Expert’s Guide to MIG Welding Aluminium

Expert's Guide to MIG welding aluminium
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As a professional or hobbyist welder, it’s important to get to grips with a range of different materials. Today, we’ll look at one of the trickiest materials of all, in our expert guide to MIG welding aluminium.

As you likely already know, MIG welding uses a feeding wire to create the initial arc. This then melts on the welding surface, forming a joint. This is the most popular form of welding, owing to its great flexibility.

Most new welders learn how to create a MIG joint on mild steel. It’s important to have this down before you move on to different materials. In particular, MIG welding aluminium presents a number of challenges.

To understand these, let’s look at some of the key differences between aluminium and steel.

Difference Between Welding Aluminium and Mild Steel

For welders, the important difference between aluminium and mild steel is the melting point. That is, it takes a lot more heat to melt aluminium. Generally, this is between 21 and 24 volts.

This creates a range of considerations.

For one thing, it means that there’s a relatively high minimum thickness for MIG welding aluminium. Typically, you shouldn’t attempt a MIG joint on a piece of aluminium which is thinner than about 18 gauge, or 1.2mm.

For anything thinner than this, you should use a TIG welder.

Additionally, when aluminium melts, you don’t get the same ‘red hot’ effect as you’d expect when you weld mild steel. People often say an aluminium weld pool looks like wet tinfoil. This means there is a bit of a learning curve for deciding when your weld pool is too hot.

As such, it can be easy to accidentally destroy your aluminium MIG welding projects, at least at first. In fact, accidental burn-through is very common when MIG welding aluminium.

Let’s look at the proper technique, so that you can prevent this from happening.

Technique for MIG Welding Aluminium

When learning to weld any new material, it’s crucial that you learn and practise the correct technique. Once you have mastered this, MIG welding aluminium is actually a fairly simple process.

Spray Transfer Welding

Most of the time, you should MIG weld aluminium using what’s known as the spray transfer method. This is when tiny droplets of molten metal are transferred across your welding arc, similar to how water comes out of a garden hose.

To achieve this, you need to use a welding arc which is above the ‘transition’ threshold of your feed wire. This can be done with either constant current or constant voltage power supplies, although CV welding is more common for aluminium.

Angle and Gas Flow

Luckily, there are still some similarities between MIG welding aluminium and mild steel. For example, there is generally no different in the best angle of attack for each material. When MIG welding aluminium, aim for a 10-15 degree attack angle.

It’s best to set your gas flow to 20-30 CFH. It’s essential to get the right gas flow when welding using the spray transfer method. If this is either too high or too low, you can end up with porosity in your joints.

Gas flow meters are vital for mig welding aluminium
Use a gas flow meter to ensure you create the perfect spray transfer weld.

Additionally, many people overestimate the right gas flow when MIG welding aluminium for the first time. However, as well as porosity, this can easily lead to weld turbulence where gas flow exceeds around 40 CFH.

Travel Speed and Direction

This is where things get difficult for many people MIG welding aluminium for the first time. One of the common rules of thumb you’ll see is that if you’re using the same voltage as you would for mild steel, you should double your travel speed.

At first, this can be very difficult, simply because it’s a challenge to move with precision at high speeds.

Additionally, selecting the right direction of travel for MIG welding aluminium is crucial. When welding any material, you have two options:

  • Pushing – where you angle the welding gun towards the direction of travel,
  • Pulling – where you angle the welding gun away from the direction of travel.

When MIG welding aluminium, you should always push. This gives you a clean, flat joint, as well as ensuring that you have adequate gas coverage and penetration. 

MIG Welding aluminium

Equipment for MIG Welding Aluminium

By now, you understand the theory behind MIG welding aluminium. However, it’s also worth noting that you might need some different equipment to MIG weld aluminium than you would for mild steel.

Let’s take a look at the key gear you’ll need.

Gas

The most common welding gas is known as C25, meaning that it’s a mixture of 25% carbon dioxide and 75% argon. However, for most aluminum MIG welding projects, you’ll need 100% argon gas.

This is necessary to ensure a successful spray transfer weld.

Additionally, you’ll need a different gas mixture if you’re welding thicker pieces of aluminium. Here, helium is added to the gas mixture to create sufficient penetration in your joints. Tis can vary between 25% and 75% helium.

Wire Feed Problems when MIG Welding Aluminium

Of course, when you MIG weld aluminium, you should set up your wire feeder for aluminium wire. The trouble is that aluminium wire is a lot weaker and softer than steel wire. It also expands much more quickly. Invest in “U” groove drive rolls and a Teflon liner for your MIG torch.  Keep your MIG torch to the shortest length possible. 

Otherwise, if you try to use “V” groove drive rolls and a steel liner which is common for mild steel wire, it will create a range of practical problems, as the wire becomes stuck in your feeder.

This can create a range of practical problems, as the wire becomes stuck in your feeder.

Luckily, there are also a number of easy workarounds for these. For example, many welders choose a larger welding tip when working with aluminium. Where a tip is rated for 1mm, they might use 0.8mm wire. 

Use the largest size wire your machine will run, this will make it easier to feed the wire.

A good option is to use a spool gun, instead of feeding your wire through a standard mig torch. This is because aluminium wire can often become jammed in the liner of your welding guns as it is fed along.

Safety Equipment for MIG Welding Aluminium

As you should already know, safety equipment is pretty much the most important investment you can make as a welder. This applies just as much to MIG welding aluminium. In fact, given the techniques you’ll use, there are a couple of extra concerns.

Specifically, spray transfer can create some additional risks.

Once a spray transfer arc is initiated, it is constantly on. This means that it is particularly important to use an auto-darkening helmet with a high sensitivity, to respond to minor variations in the light produced by the arc.

Additionally, spray transfer welds should only be performed in the flat or horizontal position. Because of this, it’s important to have a sufficiently clear workstation, and you may need additional clamps or organisational tools to achieve the right position.

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