The Ultimate Guide
Aluminium is a lightweight, thin metal that is used very commonly in welding workshops. With this metal, welding has to be done at low temperatures, or else you will just end up blowing holes in your materials.
Aluminium is quite reactive, so forms an oxide layer when it’s exposed. This poses an obstacle to welding as it results in porous welds, so you’ll need to scrub the oxide layer off using a wire brush prior to welding. However, you’ll have to work quickly as it will take just a matter of moments to form again. Learning how to weld aluminium is more challenging than welding steel, as it requires a whole lot more care, consideration, and specialized equipment.
Each different type of welding requires different equipment and techniques, but once you understand everything that’s involved, aluminium welds look clean, smooth, smart, and they’re very resilient.
Why use aluminium?
Aluminium is one of the most popular materials to use for welding, and there are several good reasons for that. Being thin, it’s incredibly lightweight, but it’s still very strong and resilient. Pure aluminium has a melting point of around 1220ºF (660ºC), and it’s highly conductive of both heat and electricity.
Furthermore, aluminium can be alloyed with many other metals, which makes them even stronger and more durable.
There are many different types of aluminum alloy, so it can be tricky trying to remember the details of each one. There is a system of classification which gives each aluminium alloy a four-digit number, and it’s the first digit which counts. Here’s a quick breakdown of what each number means:
1XXX: Aluminium alloys which begin with the number 1 are very pure. They are made almost entirely from aluminium, coming in with an aluminium content of over 99%.
2XXX: Typically used in aircraft fabrication, alloys beginning with the number 2 are usually made from copper with an aluminium cladding. They are very strong, but they aren’t very resistant to corrosion.
3XXX: Aluminium alloys beginning with the number 3 contain around 1.5% manganese. They aren’t heat treatable, but they’re very easy to work with so make a great starting point for anyone new to the aluminium and aluminium alloy welding scene.
4XXX: It’s not uncommon to find welding electrodes beginning with the number 4. This type of alloy contains silicon, which can dramatically reduce the melting point of the metal.
5XXX: Alloys beginning with the number 5 contain magnesium; they’re fairly easy to use, they’re strong, and they’re corrosion-resistant, but they’re not the strongest. Although welding aluminium usually uses relatively low temperatures, you should try to avoid temperatures which are too low with these alloys.
6XXX: These alloys are fairly strong and versatile because they are heat treatable. This is because they contain a proportion of both silicon and magnesium.
7XXX: Also used for aircraft, alloys beginning with the number 7 have very high strength. They contain zinc – and usually magnesium on top of that – which makes them heat treatable.
GMAW / MIG Welding Basics
MIG welding aluminium is possible, provided that you set up your equipment (e.g. a plasma cutter or multiprocess welder) to direct current and reverse polarity settings. This welding is possible in any position, although more challenging positions should be saved for more experienced welders.
Before you can start MIG welding aluminium, you’ll need to get hold of a tank of shielding gas. Plain argon gas is ideal, as this will result in a really smooth steady arc. However, an argon helium mix will also work effectively. The main bonus to using argon/helium gas is that it will give deeper penetration, but be careful of this if your metal is very thin. If you do decide to choose a shielding gas made up of a combination of both, something which contains around 75% helium and 25% argon should give you the best of both.
Aluminium filler wire tends to jam the system very easily, so try using either a push-pull wire feed system or a spool gun to avoid this hassle. If you are working in tight spaces or doing a lot of out-of-position work, then the spool gun should be your preferred choice.
How to MIG Weld Aluminium
Strike a clean welding electrode around an inch from the start of your weld, then quickly move it over to the point where you want to start welding.
Use a string bead technique as you move along the weld, but be careful not to change the angle of the electrode as you proceed. You should be moving fairly quickly anyway, but as you near the end of the weld, try to increase the speed. This will taper off the size of the weld pool, and in turn, reduce the amount of cracking.
It is a good idea to point the gun in an upwards direction when you’re welding in the horizontal position.
GTAW / TIG Welding Basics
TIG welding aluminium is usually preferable to MIG welding, because it gives very clean, smooth, neat results.
