MIG WELDING BASICS
MIG Welding – How it works
MIG (Metal Inert Gas) welding is an arc welding process in which a continuous solid wire electrode is fed through a welding gun and into the weld pool, joining the two base materials together. A shielding gas is also sent through the welding gun and protects the weld pool from contamination.
MIG welding can only be used on thin to medium thick metals. The use of an inert gas makes this type of welding less portable than arc welding which requires no external source of shielding gas. Producing a less controlled weld as compared to TIG (Tungsten Inert Gas Welding), MIG welding requires DC electrode positive, or reverse polarity. The polarity connections are usually found on the inside of the machine.
- Useful for automated bore welding.
- Long continuous welds.
- Once set-up, allows for more efficient (time wise) welding operation.
- Weld penetration can be an issue.
- Similar to TIG, welding in windy areas can blow the shielding gas away from the weld area.
- Least portable.
- Wire feeder, which is additional to the welding machine required.
- Time to change of welding wire increased.
Different Types of MIG Wire.
Depending on your requirements for quality, productivity, and cost, you can select from among three different gas-shielded arc welding wires for your application:
- Solid gas metal arc welding (GMAW).
- Composite GMAW (metal-cored).
- Gas-shielded flux-cored arc welding (FCAW).
- Solid gas metal arc welding (GMAW) wire.
GMAW requires a solid wire electrode or a composite metal-cored electrode. Solid wire electrodes commonly are referred to as GMAW electrodes. The mechanical properties and strength of the weld deposit depend first on the chemistry of the wire and second on the type of shielding gas used. Shielding gas is necessary to protect the weld from the atmosphere. Fabricators often prefer GMAW because the weld deposit is slagless and requires little or no cleanup, which increases efficiency.
Many classifications of GMAW wire exist, each with varying levels of deoxidizers. Highly deoxidized wire can tolerate light to medium levels of surface contaminants. For these applications, look for ER70S-6 wire, which has higher levels of silicon and manganese than an ER70S-3. You also can choose an intermediate electrode, ER70S-4, for applications requiring more deoxidizers than ER70S-3 but less than ER70S-6.
Remember to select GMAW wire from a creditable manufacturer to ensure consistent chemistry, diameter, and feedability from lot to lot or spool to spool. Some imported GMAW wires are labelled as ER70S-6, but actually conform to the European SG2 classification. Many European wires labelled as ER70S-6 do not meet ER70S-6 chemistry required by the American Welding Society (AWS) and therefore might not be suitable for your application.
On blasted plate, solid GMAW wire performs well. On a plate with heavy mill scale, GMAW wire doesn’t perform as well as metal-cored wire or flux-cored wire. Solid GMAW wires don’t deoxidize mill scale as readily, affecting bead shape and travel speeds adversely.
Out-of-position Welding. Solid GMAW wire can be used for out-of-position welding using a short-arc procedure on thin materials, which yields low deposition rates. As an alternative, pulse-spray welding with these electrodes can increase the deposition rates while still providing out-of-position capabilities.
Generally, the strength of a weld deposit made with a solid GMAW electrode is limited to the strength of the electrode. If you have a high-strength, low-alloy base material, it may be difficult to find a solid steel GMAW wire that will meet the base material requirements. For these applications, a metal-cored or flux-cored electrode might be more appropriate.
Post weld Operations.
For certain modes of metal transfer with solid GMAW wire—such as short-arc and globular—some spatter may occur that requires post weld clean-up. Silicon islands can be deposited during GMAW that might need to be removed before painting or coating. However, GMAW with solid wire generally is a clean process that requires minimal post weld operations
Composite GMAW (metal-cored) wire.
Metal-cored wires are tubular electrodes with metallic constituents in the core. Like solid GMAW wire, metal-cored wire produces a no slag weld that requires little or no clean-up. The performance characteristics also are similar to solid GMAW wire. Formerly classified as a flux-cored wire, metal-cored wire now is classified as a composite GMAW electrode.
Metal-cored wire can handle mill scale and surface contaminants better than GMAW wire because of its metallic components. These components help the metal-cored electrode deoxidize the scale better than solid wire, which is the reason this type of wire is a better choice if you don’t want to blast material before welding.
Due to the tubular nature of metal-cored wire, the current density of a metal-cored wire is higher at a given amperage than a solid wire of the same diameter. This can result in higher deposition rates at a given amperage. In some automated applications, large-diameter metal-cored wire can match or exceed the performance of gas-shielded flux-cored wire. However, large-diameter metal-cored wire might not be suitable for semiautomatic welding because of the high amperages used and heat radiated from the arc.
Metal-cored wire should be your first choice if travel speed is your primary concern. On a plate with mill scale, metal-cored wire can produce improved wetting and a flatter bead shape as well as increase productivity because of lower spatter levels (less post weld clean-up) and faster travel speed.
Although it is possible to use metal-cored wire out of position at low amperages in a short-arc mode, metal-cored wire generally isn’t used for out-of-position welding, except for in the vertical-down position. Like with solid GMAW wire, pulse welding can improve out-of-position deposition rates with metal-cored wire.
