Adhesive experts: why did this joint fail?

[BillEpstein]

JBWeld used to bond brass plate to exterior of Hammond aluminum box. After more than 3 months the vibration from drilling a thru hole caused the plate to come away clean. All the JBWeld adhered to the brass, none to the aluminum.
Did a coating get in the way? Label says product can repair "aluminum boats". Good thing I can swim!

[SY]

How warm is the circuit in this box?

[vpharris]

I really think these guys are great adhesives experts. And their specialty is epoxy, like J B Weld. http://www.westsystem.com/webpages/u.../index.htm#2.0

This section is on basic techniques, and covers bonding to aluminum. Hopefully it will provide some good hints.

[chipco3434]

The metal must have some "tooth". Rough up the surface to be bonded with 60 grit.

[Peter Daniel]

Originally posted by BillEpstein
After more than 3 months the vibration from drilling a thru hole caused the plate to come away clean.

Does that mean that you were drilling a hole 3 months after bonding and the plates came apart in a process?

[BillEpstein]

Originally posted by Peter Daniel


Does that mean that you were drilling a hole 3 months after bonding and the plates came apart in a process?

Correct.
And after reading the West Systems info, thanx to VPHarris and all who reply, I see that "tooth" is even more important in metal than wood. In fact, the West people don't spell it out, but their recommendation to :
" Aluminum-Sand and prepare with 860 Aluminum Etch Kit.
Polyester (fiberglass)-Clean contamination with a silicone and wax remover such as DuPont Prep-Sol(TM) 3919S. Sand with 80-grit paper to a dull finish"
implies that Aluminum has unique properties that must be dealt with.
I think I'm pretty knowledgeable about wood but ignorant about metal. Thanks again to this board where I can come to get technical facts and not mere opinions.

[poobah]

I am no chemist... but you can think of it this way; the less a material is subject to corrosion or rot, the more troublesome it may be to glue. There are of course a zillion exceptions. I won't risk screwing up an explanation, but google on "surface energy". It explains why thngs like teflon and aluminum don't form good "chemical" bonds.

Providing tooth for these "noble" materials becomes the logical thing to do.

Always use 2 clean white cotton rags when preparing for glue or paint... one to wet and wash... the other to dry and soak up the liquid. One rag with solvent (and no rag to soak up) really is just clever method of ensuring whatever incidental gunk you have on the surface becomes evenly distributed.

[Peter Daniel]

You also need to take into account that aluminum you were using is most likely anodized, and this is not the same as raw metal.

Creating "tooth" won't help much. The two parts fell apart not because of vibrations created by drilling a through hole, but because when drilling, a surface of metal around the hole raises and that acts as a wedge pushing the two parts to separate (you can see it in a bottom pic).

In one of my amps I'm using an aluminum-copper sandwich panels and I experienced that problem as well.

I also tested a variety of structural adhesives, like Fastweld 10 from Ciba, 3M Scotch-Weld 2216, even Lepage 12 Minute Epoxy, none of them were strong enough. Roughing a surface was not much help.

I also tested some other adhesive, readily available from Home Depot, and apparently, PL Premium was stronger than 2 parts Epoxies, but the best seemed to be an Amazing Goop (plumbing type). While none of the previously tested glues created a bond with anodizing surface, the Goop was the only one that sticks to it.

Other types: Crazy glue, Gorilla glue, Bull glue, none of them worked well either

I'm presently drilling copper first, then bond it to aluminum and drill through aluminum. This creates much less stress, especially when the holes are opened to final size in two steps.

I would be also interested in some advice on better adhesives (anodizined surface is a must though)

[chipco3434]

Here's a good adhesive that will glue almost anything together...

3m 5200. It's a urethane that I'm sure our West System advocate is familiar with... SOP for bedding hardware in marine structures which includes typically aluminum~stainless~wood~FRP. It will handle any combinations of these materials.

We call it "monkey poo'. If you get it on your hands it takes about two weeks to wear off. Downside is a very long curing time. 24 hour set, a week for strength, but you will have to hammer it off.

[dnsey]

The surface of bare aluminium is in fact a microscopic layer of aluminium oxide, unless it's just been abraded. It's notoriously difficult stuff to get paint to adhere to, and the usual solution is to use an etching primer. Perhaps such a primer would allow your adhesive to get a grip.