You need to be selective when choosing your TIG welding equipment. When used with aluminium, TIG welding must be done with AC (alternating current) – DC (direct current) simply isn’t suitable. Not all TIG welders are set up for AC, so make sure you read the specific details of each machine carefully before attempting to weld.
Furthermore, heat control is important with aluminium welding as the metal is so thin. As a result, a welder which has a pulse function is ideal as this will prevent the temperatures from rising too high.
Once again, just as with MIG welding, you will need a shielding gas which should contain argon or a mixture of argon and helium.
Distortion is a common problem when welding aluminium – due to the high temperatures – so you might want to consider tack welding before you start properly. This will save you a lot of hassle and wasted time further down the line.
How to TIG Weld Aluminium
TIG welding will require you to use both hands, so make sure that you’re wearing a suitable pair of welding safety gloves and a welding helmet to protect yourself while keeping your hands free.
On one hand, you’ll be holding the electrode holder, while the filler rod will be held in your other hand.
With scratch start and lift start techniques, there is a risk that some of the tungsten from the electrode will be left behind on the metal, contaminating the weld. In order to avoid this, form an arc on a scratch block to heat up the electrode, before breaking the arc and starting again on your weld joint.
Alternatively, high-frequency starts do not require your electrode to touch the metal at all, so check the specifications of your machine to find out whether it has this capability or not. Regardless of which start technique you use, you should always wait for a weld puddle to form before starting to move along the length of the weld.
Move along the joint, slowly moving backward and forwards a little the whole way, ensuring that your filler rod and electrode never touch.
Once you reach the end of your weld, if you just break off the arc suddenly, you will end up with cracked, defective welds. You can avoid this problem by gently reducing the current towards the end, which is very easy to do if you use a foot control.
How to Weld Cast Aluminium
Cast aluminium is much more difficult to weld than pure or alloyed aluminium, as it is already impure and porous. As soon as the metal heats up, the impurities and air pockets will bubble to the surface.
When it comes to your shielding gas, you will need to use argon gas, rather than anything mixed with helium or carbon dioxide.
TIG welding is the best welding process for these jobs, as it gives you greater control over the welds, especially in terms of heat and speed. Including elements of silicon in the job, especially in aluminium alloys, can seriously improve the final appearance of the welds, so consider using these metals in your work.
How to Weld Aluminium to Steel
Aluminium is a relatively soft, thin, lightweight metal, while steel is much tougher, heavier and thicker, so joining the two can be very difficult.
There are two easy methods for welding aluminium to steel, although both require more time and work than ordinary welding.
The first is to use a specially made piece of metal called a bimetallic transition insert, which essentially just means a small piece of metal which contains two different metallic elements. One of these materials is aluminium; the other has been welded (not using arc welding) in place already and can easily be joined to steel.
The first step in using these inserts is to weld the aluminium side of the insert to the metal part of your job; the second step is to weld the steel to the other side of the insert.
Having several steps will take more time, but it’s a really effective way of creating strong, effective welds between aluminium and stronger metals.
Alternatively, you could try dip coating the steel with aluminium, or coating it in silver solder. These techniques are more expensive and also require another challenging skill set, but they do make it much easier to weld pieces of steel to aluminium.
Other Things to Remember
Unlike steel, aluminium does not change its appearance as it heats up. It stays silver: there is no red-hot glow. As a result, you need to pay close attention to make sure that your materials are not overheating, or else it will fall apart and destroy the entire job. There is no easy, obvious way to do this – practice and experience will tell you to know from the feel of the machine, the metal and the welding that everything is getting too hot.
In addition, regardless of your welding position, you should aim to weld away from you rather than bringing the torch towards you. In other words, you should be pushing the weld rather than pulling. On the face of it, this might not seem like a big deal, but pushing will reduce the likelihood of having porous, defective welds.
By now, you should be able to see that aluminium welding presents many more challenges than welding steel or other metals. However, all the extra practice and expense on specialized equipment will pay off, as the results will look really professional. Aluminium is a great material to work with, so it’s a must-have skill for anyone who takes their welding seriously.