Porosity can be a problem in any kind of welding, but it is particularly a problem with TIG welds. This is because TIG welds are favored for work where the weld bead needs to be pretty, and TIG rods don’t contain flux that cleans the metal and fights porosity.
What are the causes of Porous TIG Welds? The top cause of porosity in TIG welds is contamination. There are only four elements to TIG welding that can allow contamination: the electrode, the rod, the shield gas, and the metal itself. However, there are lots of ways contamination can get in through these four elements.
Contamination is not the only way TIG Welds become porous. We’ve compiled a list of seventeen different causes of pores in TIG welds:
- Paint on the metal
- Grease or dirt on the metal
- Rust and mill scale on steel
- Ground-in contaminants in scratches or gouges
- Aluminum oxide
- Zinc coating on galvanized metal
- Contamination inside a pipe working up through the bead
- Too little shielding gas
- Too much shielding gas causing turbulence
- Wind or drafts
- Contamination in the shield gas
- Problems with the gas cup
- Contaminated electrode
- Dirty filler rods
- Amperage set too high, causing tungsten to blow off the rod
- Poor torch angle
- Pressure inside a pipe blowing holes in the weld
The metal for the weld itself is the most common cause of contamination that leads to porous welds. Unlike other types of welding, TIG doesn’t use any flux that can protect the weld from contaminated metal. That means all the metal has to be spotlessly clean before you crack an arc. If the metal isn’t shiny, you won’t get a good weld.
There are several different contaminants that can cause weld problems, and each has to be cleaned differently. You will most likely need to clean the metal in different ways to get rid of every last bit of contaminant on the surface. This is particularly important if your are repairing rather than fabricating. Fresh metal from the mill is usually fairly clean. Old metal in engines or on heavy equipment can get very, very dirty.
The easiest contaminant to remove is paint. If your metal has previously been painted, the paint must come off before TIG welding. Even if most of the paint is gone, you still need to clean it so that not even a speck remains.
The fix: Clean the weld surface with paint striper or acetone. Use enough thinner to remove every last bit of the paint. Acetone will also remove traces of oil from the surface, but not heavy coats. Paint thinner and acetone are flammable, so make sure to put it away before welding.
2. Grease and Dirt
If you are repairing metal rather than fabricating a new project, the metal is probably dirty. That is especially true if you are working on cars or other machinery. All the dirt, grime, and grease must go before welding.
The fix: Use a citrus degreaser to remove grease and dirt. Make sure to get all traces of the degreaser off as well. For really thick coats of grease, you may need a wire brush to knock the worst of it loose before degreasing.
The other fix: Engine parts can absorb oil deep into pores and crevices in the metal. Use a torch on a low setting to gently heat the metal and sweat the oil out. After heating, wipe down with degreaser and heat again. Keep the cycle up until the torch stops bringing up grease.
3. Rust and Mill Scale
Some arc welding rods have flux that helps burn off rust and mill scale, but TIG rods don’t. You need to remove the rust and scale prior to weld manually.
The fix: Grind the metal along the joint until it’s shiny. Depending on the size of the part, you may need a bench grinder or an angle grinder.
4. Ground-in Contaminants
If the metal has any gouges or scratches, they can harbor contaminants that will cause porosity in the weld. Wiping them with degreaser or paint thinner won’t help, and a standard grinder won’t either.
The fix: Put a thin cut-off disc in an angle grinder and use it to clean out the scratches. Just like the flat surfaces, grind the gouges until they are clean and shiny on the inside.
5. Aluminum Oxide
Aluminum is a special case. Unless it is was machined in the last day or two, all aluminum is coated with a thin layer of oxide that will ruin TIG welds. This layer has to be removed before you can get a good weld.
The fix: Clean the oxide layer off just before welding. Don’t let the clean aluminum piece sit too long. If you do, in a few days, the oxide layer will be back. There are a couple of ways to clean it:
- Scrub with a stainless steel brush. Use a new brush or one that is only used for brushing aluminum. Used brushes can leave contaminants behind that will also ruin your weld.
- Clean with a chemical agent designed for removing aluminum oxide. These are available at most welding supply houses.
When you weld aluminum, fight the remaining oxide by using alternating current (AC) instead of direct current. The quick reverses of polarity in AC have a scrubbing effect on the metal that helps break up the oxide layer. Most welders can adjust the AC oscillation rate for more cleaning, better weld penetration, or a mix of the two. Check your manual for setup details.
It is extremely difficult to TIG weld aluminum with direct current. It can be done, but the surface has to be perfectly clean and free of oxides. Any trace of oxide will cause porosity. If you must weld aluminum on DC, use a chemical oxide remover immediately before welding.
6. Galvanized Metal
The zinc coating on galvanized steel melts at 420° and actually turns to a gas at welding temperatures. This will cause a porous weld. Even worse, zinc fumes are toxic and will make you sick.
The fix: Use a grinder to get all the zinc coating off the metal before welding. Wear a mask while grinding off the zinc to keep from inhaling any fumes or dust.
7. Pipe and Tube Contamination
If you are welding pipe or tube – especially pipe that has been used to carry oil or grease – the inside can be contaminated as well. If your weld is too hot, it can cause oil from inside the pipe to migrate into the weld and cause pores to form.
The fix: Use degreaser and heat to clean the inside of pipe and tube as well as the outside. This particularly important if you think the pipe has been used to carry oil or grease. Scrub out as well as you can to keep the oil from working into the weld.
TIG welding requires the use of a non-reactive shield gas to protect the hot weld from contamination from the air. The standard gas is argon, but helium and argon-helium mixes are also used. If your shield gas isn’t working right, the weld will have problems.
