Nov 11
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What is MIG Welding?
MIG welding is an abbreviation for Metal Inert Gas Welding. It is a process developed in the 1940’s, and is considered semi-automated. This means that the welder still requires skill, but that the MIG welding machine will continuously keep filling the joint being welded.
MIG welders consist of a handle with a trigger controlling a wire feed, feeding the wire from a spool to the weld joint. The wire is similar to an endless bicycle brake cable. The wire runs through the liner, which also has a gas feeding through the same cable to the point of arc, which protects the weld from the air.
MIG welding is most commonly used in fabrication shops where production is high, and the possibility of wind blowing away your gas shielding is unlikely.
MIG Welding Names
Those seeking a job as a welder would be advised to know all of the names by which this process is known. Employers may use other names in the classifieds or on a written test.
When it was first developed it was called (GMA) Gas Metal Arc. It is also known as; GMAW or Gas Metal Arc Welding. Technically the differences in the names are the type of gas used, Inert gas versus non-inert gas.
How MIG Welding Works
MIG weld welding requires three things, electricity to produce heat, an electrode to fill the joint, and shielding gas to protect the weld from the air. MIG welding is done using a very small electrode that is fed continuously, while the operator controls the amount of weld being done. In some cases when a robot takes over this process, it becomes automatic welding.
MIG Voltage Type and Welding Polarity
MIG welding unlike most other welding processes has one standard voltage type and polarity type. The voltage used is D/C direct current, much like the current in a car battery. Direct current flows in one direction, from the negative (-) to the positive (+).
The polarity used is also standard and that is D/C electrode (+) positive. This means that the handle is the positive side of the circuit, or it may be said, the electricity flows from the metal in to the welding handle.
The power source used for MIG welding is called a “constant voltage power supply”. In MIG welding the voltage is what is controlled and adjusted. When comparing MIG welding to Arc or TIG welding, MIG welding machines use voltage settings to set the machine. TIG and Arc welding machines use amperage to set the machine or a “constant amperage power supply”.
MIG Electrode Types
When choosing the proper MIG wire or electrode you need to match the type of wire to the type of metal being welded. Some other considerations are the type of transfer, position to be welded, and resistance to abrasion. Most of the times when working as a welder the welding engineers specify the weld size and electrode type to be used.
The most common wire used for welding carbon steel is ER 70S-6. In some cases you can weld two different metals together. An example of this is welding 304 stainless steel to A36 carbon steel using an electrode made of 309 stainless steel “ER 309L”.
Typical MIG welding electrodes are a solid wire ranging from a thickness of .023 to .045. Some are much thicker for heavy industrial applications. The most common sizes are:
- .023
- .030
- .035
- .045
The manufactures of these electrodes use a standard code to identify the type of electrode. For example the code on the label ER 70S-6 represents the following:
- ER- An electrode or filler rod that is used in either a wire feed or TIG welding.
- 70- A minimum of 70,000 pounds of tensile strength per square inch of weld.
- S -Solid wire.
- 6- The amount of deoxidizing agent and cleansing agent on the electrode.
Common MIG Welding Gasses
Gas for MIG welding is what makes MIG welding possible. The name informs us of this; “Metal Inert Gas welding”. The gasses used are what shield the weld from oxygen in the air. When Carbon Dioxide or Oxygen is added to the mixture, MIG welding is not technically MIG welding anymore. This is due to the fact that both Carbon Dioxide and Oxygen are not inert gasses. The process then becomes GMAW or Gas Metal Arc Welding.
The type of gas uses also determines:
- How deep the weld penetrates the metal welded
- The characteristics of the welding arc
- The mechanical properties of the weld.
When choosing the type of gas to be used, it is best to seek input from a welding supply store. The store will recommend the proper gas to match the welding wire to be used. Or, one may utilize the welding wire manufacturer’s recommendation. Typically, the manufacturer will provide a few choices ranging from the best choice, to something that will provide the minimum acceptable results. The final choice in the type of gas, is based on cost.
The four most common gasses used are;
- Argon
- CO2 / Carbon Dioxide
- O2 / Oxygen
- Helium (the least common)
Most of the times these gasses are used in a mixture form, typically consisting of carbon dioxide, and Argon or Oxygen. Oxygen is what causes most weld defects, however, in small percentages, mixed with other gasses, it improves the arc characteristics. Argon and Carbon Dioxide can be used by themselves. In some case there is tri-mix shielding gas containing Argon, Carbon dioxide, and Helium, or, Argon, Carbon dioxide, and Oxygen. I recently used this last mixture, when taking my
The more common mixtures and gasses are.
-
- C2 or 2% Carbon Dioxide and 98% Argon
- C25 or 25% Carbon Dioxide and 95% Argon
- 100% Carbon Dioxide
- 100% Argon
Some basic guidelines for choosing the proper gas are as follows:
Welding of carbon steel can be done with Carbon Dioxide alone and it produces the deepest penetration, the most smoke and the roughest weld. A mixture of gasses ranging from 2% to 25% Carbon Dioxide and the rest Argon can be used. However, a higher percent of Argon will result in a smoother, better looking weld, and improves the arc characteristics.
Welding of stainless steel is typically done with C2 or 2% Carbon Dioxide and 98% Argon. In some cases there is tri-mix shielding gas containing 90% Helium, 7.5% Argon, and 2.5%Carbon dioxide.
