How to MIG Sheet Metal for a Perfect Weld
May 6, 2021
MIG welding sheet metal can be harder than TIG, thanks largely to the fact that there’s more heat involved in the process. The number one thing you’re trying to do is minimise the heat input to your metal so that it doesn’t warp or blow out.
The key to MIG welding sheet metal is to run it on the same settings as though you were doing a vertical up weld. However, don’t forget that if your machine is running too cold to properly melt your filler and penetrate the metal, you’re going to have to spend extra time in each spot, which adds more heat. The trick to sheet metal is finding the right balance.
Filler
You can do a couple of things to combat this extra heat, the first of which is picking the right wire size. In general, don’t use a wire that is thicker than your sheet metal.
If you’re welding 1mm thick steel sheet metal, then you’ll want to use an 0.6mm wire for the best results, although 0.8mm would also work. The thinner your wire, the less heat you’ll need to melt it, and so the less heat you’ll put into the sheet as you work.
For the same reason (keeping your heat level down), you don’t want to use flux-cored MIG wire, as it requires more heat and more post-weld work.
Gap or No Gap?
There are two ways that you can fit up your metal: with a gap or without one. There are pros and cons to both ways, but if your metal is thinner than 3mm, you won’t need a gap.
Gap
When fitting your sheet metal together, you can leave a small (like 1mm small) gap between the two pieces.
When you go to weld, you’ll fill this gap with the filler wire, which allows for better penetration as there’s space for the wire to fill (which can help as MIG often deposits more metal).
The problem with leaving a gap, especially if you’re still learning, is that it can be difficult to properly fill. Plus, if you get your machine settings wrong and do blow through the metal, then you’ve got an even bigger hole to fill back in.
If you work with a gap between your metal, a good technique you can try is to start with your torch at a steep angle (around 45°, nozzle resting on the plate). Angling the torch means you can see what you’re doing.
As soon as it’s puddled, pull the torch up, back to 90° (vertical). The entire movement should occur over the span of 1-2 seconds, so you don’t blow through.
You’re still making a tack weld, but doing it this way basically ‘leads’ the puddle from one edge to the other, giving you full penetration. It also leaves a relatively flat bead, so you’ll have less grinding to do later on.
No Gap
If you don’t leave a gap and press the two pieces directly together, then you’ll need to run a bit hotter than if you were leaving a gap. The reason is, with no gap, you’re trying to melt the two pieces directly together, so you need them both to have melted. No gap does mean you can achieve flatter welds if you have your machine set to the exact settings.
Settings
When you’re working with sheet metal, the best thing you can do is get some extra metal that’s the same thickness and play around with the settings (and practice your speed). That way, you’re not trying to figure it out on the main piece, and you can tweak them as needed.
If you’re working on a car panel, finding out you’re running too hot and blowing a hole through the actual piece is not how you want to learn.
Grab some spare metal, and make sure that your machine is running hot enough to form a puddle and penetrate properly before you start on the real thing.
If your weld bead is crowned and you’re not getting full penetration, you can turn the volts up and/or wire speed down. You want the weld as flat as possible, so minimal grinding is needed at the end.
Making the Weld
Once you’ve got your machine all set up, it’s time to weld. The two most common techniques are a spot/tack weld and a stitch weld.
To do a spot weld, hold your torch over the joint, press the trigger for 1-2 seconds until you can see a puddle form. As soon as you see this puddle, you can release the trigger. That’s your spot (or tack) weld done. It’s exactly like if you were making a tack on a normal weld joint to hold the two pieces together.
To weld a full sheet, you make a series of spot welds, with each new spot overlapping the previous weld by about half. Welding with the overlap ensures you don’t leave any gaps.
When making a series of tack/spot welds, you only want to do about 3 or 4 overlapping tacks in one spot, as any more and you risk burning through the now heated plate. Then move to the other end of your weld and make another series of overlapping spot welds.
A stitch weld is a continuous weld that is short, only 2cm-3cm long. Rather than having a series of overlapping tacks, you’d instead have a small run. If you’re going to be stitch welding, make extra sure that your settings are spot on.
Like with tack welding, you don’t want to make your stitch welds directly next to each other. Start at one end and swap to the other so the metal doesn’t overheat in one spot.
The idea is to move around the plate as you go, to evenly distribute the heat and stop it from warping. Don’t go back to a previously welded area until it’s cooled down. You’ll know it’s ready to be welded on again if you can touch it with your bare hand.
Welding sheet metal can take a lot of time (and patience), especially if you’ve got a long panel to weld, but if you rush through it and try to run a long bead on it, you’re just going to end up with problems that’ll need fixing which will take even longer, so take your time.
Quick tip: keep an eye on your MIG wire stickout each time you pull away, as you want to trim it whenever a ball forms on the end. The ball that forms is thicker than the wire, which means you’ll need more heat to melt it in. If it doesn’t cause you to blow through your sheet, you’ll have to grind the excess metal off later anyway, so it’s best to just trim it.
A good way to keep from accidentally blowing through is to insert a copper backing plate behind the joint (one that you can remove!). The copper helps to absorb the heat being pumped into the metal, and because it’s a dissimilar metal, the weld won’t stick, so you can pull it out when you’re done.
Post Weld
When you’re working with sheet metal, especially if it’s for car panels, there’s usually going to be some post weld work, especially when you’re MIG welding. You need to make the weld flush with the base metal so that when you paint it, there aren’t any obvious areas that stick out.
Because you’ll be grinding the top part of the weld off, it’s super important that you get complete penetration. If your weld hasn’t gone all the way through to the back, when you grind down and smooth out the top of your weld, you’re losing most of it.
The other thing to keep in mind, and why you’re trying really hard to make your weld sit flat in the first place, is that you have to grind it flush once you’re done. The flatter your final weld, the less grinding you’ll have to do after.