MIG Welding Section
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 is by far the easiest to learn. With a few minutes of practice, you'll be making professional welds. MIG welding is similar to arc welding, but the wire is automatically fed from the end of the gun at a preset rate. A gas bottle provides gas, which is expelled from the end of the gun to shield the weld from the ambient atmosphere and avoid the oxidation caused by oxygen. You can make unshielded welds (without gas), but the welds will be highly oxidized which will make them weak and brittle. If your system is not equipped with gas, you must use flux cored wire to avoid highly oxidized welds. The key to good MIG welding is to set the wire feed-rate and the electric arc intensity at the correct values for the material. You can do this only by experimentation.
MIG Welding Names
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 Welding Equipment and Accessories
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.
MIG Welding Gasses
MIG welding requires a shielding gas to be used. As the name states “Metal Inert Gas Welding” there is no shielding on the electrode or filler wire. MIG welding would not be possible without shielding gas. The way the shielding gas works is it is feed through the MIG gun and it literally suffocates the weld area from any air. This provides an air free zone where the welding arc and filler wire can do their work to get the joint welded.
MIG welding typically three types of gas for shielding and they are:
Argon
Carbon Dioxide
Helium
These three gases are typically used as a mixture depending on the metals that are being welded. The shielding gas needs to be matched to the electrode and base metal. If they are not compatible then the welds will either not be strong or it just won’t weld properly.
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.
Transfer Types
MIG welding has four ways of transferring the wire to the joint.
Short circuit
Globular
Spray
Pulsed spray
The transfer types used to MIG weld are determined by the metal type, shielding gas used and machine settings. MIG welding transfer types are more of a machine set-up issue then a welding issue.
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.
Joint Setup and Preparation
Metal Preparation Unlike Stick and Flux-Cored electrodes, which have higher amounts of special additives, the solid MIG wire does not combat rust, dirt, oil or other contaminants very well. Use a metal brush or grinder and clean down to bare metal before striking an arc. Make sure your work clamp connects to clean metal, too; any electrical impedance will affect wire feeding performance. To ensure strong welds on thicker metal, bevel the joint to ensure the weld fully penetrates to the base metal. This is especially important for butt joints.
With MIG welding it is very critical that the weld area is clean. MIG welding will not be successful with a dirty joint. Unlike some stick welding / SMAW rods that can burn through rust, MIG welding has a lot of difficulties welding dirtier metals.
It also does not have slag to protect the weld when the gas is gone. When MIG welding make sure you have a clean joint by removing any foreign substance. With MIG welding a slight bit of dirt or rust is Okay but anything more is asking for trouble. MIG welding painted or coated metals does not work well at all.
MIG Equipment Preparation
• Check your cables: Before striking an arc, check your welding equipment to make sure all of the cable connections are tight fitting and free of fraying or other damage.
• Select electrode polarity: MIG welding requires DC electrode positive or reverse polarity. The polarity connections are usually found on the inside of the machine.
• Set gas flow: Turn on the shielding gas and set the flow rate to 20 to 25 cubic feet per hour. If you suspect leaks in your gas hose, apply a soapy water solution and look for bubbles. If you spot a leak, discard the hose and install a new one.
• Check tension. Too much or too little tension on either the drive rolls or the wire spool hub can lead to poor wire feeding performance. Adjust according to your owner's manual.
• Inspect consumables. Remove excess spatter from contact tubes, replace worn contact tips and liners and discard the wire if it appears rusty.
Wire Selection
For steel, there are two common wire types. Use an AWS classification ER70S-3 for all- purpose welding. Use ER70S-6 wire when more deoxidizers are needed for welding on dirty or rusty steel. As for wire diameter, .030-in. diameter makes a good all-around choice for welding a wide range of metal thicknesses in home and motorsports applications. For welding thinner material, use a .023-in. wire to reduce heat input.
You have to wait 30 seconds here.
For welding thicker material at higher total heat levels, use .035-in. (or .045-in. wire if it's within your welder's output range).
Gas Selection
A 75 percent argon/25 percent CO2 blend (also called "75/25" or "C25") works as the best "all purpose" shielding gas for carbon steel. It produces the least amount spatter, best bead appearance and won't promote burn-through on thinner metals.
100 percent CO2 provides deeper penetration, but also increases spatter and the bead will be rougher than with 75/25.
Voltage and Amperage
How much voltage and amperage a weld requires depends on numerous variables, including metal thicknesses, type of metal, joint configuration, welding position, shielding gas and wire diameter speed (among others).
MIG Wire Stick-out
Stick-out is the length of unmelted electrode extending from the tip of the contact tube, and it does not include arc length. Generally, maintain a stick-out of 3/8 in. and listen for that "sizzling bacon" sound. If the arc sounds irregular, one culprit could be that your stick-out is too long, which is an extremely common error.
Push or Pull?
The push or forehand technique involves pushing the gun away from (ahead of) the weld puddle. Pushing usually produces lower penetration and a wider, flatter bead because the arc force is directed away from the weld puddle.
With the drag or backhand technique (also called the, pull or trailing technique), the welding gun is pointed back at the weld puddle and dragged away from the deposited metal. Dragging typically produces deeper penetration and a narrower bead with more buildup.
There's an old saying that goes, "If there's slag, you drag," which means use the drag technique for Stick and Flux Cored welding. When MIG welding mild steel you can use either technique, but note that pushing usually offers a better view and enables you to better direct wire into the joint.
Travel Angle
Travel angle is defined as the angle relative to the gun in a perpendicular position. Normal welding conditions in all positions call for a travel angle of 5 to 15 degrees. Travel angles beyond 20 to 25 degrees can lead to more spatter, less penetration and general arc instability.