By Jayden Mark
When people start welding, they almost always learn MIG welding first. This welding method is popular with do-it-yourselfers and fabricators because it offers a great balance of power, flexibility and ease of use. While learning the right techniques may be easy, setting up a MIG welder can be difficult.
With so many choices in wires, gases, settings and machines, it can be hard to find the right setup for your needs. Here’s a breakdown of everything you need to know to get started, from getting the right machine to solving problems with poor welds.
What is MIG Welding?
“MIG” stands for Metal Inert Gas. This is a type of welding that uses a wire electrode and gas to bond materials. It’s also referred to as metal arc welding (GMAW), wire welding and wire feed welding.
The welding machine feeds a wire electrode from a spool to the welding gun. An electric arc between the electrode and the piece being bonded creates the heat needed to melt the wire. In most types of MIG welding, an inert shielding gas envelopes the electrode and weld pool, protecting them from oxygen contamination. Reactive gases are also used in some situations, while flux core wiring doesn’t need shielding gas.
MIG Wire
In MIG welding, the filler wire is the electrode. This consumable is stored on a spool that’s either on the side of the power supply or on the gun. Pushing the button on the gun sends current through the welder and turns on the wire feed. The feed speed must be adjusted to keep a steady supply of wire running to the weld pool. Some new welders can adjust the wire speed automatically, making it easier to maintain a steady arc.
Solid MIG wire is little more than filler wire. Since it doesn’t have the additives of flux core and stick electrodes, it’s not as good at overcoming rust, oil, dirt and other contaminates. However, it handles these contaminates better than TIG welding. For the best results, the surfaces being welded need to be clean. Wire formulas are available that have added deoxidizers, which helps the weld penetrate rusty metal.
Wire is available in a range of thicknesses. Increasing wire thickness increases the strength of the weld, but it also increases the heat transferred to the metal. 0.030 inch wire is the most common size for general purpose welding, while 0.023 inch wire is used for sheet metal. 0.035 and 0.045 wires are used for welding thick metal parts.
The wire feed setting is in Inches Per Minute (IPM). Increasing the wire diameter decreases the speed it needs to be fed into the weld pool, while increasing the amperage increases the feed speed needed to maintain the arc. 0.023 inch wire needs a feed speed of about 3.5 IPM per amp, while 0.045 inch wire only needs a feed speed of 1 IPM.
Welding wire uses a code to indicate its characteristics. Different code formats are used depending on the metal the wire is designed to weld. All wire codes start with “ER,” which stands for Electrode Wire.
Carbon Steel Wire
For example, ER 70S6 is a solid electrode wire with 70,000 lbs. of tensile strength and level 6 cleaning agents, which is high. This formula is used for welding carbon steel.
Stainless Steel Wire
The most common type of stainless steel wire is ER 309L. It can weld stainless steel, and it can bond stainless and carbon steel pieces together.
Aluminum Wire
ER4043 is a common wire made for 4000 series aluminum. Wire used for aluminum welding is soft and easy to bend, so it requires a special feeder.
Shielding Gasses and Flux
MIG welding uses carbon dioxide, argon or helium as a shielding gas. Technically, any welding done with CO2 or oxygen is Gas Metal Arc Welding (GMAW), because these gases aren’t inert. However, GMAW uses the same equipment as TIG welding.
Most MIG welding uses a 75% argon/25% carbon dioxide mix. This is sometimes called “75/25” or “C25.” This mix reduces burn through and spatter.
100% CO2 increases penetration, but it increases spatter and leaves behind a rougher bead.
100% argon is used to weld most non-ferrous metals, including aluminum, copper, magnesium, nickel and titanium.
Tri-Mix is 90% helium, 7.5% argon and 2.5% carbon dioxide. It’s used to weld stainless steel.
Small amounts of oxygen may be added to helium and argon blends to increase weld penetration. However, too much oxygen can lead to defects from oxidation.
Flux core wire doesn’t require a shielding gas, but it can only be used on carbon or mild steel. Other types of MIG welding aren’t suitable for outdoor work, because wind can blow away the gas, leaving the weld pool exposed.
For most welding, a flow rate of around 20 cubic feet per hour will keep the weld covered long enough to create a solid bond. More or less gas may be needed, depending on the transfer type being used. MIG welding tolerates drafts better than TIG welding.
Power
MIG welding uses DC electrode positive current. That means electricity flows from the metal to the welding gun. While TIG and arc welding machines directly control amperage to control current flow, MIG machines use voltage.
Adjusting the voltage changes how the electrode welds to metal. Increasing voltage increases weld penetration, but too much power can cause blowouts. The right settings depend on many factors, including the welder model, gas, wire gauge, joint type and metal thickness. Most welders have a built-in chart with setting recommendations. More advanced machines can adjust automatically once the wire and metal thickness are set, and they have presets for common welding jobs.
