Not so long ago, we had the case where an old welded, mild steel support structure showed some quite ugly weld beads. As we are a mainly TIG process factory, the question was:
Can we TIG weld over MIG welds? Generally speaking, yes. TIG welding makes it possible to melt a weld without adding filler material. This makes it possible to smoothen a MIG weld with only existing material or adding small amounts of extra filler material where necessary.
If you are still unsure if you should give it a go, you might be interested in the more detailed answer, as every welding application is different. Below I put together some info you might find helpful to decide if you are going to TIG weld over your MIG weld.
Reasons for TIG welding over an existing MIG weld bead
First question you should ask yourself is: What are the benefits of doing this? And there are a couple of reasons why it could be worth a shot to start your TIG welding plant:
Bringing your best foot forward: If you are looking at an o.k. weld, but the weld aesthetic really counts, TIG welding might allow you to smoothen the weld bead to a nice looking and evenly defined weld wave. TIG welding is much preciser in the heat input location, smaller in the weld focus and allows you to not add any additional material if the current weld material is already sufficient. This means after having molten up the weld bead again, you push the chewy and liquid metal around where you need it to be. The risk to blow through your work piece is much lower than with MIG because of the mentioned denser heat input. That allows you to bring in the heat more directed and surgical. All these points are advantages from TIG over MIG when cleaning up, and might make it a worthy investment of your time.
Accessing the awkward places (again) – Accessing harder to reach places on your work piece with a smaller in size TIG torch can be enough to either touch up small holes or clean up a weld as mentioned before. TIG torches are usually smaller in size because the filler rod is not part of the torch head but added “cold” from the side into the puddle when necessary. This allows a more compact overall built.
Increase the penetration and effective bonding zone – TIG welding will allow you to increase the bonding depth in certain areas when targeted or will increase the area of bonding with careful weaving and more evenly distributing of the weld bead material. When done carefully, this can improve your structural strength of the weld. If you are welding mild steel, which is more forgiving, the results are more likely better than for example with stainless steel. Heating up stainless steel again can cause excessive heat input and result in heat cracks. More risks are listed in the next section.
4 Risks when TIG welding over a MIG weld
Just for aesthetics? Worth the risk? “What could possibly go wrong?” here are four common risks:
Risk 1: If the weld is not properly cleaned, you might bring in contamination to the weld such as porosity or distortion that will cause pulling or contracting due to the heat input. Especially if you are re-welding thin plates or work pieces, going again might not be worth the risk of distortion. Also undercutting or sweeping to far away from the center and thinning out the weld material too much can do more harm than good.
Risk 2: If the weld is really messy, starting new might be the best option to be sure about the structural integrity of the joint. That means grinding away the weld bead, cleaning the surface and have another go with the MIG welder. This will prevent adding excessive heat into the part which might cause heat cracks.
Risk 3: Is it worth it? It will take you some time to clean up the weld and some good weaving skills to make the appearance look better than before. If you don’t have access to a TIG welder, you also have to consider the borrowing or purchase cost.
Risk 4: No spare hand! Going from MIG to TIG will occupy your supporting hand if you intend to add some filler material. In contrast to MIG welding, in TIG welding, the filler rod is fed from the side into the weld puddle. With your second hand! For a straight forward job, this should be no concern, but when welding something more demanding or in a awkward position, “loosing” one hand might be something you have to get used to.
Switching from MIG welding to TIG welding without much prior experience might prove to be difficult. TIG welding poses some different requirements in welder skills than MIG.
What kind of TIG welding plant specs will I need?
As mentioned before, it is probably not worth buying a separat welding plant for touching your MIG welds up. But if you are looking for a new TIG plant anyway or are curious if the one you have will do, here are some rule of thumb figures for you:
DC or AC/DC? – AC is not necessary for touching up MIG welds, because you probably mainly welding different kinds of steel at the moment. When you are just dealing with steel, and not having the odd Aluminium welding job occasionally, DC is sufficient.
HF or scratch start – The high frequency start capability makes TIG much easier to use and would be especially handy if you are not used to TIG welding. It takes a bit of the hassle out of the process and therefore might be worth having. However not necessary for welding over MIG. Definitely something to safe with scratch start when on a budget.
Amps – Totally depends on the thickness of your material. But since you are only touching up, a welding plant with capability somewhere around 150 A would be sufficient for 4-5 mm steel thickness. This is a cost versus utility decision.
Water/Gas cooled – When you are dealing with hard to access welds, a water cooled torch can be helpful as the size is reduced due to the good heat deduction. But also more expensive. If you where able to access the weld with a MIG torch, there should be no reason why you can’t get there with a “cheap” air-cooled TIG torch.
If you start out on TIG, a “just-enough” welding plant might be the right entry point for you to get familiar with the different process and TIG welding techniques. Otherwise investing a bit more can be a good choice, as they say “buy cheap, buy twice“.
This is a general answer to the question, however there are a lot of factors that influence a good weld. Metallurgy and material would be two major ones to mention. Contamination in the existing weld bead and materials that need special attention, e.g. preheating before welding, might sabotage your efforts when welding over. So keep that in mind and inform yourself when in doubt about your application.
I hope this article shed some light on the question for you. If you have any additional questions, feel free to contact me under alex@[domainname].com. Best of luck with your weld project!