Metal-cored electrodes are available for high-strength, low-alloy applications. Low-alloy metallic components can be added to the core to achieve the desired mechanical properties. This ability to add components can make it easier to produce the desired mechanical properties with metal-cored electrodes than with solid wires of comparable strength.
Post weld Operations.
You may need to do some post weld clean-up to remove the silicon islands deposited from metal-cored electrodes before painting or coating the base metal. However, clean-up time may be reduced overall because metal-cored wire typically produces little spatter.
Gas-shielded flux-cored arc welding (FCAW) wire.
These tubular electrodes contain fluxing agents in the core, as well as deoxidizers, to provide additional protection from the atmosphere. The flux ingredients can be engineered to enhance the mechanical properties of the weld deposit. FCAW electrodes are available for both out-of-position and in-position welding.
FCAW wire is the most tolerant of the three types of wires for welding dirty base metal. Because it has a flux and is used with a shielding gas, it offers an added layer of atmospheric protection.
For high-deposition applications, large-diameter, gas-shielded FCAW wires often can deposit more pounds per hour than solid GMAW or metal-cored wire.
An exception to this rule is tandem GMAW, which uses two solid wires in one weld pool. Tandem GMAW offers advantages similar to automatic metal-cored welding, often exceeding deposition rates of gas-shielded FCAW wire.
To obtain high deposition rates for out-of-position welding, choose small-diameter—0.035- to 116-inch-diameter—gas-shielded FCAW wire. Wires such as AWS E71T-1 or E71T-12 offer high deposition rates when used out-of-position. The slag from these products is designed to support the puddle when welding vertical-up or overhead.
Because slagging agents and other components are added to their cores, FCAW electrodes can achieve good mechanical properties. FCAW electrodes can be used for many high-strength, low-alloy applications.
Post weld Operations.
FCAW wire requires the most labour-intensive clean-up because of the slag it leaves on the weld. You will need to remove slag between passes in multiple-pass applications and before painting or coating.
MIG Welding Set-Up.
The set-up for MIG welding usually consists of the following parts
- Welding gun/ handpiece.
- Earth Clamp.
Inside the welder you will find a spool of wire and a series of rollers that pushes the wire out to the welding gun. There isn’t much going on inside this part of the welder, so it’s worth it to take just a minute and familiarize yourself with the different parts. If the wire feed jams up for any reason (this does happen from time to time) you will want to check this part of the machine out.
The large spool of wire should be held on with a tension nut. The nut should be tight enough to keep the spool from unraveling, but not so tight that the rollers can’t pull the wire from the spool.
If you follow the wire from the spool you can see that it goes into a set of rollers that pull the wire off of the big roll. This welder is set up to weld aluminium, so it has aluminium wire loaded into it. The MIG welding I am going to describe in this instructable is for steel which uses a copper coloured wire.
Assuming you are using a shielding gas with your MIG welder there will be a tank of gas behind the MIG. The tank is either 100% Argon or a mixture of CO2 and Argon. This gas shields the weld as it forms. Without the gas your welds will look brown, splattered and just generally not very nice. Open the main valve of the tank and make sure that there is some gas in the tank. Your gauges should be reading between 0 and 2500 PSI in the tank and the regulator should be set between 15 and 25 PSI depending on how you like to set things up and the type of welding gun you are using.
**It’s a good rule of thumb to open all valves to all gas tanks in a shop only a half turn or so. Opening the valve all the way doesn’t improve your flow any more than just cracking the valve open since the tank is under so much pressure. The logic behind this is so that if someone needs to quickly shut off gas in an emergency they don’t have to spend time cranking down a fully open valve. This might not seem like such a big deal with Argon or CO2, but when you’re working with flammable gases like oxygen or acetylene you can see why it might come in handy in the event of an emergency. **
Once the wire passes through the rollers it is sent down a set of hoses which lead to the welding gun. The hoses carry the charged electrode and the argon gas.
Welding gun/ handpiece.
The welding gun is the business end of things. It’s where most of your attention will be directed during the welding process. The gun consists of a trigger that controls the wire feed and the flow of electricity. The wire is guided by a replaceable copper tip that is made for each specific welder. Tips vary in size to fit whatever diameter wire you happen to be welding with. Most likely this part of the welder will already be set up for you. The outside of the tip of gun is covered by a ceramic or metal cup which protects the electrode and directs the flow of gas out the tip of the gun. You can see the small piece of wire sticking out of the tip of the welding gun in the pictures below.
The ground clamp is the cathode (-) in the circuit and completes the circuit between the welder, the welding gun and the project. It should either be clipped directly to the piece of metal being welding or onto a metal welding table like the one pictured below (we have two welders hence two clamps, you only need one clamp from the welder attached to your piece to weld).
The clamp must be making good contact with the piece being welded for it to work so be sure to grind off any rust or paint that may be preventing it from making a connection with your work.