Working with Aircraft Aluminum tubing?

[rwrogers]

I am rebuilding a Quicksilver MX II and would like to know if TIG welding and Powder Coating affect the strenght of the tubing. Are there any required step, preparation or whatever for TIG Welding this tubing or is is a bad idea?

[Dave Prizio]

Only certain aluminum alloys are weldable at all, so first you will need to determine what alloy you are working with. The most common weldable aluminum tube is 6061. Unfortunately the heat treat is lost when you actually weld the material. For example 6061-T6 with a tensile strength of 42,000 psi goes to 6061-0 with a tensile strength of 18,000 psi. This must be taken into account before you decide to weld, which is why most aluminum tube airframes use mechanical fasteners that will not change the heat treatment of the material.

[cub builder]

Additionally, the heat applied to powder coat aluminum also contributes to aging the aluminum. Aging is a part of the hardening process. Again, 6061 is a prime example. It goes from 6061-T0 to 6061 T-4 through a heating and quenching process. Taking it to T-6 is done via aging, which is a much lower temperature heating process that is in the same range as the heat used in powder coating. It is possible to go past the aging curve and lose the T6 toughness of the 6061 tubing by spending a significant amount of time in the powder coating oven. If you plan to powder coat aluminum, make sure you look up the aging process curve so you know how long it can spend in the oven to melt and cure the powder coating without damaging the strength of the aluminum. If you're not sure, you're better off to prime and paint.

-CubBuilder

[rwrogers]

Thanks for the information, you give me food for thought. Bob.

[WLIU]

If I may ask, what parts are you interested in welding?

This is not the forum to start doing structural analyses, but it appears that a lot of the lighter aircraft connect aluminum tubing using rivets and bolts, which suggests that the actual 6061Tx type of tubing used is not the limiting design factor. Joints that use rivets and bolts require the designer ensure that there is adequate bearing strength between the components of the joint and if you can talk to an engineer familiar with those sorts of structural issues, you may find that a welded connection that is analyzed using the assumption that the material is now 6061T0 is good enough. But you have to do the math.

That said, if the weld in question will be in parts of the aircraft that are not primary structure, welding may be just fine. As an example, I once fabricated the frame for a Cessna door that is opened in flight from lengths of 6061 bent into "hat" shaped channel sections. The lengths of hat sections were formed using a press brake and then cut to length and welded into the outline of the door on a jig. I do not have the paperwork in front of me but I think that 6061T4 as used. In this application the strength of the joints is adequate with the resistance to racking and overall structural stiffness being provided by the "hat" shape of the formed channels and the skin riveted onto those channels. The point here is that you are best served to start with an understanding of the loads involved so that you can make an educated decision about materials and the strength required of the joints.

Best of luck,

Wes
N78PS

[hogheadv2]

Anodizing may be worth looking into, no heat issues and no added weight.

[Matt Gonitzke]

Anodizing may be worth looking into, no heat issues and no added weight.

I'm pretty sure anodizing reduces the fatigue strength, so that may not be wise, either.

[1600vw]

Sounds like one does not want to weld or heat aluminum tubing at all. One should paint and not powder coat or anodize any of this material.

Can any paint and primer be used on this material or is there some that one must avoid?

[1600vw]

I fly a wooden airplane but there are a lot of aluminum pieces and I thought about removing them one by one, or anytime I removed something aluminum go ahead and powder coating all these pieces. Like wing jury struts, rudder control bracket or whatever its called, but now you get the idea of what I wanted to do.
After viewing this thread I will leave these alone. I was not aware that just putting some of this stuff in an oven will change things that much. There are a lot of folks running to harbor freight buying these powder coating kits and powder coating smaller parts. I have a friend whom did just this thing on every small part on the plane he built. It turned out to be a really nice airplane, its built and ready for flight. All small parts are powder coated an this airplane.

[Aaron Novak]

1600,
Do not powdercoat them. Stick with liquid paints, and proven materials. Etch, Alodine, epoxy prime, then topcoat. Powder offers little to no corrosion protection itself, hinders inspection, and the heat cycle is of detriment to some alloys as has been mentioned.

[cub builder]

Alodine is a great way to corrosion proof your aluminum parts without any adverse effects to the base material. It is also a nice prep for primer/paint, although you can do just fine with just the acid etch part of the alodine process if you plan to prime right away. Anodizing and powder coating can both be done safely, but there are caveats you should understand before using these processes. Anodizing is a very hard surface that plates onto the metal. Hard also often times means brittle, so if you are anodizing sheet aluminum, you can create a brittle surface. But anodizing something like a solid spacer isn't going to have any kind of significant adverse impact to the base material and does a nice job of corrosion proofing the base metal by encapsulation.

Heating to melt and set powder coat will artificially age aluminum. However, if you look at the phase diagram for the aluminum you are using, it might be that it won't age it enough to adversely impact the base material. It is ultra-safe to avoid using these processes until you understand possible adverse impact to the base materials, but it doesn't mean they aren't safe to do under the right circumstances, which may well be *most* of the time.

-CubBuilder