Composite hollow rod to metal joint

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#1

Hi All

I need to join an Aluminium part with a hollow carbon fiber square rod of about 25mm diameter.

It is for arm of a Drone/multirotor/Quadcopter.

Most designs in the hobby industry use clamp type designs.
Some professional companies seem to use rivets (or other types of fasteners.

My concerns:

  1. Clamps may work for round hollow rods but for square/rectangular ones, there is risk of crushing by oversampling

  2. Not sure about rivets , but screws/bolts passing through carbon sheets/rods tend to eat away carbon fiber (the holes in carbon fiber enlarges)

Solution:
Bonding and slight tapering of mating surfaces

If I keep the metal part slight tapered and cure the CF rods directly over the part , such that the taper mechanically locks them against any pulling force, will that work?
About 1-2 degree over a 30mm length of mating area will be enough?
Square rods will also prevent any twisting/slipping

Also I read online that Carbon fiber corrodes aluminium by Galvanic corrosion
How much should I be worried for it? What if Aluminium is already anodized before coming in direct contact with carbon fibers?

#2

Hello,

I have attached a sketch of a quick example of what I think you are implying. Did I assume correctly?

Do not use nuts and bolts. As you previously stated, the nut and bolts will over time loosen and will enlarge the hole. A pull action blind rivet would be ideal in this situation. I recommend some type of fastener in addition to bonding these two faces together. The weight of a few fasteners are negligible.

If you use aluminum with carbon, it must be anodized. You can absolutely bond aluminum to carbon. Aerospace does this all the time. As long as you don’t have a dielectric constantly contacting both surfaces, you should be just fine. With this being a UAV application, I seriously doubt it will be in contact with water at all.

I hope this helps.

#3

Hi NeuHaus,

Thanks for your reply.

I guess your sketch is not what I meant.
Check the image attached for a better idea.(I have modified the design since my last post).
The square CF rod will be inserted into the metal piece and then the small flanges on the sides will be screwed in to tighten the joint mechanically (besides the bonding). (Initially there wasn’t any flange for tightening)

Do you think this will be safe enough?
Rivets is very tempting but that will a form of trial and error for me.
My friend and mentor had previously tried rivets but usually ended up breaking the CF.

Bonding plus bolts and nuts may work out I guess if rivets don’t.

Regarding corrosion, well my workshop (my friend’s garage) is very close to sea and is surrounded by marshy areas. We have a lot of corrosion on parts (his helicopter components , spare nuts and bolts etc). Even my USB cord got rusted over time.

I am guessing anodizing should be enough. Fingers crossed

#4

If you use bonding adhesive, there would be no need for a bolt.

You could also co cure the laminate to the metal no problems. You just need to properly prepare the bonding surface; there are a few methods to do that.

We normally use a single ply of glass to start, as a barrier to prevent galvanic corrosion. There are also coatings that could be used. Not sure if 3M AC130 is for prevention of corrosion, but is one product that I’ve worked with for bond promotion.

http://solutions.3m.com/wps/portal/3M/en_US/GovernmentSolutions/Home/ProductInformation/ProductCatalog/~/3M-Surface-Pre-Treatment-AC-130-2-50-ML-Kit-10-per-Case?N=8699174+3294568463&rt=d

#5

annodizing and a glass bubble filled adhesive should be fine. A layer of glass on top of the carbon would be perfect but I think not really nessecary.
I like the 3M DP 490, but if I am right it is not available in US, another one I use is Adekit a 170. But I think you can mix an adhesive on your owen with bonding adhesive, thoxo, cotton flocks, micro bubbles and black pigments.
You should have enough surface that it should work.

#6

You want to use calibrated beads in a bonding adhesive to maintain proper bond gap. Micro balloons are not for this purpose. You can buy adhesive unmixed and add beads or you can also buy adhesive that has beads premixed in if you know what bond gap you need.

#7

@sammymatik & ddcompound

I am not sure about micro balloons (aren’t they used in foams for a different application)

Is it really necessary to add glass beads or barrier?
On one end I am using my own cured/made part so adding a glass layer should be fine.
On other end I am using already fabricated hollow rods. Not sure if the gelcoat/finish coat on the rod will bond with my glass fiber layer. Tolerances will also be an issue if I try to add layers to it.

I have found a local anodizing service which will guarantee resistance against galvanic corrosion.
I will let everyone know once I start manufacturing