MIG vs TIG welds August 12 2017
You may have seen products that feature MIG or TIG welded construction, but what are the differences, and which one is better? We'll explain some of the basics behind MIG and TIG welding and compare them. This is not really a guide for people looking to get into welding, but rather a guide for consumers who have never welded before and would like to know the pros and cons between MIG and TIG welds.
Let's talk about what MIG welding is first. MIG welding is like the hot glue gun of the welding world. You pull the trigger on the gun, and an electrical arc is initiated between the base metal and the wire at the tip of the MIG gun. This forms a puddle of molten metal, and the gun automatically feeds the metal wire into the weld puddle. As the wire is fed in, it melts into the puddle and the operator moves the gun along the joint and creates the weld. A MIG welding machine will typically have adjustments for wire feed speed and electrical current. A shielding gas is dispensed out of the front of the gun to displace the oxygen in the vicinity of the weld. Keeping oxygen away from the molten weld puddle is important to prevent porosity and oxidation.
Now, let's talk about how TIG welding works. TIG uses a non-consumable tungsten electrode to make an electrical arc that melts an area of the base metal into a puddle. The operator uses their other hand to dab a rod of filler metal into the weld puddle to add metal to the joint. A foot pedal modulates the amount of current output, and thereby the amount of heat. The operator moves the torch along the joint, dabbing the filler rod into the weld puddle at a steady rate. Most people feel that TIG requires more skill because you are using both hands and your foot all at the same time and making constant adjustments, while still trying to maintain consistent puddle width and heat input. Shielding gas is dispensed out of the front of the torch just like a MIG welder.
TIG wand and aluminum filler rod
So what are the advantages of MIG? MIG is typically 20%-50% faster than TIG. Some also feel that MIG also has a shallower learning curve. MIG welding machines are also typically less expensive than TIG welding machines. These advantages translate to higher productivity, and lower cost, which is great.
Perhaps the most significant advantage of MIG is the ability to automate the process. In large production runs, employing robots or machines to weld is often faster, cheaper and more consistent.
MIG weld on plain carbon steel
Lastly, MIG is a bit more forgiving than TIG in terms of metal preparation and fitment. I won't go into detail of how the difference in polarity causes this, but MIG can tolerate small amounts of rust, oil and mill scale on the metal, whereas TIG will tolerate much less before it starts to create soot, lose penetration and create porosity. MIG can also fill gaps between metal easier than TIG. For TIG, the fitment of the metal pieces has to be tighter, with little to no gap between the pieces. The take-away is that MIG typically requires less prep work to get the parts into a weldable state. The flip side is that TIG is less forgiving to contaminants and gaps between pieces, so a TIG weld implies that a minimum level of cleaning and fitment has been achieved prior to welding.
Now, let's talk about the advantages of TIG. Making a good weld is a balancing act between weld puddle speed, filler material addition rate, and electrical current. A TIG operator can control all three independently on-the-fly. A MIG operator can control the travel speed of the gun, that's about it. They can't control wire feed rate or electrical current once they initiate the weld, unless they have a buddy adjusting the controls. This translates to TIG welds typically having more consistent and accurate penetration, where just the right amount of base metal is melted. This matters more on parts thinner than 0.125 inches, and on metals with high thermal conductivity, like aluminum.
TIG weld on plain carbon steel
TIG welding also creates the stack-of-dimes aesthetic naturally, which many people like. Some MIG techniques can create stack-of-dimes aesthetics, but that requires more operator skill and can produce weak welds if improperly done. Therefore, this is not typically done.
TIG also creates less weld spatter than MIG welding. A TIG torch produces barely any sparks, whereas MIG usually has sparks flying all over the place. You may have noticed some tiny balls of metal near a weld, and these are from weld spatter that has fused to the base metal. They look bad and can snag skin or wiring if they are not ground off.
Weld spatter from a MIG weld
Structurally speaking, TIG welds in general have less porosity than MIG welds. Porosity reduces the strength of the weld because the tiny voids can't carry any load, and more significantly, they create tiny stress concentrations where cracks can form and propagate.
For aluminum welding, TIG is usually the answer, for several reasons. First, aluminum conducts heat almost 5 times better than steel, so the part heats up quickly and the amperage needs to be reduced as the weld continues. This also means that the area surrounding the weld is hotter, which makes accidentally melting a hole through the part easier. Second, melted aluminum is less viscous and has less surface tension than melted steel, so it is easier to burn a hole through the part if the heat input isn't spot-on. TIG allows for more precise heat input, and for amperage pulsing techniques that are great for preventing melting through the part.
So here's the bottom line: MIG and TIG can both create beautiful, structurally sound welds, but TIG welds are generally regarded as being slightly higher in quality. You can see gorgeous examples of both MIG and TIG welds, but most people like the TIG look more, and can appreciate the skill that goes into creating a TIG weld. The same goes for proper weld penetration and porosity. The average TIG weld is going to have more accurate and consistent penetration and less porosity than the average MIG weld.
Other times, TIG welding allows for joining thinner parts, or parts made of aluminum - welds that would otherwise be very difficult with a MIG setup. So sometimes TIG is a requisite for lightweight parts.
So now you know a little about TIG and MIG welds. A lot of it comes down to preference and what your priorities are. MIG typically has incrementally lower cost, TIG typically has incrementally better strength, and can allow for lighter weight construction. As far as looks, it is a matter of personal preference.