Spot Weld Failures in Zinc Galvanized Frame 2

[MadMango]

I'd appreciate a basic education here prior to me contacting our vendor on this subject. We have a 16ga frame made with individual parts that are mild steel and galvanized zinc coated. It is spot welded in the corners, with two spots at ~.30" dia in each corner. We are getting failures upon receiving where some parts have failed welds. Can this be corrected with higher current, better grounding, or larger spot diameters? Is this be caused by the galvanized zinc coating or just poor spot welding?

Thank you in advance.

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[metengr]

Yes, it can be corrected with better control on spot (resistance) welding essential variables provided the weld size is correct for the application, and your material is weldable. This is not an easy solution because multiple factors can work against you.

The galvanizing should be done after spot welding and would have no effect on the weld quality. If the spot welds are sized properly, the failure could be lack of consistent controls on one or more of the following essential variables; current/voltage, electrode projection, control of initial current and clamping force, etc.


Here is information on type of spot weld failures along with technical details-

http://www.me.sc.edu/fs/pdf/STWJ paper published.pdf

 

[metengr]

Forgot to include the second education piece

http://www.millerwelds.com/pdf/Resistance.pdf

 

[MintJulep]

Galvanized steel can be spot welded, but it requires different magic than used for uncoated steel.

 

[MiketheEngineer]

Galvanized steel can be a bitch when welding. Get with vendor and work out details....

 

[metengr]

If you are going to spot weld galvanized sheet, which is not really a good idea, I would locally remove the galvanized surface to be spot welded and after spot welding spray apply a zinc primer repair coating to restore corrosion resistance. Otherwise, spot welding on galvanized sheet can be difficult because molten zinc during welding can penetrate the weld nugget causing cracks.

 

[berkshire]

Mad Mango,
Check out page 10 of the miller handbook Metengr posted for you.
In addition to that, causes of quality problems with spot welding galvanized sheet are: The zinc alloying with the copper tips making the tip grow in diameter. Pressure and heat cycles not set properly.
Ideally on galvanised the heat cycle should be 2 stage, one low heat high pressure to melt the zinc and squeeze it away from the tips, followed by welding heat with enough after squeeze time to ensure the nugget is solid.
Make sure that the operator keeps the tips properly dressed, the growth of the diameter of the tip from zinc alloying is the biggest cause of bad welds on Galvanised sheet metal.
B.E.

 

[MadMango]

Thank you for the great feedback all. It seems that I'll have to talk with our vendor, and if they cannot improve their process, we will have to ask for the entire frame to be galvanized post weld.

"Art without engineering is dreaming; Engineering without art is calculating."

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[CoryPad]

Automobile structures use spot welding to join galvanized steel panels. It does require different welding parameters. Photographs of fractured welds would help to determine the root cause(s).

 

[dhengr]

MadMango:
AWS has some pretty good general info. on spot welding in their “Welding Handbook” series of books. Spot welding galvanized sheet metal is not uncommon as CoryPad mentions, but it does sound like your vendor should clean up their process and equipment to accomplish what you want. Remember that these welds should primarily be loaded in shear in the faying surface of the joint btwn. the sheets. And, in that respect your design should be checked to make sure it meets this most favorable loading orientation. What do these frames/joints look like, how are they loaded, dropped, mishandled; how is that joint loaded which is causing it to fail? Two spot welds may not be enough in that joint, you should be able to come up with a shear force per spot weld as an allowable loading for their process or for your needs. A joint can be made to take a moment, just as a bolted or riveted joint would, but maybe more spot welds or more favorable distance btwn. the welds is required. The spot welds are just not very good in prying or tension loading perpendicular to the faying surface.

 

[MadMango]

Here are a few images of the joint (hope they post for everyone):

Overall view of the frame in question. We are spot welding in all four corners, the frame is about 14"x14"x6".

Front side of welded corner.

Backside of welded corner.

And here is a photo of the actual weld spots. I think the welds look sloppy with the multiple hits, hints to me that the vendor process still needs tweaking.

Our vendor is suggesting we switch to a galvaneal (annealed after galvanic bath) material, which seems to be a 2-step process. I'm thinking it will just be easier/cheaper to weld first, galvanize last. Thoughts? Seems to me our sheet metal vendor is not that proficient with spot welding, and have yet to determine if they are sending this to a 3rd party to finish.

"Art without engineering is dreaming; Engineering without art is calculating."

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[MintJulep]

Don't weld.

http://www.assemblymag.com/articles/83301-in-a-clinch

http://www.btmcorp.com/

http://www.tox-us.com/us/products/joining-systems/clinch-technology.html

http://www.clinchsystems.com/

http://www.boellhoff.com/en/de/assembly_systems/rivclinch_clinching.php

 

[MikeHalloran]

Galvannealed steel sheet is supplied with the galvanizing already applied, in sheet form.
Galvannealed sheet is normally a flat gray color, and free of spangles. ... like the narrower part in the photo.
The larger, bright part in the photo appears to have been electroplated with zinc, not galvanized.
Galvanneal takes paint much better than dip- galvanized steel does.
Forming tools may burnish it a little, leaving a glossy witness mark, but don't typically remove the zinc.
The sheared edges will be clean ungalvanized steel, and the steel itself is annealed, so it may act a little gummy.
The zinc still protects the exposed steel, mostly, depending on the environment.
If it hasn't aged too much in storage, it is solderable.
I have no usable experience with spotwelding galvanneal.

The joint in the photo may be a candidate for rivetless riveting, like this:

http://www.wh-rivet.com/erivetless.htm

I forget the zoomy process name associated with the original patent.







Mike Halloran
Pembroke Pines, FL, USA

 

[dhengr]

MadMango:
The bent tab which gets the two spot welds is a continuation of side “A” (on the left in the photo) and is about 1" x 1", and it is positioned inside of the frame side “B” with the long vert. slot. The frame side “B” which has the long vert. slot also has a narrower and longer vert. bent tab which could be extended and made wider/longer in the area of the welding and be made to fit outside of the frame side “A.” Then a couple spot welds could be made one both sides “A & B” of the joint, which would eliminate any tension on the spot welds. Nice tight bends and an 89-91° angle on the bend on those tabs are important to get good fit-up for the spot welding. Certainly, 4 spot welds, or at least two out near the corner, and one in further (to the right) on the tab would be stronger. But, I think this is mostly a fit-up, clamping pressure, and weld process problem. If their spot welding equipment doesn’t hold the clamping pressure long enough for the weld nugget to solidify you could get some funny weld cracking or spring-back forces on the bent tab.

 

[berkshire]

Mad Mango,
Looking at your picture of the spotwelds, it looks like the operator is not holding down long enough to let the weld cool.
He is also welding over a prior spotweld and damaging the electrode. This may be improper education at your vendor, or guys on piecework trying to make bonus.
Galvaneal is much easier to spotweld if you can stand the reduced thickness of galvanizing. If you are painting over the galvanizing then Galvaneal is the preferred coating.
B.E.

 

[CoryPad]

Galvanneal produces a zinc-iron layer on the surface which improves weldability.

 

[MadMango]

Thanks for the additional info. One of our QA reps is going onsite to the vendor to resolve this. I have armed him with all the above information.

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[MadMango]

Just a follow-up, our rework was to use a blind rivet in each corner. By adding the required holes at fabrication, it is easier for this to be the go forward action and abandon the spot welding.

"Art without engineering is dreaming; Engineering without art is calculating."

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[berkshire]

Mad Mango,
Good now you have pre indexed holes from the CNC press. That should get rid if the mis alignment problems I saw in your spotwelded part.
B.E.