Aluminum Alloys

[ZACH P.]

What's a good aluminum alloy for use in chassis fabrication? Obviously it would need to be light, strong, etc. I want the best. Help me out.

[Cory]

I think that most teams use 6061. It is easily weldable, and strong. I believe 7075 is much harder to weld than 6061, but I may be wrong.

Cory

[Paul Copioli]

6061 Aluminum is a really common blend that has good weldability characteristics. The -T6 nomenclature is the temper of the aluminum. -T6 is a solution heat treat (I.E. put in a hot liquid) that, without going into too much detail, adds yield strength to the 6061 aluminum. Our team uses only 6061-T6 aluminum for our aluminum parts.

-Paul

[ZACH P.]

Thanks a lot guys.

[Ken Patton]

You could also use 2024-T4. We pretty much interchangeably use 2024, 6061, 7075 depending on whats readily available and what shapes are available.

The 7075 makes for a nice surface finish if you are going to go crazy polishing it.

If you are going to be bending thin sheets of it into different shapes, something more bendable like 3003 series is better - it won't crack as easily as the others.

If you are going to weld it, make sure you periodically check the highly-stressed areas of the structure because they will probably crack.

Ken

[Elgin Clock]

Is chro-molloy (sp) an aluminum alloy??

We have used that for the past couple of years for various things, but just this past year for our frame. It was a tubular frame, kind of like a dune buggy!!

[Paul Copioli]

Elgin,

Chrome Moly is a slang term for for an Alloy steel class called "Chromium-Molybdenum Steels" Those steels are the AISI-SAE 41XX class of steels. Bike manufacturers use the term chrome-Moly all the time. 4140 is a common form of a chrome-moly steel and we use it in the industrial robotics world for hardened cam follower ways. 4130 and 4150 are other common Chrome-Moly Steels. So, to answer your question directly: No, chrome-moly is not an aluminum alloy.


-Paul

[Ken Patton]

Chrome moly tubing is a great high strength material for tubular steel frames. Great for race cars. Usually overkill for FIRST robots, but if you had a very-highly-stressed component it would be a good choice.

Some generic strength values for aluminum alloys (all values tensile yield strength):
2024-T4 aluminum 47,000 psi
6061-T6 aluminum 40,000 psi
7075-T6 aluminum 73,000 psi
3003-H14 aluminum 21,000 psi

(source: Smith, W.F., Sturcture and Properties of Engineering Alloys, text book for Michigan Tech Univ materials science class MY471 in the mid-80's)

Ken

[Gary Dillard]

Be careful about welded 6061 for chassis construction - the strength in the heat affected zone (within about 1 inch of the weld) reduces to about 14 ksi because it loses it's temper. 5456 and 5086 are better choices if strength is a concern - they only lose a couple ksi as-welded - but they're a little harder to weld because they're not as ductile.