Weld over existing spot weld — three layer weld
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  1. #1
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    Weld over existing spot weld — three layer weld

    I've got a question about how to attach a new sheet to an existing pinch weld, where a pair of 22 gauge sheets are spot-welded together. I want to weld a third sheet over the top of this pinch weld, and effecting make it a three-layer pinch weld.
    I need to trim the lower sheet off, and attach a new sheet.

    My two options are:

    (A), trim the lower sheet, and weld the new sheet on top of the existing pinch weld, making sure to align the holes of the new sheet with the existing spot welds, and rosette weld these with my TIG.

    (B) trim the lower sheet a bit longer, and weld the new sheet to the bottom layer. This makes a pair of double layer pinch welds connected by a single sheet.

    I think option (A) is a stronger, but are there any issues with this approach?

    this is an automotive application, so strength is kind of important.

    I know that in industry, three-layer spot welds are pretty common.

    Would there be an issue with weakening or hardening the existing weld if I weld over it?











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  2. #2
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    Re: Weld over existing spot weld — three layer weld

    A concern I see might be popping an existing spot weld since you dont have clamping pressure. If it were me, I would do option A, however off set the new spots to not be on the original pinch welds. I would also drill through what would be the middle sheet when making the new spot welds so you would be welding from the bottom sheet up, if that makes sense.
    -Aaron

  3. #3
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    Re: Weld over existing spot weld — three layer weld

    Quote Originally Posted by makoman1860 View Post
    A concern I see might be popping an existing spot weld since you dont have clamping pressure. If it were me, I would do option A, however off set the new spots to not be on the original pinch welds. I would also drill through what would be the middle sheet when making the new spot welds so you would be welding from the bottom sheet up, if that makes sense.
    -Aaron
    That makes a lot of sense. I do actually have a pretty solid clamp I’m making to cinch the panels together. That’s I think the safer routes to offset the new welds because that way, you wouldn’t have any danger of mucking up existing welds.

    Here’s a panel clamp I’m making, found this C shaped piece of 1” plate.





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  4. #4
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    Re: Weld over existing spot weld — three layer weld

    Since you say automotive + strength, throw out proposed design B, it is weak and adds stress to the existing 2 layer assembly. Probably adds stress fracture downline from vibration and flexing too.

    Design A is good if you don't have rust in the existing assembly. Drawback is in aligning the hole in the green piece to the spotweld in yellow.

    If your plan is to use a spot welder you'll play hell. If you'll be MIG welding you can put holes thru the green piece and MIG to the original welds. I'd probably use a rivet shaver to gain on penetration into the original weld in the yellow assembly. Right size countersink might work well too.

    I wouldn't stagger off from the original welds to minimize stress fracture potential.

  5. #5
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    Re: Weld over existing spot weld — three layer weld

    Quote Originally Posted by Virgil5 View Post
    Since you say automotive + strength, throw out proposed design B, it is weak and adds stress to the existing 2 layer assembly. Probably adds stress fracture downline from vibration and flexing too.

    I wouldn't stagger off from the original welds to minimize stress fracture potential.
    Thanks, I’m planning on drilling holes in at least the new green piece, and TIG welding rosettes to weld them together.

    I could use a spot weld cutter to cut through the middle piece, and rosette weld all the way through to the bottom one.

    A lot of people say to stagger the welds, offset from the original. I think this makes a lot of sense as it avoids the possibility of mucking with the existing weld chemistry, and potentially embrittling it or something.




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  6. #6
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    Re: Weld over existing spot weld — three layer weld

    Quote Originally Posted by Andy Somogyi View Post
    Thanks, I’m planning on drilling holes in at least the new green piece, and TIG welding rosettes to weld them together.

    I could use a spot weld cutter to cut through the middle piece, and rosette weld all the way through to the bottom one.

    A lot of people say to stagger the welds, offset from the original. I think this makes a lot of sense as it avoids the possibility of mucking with the existing weld chemistry, and potentially embrittling it or something.




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    2 ways of looking at it when vibration and automotive are invilved, probably more than 2 if beer gets into the discussion.

    My view is the spotwelds need to be monolithic and you ain't going to embrittle anything by melting and resolidifying. You also minimize stress potential between the layers.

  7. #7
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    Re: Weld over existing spot weld — three layer weld

    For those of you who said to use option (A), and offset the new welds, you're right!

    Here's testing that shows triple layer spot welds with offset welds are stronger.

    However spot welds placed on top of existing welds are weak and lead to metal tearing.

    https://i-car.co.nz/wp-content/uploa...anFeb-2010.pdf


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  8. #8
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    Re: Weld over existing spot weld — three layer weld

    Hey even a broken clock is right twice a day

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