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TIG welding is popular because it can make clean, attractive welds in a variety of metals. You can even use TIG welding to join different types of metal. If TIG has a weakness, it is penetration — it is harder to get good joints on thick material with TIG than other welding processes.
How thick can a TIG welder weld? TIG welders can usually weld one thousandth (0.001) of an inch of mild steel per amp. A 100-amp TIG welder can weld steel up to 0.10 inches thick. A 200-amp welder will weld steel up to 0.20 inches thick. Other metals will have slightly different requirements.
You can get more precise estimates with this online calculator from Miller Electric, but we are going to go over the basics to help you understand those estimates.
Welding Stainless Steel
Stainless steel does not dissipate heat as well as mild steel, so you can go a little thicker. The rule of thumb is that you need about 10% fewer amps to weld stainless than mild steel. That means that the thickest stainless steel you can weld will be about 10% thicker than the biggest piece of mild steel your machine will handle.
Aluminum dissipates heat much better than mild steel, so the thickness of aluminum a particular machine will weld is 20% less than the thickest steel. For a 100-amp welder, you are limited to aluminum up to 0.08 inches thick. For a 200-amp welder, you can go up to 0.16 inches thick.
If you need to weld material thicker than your welder is rated for, you aren’t out of luck. There are techniques that let you weld thicker stock. With a little knowledge, you can weld stock so thick you will need a forklift to move the finished product. There are two problems you have to solve when welding thick material: weld penetration and overheating.
If the weld doesn’t penetrate, the joint will be weak. The key to getting good penetration is moving slowly and letting the heat build up along the joint. Deep penetration heats the metal along the bottom of the joint enough to flow into the bead and make a strong joint. If the bead doesn’t reach all the way to the bottom of the joint, it will be brittle. However, deep penetration leads to overheating.
When the metal along the joint gets too hot, it will warp. The difference in temperature between the joint and the rest of the piece cause the metal to expand unevenly and change shape. When you move slowly to get more penetration, the buildup of heat along the joint leads to warping and an uneven joint surface.
Since you need to balance penetration and warping, you can’t make joints with thick metal the same way you would in thinner stock. There are some tricks that will help you get good welds on thick metal without warping or cracking.
One secret to getting good weld penetration on very thick metal is beveling the joint. Instead of welding two square edges, grind back the corners along both edges of the joint. Rather than welding a flat crack, you are now welding a V-shaped valley. You should also leave a gap about the thickness of your welding rod between the pieces.
With a bevel joint, you make more than one pass to join the pieces. This is called root-and-cap welding. The first pass, called the root bead, fills the gap and the bottom of the V. Once the bottom is full, you make one or more passes along the top of the first bead. This is called the cap. You make cap passes until the bevel is filled with filler, and the joint looks like a flat butt weld.
Using a root-and-cap technique along a beveled edge guarantees that you will get good penetration the full thickness of the metal. Since you are layering multiple beads along different depths of the joint, everything gets welded together, and the joint will hold.
For metal between ½ and ¾ of an inch thick, you can use a double bevel. In a double bevel weld, you bevel the front and back of the weld joint and weld it from both sides. This gives even better penetration of thick material than a single bevel joint. It’s the same process, but you build out from the middle instead of up from the bottom.
To prevent overheating as you weld, backstepping is used. To backstep weld, you work in small sections that flow in a direction opposite of the overall weld. Let’s say your overall weld direction is left to right. In a backstep weld, you would start about an inch in from the left edge. You weld right-to-left from the starting point to the left edge. Your next bead would start an inch to the left of the first, then work back toward it. You keep working back toward the previous weld, like this:
| ?1 ?2 ?3 ?4 ?5 ?6 |
You weld from #1 to the edge, then #2 to the start of weld #1, then #3 to #2 and so on. Working this way keeps the heat from building up too much along the weld, so the metal doesn’t warp.
Another way to keep the heat from warping the work piece is preheating. By getting the entire workpiece hot, you reduce the temperature differences that cause warping. You can use a propane torch or (carefully) use an acetylene torch to preheat the metal. Use an infrared thermometer or a thermo crayon to determine when the metal reaches your desired temperature.
Once the metal is hot throughout, you can start welding. The piece is less likely to warp because the temperature difference is smaller. You will make your weld the same way you always do but be careful about where you rest your arms — the workpiece will burn you if you don’t watch out.
There are some welds where preheating is required. Those include pressure vessels, piping, and structural work. Follow the ASME or American Welding Society regulations for those cases.
Change Shield Gas
Argon is the most commonly used shield gas for TIG welding, but helium is also used at times. One reason to use helium is that it transmits heat better than argon. (The main reason not to use helium is price — it’s more expensive than argon.) Switching your shield gas from pure argon to a mix of helium and argon, or even pure helium, will make your welder work hotter.
Using helium is like buying a few extra amps to add to the welder. Welding pros feel that using pure helium produces an arc that works like one with about thirty more amps. Your results may vary, but you will get more heat to the weld with aluminum.
Skip the TIG
Ultimately, TIG welding is limited in the thickness of the metal you can weld. TIG is a good process for getting pretty welds, but sometimes you need a process that will really push heat deep into the weld. Almost all TIG welders also have the ability to stick weld as well. When you need to join thick pieces of metal, it can be better to switch over to arc welding.
The arc function of home TIG welders can handle metal up to an inch thick if you make multiple passes. That’s far more than a TIG welder will do. If you need to get really thick metal together, arc welding is the way to go.
Put On Your Thinking Cap
The thickness of metal you can TIG weld is limited by the amperage of your welder, but there are ways to extend that capability. Part of the fun of welding is figuring out how to solve new problems and work around the limits of equipment and materials. Ultimately, the thickest metal you can weld isn’t determined as much by your equipment as by your brain. Put on your thinking cap and get to the shop!