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OT? Aluminum Metallography?

Halcohead

Stainless
Joined
Apr 10, 2005
Location
Bay Area, Ca
Has anyone here done in-house metallography on aluminum? Any tips for sample prep and a (relatively) safe etching solution?

I have a number of weld specimens in 1/16" aluminum I'd like to do metallography on to examine grain structure and look for voids and areas of poor bonding. There are strong reasons to believe these defects are there, however my current attempts at polishing have not revealed any, even under 40x magnification.

So far I've milled the sample with a sharp endmill, then used progressively finer sandpaper up to 3 micron to clean up the surface. There is a fine polish at this point with minimal scratches. For some reason when I go to 1 micron I get severe scratching, even with brand new sheets. The sheets are backed by a surface plate.

So far haven't messed with etching since I every etchant I've found includes HF, which I'm not equipped to handle (and don't want to be). Would regular hydrochloric acid be sufficient to reveal grain structure and defects?

Of course I can send samples out, but lead times are 2-3 weeks which really slows development.
 
For *welding* purposes, see Jody Collier's welding tips and tricks, which suggests polishing with sandpaper, scotch brite, etc., and then I think it's easyoff (I'd have check.) But that's for finding voids and level of penetration for *welds*, it sounds like you are after something rather different???
 
Haven't done this, but aluminum oxide paper is a disaster for fine polishing. Have to use silicon carbide or diamond. I'd think there are all sorts of reasonable etchants for aluminum, though few show up in a search. Try here- file:///C:/Users/user-107184/Downloads/AlEtchExperiment.pdf and look for the Weck's formula.
 
What alloy ? If the alloy contains silicon, then you probably cant avoid HF. Googling HF makes you want to do something else, its a stuff from hell. Melts your bones and sends reminding pieces to blood circulation until they stuck behind heart valves.
 
I did metallography on 1100 aluminum in the 1960's. The only etchant I found to work was concentrated HF and nitric acids. They would only work for a short time before needing replacement.

Tom
 
Has anyone here done in-house metallography on aluminum? Any tips for sample prep and a (relatively) safe etching solution?

I have a number of weld specimens in 1/16" aluminum I'd like to do metallography on to examine grain structure and look for voids and areas of poor bonding. There are strong reasons to believe these defects are there, however my current attempts at polishing have not revealed any, even under 40x magnification.

So far I've milled the sample with a sharp endmill, then used progressively finer sandpaper up to 3 micron to clean up the surface. There is a fine polish at this point with minimal scratches. For some reason when I go to 1 micron I get severe scratching, even with brand new sheets. The sheets are backed by a surface plate.

So far haven't messed with etching since I every etchant I've found includes HF, which I'm not equipped to handle (and don't want to be). Would regular hydrochloric acid be sufficient to reveal grain structure and defects?

Of course I can send samples out, but lead times are 2-3 weeks which really slows development.
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aluminum sticks and buildup can cause scratches. wet polishing is often done same as coolant is used in milling to prevent chips sticking to cutting tools.
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buffing compound is basically wax and abrasive. wax forms lubricant layer preventing sticking
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some use kerosene as lubricant basically a very thin oil.
 
Has anyone here done in-house metallography on aluminum? Any tips for sample prep and a (relatively) safe etching solution?

I have a number of weld specimens in 1/16" aluminum I'd like to do metallography on to examine grain structure and look for voids and areas of poor bonding. There are strong reasons to believe these defects are there, however my current attempts at polishing have not revealed any, even under 40x magnification.

So far I've milled the sample with a sharp endmill, then used progressively finer sandpaper up to 3 micron to clean up the surface. There is a fine polish at this point with minimal scratches. For some reason when I go to 1 micron I get severe scratching, even with brand new sheets. The sheets are backed by a surface plate.

So far haven't messed with etching since I every etchant I've found includes HF, which I'm not equipped to handle (and don't want to be). Would regular hydrochloric acid be sufficient to reveal grain structure and defects?

Of course I can send samples out, but lead times are 2-3 weeks which really slows development.

Your 1 micron paper is contaminated - the entire lot will be bad. I use diamond suspension and a nap cloth from 1200 grit wet SiC paper. Use 9 micron, 3 micron and finish on 1 micron followed by 0.04 micron colloidal silica. You can use the same nap cloth - the diamond quickly works off the nap. Colloidal silica (and diamond suspension) can be ordered from the metallurgical supply houses. USE GLOVES - the stuff that forms the colloid is rough on the skin. And you're right about the aluminum etchants, all the good ones have HF at low levels. You can buy Keller's etch premixed so that you don't have to be exposed to concentrated HF. Gloves, mask, and fume hood are all advised for etching with Keller's. I do this for a living - PM me if you'd like more detailed instructions.
 
Bryan, you're correct, these are friction stir welds, not conventional GTAW. They are visually full-penetration, but tensile (and bend) tests can often tell a different story. Material is 6061-T6 before welding. Can't talk much about application, unfortunately.

Your 1 micron paper is contaminated - the entire lot will be bad. I use diamond suspension and a nap cloth from 1200 grit wet SiC paper. Use 9 micron, 3 micron and finish on 1 micron followed by 0.04 micron colloidal silica. You can use the same nap cloth - the diamond quickly works off the nap. Colloidal silica (and diamond suspension) can be ordered from the metallurgical supply houses. USE GLOVES - the stuff that forms the colloid is rough on the skin. And you're right about the aluminum etchants, all the good ones have HF at low levels. You can buy Keller's etch premixed so that you don't have to be exposed to concentrated HF. Gloves, mask, and fume hood are all advised for etching with Keller's. I do this for a living - PM me if you'd like more detailed instructions.

This is hugely helpful, thanks so much! I figured you should be able to get Keller's or Weck's premixed, but it still sounds like I'm not equipped to handle them (no fume hood, barely more chemical training than highschool). Luckily I've heard porosity is often easier to find without etching?

I sent a few samples to a professional metallography house and they were able to find porosity without etching, but only under 200x magnification. So it sounds as if at the very least I need more magnification for these.
 








 
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