Welding Technique
Wire stick out is the length of electrode wire extending past the tip of the welding gun. Most welders work best with 3/8 inches of stick out. Get too close, and spatter can weld the electrode wire to the contact tip. Get too far, and the shielding gas won’t cover the weld, resulting in poor bonding.
The direction of movement influences the resulting weld:
In most cases, the gun should be kept at a 5-15 degree angle for the best results. Too much angle can force the electrode away from the puddle, resulting in more spatter, less penetration, and arc instability.
Weld Transfer Types
There’s more to MIG welding than pushing the button on the gun and making an arc. Varying gun position, amperage and gas flow changes how the weld pool forms and how it cools, resulting in different types of welds. Each method has a distinct sound. Welders can often hear sound changes before they see changes in the weld, helping them make corrections sooner.
Short circuit transfer uses fast, intermittent pulses to melt metal in short bursts. By spacing out heat transfer, this method reduces warping on thin metal. High carbon dioxide gas blends are used with this method, because it reduces spray. This method is mostly used to weld body panels on cars. However, even with reduced heat transfer, these welds are usually spaced out to keep metal temperatures down. The welder makes a series of short welds across the panel, then goes back to fill in the spaces. When performed correctly, this type of welding makes a high-pitched crackling sound.
Globular transfer uses arcing between the electrode wire and the metal to form a glob of filler that drops into the joint. This type of welding uses shielding gas with a high amount of argon to increase spray, which pushes the weld pool into the joint. The weld starts with a high wire speed to establish the metal blob. From there, slowing the feed speed lets the blob drip across the joint. This makes a slow popping sound.
Spray transfer is used on flat, horizontal joints. This uses a high wire speed to create a continuous pool with little spatter. Typically, a 75% argon gas blend is used when welding ferrous metals with this method, while aluminum welds use pure argon. This welding method makes a steady hissing sound.
Getting a Good Weld
Increasing the area of contact increases the strength of the weld. For the best results, the pieces being bonded should be beveled around the welding area, especially if you’re making a butt joint.
MIG welding usually uses DC electrode positive, AKA reverse polarity, while other types of welding may use DC electrode negative or AC welding. It’s important to check the polarity on machines that support multiple welding methods.
Porous welds are usually caused by using the wrong amount of shielding gas. If there’s too little gas, the weld oxidizes before it can cool. If there’s too much gas, it can cause turbulence around the weld, mixing the gas with the surrounding air. Using too much stick out can keep the gas from enveloping the weld.
If you’re having feed problems, start with the gun. Often, spatter can melt onto the tip, which keeps the wire from exiting smoothly. Check the tension setting on the feed rollers, and make sure the rollers are sized correctly to your wire. Sometimes, the wire can tangle on the spool. If you notice the feed only has problems with the gun in certain positions, the wire may be binding inside the cable. If this is the case, you need to replace the liner.
What to Look For when Buying a Welder
MIG welders are easy to learn, have reasonable requirements for metal preparation, and can be used on almost any type of metal. This makes them a popular option for all segments of the market, from hobbyists to full time fabricators. As a result, the market for these welders stretches from models that cost a couple hundred dollars to several thousand.
120 volt welders have enough power to weld thin metal, so they’re suitable for most hobbyist work, including auto body repair and basic fabrication. These welders can typically handle metal up to 1/16 or 1/8 inch thick. Keep in mind that the rated power output is often only available when connected to a 20 amp outlet, while most household outlets top out at 15 amps. If you’re adding a welder to your shop, consider having a 20 amp circuit installed. 20 amp outlets are easy to identify, because one of the socket holes is T-shaped.
240 volt machines can handle long periods of high amperage work, and they have the power to weld thick metal effectively. Some 240 volt machines can run on 120 volts. This lets you use your welder away from your 240 volt outlets, albeit with less power available.
The cheapest machines don’t use shielding gas. This limits them to using flux core wire. Welds made using self-shielding wire won’t be as strong or leave behind as smooth of a bead as gas-shielded welds. However, this makes the welder easy to move around, and the lack of gas means wind is never a problem. This makes these welders a great choice if you need something that can be used on the go, drawing power from a household outlet or a generator.
Some welders prefer a spool gun for welding aluminum. By placing the spool on the gun, thin aluminum wire has a much shorter path to the tip, reducing the chance of binding. However, it also makes the gun heavier.
Air-cooled MIG guns don’t heat up as fast as TIG torches, but they can still get hot over prolonged periods of welding. Liquid-cooled guns can stay at a comfortable temperature for long jobs, although they’re very expensive.
Duty cycles are stated as a percent of welding time versus cooling time. If a welder has a 60% duty rating, it can be used for 6 out of every 10 minutes. A higher duty cycle makes it easier to do lengthy jobs.
The Advantages of MIG Welding
The Disadvantages of MIG Welding
Where is MIG Welding Used?
The flexibility, small learning curve and small amount of setup needed for MIG welding makes it a great choice for all types of general repair work.
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