Spotweld vs adhesive on galv steel

[griffengm]

I have an assembly of galvanized sheet steel and the option of joining it with an adhesive or spotwelds. If adhesive, we eliminate a caulking operation that we need if we use spot welds.
The big guestion is service life of the adhesive compared to the spotwelds. Adhesive manufacturer has done environmental testing to 1000 hours but we have concerns as to longer term durability in outdoor use.
Anyone have or know of any comparative information beyond 1000 hour testing?

Griffy

 

[MikeHalloran]

Freshly galvanized steel doesn't take paint very well, so I wouldn't expect it to behave any better with adhesive.

If you spotweld it, are you melting the steel, or the zinc?

IMHO, you have apparently been tasked with selecting between two of the worst possible joining methods for the given situation. It may actually be a test, in which case the correct answer is "none of the above".

There are literally thousands of mechanical joining methods for galvanized, including lockseams and locking|separable dimples and variations, metal stitching, and solder. Most of them are faster and stronger than adhesive or spotwelds.

Choose ANYTHING but the choices given.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

Loctite makes a primer specifically for zinc plated or galvanized surfaces.

I was reviewing the TDS in relation to a vendor proposal for which it clearly was not suitable, so I didn't pay that much attention. Sorry that I don't recall the product number.

Give Loctite a call, if they have the primer they probably have an adhesive to go with that might work for you.

 

[CoryPad]

Is this Hot Dipped Galvanized steel? What is the thickness? If it is very thick, then Mike is correct, it won't spot weld well. However, thin galvanizing and galvanneal coatings do have acceptable spot weld performance.

With proper cleaning, an adhesive can be used to produce high joint strength. In fact, you may be limited by the zinc-to-steel adhesion strength.

Try Loctite and 3M for adhesives.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[griffengm]

The adhesive is a Loctite product. As to spotwelding galvanized steel, we've done it for years with reasonable success. The issue here is whether the adhesive joint will standup as well or better than the spotweld.
Mike,
You may be correct about Better options but there are practical reasons having to do with the application for the decision.
As I said in my initial post, the adhesive would eliminate a costly secondary step if it works. Of the options you list, only solder comes close the the continuous seal required. That said, I may just check it out as well.

Griffy

 

[MikeHalloran]

Loctite's stuff works as advertised, if your process stays under control. Still, risking your business on someone else's short term test is ... probably outside the scope of your authority, when the peanut butter hits the fan.

While you're evaluating options, look at the mechanical lockseam technology used on steel cans (e.g. for paint and for food), and also the associated plastic coatings that have mostly replaced tin in 'tinplate'.



Mike Halloran
Pembroke Pines, FL, USA

 

[attilon]

Hi,

You can try Sika Force 7780 (bi-component structural adhesive). This is what I use for our asian client for bonding the zinc coated sidewall plates into the sidewall frame for some goods transport wagons.
Before that we used electro-rivetting (this is a MIG-MAG kind of spotwelding applied with a temporizing device). At this second solution the weld is as good as the filler metal characteristics can be, but more problems will rise because of corrosion.
The adhesive solution has much lower stress resisting characteristics, but in my case it only fixes the zinc coated plates, as the frame takes over the stresses, so in my case I don't have any problems.
The main problem is if there are any shear stresses, vibrations, tensile stresses or your plate works under compression, and so on....
Generally todays best adhesives have their max tensile stress at 1/10 of the normal steels ( for example S235J2G3), so what you choose very much depends on your application. I was lucky, I could use both.