PDA

View Full Version : How to test aluminum



quadrod
06-01-2010, 11:27 PM
Is there any way to test the hardness of aluminum in the home shop with out a hardness tester.

PaulT
06-01-2010, 11:32 PM
I take a fine file and use the edge of the file on the edge of both unknown steels and aluminums. It gives me a basic idea on what type of metal I'm dealing with and how well it would machine and it will also give you some idea on how it will weld.

Grab some scraps of known metals and alloys and play with a file on them a little and I think you'll see what I mean. In the case of aluminum its pretty easy to tell the difference between 6061 and the softer stuff. 2024 has a lot more copper in it than 6061 while 7075 has a lot of zinc, giving these alloys slightly different visual tones. This isn't a bulletproof method of identication for a part going on the space shuttle, but once you get used to what they look like it can help you in your home shop.

Paul T.

Evan
06-01-2010, 11:53 PM
Is it important that you know the actual hardness? Or, is it the yield strength you need to know?

If you are trying to indentify what series and temper a piece is you can only expect to place it in a very general category unless you already have a great deal of experience with a wide range of alloys. If you are trying to sort out 1075 T6 from 6061 T6 from 2024 T3 or T4 from 1100 electrical aluminum then that is fairly easy. 1100 aluminum has a yield strength of under 10,000 psi and can be bent almost indefinitely without work hardening or cracking. It is also a betch to machine.

6061 T6 is the universal weldable and heat treatable aluminum for jobs that need both strength and light weight. A thin sheet, say .040" thick can be bent to a sharp radius without breaking but only once. It machines nicely and the chips don't cut you easily.

Next is 2024 T3. This is aircraft aluminum and is often found as Alclad. That means it has a thin layer of pure aluminum on each side of a sheet that provides excellent corrosion resistance for the alloy in between. The corrosion resistance of 2024 is relatively poor and it forms obvious white oxide patches if left in the weather for a long time, especially near salt water. A thin sheet of .040" 2024 T3 or T4 will crack if bent to a nil radius. It is also amazingly strong and very hard to bend. When you work with 2024 you will realize that this isn't the stuff that they sell for countertop molding. It machines beautifully without burrs on holes and is easy to machine to a mirror finish. It is as strong as steel of the same physical dimension but weighs only a third of what steel weighs.

7075 T651 is one of the most common of the aluminum super alloys. It is about 20% harder and stronger than 1018 steel and doesn't machine like other grades of aluminum. The swarf will cut and stick you and when drilling it it will drill to exact dimension. However, never drill close to the desired dimension and then try to open it one more size up. It will grab the bit and won't give it back. If you try to bend an .040" sheet of 7075 it will snap before it gets to 90 degrees. It's the grade that aluminum keys are made from. It also turns gray when left in the weather.

Paul Alciatore
06-01-2010, 11:58 PM
I believe that hardness testers just make a dimple in the material being tested with a known force and then measure the depth of that dimple. The deeper the dimple the softer the material. A standard automatic center punch (the kind you push and it goes "click") will produce a fairly repeatable amount of force. Not highly accurate, but fairly consistent.

So, if you have a punch and some samples of known aluminum alloys of known hardness, you could make punch marks on the known alloy and the unknown one and compare them with a magnifier. Equal hardness should produce equal diameter punches. Be sure to adjust the punch so that the depth does not exceed the diameter of the tip's cone. Rough measurement, but probably usable.

Perhaps some of the old hands have a better way.

fishfrnzy
06-02-2010, 12:20 AM
In the US the color marking on aluminum is uniform. If there is still some paint or line marking it is a good guide.

blue= 6061
black= 7075
red= 2024
brown= 2011
green/yellow= 6063

These ar the most likely alloys you'll run accross exept in sheet where you'll find 5052 and 3003 ( both relitively soft )

Paul's idea is comparative but the only one I know of for the home shop.

Aluminum also gets a little harder with age ( time ) from when it is made for about 1 year. Not all alloys behave the same.

Rich Carlstedt
06-02-2010, 12:50 AM
Is there any way to test the hardness of aluminum in the home shop with out a hardness tester.

Yes, quite easily done if the material is all the same thickness and you are looking at relative comparisons
Its called a " bend" test
Take strips of the same thickness and roll them around a given radius, like a 1/2" dowel pin.
The more they bend without fracture...the softer they are
Width of the strip is irrelivant.
Mark the failure point.
There is a better variation of this where you do not go to failure, but bend say 30 degrees and then let the material return by itself. It is called a "springback" test and is extremely accurate with a calibrated chart behind it

Now if this is for solid materials , place them on a stout bench and press down with a spring loaded centerpunch.
comparing the upset with a magnifier will give you a relationship.
A pocket comparitor is really the thing to use here !

Rich

radish1us
06-02-2010, 08:03 AM
If the ally is a piece you can pick up and hold with one hand, then hold something hard in the other hand and give the ally a smack.

The tone that the piece of ally gives out, will tell you how hard it is.

If it's a sodden dull sound, then it's a piece of soft junk.

If it's a clear crisp ring type tone, then it's a good hard bit of ally.

Try it out if you don't believe it, it sure works for me at the scrap metal merchants.

This simple test can be done on ANY material, it all works the same way whether it's steel, brass or ally, if it's a dull tone, it's crud, good sharp tone, then it's a good bit of whatever you just hit.

hojpoj
06-02-2010, 11:24 AM
Thanks for that rundown, Evan! I have access to some of the recycle bins here at work, and since we do helicopter stuff there's a lot of 2024, 7075, and a little 6061. Some of it still has the mill markings on it, the small bits not so much. What behaviors to look for when I start hacking away at it is good to know :D