Hardness Test

On 15 Dec 2012, I posted about some aluminum rod stock that was supposed to be 7075, but might be something else because it did not anodize properly. I theorized that it might be 2024 (a known poor anodizer) because the vendor also stocks 2024. How might I easily identify non-conforming material when I have to order more 7075 rod? It is now time to order so the problem becomes urgent.

Last winter I sold several reels and so had some funds to pour back into this consuming hobby. I bought two items, a load cell (Ebay NOS) and a microscope with a measuring reticle. These are good general purpose items, but I also thought that I might be able to do a hardness test with them.

In a Brinell test, you push a hardened ball into a material sample with a known force. The diameter of the indentation is then measured. The Brinell Hardness Number (BHN) is calculated as

BHN = F / (Pi/2 * D * (D – sqrt(D*D – I*I)))

F = Force applied (Kgf)
D = ball diameter (mm)
I = indentation diameter (mm)

A standard Brinell hardness test involves a 10 mm ball and a force of 3000 Kgf. I have a “1 ton” arbor press and a 1/4 inch (6.3 mm) diameter material sample. Clearly, I will have to do a non-standard hardness test. I selected a 4 mm ball and 200 Kgf (440 lbf) because the expected indentations would then be in the 1 to 2 mm range. I made test samples of 6061-T6 (95 BHN, expect 1.60 mm indent), 7075 (150 BHN, expect 1.29 mm indent), and of the mystery material. If it is indeed 2024-T4, then it will be 120 BHN and will have a 1.43 mm indentation.

Here is my load cell, resting on an aluminum disk of the same diameter, and both resting on the arbor press.

I have placed a bronze disk, with a drill point dent, on the crowned button of the load cell. The ball rests on the dent without rolling away. A Delrin spacer retains the disk in position but does not interfere with pressing.

Here I have placed a test sample on the ball, guided by a plastic sleeve.

I had a few false starts in the pressing. It was easy to overshoot when cranking the arbor press up to load. Once I had my technique established, I produced these three indented samples.

The 6061 sample has a 1.75 mm indent (vs 1.60 expected). The 7075 sample has a 1.22 mm indent (vs. 1.27 expected). The mystery material also has a 1.22 mm indent. By this test, it is indistinguishable from the 7075 that anodized properly. So perhaps it is not 2024, but what could it be? When I buy more rods, I guess that I am stuck with anodizing samples of each to check for unsatisfactory material. (Update 12 Sept 2012: Ordered 10 rods at 2 ft. each; all anodized properly.)

Remarks on the test:
I am a 150 lb weakling and 440 lbf is about the largest force that I can achieve with adequate control (i.e., no overshoot) from this “1 ton” arbor press.
Under magnification, the edges of the indents were quite sharp and easy to measure with the reticle.
My microscope has 3 objective lenses for 20X, 40X, and 100X magnification. I used 40X here, and was able to resolve indentation diameter to .025 mm (.001 inch).

I suspect that the greatest weakness in my test procedure is control of the force. Obviously it would be better to have a purpose-built Rockwell tester (a heavy, expensive unitasker). But had the problem material been 2024, I think that my test would have adequately shown the 120 BHN hardness.

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