carbon fiber and aluminium

[Bizcocho]

Hi again!

I would need some recommendations for a problem that I have with an aluminium frame that has to be attached to a carbon fiber laminate wall. I have realised that three main problems may occur: galvanic corrosion of the frame, high stress on materials due to thermal properties mismatch and risk that the adhesive bond will not have adequate strength.
I still have to use those materials together. Could somebody give me some hints about how to minimize the risk of failure? (best bonds, aluminium sealing..?)

Thanks in advance!

 

[ewh]

One thought... Use a ply of glass between the carbon and aluminum to limit galvanic corrosion.

 

[samplesecond]

...another benefit of using fiberglass as an intermediary ply is that it has a CTE rougly in between that of carbon and aluminum-which will reduce internal stresses in the assembly during temperature variation when compared to the direct bond of aluminum/carbon.

 

[Compositepro]

Use an elastic silicone adhesive, and a thick bond line (0.05" to 0.1"). This will allow both materials to move, within limits, and electrically insulate them from each other.

 

[Joshmo]

Etch and prime the aluminum for repeatable bond strength. Typical acid etching works with a BR-127 primer. Additionally, another benefit of this method is that the aluminum part can be treated then stored for use versus sand blasting or hand abrading which requires bonding within 8 hours. Regarding adhesive, my personal favorite is EA 9394 which is an aluminum filled two part adhesive from Hysol or Henkel with a room temp tensile lap shear strength of 3 ksi.

 

[Compositepro]

If you are building an airplane and need to carry primary structural loads across the bond then you should use BR-127 primer. Otherwise don't even consider it. It is a chromated primer which is probably prohibited in most non-aerospace applications and needs to be applied at 0.2 mils thick. There is a big learning curve to using it correctly. And Cytec probably wouldn't even sell you any.

It sounds to me like you have a panel that is carbon fiber mainly for decorative reasons and the aluminum frame carries most of the loads. The question that you need to answer is what loads do you need to carry.

 

[Bizcocho]

Hi all,

The application of this part will be in the astronomic field, for an instrument used in huge telescopes. It is obviously far from the tight requirements of the airplane's components. Both carbon fiber wall and the aluminium frame have to be properly attached together to avoid the bucking of the wall that otherwise would occur. The external loads and the stresses aren't high.
My concerns are:
- avoid corrosion, which you have already answer me how to do it.
- limit the stresses that could appear between carbon and aluminium because of different CTEs. I have noted it can be solved with a ply of fiberglass, an elastic silicone adhesive or the EA9394 adhesive. My idea is that the stresses will not be too high for either aluminium or carbon but could be too much for the bonding. Would a bolted solution be more appropriate in this case?

thank you very much for your responses!
Bizcocho

 

[MechanicalChef]

Fasteners will certainly make your job easier. Depending on how serviceable this part will be once it's installed, another option is to add a replaceable part in case corrosion exceeds the isolation potential of the glass layer.

Example: The 777 uses a composite C-channel floor-beam. Connecting the aircraft frame to the floor-beam is an aluminum plate which is easily repleacable if galvanic corrosion begins eating the aluminum. With a replaceable part and a glass isolation layer, I think that might take care of this issue.

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Regarding the CTE, what type of thermal gradient do you expect the part to experience in service? Also, is aluminum a solid requirement? Carbon steel has a CTE half that of Al. Around here, for our production composite tooling, we use Invar. Very low CTE. However, this comes at the expense and cost and weight.

 

[Joshmo]

Can you use fasteners with slots in the direction of thermal growth? If the load direction isn't in the same direction as the slots this could be a viable solution.

 

[Bizcocho]

Hi,

I expect to have a gradient about 50K maximum. The aluminium is a solid requirement mostly because the parts are already manufactured and the time is too short to change the material, unfortunately.
I will study the use of fasteners, thank you very much

 

[Joshmo]

This is off topic from your questions but some applications of composites used in optical structures require low outgassing resins to avoid organics depositing on lenses or mirrors during the structures service life. This might not be a requirement but something I have encountered before but mostly for space rather than teresterial structures. We used mostly cyanite ester resins to meet outgassing requiremnts.

 

[RPstress]

Dare I suggest an analysis to quantify the part's stresses in service? From the sound of it your main loading is a temperature change. Analysing bond lines can have a few subtleties, but it's not that hard.

 

[PanelGuy]

Huntsman's Araldite 2040 is a two part Urethane used for bonding differing materials. The cool part is that it has a 400% elongation at failure. Combine this adhesive with the 0.050-.100 bondline suggested by CompositePro and you will be most of the way there...The bondline will provide insulation.

Now then... I would recommend a 20 minute clear anodize (unsealed) on the aluminum...consistent surface and bond enhancing. Do not let them seal it, just etch and convert.

As well...consider that that galvanic corrosion requires opposed charges in materials whilst connected with a salt solution for conductivity...If you have no salts present and/or seal the materials to the environment it should not be a problem. I am assuming you are not in the sea and most likely sheltered...

Let us know how it goes.

 

[entx]

I read though this post and I have the same question...I see three main suggestions...

1. use fiberglass as an insulator between the CF and Al. Is there any requirements on the FG fabric, density or anything...could this just be a veil coat(1 oz fabric)

2. use an adhesive to bond the AL to act as bond and insulator

3. Anodize the part, unsealed...I'm unfamiliar with the 20 min anodize...typically a non-hard coat anodize is specified by thickness typically .0002" thick.

Is there a galvanic chart that include composite fabrics with the typical chart of metal?

 

[Higgler]

Regarding;
"The application of this part will be in the astronomic field, for an instrument used in huge telescopes."

Is this an optical telescope or antenna based radio telescope? Just curious.

If it's RF, they might have a "don't use dissimilar metals" requirement to minimize self generated rf signals (intermodulation distortion).

kch

 

[Bizcocho]

It is an optical telescope, but anyway the part belongs to an instrument placed on the telescope focus. Several instruments can be mounted on the same telescope.

The part has already been manufactured and everything has gone well until now. I have used a veil fiberglass coating between the carbon fiber and the alluminium frames. I think a adhesive bond layer will isolate as well. For the screws I have used an anti-corrosion spray, Lanocote, on the metallic surfaces.

Bizcocho