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I recently received an order of extruded aluminum strip 3/32" thick that supposed to be 6061 hardened to specification ASTM B221. However, when we tried to bend it, it broke and the interior looked like cast zinc coated with aluminum. Is 6061 supposed to look that way and be unable to bend 90-degrees?

How can I verify that I have solid aluminum and not some fake alloy with zinc in it?

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    $\begingroup$ 6061 isn't a great alloy for forming. try 5052 for a sheet metal aluminum $\endgroup$
    – Pete W
    Oct 26, 2021 at 22:09
  • $\begingroup$ Many aluminium grades harden in storage, so you often need to do a heat treatment the aluminium before forming even if you bought it in soft state simply because it didnt get used immediately after the heat treat. $\endgroup$
    – joojaa
    Oct 27, 2021 at 6:29
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    $\begingroup$ Next time, just purchase it from a reputable supplier. $\endgroup$
    – D Duck
    Oct 27, 2021 at 8:02
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    $\begingroup$ ASTM B221 is a collection of specifications. You got 6061 hardened to one of those specifications. Which one? It's expected that extremely hard alloys will be more brittle. If this is for any type of production the most obvious test should be to verify that the mechanical properties of the alloy meet the required specification (ie: does it have the yield strength it should? etc?). Do you have access to appropriate test equipment? $\endgroup$
    – J...
    Oct 27, 2021 at 17:04

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I figured out a simple test.

Since the density of 6061 is 2.5 g/cm3 and the density of zinc is over 7 g/cm3, all I had to do was measure the density of the material.

This showed that indeed it was aluminum. So, its brittleness is probably just because it was hardened.

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    $\begingroup$ Another way you can tell is by looking at the prices of zinc and aluminum (today's prices are \$3500 per metric ton for zinc vs. \$2900 for aluminum). $\endgroup$
    – Biswajit Banerjee
    Oct 26, 2021 at 20:21
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    $\begingroup$ @BiswajitBanerjee LOL, I had no idea aluminum was cheaper than zinc. $\endgroup$
    – Wallace Park
    Oct 26, 2021 at 20:42
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    $\begingroup$ And that's the per weight price- if you compare the volumetric price it will be more than double again. $\endgroup$
    – Spehro Pefhany
    Oct 27, 2021 at 11:28
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    $\begingroup$ And if you compare extruded formed metal, the aluminium is massively cheaper again. Your basic premise is flawed, it's like complaining that you think your Brass doorknobs have been adulterated by replacing most of the Brass with Gold. $\endgroup$
    – PcMan
    Oct 27, 2021 at 14:38
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    $\begingroup$ The only cheaper metal (by weight or volume) is steel. $\endgroup$
    – fraxinus
    Oct 27, 2021 at 14:45
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You can confirm the alloy and heat treating condition by measuring conductivity. It's like a fingerprint for any given combination of alloy composition and heat treatment. Reference values can be found here, for example: https://content.ndtsupply.com/media/Conductivity_Al%20Reference%20Chart.pdf

If you are faced with this task more often, you might want to get a eddy-current based conductance measuring device (eg. "Fischer Sigmascope"). As far as I remember, they are not exactly cheap (around $3000), but very reliable and can even measure through paint or metallic coatings

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    $\begingroup$ A benchtop XRF spectrometer might also be useful for frequent use. $\endgroup$
    – Andrew Morton
    Oct 27, 2021 at 12:03
  • $\begingroup$ Be cautious when using contact probes to measure conductivity/resistivity, as Al forms an oxide layer pretty quickly. $\endgroup$
    – Carl Witthoft
    Oct 28, 2021 at 13:37
  • $\begingroup$ Yep, that's why you usually use the eddy current induction method. Works even through coats of paint. $\endgroup$
    – haarigertroll
    Oct 28, 2021 at 13:47
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Unlikely you have anything except aluminum alloy. What is the heat treatment supposed to be ; what grades are offered in the spec? On the net, I see reference to T-5, that is- as extruded, then aged; lowest cost high strength option. I see elongations around 10% in ASM ( vol. 2, 8th ed.), so, depending on radius, it could easily fracture in a 90 degree bend. I expect a relatively rough grey, granular fracture surface at that limited ductility. Solution anneal and age T-6, should be slightly more ductile. You may need a larger bend radius and heating would help.

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    $\begingroup$ Heating will also affect the heat treatment, so this should be done very carefully. There's no point buying nice hardened material only to remove the hardening with heat because you need it to be bendy. $\endgroup$
    – J...
    Oct 27, 2021 at 17:10
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The standard simple test is to do vickers hardness testing, at a fixed load. the HV should be specified for the material and condition. I'm assuming you are using something like 6061 in the T6 condition. Random online papers suggest that the hardness should be 95HV (load not specified).

Your density test won't tell you too much about the Al grade, as the alloying elements are usually dilute, so don't change the overall weight to more than a few %.

The more complicated ways to confirm the material is to do either XRF, EDS or wet-chemical testing (in order of increasing cost), which can be used to confirm the chemistry, but not the mechanical properties. XRF and EDS are quick to do, but all these things require specialised equipment.

When you say that it wont bend 90*, you've not specified the important parameter, which is the bend-radius, and how it was bent. A two point bend-test and a three point bend-test will give different results for cracking, as will whether you do it at low or high temperature. If you do it at high temperature for any period of time (>150C) then you will likely invalidate the material condition by forcing ageing, and internal precipitation.

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