Sherlock Holmes, Metallurgist

Recently I anodized two reels. All end plates and pillars (6061 aluminum) look right. The feet (7075 aluminum) look right. Of the 20 screws (all 7075 aluminum), 14 look right but 6 of them are dark and dingy. What happened? Each reel frame was anodized as an assembly, so the dark screws did not get a different process

I had to put this aside and work on other parts while I thought about it. After a few days, I had a theory. I had bought 10 rods (0.25 inch diameter) of 7075 to make screws. By the time I did these reels, I was on the 4th rod and had had no problems. Part way through making these 20 screws, I started on rod 5. Was this rod really 7075, or perhaps something that does not anodize well, like 2024?

So I made small anodizing samples from each of the 5 rods yet unused. I also made a sample from a 6061 rod, and one from the partially used rod (#5) that may have been the source of the 6 bad screws. I marked the samples with a file so I could match them to the rods.

And when done anodizing the samples, I had 2 that were dark and 5 that were correct. The two dark ones were from rod #5 and from one of the 5 unused rods.

So now I knew which rods I could use to make replacement screws. The new screws all have proper appearance after anodizing.

My conclusion is the the vendor sent me 8 rods of 7075 and 2 of something else.

Update 12 Sept 2012: Further tests on the unknown material posted 5 Sept 2012.

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5 Responses to Sherlock Holmes, Metallurgist

  1. Jerry Hilgenberg says:

    Dave, I’m impressed that you make even the screws yourself. I am assuming you turn the metal to form the countersunk head, and saw the slot, but what about the threads? Do you have some sort of roller die that will cold form the threads onto the blanks?

    What sort of equipment do you have in your shop to do the anodizing? Was this covered in an earlier post that I missed?

    Deborah and I are in LaPaz, BCS, Mexico, for the winter. That’s right, we’ve now lived in the south long enough that 40-degrees feels really cold to us, so we skipped off to southern Baja until March. So far the results are mixed. The days are sunny and clear, but the nights are windy and cold. And this house isn’t heated – so we may have been better off staying put in Chapel Hill.

    But I suppose things could be worse: We’re becoming experts on fish tacos and margaritas.

    We’ve talked about making a blog about our experiences here – the drive down Baja, the people, the gringos who live here year around — but it all just seems like so much work!

    Hope all is well with you.

    • dave49 says:

      Hi Jerry,
      Good to hear from you.
      Both anodizing and screw cutting have been blogged. You can quickly find these on the “Categories” list at the right.
      Screw cuting is done with round dies. The hex dies that you see at Sears are good for thread repair only.
      Aluminum screws are almost impossible to find, and I am paranoid about electrochemical corrosion.
      Anodizing takes just a power supply and battery acid.
      If you blog on Baja, send me a link.

  2. Rich Bahl says:

    Dave – I got to thinking (a dangerous thing – I know). Could you determine the alloy by weighing samples and calculating the weight/per unit volume. Since the alloying metals are different for 2024 vs 7075 the weight should be different. High quality digital scales are available for a reasonable price. Short of a mass spectrometer I’m can’t think of any other method.

    Rich Bahl

    • dave49 says:

      I just looked for published data on alloy density. Found 0.102 lbm/in^3 for 7075 and 0.100 for 2024. Since 2024 has more Cu, I would expect it to be heavier. Probably inconsistent data from different sources.
      So if we start with the percentages of alloying agents, of course these are ranges. I suspect that if we look at the extremes of the ranges, there is overlap of the possible densities of the two alloys.
      Finally, any sample that you would weigh would have to be precisely dimensioned. The weight change from 0.250 to 0.252 diameter is 2%.
      I can’t think of anything short of an anodize trial to distinguish them. Never met a QC man who did any more than look at certs.

  3. Rich Bahl says:

    I kind of thought about that and wondered how much of a weight difference there would be. Not much. It would take a very fine analytical balance to detect on the small samples you have available. Some companies do do incoming random sampling for complete compliance verification. It is rare however. I know I was blind sided a number of times with non compliant material or parts.

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