8. Not Enough Gas
If you don’t have enough gas flowing through the torch, the weld can corrode or develop pits and holes. Not enough gas is usually easy to fix. Here are some things to check:
The fix: Make sure the gas is turned on at the tank and the torch, and make sure the gas is flowing. Check the hose for kinks, too.
The fix: Verify that the tank isn’t empty. You will run out of gas sooner or later….don’t try to keep welding when the gas is gone.
The fix: Check the regulator to assure that the gas flow is set to the correct rate. Check your welder’s manual for appropriate gas flow rates and adjust the regulator properly.
9. Too Much Gas
You can use too much gas, especially for aluminum. Fast-flowing shield gas causes turbulence that can disturb the weld pool and cause bubbles or ripples in the bead.
The fix: Check the manual to make sure the gas flow rate is correct and adjust the regulator.
The other fix: If you really need a high gas flow rate, install a gas lens in the cup of the welding torch. A gas lens is a small screen of steel or stainless mesh that breaks up a fast gas stream into a smooth flow. Using a lens reduces turbulence with high gas flow rates and allows for smoother beads even with high gas flow.
10. Wind and Drafts
Another common cause of problems with shield gas is wind and drafts. TIG welding only works with still air around the weld so that the shield gas flows smoothly around the arc and weld bead. Welding outside or in drafty conditions can disrupt the shield gas flow and lead to weld problems.
The fix: Make sure there are no drafts or wind. If you have to weld outside, rig up a wind screen any way you can. Use tarps, cardboard, your body, or anything else you can find to block the wind. If you are welding inside, check for drafts. Electric fans are a common problem inside. Turn them off and find a different way to deal with the heat.
11. Contaminated Gas
It isn’t supposed to happen, but sometimes the shield gas itself is contaminated. The problem can be with the gas itself, or with your setup. Either way, if there is moisture in the cylinder or the hose, the water will react with the weld bead and create bubbles.
The fix: Make sure there are no leaks in your gas setup. When the gas is flowing, small holes can actually pull air into the gas line and cause problems. Spray all the connections and the hose itself with soapy water. Leaks or pinholes will show themselves by making bubbles in the soap.
The other fix: It is possible for the gas inside the cylinder to be contaminated. If you think this is the case, return the cylinder to your gas supplier and ask them to check it.
12. Gas Cup Problems
You can also have problems with the gas cup over the torch. Make sure it is in good condition, with no cracks. Check the manual to make sure you are using the right size cup for your application.
The fix: If you aren’t using the right sized cup, or it is damaged, use a new gas cup that meets the specifications for the project you are working on.
TIG welders use a tungsten electrode to carry current from the welder to the workpiece. Problems with the electrode can cause your weld bead to become porous.
13. Contaminated Electrode
If your electrode has something other than tungsten on it, the other metal can drip off the electrode into the bead. This will cause pitting and porosity to form. Always make sure your electrode is clean and ground to the proper tip shape for the weld you are making.
If you happen to touch the electrode to the bead or the workpiece, stop immediately. You will need to grind the electrode again to remove any contamination from the weld pool on the tip. Some welders like to have a number of electrodes prepared so that they can just swap out a contaminated electrode instead of stopping to re-grind it.
The fix: Use clean electrodes. Stop and re-grind if necessary.
14. Excessive Amperage
If you use too much amperage, or an electrode that is too small for the amps used, the electrode overheats. This causes bits of tungsten to blow off the end of the electrode and contaminate the weld. You will know this is the cause of your pores because the sharp point of the tungsten rod will get round.
The fix: Turn the amperage down or increase the size of your electrode by 1/32 of an inch.
15. Rod Contamination
The last place contamination can be found is on the welding rod. It doesn’t happen often, but sometimes rods do get dirty. A dirty rod will cause the same problems with your weld that dirty metal causes.
The fix: If you can see contamination on the rod, clean it the way you would clean metal. The easiest way is to wipe down the rod with acetone before you start welding. This is helpful if the rods have been sitting around your shop loose as well, even if they don’t look dirty. You never know what they might have picked up.
The other fix: If you are just practicing laying beads, it is fine to use any old bit of metal as a filler rod. However, this is more likely to lead to porous welds. If you are welding a project, use the correct TIG rod for your application.
If you have cleaned the metal to a bright shine, verified that the gas is flowing properly, re-ground the electrode to a perfect point, and pulled out a fresh rod, but you are still getting porosity in the bead, what else can you do to fix it?
16. Torch Angle
TIG torches work best when held perpendicular to the weld surface, or halfway between the elements if you are welding an inside corner. This angle provides the best coverage of shield gas over the weld bead. If you hold the torch too flat, the shield gas won’t cover the arc, and the weld will oxidize.
The fix: Hold the torch at 90° to the weld. Don’t let it tip over too much. Even if you start with good torch position, long welds can lead to awkward body positions that make you tip the torch over. Make a practice pass with the torch before cracking an arc to see if you will accidentally lean it too much and get a bad bead.
17. Blowing Bubbles
If you are welding a tube or pipe that is sealed, the air inside can heat up and increase the pressure from the back of the weld. This pressure differential will cause the air to push through the end of the weld while it’s still hot and make a hole in the bead.
The fix: If you think air is pushing out of the weld, pre-heat the inside of the pipe to expand the air before welding. The weld heat won’t cause the air to expand further, so you will never get the pressure that causes the leak.
The other fix: If pre-heating the pipe isn’t feasible, drill a tiny hole in the pipe near where you will finish the weld. This will allow air to escape from the pipe, equalizing the pressure without blowing out the weld. Once the bead is cool, you can fill in the hole quickly without heating the pipe too much to seal the pipe up again.