Dan-Thanks for following that up. Congradulations on some successful elk hunts. Here is a little more info to chew on when you are sitting in blind or glassing a canyon. I am always learning...makes me better at whatever I'm doing.
7075 is an aluminum alloy with zinc as the primary alloying element. It also contains magmesium and copper. The addition of these alloying elements is what makes 7075 respond to heat- treatment.
A little rabbit trail to help understand the properties of materials.
If you were to pull a sample of any material (tension), it would resist movement. As more and more force is applied, the material would eventually begin to elongate and neck down in the center of the sample. Then at some point, the sample would snap in half. The peak amount of force exerted on the sample divided by the cross-sectional area of the sample would be the ultimate tensile strength (uts). If the force is measured in pounds (p) and the cross-sectional area is expressed in sguare inches (si), the uts would be psi, and if the force is per 1000 pounds (k), then the uts would be ksi.,(ex. 63.8ksi=63,800psi)
Mild steel (ex.alloy 1018) 63.8ksi.
Pure aluminum (ex. 1100) 13ksi.
6061-0 18ksi.
6061-T6 45ksi.
7075-0 33ksi.
7075-T6 78-83ksi.
The "O" designation refers to the annealed state. The material is heated below the melting point but hot enough for everything to be uniformly distributed throughout the material. Then it is cooled slowly. The result is a relatively soft material that is "relaxed". A piece of bailing wire or copper electrical wire are examples of something in an annealed state.
If a material responds to work-hardening, then it will get stronger the more you stress it. Bending, hammering, or rolling will cause some materials to get stonger. Take a piece of copper wire and bend it, then try to straighten it back out where you just bent it. The bend will be harder then the unbent section. If you were to heat the bend with a torch, it would become soft again.
If a material responds to heat-treatment, then it will gain strength through a process of heating, quenching, reheating, and cooling. What is causing the increase in strength are internal changes in the microstructure and composition of the material. The annealed material is heated to a predetermined temperature and allowed to stay at that temperature until everything is uniform internally. This can be done in a furnace or salt that has been heated until it melts to a predetermined temperature. Then the material is quenched to remove the heat and "freeze" everything in place. Water, oil, or even air can cool a particular material fast enough to get the desired properties. At this point the material is very hard, but brittle. An example of this would be a metal file. If you hit a file hard on a table, it will chip or snap in half.
Then the quenched material is then reheated to a predetermined temperature, (tempering temperature). The material responds to the temperature and the result will be a material that is slightly softer but much tougher. An example of this would be a spring, knife, or lawn mower blade. The hardness of the material will be directly proportional to the strength of the material. The hardness is tested by pressing a hard tool into the surface of the test material with a uniform amount of weight. The depth or size of the indentation is measured and that corresponds to a hardness value. That is why sometimes if you look closely at a knife blade or a heat treated aluminum part you may see a tiny indentation left from a hardness test. That is done to verify that the material meets specs.
In aluminum alloys, an "H" designation indicates work/strain hardening and a "T" designation indicates heat treatment.
Enough of the metallurgy lecture. Just some background information to help understand metals and expand your knowledge of the process. The important thing to realize is that pure aluminum is very soft and weak, (ex. aluminum wire). But when alloyed with other elements, the strength can be greatly increased through work hardening or heat treatment. 6061 can be heat-treated to a high strength. 7075 can also be heat-treated to a high strength that is almost double that of 6061 and is approaching or exceeding that of mild steel. 7075 is also more expensive then 6061 and the heat-treating process adds extra steps but is necessary to achieve optimum strength.
Drilling a hole through anything will "weaken"
it to a degree, but sometimes the loss of strength still exceeds the minimum requirements for good performance.
As you correctly point out, the true test is in the field. You have obviously done your homework on your material selection. The only thing stronger while still maintaining the light weight would be titanium. If it proves that your new design can perform reliably every time even when conditions are less then ideal and the result is more accurate shot placement and more animals recovered then it is all worth it. After all, what good is technology unless it can help me be a better hunter or catch more fish! I wish you all the best for future success with your new broadhead.
