What is MIG welding and how does it work?

Metal inert gas (MIG) welding joins together two pieces of metal, using an electrical supply to fuse the base metal with a consumable electrode filler.

Also known as gas metal arc welding (GMAW) or simply wire welding, the MIG process differs from TIG in that it uses a consumable wire that acts as both the electrode and the filler material. Whereas TIG welding simply relies on a tungsten tip to heat and join the metal surfaces directly.

In this blog, we answer some frequently asked questions, such as ‘what is MIG welding’, while exploring pros and cons of the process.

What is MIG welding?

MIG welding uses a constant voltage power supply to create an electric arc that fuses the base metal with a filler wire that is continuously fed through the welding torch. At the same time, an inert shielding gas is also fed through the gun, to protect the weld pool from atmospheric contamination. (Inert gases do not react with the filler material or the weld pool.)

MIG welding is generally used for heavy duty fabrication projects such as metal gates. Compared to TIG welding, it is much faster resulting in shorter lead times and lower production costs. Additionally, MIG is easier to learn and produces welds that require little to no cleaning and finishing. However, TIG will always be the first choice where precise, strong and very clean welds are primary concerns.

The MIG welding process

The welding process creates powerful joints for structural efficiency and integrity, and there are many different techniques. With MIG, the process starts with a pre-weld clean up: MIG electrodes are not resistant to rust, dirt, oil, etc, so it’s vital to start with a contaminant-free area.

Here are 3 top tips for how to MIG weld:

1. Position and angle

Getting the right position and angle of the gun is vital for successful MIG welding to avoid the filler sagging or rolling over the side of the weld joint. MIG can weld from flat, horizontal, vertical and overhead positions. According to the type of weld and metal involved, you also need to take work and travel angles into consideration:
Travel angle: defined as the angle relative to the gun in a perpendicular position. Normal welding conditions in all positions require 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.)
Work angle: this is the gun position relative to the angle of the welding joint, and it varies with each welding position and joint configuration.

2. Speed

The rate at which you move the gun along the join influences both the shape and quality of a weld bead to a significant degree. Many experienced MIG welders determine the correct travel speed by judging the weld puddle size in relation to the joint thickness.

3. Modes of metal transfer

Metal transfer describes how the filler metal (electrode) is processed through the welding arc and into the weld puddle:

• Short-circuit welding: uses the lowest current and voltage ranges, resulting in low weld heat input. It is typically used with smaller diameter filler wire, producing a relatively small and easily controlled weld pool. It’s well-suited to out-of-position welding and joining thin sections. The low heat input makes short-circuiting transfer susceptible to incomplete fusion (cold lap) defects, especially when welding thick sections or during multipass welds.
• Globular transfer: requires higher current and voltage levels than short-circuiting. It is characterised by large, irregular drops of molten metal. Since the force of gravity is critical for drop detachment and transfer, globular transfer is generally limited to flat position welding. It can create inconsistent penetration and uneven weld bead contour leading to defects.
• Spray welding: increase the welding current and voltage further to cause a high deposition rate similar to a water hose. This technique is used for joining thicker materials, as the tiny little droplets of molten metal lead to greater penetration. Spray welding offers strong, aesthetically good-looking welds with little spatter.
• Pulsed mode: lower heat input means this mode is ideal for welding thinner materials. The material is transferred in a controlled droplet form while the pulse mode creates spatter-free welds.

Materials used in MIG welding

Let’s take a look at MIG equipment and materials and their role in the process:

Wire electrode

With MIG welding, the electrode carrying the current, also acts as a filler. There are 2 basic types of wire electrodes available according to the projects and metals involved:

• Hard wire: used by most MIG users due to its affordability, ease of control and versatility at different angles. Typical hardwires comprise a 75:25 ratio of argon and carbon dioxide.
• Flux-cored wire: the flux built in to the wire itself negates the need for shielding gas. This, in turn, makes MIG welding more suited to working outdoors and in windy conditions. On the downside, flux-covered wires create slag during welding which needs cleaning off.

Inert gas

As the metals fuse in the welding pool, shielding gas is supplied through the welding torch keeping the weld pool free from contamination without affecting the metal’s properties. Shielding gas selection in MIG welding depends on the material being welded and the application. The most commonly used are argon and helium sometimes mixed with other (semi-inert) gases, such as carbon dioxide, to reduce cost.

Cheaper than argon and helium, carbon dioxide allows for deeper penetration but also creates more spatter in the weld pool. In specific situations, non-inert gases are used in very small quantities to further increase metal penetration. However, the resulting oxygen creates rust and oxidation on the weld can affect its quality.

