I completely forgot about this topic.
Albeit late, I'll post his answer anyway:
"...First of all, aluminum and carbon-fiber are used extensively in the interface and power plant.
For the legs and chassis The Alpha Leg is made of mild steel and the final machine will be Chromoly. There are two main reasons I've chosen steel.
1) Fatigue. Aluminum has no lower fatigue limit. This means that parts made of aluminum *will* break, it's just a matter of when. Yes, you can push that out to "100 years" but weight savings will diminish.
2) Repairability and ability to modify. Aluminum needs to be heat treated after welding to reach max strength. This is an expensive process that requires putting the part in an oven for, literally, days. This makes in-field repairs or mods impossible (or extremely risky)
Of course, if I had a million bucks, I'd make the whole thing off titanium."
1) I'm not quite sure what the issue is here, yes aluminum is more susceptible than steel to repeated stress fracturing. But how long are you expecting this robot to walk for 100,000 miles? I thought it was a racing robot, the thing should just barely be holding itself together by its bootstraps. On a more serious note lowering the mass lowers all of the forces involved in that walking motion. This is what engineering is about, applying steel where the forces involved necessitate it and using aluminum where you can.
One of my preferred methods of construction is carbon fiber coated aluminum. You use thinner aluminum tube and layup carbon fiber over the assembled structure. The aluminum dampens out oscillations keeping it from wiggling so bad, while the carbon fiber adds gobs of strength.
2) Yes to maximize the strength of aluminum, annealing is a good idea. But I don't really think they are that close to the safety margins that welding without heat treatment will compromise the integrity of the vehicle. Granted, Over time the vehicle will need to be overhauled to replace things that are getting dodgy. But saying you have to heat treat
every time you weld aluminum in this realm of application is not true in the slightest.
But welding isn't the only way to assemble aluminum. Make the root fixtures out of steel, then thread the inside of the steel tubing, and each end of the aluminum tube; coarse class 2 threads would be the way to go to make it come together easy. Hard anodize the threads if you are going to take them apart frequently, and add some thread locker, and now you have a screw together robot, that if you break a tube you can swap it out in a few minutes and you still can weld the steel together. Everyone wins!
It will take a bit more to get everything fabricated since each thread takes about 20 minutes of lathe time, annodization sounds expensive but you can do it in a plastic bucket in your shop if you are determined. But you should be able to shave off quite a bit of mass overall, and increase repairability.
I would encourage them to look around more more innovative construction methods than triangle frame tubular construction. There are a million ways to assemble that bot, and many of them involve a lot less weight.
