The sound of $$ may make them a bit more responsive.

Can you live with the distortion (you said the panels passed proof testing)? I would have thought that the quench distortion would flatten out somewhat during your stamping process.

I agree the shipping of T0 condition sheet can be a pain in the a$$, but Alcoa ought to have the logistics worked out by now, or at least agree to buy back the scrap if the sheet has aged prematurely.

You may have better luck with the 71xx alloys just because they are a bit more corrosion resistant and less susceptible to SCC (than 7075), and therefore see more usage in the industry (thus the process knowledge available in the industry may be greater due to sheer volume of material moving).

 

[andrewjmorin]

"-T0" means annealed, no heat treatment. "-T6" means solution treated at 870degF for 60+ minutes, drop-tank quenched (not spray) in water and artificially aged at around 250degF for 24 hours. "Natural Aging," begins immediately at room temp, but can be held for a few days if kept cold.

We buy the plate/sheet cut to the size of our stamping blank (aprox 2' x 5', 1/4" thick)in full ingot quantities (10k#).

Our 1st plan was to stamp the parts in the annealed (t0) state, taking advantage of the softness. Then send the parts for heat treat to -t6 where the yield gets up around 70ksi. The parts distorted, in spite of several attempts using mechanical fixturing to prevent it. Ultimately, we were able to straighten some while others were beyond remediation; About a 50% scrap rate.

So somebody suggested we heat-treat the flat blanks to the '-W' condition, which is unstable in that it will gradually progress (age) toward -t6 over time. Cold temps retard the aging. The theory was that the flat plates had little or no internal stress that could lead to distortion, and if you could form it before it aged you'd have a winner. However, the flat blanks distorted pretty badly, and our heat treater jumped the gun and baked our test parts before our ice-box was in place...the parts were allowed to natrally age (on a 100+ degree day, no less) for several hours before we chilled them, and they subsequently cracked in the stamping die.

I'm looking into the 'straightening' process for the '-W' condition blanks, but the thermal considerations raise a lot of doubts: I can't let the blanks warm up before I stamp them.
I agree that non-uniform cooling is the issue: the bottom of the part hits the quench several seconds before the top, and that's the source of the distortion, no doubt.

So I'm definitely hoping that the 71xx series is known for having fewer distortion problems, but I havn't found anyone willing to go on record regarding that fact.

 

[andrewjmorin]

oh, and the distorted blanks seem to lead to distorted parts after stamping. They did so on our snafu'ed '-W' condition blanks. There's still some hope that true W's would not do that.

Here's hoping,
andy

 

[kenvlach]

Symmetric material such as rectangular plate shouldn't warp during quenching; warping indicates somewhat asymmetric quenching as already mentioned.

The residual quenching stresses (compressive at surface, tensile in interior) combine with forming operations to create warping. Note that ASTM B597-92(1998), "Standard Practice for Heat Treatment of Aluminum Alloys"
[Withdrawn 2002; see B918-01 Standard Practice for Heat Treatment of Wrought Aluminum Alloys] recommends mechanical stress relief: After solutionizing & quenching, stress relieve to W51 temper by cold-stretching to a permanent set of 1 to 3%.
Small heat treater may not be able to stretch relieve, but Alcoa certainly can.

As an alternative to 7075, consider 7050. This alloy was developed (in part) to be less sensitive to quench rate than 7075. As strong (maybe a little stronger) as 7075 for equivalent temper. More importantly, it can reach 95% of its final aged strength when quenched at 20^oF/sec, while 7075 requires quenching at 100^oF/sec to end up at 95% of its max. If quenched at 30^oF/sec and overaging, 7050-T73 has a UTS of 77 ksi while 7075-T73 has a UTS of 68 ksi.
From Aluminum: Properties and Physical Metallurgy (1984);

http://www.MatWeb.com

has similar data although only for recommende quenching (max. 30 sec to cool 7075 from solutionizing T to 100 F).

Summary: Switch to 7050 and/or stretch-relieve after quenching (for temper W51). If necessary, 7050 can be quenched at a lower rate. If slower quenching, do some hardness tests to check UTS & maybe pull a tensile specimen (both, after aging).

 

[andrewjmorin]

kenvlach,

are you employed in such a capacity that my company could briefly contract your services?
NO travel would be required.

-andy

 

[kenvlach]

Thanks for the compliment. My experience with aluminum is mostly in other areas (anodizing, corrosion, phase equilibria, high T oxidation). Very little mechanical experience (haven't personally rolled any Al except small specimens as a teaching assistant); that info's from reference books. Did QC many warped parts that warped from residual stresses -- sometimes machines shops tried to blame on electroless nickel plating!