Welding torch

A welding torch, or gun, is a specialised tool for fusing and melting metals. MIG torches are versatile in their application to a variety of metal types and thicknesses. MIG torches are divided into two groups:

• Gas-cooled welding torches: overheating can become an issue so these torches are generally limited to minor projects.
• Water-cooled welding torches: can be used at higher amperages and provide more power. They also offer smoother control over the contact tip of the nozzle. However, they cost 20-30% more than gas-cooled torches and require a water-cooled system to be installed in the welding machine.

There are 4 different nozzle types used in a welding torch: recessed, flush, protruding and adjustable. The choice is determined by the type of wire electrode being used.

Power source

Modern MIG welding equipment auto-corrects the current when the arc length and wire feed speed change, creating a stable weld puddle. These are your choices for both TIG and MIG welding:

• DC positive polarity: in DCEP (direct current electrode positive) or reverse polarity, the electrons flow from the contact tip of the electrode to the base metal. This is the most widely used setting since it offers a stable arc, ensuring better bead quality, weld penetration, and less spatter. Suitable for welding both thick and thin materials.
• DC negative polarity : in DCEN (direct current electrode negative) or straight polarity, the electrons travel from the base metal to the tip of the electrode wire. This method offers faster deposition rates than DCEP but it has several drawbacks such as lack of penetration and not enough heat in the weld pool. Not suitable for thicker materials but is sometimes used for welding thin metals.
• AC power: AC is hardly ever used in MIG welding. TIG welding of aluminium requires AC in which the machine is constantly switching between positive and negative currents. The alternating current provides a cleaning action to remove the oxide film present on the surface of the metal.

Advantages and disadvantages of MIG welding

First, let’s look at the advantages of MIG welding:

1. MIG welding is a versatile technique suitable for various types and thicknesses of metal
2. The continuously fed wire enables fast, uninterrupted welding resulting in shorter lead times and lower production costs
3. MIG welding is a relatively clean welding processes, leaving little slag and minimal spatter
4. A MIG torch handles horizontal, vertical or flat welding positions with ease
5. MIG welding is one of the simplest welding techniques to learn

Disadvantages of MIG Welding can be described as:

1. Though cleaner than most welding processes, MIG cannot better TIG for ‘aesthetic-quality’ projects involving thin sheet metal
2. MIG welding equipment has a relatively high initial set-up costs
3. With limited deoxidants available in the process, all rust must be removed from the workpiece before MIG welding can commence.

Frequently Asked Questions

These are FAQs relating to MIG welding:

Where is MIG welding used?

MIG welding is a cost-efficient and diverse welding process, making it one of the most attractive welding methods, especially in industrial environments. It is used extensively in fabrication with thicker materials.

Is MIG welding expensive?

A good MIG welding machine is an investment. Set-up can be expensive but shorter lead times and lower production costs are can also be key outcomes.

Why is there spatter in MIG welding?

MIG welding uses electrode wire that bonds with the metal to form the weld. Spatter is the excess molten droplets wire that bonds generated around the welding arc. Once these molten droplets land and cool, they create spatter around the worksurface and sometimes on the product you are actually welding. Good practice can reduce spatter however.

In MIG welding, which gas is used?

Active shielding gases used in TIG welding are carbon dioxide or mixtures of argon, helium, carbon dioxide and oxygen, depending upon the project.

Is MIG welding easy or hard to learn?

Successful MIG welding calls for skill and experience, like any welding process. That said, it is one of the easiest welding techniques to learn.

Is MIG welding DC or AC?

The power source in a MIG welder is mostly set to DC as it offers constant voltage. This is the most widely used setting since it offers a stable arc, ensuring better bead quality, weld penetration, and less spatter. AC is hardly ever used in MIG welding.

In MIG welding strong?

All welds are stronger than the base metal because they are fused with filler material. However, if the metal is not clean, the weld will be weak or brittle. You can make a MIG weld even stronger by controlling the speed of the weld, the level of voltage and the depth of penetration.

Will MIG welding stick to brass?

Yes. You will need to use a shielding gas that comprises argon on its own or mixed with carbon. For example, a 75:25 ratio of argon to carbon dioxide gives excellent results.

Can MIG weld stainless steel?

Yes. Cleanliness of materials and workspaces is particularly important in welding stainless steel. Stainless steel is corrosion resistant, but it’s also susceptible to contamination so avoiding even the tiniest particulate is vital.

MIG welding: a cost-efficient and versatile process

MIG is a fast and versatile welding process, especially suited to fabrication projects where speed and cost are key considerations. Let Alroys metal welding experts help you decide if and how MIG welding is the best option for your fabrication projects.