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Old 06-17-2017, 07:22 PM   #11
Hanaldo

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What adhesive are you using Zebra? Carbon and aluminium is easy, but you need to be using a suitable adhesive. Generally epoxies work the best, so something like Huntsman Araldite 2015 will work perfectly assuming your prep is good, you'll break the laminate before you break the bond. Whereas a PU adhesive like Araldite 2018 wouldn't be very good at all, even with proper prep. There will be lots of good adhesives, but I know the Huntsman range quite well and use them myself, so it's just a matter of looking at the data sheets and application guides.

It sort of sounds like you are trying to bond the ali using regular laminating resin, which while it can work, it isn't really ideal. A good bond needs a good bond gap, which means having an adhesive that stays where you put it. Laminating resin, even higher viscosity ones, are simply too thin and will likely leave the bond area on the dry side unless you have a way to lock it in place, as you mentioned.

Adhesion promoters are largely unnecessary for most applications unless it has been spec'd into the engineering. Unless you're building aircraft for Boeing, you won't need it if you pick the right adhesive.
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Old 06-18-2017, 08:01 AM   #12
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Quote:
Originally Posted by Zebra View Post
Would using C-130 allow a strong bond between a flat aluminum surface and a flat cf part?
I'm intrigued to try C-130 to see if it will solve my issue now.
You must deoxidize the aluminum before applying the pre-treatment and the primer that is included in the AC-130 kit. The aluminum oxide layer is the number one reason why bonds to aluminum fail. As for adhesive, Hanaldo gives great advise, though I believe that laminating resin will work just fine as long as you use something like microspheres for bond line spacing.
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Old 06-18-2017, 08:12 AM   #13
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Yes nothing wrong with thickening up laminating resin with microballoons, fumed silica, calcium carbonate etc. That's essentially all a toughened adhesive is really, so there's definitely nothing wrong with that.

But without any fillers, a laminating resin is just too thin to be reliable.
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Old 06-18-2017, 09:29 AM   #14
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It's epoxies that I have had issues with for bonding to aluminum but I never considered trying one of my thick epoxy resins as a solution. I have a number of thick resins on my shelf so trying one is an easy for me. I guess I avoided using them for bonding because they aren't always the best for achieving a flat surface.

I sand the aluminum immediately before bonding to remove the aluminum oxide layer. I haven't tried a chemical remover but it sounds like a good idea. Sanding hasn't proved to be a reliable method for large flat surfaces.

I have crates of glass micro spheres here from my home theater business so I can try using them with the resin too.
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Old 06-18-2017, 05:49 PM   #15
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Aluminium has a high oxide content (especially if it is anodised) and surface oxide forms almost as soon as you have finished sanding it, so you need to be sure to give it a really good key, and bond it virtually straight away after cleaning with acetone. Realistically unless you are chemically preparing it, you will always have reduced lap shear strength due to the oxide layer, it just depends on whether the load experienced is actually enough to make that an issue. In most cases the bond is still strong enough.

In the case that it isn't, chemical etching using some form of strong acid like sulfuric will sort it out. A simpler, less effective, solution is citric acid. Citric acid will also etch aluminium to a degree, but obviously not as well as a stronger acid.
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Old 06-19-2017, 12:30 PM   #16
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No other mechanical fasteners involved to stop the alloy rotating BTW, and if you have issue with oxidisation, abrade through the resin!


I disagree with the statement that toughened epoxies are just those mixed with normal fillers.

This is wrong, a toughened system is modified in such a way that its properties and resistance to fracture and impact are increased significantly

Mixing powders gives a modification in the properties, but none that really enhance the toughness.

Adding silica will make the resin even more glass like, very brittle! Not good in combination with any flexible metal

As a starting point try Araldite 420 a/b.

Araldite 420 AB.pdf
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Old 06-19-2017, 01:59 PM   #17
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Zebra - Just a thought... Are you post curing before the previous tests you have tried?

If so, what is/are the cure cycle/s.

Thanks.
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Old 06-19-2017, 08:26 PM   #18
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Quote:
Originally Posted by Astute View Post
No other mechanical fasteners involved to stop the alloy rotating BTW, and if you have issue with oxidisation, abrade through the resin!


I disagree with the statement that toughened epoxies are just those mixed with normal fillers.

This is wrong, a toughened system is modified in such a way that its properties and resistance to fracture and impact are increased significantly

Mixing powders gives a modification in the properties, but none that really enhance the toughness.

Adding silica will make the resin even more glass like, very brittle! Not good in combination with any flexible metal

As a starting point try Araldite 420 a/b.

Attachment 3894
Sorry, that wasn't what I meant by that comment. They certainly aren't the same thing by any means.

What I meant is that a toughened adhesive is a modified epoxy designed to have certain properties, one of which being a viscosity suitable for maintaining ideal bond gaps. If you have a base epoxy resin that has all the properties you need but isn't viscous enough, then you can certainly just modify it with fillers to make it more suitable. Even if you need to give it more strength, there are fillers available that will do that too.

But no, an epoxy laminating resin with some filler powders in it is not the same thing as a manufactured toughened adhesive.
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Old 06-19-2017, 11:46 PM   #19
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No worries Hanaldo...

Zebra -

I see you are in the U.S, so sourcing a Loctite product will possibly be easier.

This would also be a good starting point:

Loctite EA 9330

LOCTITE EA 9330 AERO-EN.pdf

If you take a look at the data sheet, you will see that the results are given for Aluminium, and include the popular preparation methods.

The Araldite and the Loctite are comparable system, with very good bond characteristics to both composite and alloy.

If these do not work, then you could try a methacrylate adhesive such as Plexus.
They have a primer specifically for anodised alloy and it is very tenacious.

If your preparation is meticulous when bonding to composite, you will disbond the first ply it is attached to, before you separate the alloy off the surface.
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Old 06-20-2017, 11:54 AM   #20
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Quote:
Originally Posted by Astute View Post
Zebra - Just a thought... Are you post curing before the previous tests you have tried?

If so, what is/are the cure cycle/s.

Thanks.
I usually do post cure but I have pretty much given up on bonding flat cf parts to a flat aluminum surface until this thread. The results I was seeing were not just a low strength bond. It was more like no bond at all. I could easily peel the CF off the aluminum as if it was silicone if there was no mechanical lock.

I have had much better results adding aluminum inserts to cf tubes by wrapping the inserts in cf yarn or some other mechanical lock.

I have a little more hope now that a flat to flat bond is possible.
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