Welding of aluminum is typically done with Argon alone, with one exception. If the aluminum being welded is thicker then ½ of an inch there may be Helium added to the mix.
Argon gas by itself works well on most exotic metals like:
- Copper
- Copper Alloys
- Magnesium
- Nickel
- Nickel Alloys
- Titanium
MIG Welding Transfer Types
MIG welding has four ways of transferring the wire to the joint.
· Short circuit
· Globular
· Spray
· Pulsed spray
Short Circuit Transfer
Short circuit transfer is a transfer used when a lower voltage is used for MIG welding. Short circuit transfer occurs when the wire arcs and contacts the metal creating short circuits. During this short circuit, the wire contacting the metal heats up and drips into the joint by creating a puddle. Then another arc begins and the process keeps repeating many times a second. The easiest way to tell if the transfer is short circuit is by the sound. The sound greatly resembles, an egg hitting an extremely hot frying pan. It is a very crisp and fast crackling sound. Typically short circuit transfer is used on thin metals or sheet metals.
Globular Transfer
Globular transfer is similar to short circuit transfer. Globular transfer like short circuit occurs when the wire arcs and contacts the metal creating a short circuit while melting the wire but the main difference is the wire melts for a longer period of time creating a glob that hangs off of the wire. Then the glob falls to the metal filling the joint and then the process begins again. Globular transfer has a popping sound to it. It has a few pops per second and many times you can actually see the glob being formed and dropped.
Spray Transfer
Spray Transfer is a transfer where a higher voltage is applied. The transfer is exactly like it sounds. The wire sprays or has very fine mist transferring to the metal. The way to tell if you using a true spray transferring are it has a fine hissing sound and no crackles or pops. You can also see a clean arc to the metal and will not have any spatter.
Pulsed Spray Transfer
Pulse transfer is a transfer that is done with special equipment. The equipment pulses the voltage many times a second, allowing the arc to go between spray transfer, to globular transfer, back to spray transfer and so on.
How to Set Different Transfer Types
If you have never MIG welded it would help to know that there are no settings on a MIG welding machine offering a choice of transfer type. The ways a different type of transfer are set have two ingredients. The first is the voltage settings and the second is the type of gas used.
Most short circuit transfer welding is done with lower voltage and usually using Carbon Dioxide or a Carbon Dioxide gas mixture.
Globular and spray transfer use a higher voltage setting and commonly use Aragon gas. The Argon gas may contain a percentage of Carbon Dioxide, Helium, and or Oxygen.
Almost any metal may be MIG Welded
MIG welding is a welding process that can weld almost any metal. It may not always be the best choice for weld quality but MIG welding is a fast, cost efficient, and produces results that are more than acceptable for most manufacturing and fabrication needs! Not everybody is building a space station. The three most common metals welded with a MIG welder are:
· Carbon steel.
· Stainless steel.
· Aluminum, with a special feeder because aluminum wire is very soft.
MIG Welding Carbon Steel
Carbon steel welds are almost flawlessly done with a MIG welder. There are very few problems, beside the downside of the design of a MIG welding machine. The wire stiffness is just right to pass through the liner from the machine with minimal friction to cause problems and has enough stiffness to be feed without coiling up. Depending on how much voltage the MIG welding machine is running at, the weld can be set to one of three transfer types, short circuit, globular, or spray.
MIG Welding Stainless Steel
Stainless steel MIG welding does not require any special equipment. In the case of welding stainless steel, the biggest problem comes from the cord or liner of the welder. The trick when welding stainless steel is to keep the cord as straight as possible. Otherwise, the wire feed that feeds the weld joint will have too much friction due to stainless steel being stiffer then carbon steel. Think of trying to shove a wire hanger through a bent garden hose. Chances are if the hose is straight you can easily put it through. If the hose is bent, you will have difficulty. That is how stainless steel wire is in the liner of the welding cable. What happens is the wire is so stiff in the liner that it causes so much friction that the wire stops the feeding wheel from feeding the joint (literally spinning its wheels). The result is a fused MIG tip. That is when the wire stops or slows down to the point that the arc melts the wire up to the MIG tip and it welds itself to it. The other major problem is again the liner. If you bend the cord to much the friction stops the wire and the wheels that are feeding the MIG gun push the wire so hard that the wire having no place to go spooling up like a birds nest before it gets pushed into the liner.
MIG welding stainless steel is mostly done in a spray transfer or globular borderline spray. The reason behind this is that stainless steel does not do to well with short circuit transfers. The short circuit transfer and globular have the wire actually hitting the metal, splashing and splattering, and that also causes friction on the liner.
Welding stainless steel requires a high percentage of Argon used in the gas mixture. If it is a critical joint that will be x-rayed, any disturbance in the transfer will cause flaws. Spray is a smooth crackle-free transfer that has no metal being violently transferred. Spray transfer also keeps spatter to the bare minimum. With stainless steel spatter is extremely difficult to clean. The pictures below on the left is a weld I did using ER 308L wire, C2 gas, with a globular boderline spray transfer. I also used the same settings for the picture on the right. One of the great things about MIG welding is, I did not have many restarts. The long welds are about 3 feet between the restarts.
- Nickel
- Nickel Alloys
Titanium