Bonding 303 SS to anodized 6061 Aluminum - Am I on the right track?

[AllAngles]

I'm adhesives dumb, so please bear with me!

I'm working on a next-gen prototype of an automotive/powersports device. It will see lots of vibration, and some thermal variation (maybe up to 300F tops). Functionally, the aluminum part (4" disc 0.050" thick) has become obsolete, but is needed for protection and aesthetics. I no longer have room to use fasteners to attach it to the expensive stainless piece below (4" disc, 0.25" thick). The aluminum piece will have basically zero load on it, but MUST NOT come off.

So, I'm thinking an adhesive bond might be the answer to my dilemna. The only trick is that I need to have the option to be able to - in pretty rare cases - break the bond and re-use the stainless piece. In this unit, the stainless part is very expensive, the AL piece costs almost nothing - but it could be damaged and need to be replaced. Having to replace the whole unit (stainless and AL together) would not be economical.

For production purposes, I would really like to avoid any elaborate surface preparation on the pieces if possible. The aluminum is anodized, but not sealed (so far). If we could just do some kind of dip/dry clean cycle that would be ideal.

I am looking at a LORD 2-part acrylic as they seem to have the strength and work-times we would need. But - can they be disassembled and, with some work, re-assembled? The AL piece will be destroyed during removal, that's fine, I just want to make sure I can get any leftover glue back out of the stainless part if I need to.

Or, am I barking up the wrong tree all together?

Thoughts?

 

[CoryPad]

My thoughts are that your wish to avoid elaborate surface preparation is unrealistic and that beyond hot melt adhesives, reuse is tricky at best. You may be able to use heat to degrade the adhesive enough for reuse - you will need to work with an adhesive source to get a suitable product to meet your needs.

 

[Compositepro]

If the parts do not see oils and high temperature 3M VHB adhesive tape may work for you.

 

[AllAngles]

CoryPad - Just to be clear, when I say reuse, I only mean of the stainless piece - not the adhesive itself. Maybe that's how you understood it, just wanted to be certain.

Compositepro - Interesting thought - tape would certainly make it easier to keep the adhesive where we wanted it.

I will definitely work with an adhesives supplier if we go down this path, just trying to see if this is a viable option at this point. Maybe some testing is in order.

 

[CoryPad]

I knew what you meant. Tape isn't a bad idea.

 

[AllAngles]

Cool - thanks Cory, just making sure.

 

[MikeHalloran]

My experience says that tape of any kind does not meet the 'must not come off' requirement. ... unless it's used to hold the pieces together while some RTV cures.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

Sounds like a tough application.

Both stainless and anodized aluminum are difficult to bond. A good bond with stainless really needs an acid etch to activate the surface. Anodized aluminum needs something too, but I don't remember what.

Assuming that the low bound of your temperature is "ambient", your temperature range is large enough that you need to worry about differential thermal expansion. That means that the adhesive thickness must be sufficient to keep the internal sheer stress within the goop's limits.

Strong adhesives are typically difficult to remove cleanly, and a field re-do would certainly not ever have the same strength as a production part.

Why not just finish the stainless part so that it looks pretty?

 

[AllAngles]

The main reason we can't just make the stainless part pretty is weight. The top of the stainless disc is hogged out on the milling machine so that only about 25% of the surface area remains intact. The bottom of the disc must remain smooth. And yes, ambient would be our lower temperature bound.

How big of a deal is the acid etch process? Can it be localized so that - in our case - only that 25% of the top surface that remains gets etched and the rest of the stainless piece is unaffected?

As far as removal and re-application, this would be something that customers would send the unit to us to re-do. So, we can put in place whatever process is required to break the bond, clean the stainless parts, and re-bond them.

The more I think about it and get feedback, the more I'm thinking that
1) Adhesive is going to be the way to go - it greatly simplifies the manufacturing of this part (drilling and tapping tiny blind holes in stainless is not cool)
2) The most important feature of this adhesive bond is that it's got to stay put
3) If it's not feasible to come up with an adhesive that satisfies #2 and can be removed and cleaned off, then so be it. That is an economic need and I can find other solutions.

Thanks for the feedback so far guys - it is sincerely appreciated.

 

[blakmax]

So you are looking for an adhesive to operate from room temperature to 300F? In my experience 300F is way above the glass transition temperature for even the most high performance aircraft grade adhesives, and certainly well above the capacity of most tapes. Above Tg, the adhesive becomes a soft weak rubbery material which does not transfer much load.
Also, be careful. The anodize for adhesive bonding is not the same anodize on commercially available aluminium stock.
Acid etching is not the only process available for stainless. You could look at the sol-gel process.
Also, if the bond is effective, it will be difficult to separate the parts. You may need to use sodium hydroxide to eat away the aluminium part to recover the stainless steel part.
Then there is the bond design itself. You need to be sure that the adhesive is strong enough at all temperatures, and that you have an adequate overlap at all temperatures. Provided you have the correct properties of the adhesive, these can be calculated. Please note that the lap-shear strength (average shear strength on a 0.5 inch overlap) is NOT the parameter needed to correctly design a joint.

Regards

blakmax

 

[unclesyd]

Depending on your anticipated return rate you may want to use a product from Grip Nail, like the Nameplate Fasteners.
Probably you best bet would would be give them a call with your specific requirements.

http://www.gripnail.com/nameplate.html

 

[Compositepro]

VHB tape will not work at 300 but silicone will. You can use VHB tape to fixture the parts while dabs of RTV silcone cure. RTV cures by reacting with moisture in the air so you want to apply it in dabs with gaps between them to allow it to cure properly. Silicone is relatively easy to remove for repair.
The bond will be flexible and about 0.040" to 0.060" which make it tough and won't crack due to thermal stresses.

 

[AllAngles]

3M's product literature says 300F is within VHB tapes temp range for short term - which they define as minutes/hours. I think my application would definitely fall into that category.

Again, this part has almost no real loading on it, so I'm not concerned with absolute strength - just don't want it to fall off due to vibration.

0.040 is too much thickness to add, otherwise sounds like silicone might work well.

The gripnail products look very interesting - had not seen those before. Dimensionally they are right in the range of what I have to work with, it's worth chatting with them anyway.

 

[DesignerGuy16]

(References from Loctite's catalog for structural bonding)
For Steel, aluminum, stainless steel:

They recommend: E-3OUT, H8000, H3151, 9460, E-60HP

Varying fixture times, but strength should be plenty for your application. Once cured temperature should be a complete non-issue as they're all basically permanent set epoxy. Just remember fixture time does not equal full cure strength. Pick your poison from those. Look up the Loctite adhesives sourcebook.

James Spisich
Design Engineer, CSWP