On paper, the 2024 looks like a much stronger choice, however, you are looking at the ratings for virgin material. Once you begin to look at the stress life data curves or fatigue life cycles charts on 2024? Even in an over-aged temper it looses more of its original tensile and yield properties than any other high-strength aluminum I have researched. And that is at ambient temperature. When the testing temperature is elevated, it looses a MUCH higher % of its strength than 6061. At a point in the life cycle testing, the curves cross and the 6061 is now stronger. I have stress life data curves provided by Westmoreland Mechanical Testing & Research. Depending on the shape of the test sample and application of test forces, some of the results are shocking, but all of them are at least troubling. In worst case, the 2024 test sample had lost about 75% of its original strength, and that was at ROOM TEMPERATURE testing! This potential loss of strength MUST be taken into account in the engineering stage.

If you are not familiar with this type of test, the sample is continually stressed short of yield. As the test cycles accumulate, the stress levels are lowered appropriately to ensure that the material is never stressed to yield. Again, that is keeping short of yield; not approaching UTS. After only 20,000 cycles the 2024 was already under 30,000 psi (calculated), at AMBIENT temperature. When these tests are performed at elevated temps things get really scary. I have results showing 2024 loosing about 85% of its original strength at the end of the life cycle test when performed at 300* F.

After giving my metallurgist the worst case temperature that the brackets would reach on the race track (hard data accumulated from actual track testing), he suggested using 6061 and simply beefing up the bracket and lab testing again (his was the second professional recommendation I received to do that). And he was in charge of Kaiser’s 7068 project. He really wanted me to use the 7068 initially, until I was able to give him the data on maximum temps. That is (was?) Kaiser’s strongest, newest aluminum alloy: 100,700 psi Ultimate Tensile and 93,600 psi Tensile Yield. He had even offered to get me two 12’ bars to perform sample runs, until he determined that the alloy was not appropriate for my application. Of course, they manufacture 2024, 7075, and other high-strength alloys as well, but he recommended against their use also, as their initial strength and remaining strength after fatigue life cycle testing were both inferior to 7068. The 7068 also has a greater % property loss from a high heat environment than 6061. During the months I was researching alloys, the price of the 7068 kept escalating, but before strength concerns arose, I was committed to use it even when it cost about 15% more than 7075 (and more than 2024).

Then there is the problem of corrosion. 2024 is far inferior to 6061 in corrosion resistance. And even more so in the T3 temper. The 2024 needs protection from the environment. Powder coating will provide protection as long as it is not violated. When you torque down the 14mm bolts to 120 – 140 lbs/ft, there is a good chance that the powder coat will crack at the bolt or washer edge. Then you have no protection for the corrosion prone 2024, and even worse, this is an area of high stress.

I choose clear HARD COAT anodizing after doing research and getting professional advice on the subject. The anodize is not just an exterior coating. It penetrates as deeply under (into) the metal surface as it builds up thickness on the top of the surface. It further increases abrasion resistance and measurably increases surface hardness. And in any case, 6061 is naturally very corrosion resistant, even without anodizing. 6061 is more environmentally stable (corrosion resistant), more temperature stable (weakens less from heat), and long term stress strength stable than 2024. In the correct environment and application, virgin 2024 is obviously stronger, but we can never be sure of how many years someone may leave these brackets on their car, or how hard they will be used?