Aluminium is known for its lightness and strength and is therefore a sought-after material in areas such as the construction of vehicle bodies and space equipment. However, welding aluminium can be difficult due to its rapid oxidation on contact with air.
Aluminium and aluminium welding: what you need to know
In contrast to steel welding, there are more special features to consider here. One of these is that welding is carried out with inert gas. This means that the shielding gas used does not react with the environment. In Europe, argon is usually used when welding sheet metal up to 12 mm thick. Otherwise, an argon-helium mixture is used. The oxide layer must also be removed before welding. The best way to do this is described below.
What does aluminium welding mean?
The term aluminium welding initially indicates that it is the welding of aluminium materials using a selectable process. Various processes can be used for this. For example TIG or MIG. Other processes are not considered here. A distinction is also made between joint welding and deposition welding. As the name suggests, joint welding involves joining two parts together. These can be pipes or sheet metal. In deposition welding, layers are applied to a metal sheet by welding in order to change the surface properties.
Aluminium vs. steel: differences in welding
Welding steel is relatively simple compared to welding aluminium. There are fewer points that need to be considered when fusion welding steel. As already mentioned, the welding gas and the oxide layer must be taken into account.
Types of aluminium alloys and their properties
Aluminium alloys are very diverse and are produced by adding various elements such as magnesium, silicon, manganese, copper and zinc to pure aluminium (Al99.5) in order to achieve certain physical and mechanical properties. These alloys vary depending on the additives and have specific areas of application. There are age-hardenable and non-age-hardenable (naturally hard) alloys as well as wrought and cast alloys. Wrought alloys are processed using forming processes such as rolling or extrusion, while cast alloys can hardly be subsequently formed. Wrought alloys include pure and ultra-pure aluminium as well as various alloys such as AlMgMn, while cast alloys include AlSi and AlCuTi. Special alloys are used in aviation. The various alloying elements improve properties such as strength, hardness and corrosion resistance, but also influence aspects such as seawater sensitivity and weldability, whereby some, such as copper, can reduce corrosion resistance but increase strength.
Tips for gas-shielded welding of aluminium
Please pay attention to cleanliness when welding aluminium. Contaminated material, dirty grinding discs or a damp gas hose can quickly have a negative effect on the result. If black marks appear on the weld seam, this is often an indication of unclean work.
MIG welding of aluminium: a detailed look
The MIG process is a popular method for welding aluminium. Here, an endless electrode, which is rolled up on a reel, burns in the arc between the workpiece and the torch. Gas protection is provided by the gas nozzle on the torch. MIG welding machines are very universal and can also process steel after changing the welding wire, gas and wire flux.
It also makes sense to use a welding power source that can be operated in alternating current mode. This leads to fewer problems with the oxide layer, as this is always broken up during the negative half-wave.
How does MIG welding work with aluminium?
In the MIG process, it is important to incorporate a Teflon wire core into the hosepack. This is because steel wire cores would cause too much abrasion in the hosepack. A suitable shielding gas must also be used. A triangular shape should be selected for the feed roller, as this can transport the soft aluminium wire better.
The advantages and disadvantages of MIG welding with aluminium
The main advantage is obvious. MIG is a very fast process with relatively low heat input. Purchased welding power sources can also be used for steel or stainless steel with minor modifications. However, MIG is not the cleanest process. If fine seams are required, the TIG process may be more suitable.
Choosing the right welding wire and filler metal for aluminium
In contrast to other welding processes, a welding wire is always used. Advice from a specialised dealer should always be part of the welding task. There are thousands of filler metals on the market. The wrong filler material can lead to negative results such as cracks in the weld seam.
Welding aluminium alloys with MIG: Tips
- Preparation: Thorough cleaning of the material with a hot air gun, stainless steel brush and acetone is essential to obtain a clean welding surface.
- Fire protection: The use of acetone instead of thinner prevents the risk of fire as it evaporates more quickly. Remove soiling during welding with a stainless steel brush.
- Better handling: Place the welding torch on your lap to distribute the weight and choose the right filler metal for the material.
- Weld efficiently: Work from the lowest point and press the wire firmly into the weld pool. Apply enough material to have a buffer later when grinding and use double welding for stability.
- Adjust nozzle size: Select smaller nozzles for narrow areas to have better freedom of movement and visibility of the weld pool. Increase the amount of gas accordingly.
- Adjust amperage: Select amperage according to material thickness and adjust during welding if necessary to avoid overheating.
TIG welding of aluminium: How to do it right
In contrast to the MIG process, TIG uses a non-melting electrode. However, this also means that the wire has to be fed in by hand. The filler metal is sold in the form of handy wires in specialised shops. A little tip: Welding rods for TIG can be used for oxyfuel welding, but not the other way round. Pure argon 4.6 can also be used.
How TIG welding of aluminium works
Practise welding on scrap metal first to gain experience with handling the welding machine and temperature control. Thoroughly clean the work area of paint and rust before welding. Heat up the aluminium welding machine and insert the welding rod evenly into the metal to avoid holes. After welding, allow the metal to cool and grind off any unevenness.
Advantages and challenges of TIG welding aluminium
In addition to a welding machine, a certain amount of manual dexterity is also required. As with many other activities, it doesn’t hurt to have previous welding experience. It is not possible to simply correct a welding error. Compared to welding steel, aluminium welding work is often finer and more detailed. It is therefore important, as an inexperienced welder, to carefully consider how to approach this task. It is possible that someone in your personal environment knows someone with the relevant experience who can help. In principle, it is possible to weld aluminium without extensive prior knowledge. Especially in model making, you can try it yourself. To be on the safe side, you can perhaps get one or two additional aluminium sheets. This way, you can continue after a failed attempt without having to take a break.
Choice of tungsten electrode and filler metal for TIG welding
The following tungsten electrodes are usually used for aluminium welding:
- TIG electrodes: The addition of zirconium in welding materials reduces the risk of fusion impurities. Tungsten electrodes used for AC welding of aluminium alloys benefit from this property. However, these electrodes are only suitable for DC welding to a limited extent
- WT electrodes: Lanthanum oxide increases the ignitability of electrodes that are suitable for both DC and AC welding. These electrodes can be used for a wide range of materials, including unalloyed and high-alloy steels, aluminium, titanium, nickel, copper and magnesium alloys as well as for microplasma welding. They are particularly ideal in the low-current range and are a good alternative to WT electrodes.
- WC electrodes: Cerium oxide improves the ignition and re-ignition capabilities of electrodes that are suitable for welding with direct and alternating current. These electrodes are suitable for a wide range of metals, including unalloyed and high-alloy steels as well as aluminium, titanium, nickel, copper and magnesium alloys, especially in the low and medium current range
- And possibly WS electrodes. Follow the manufacturer’s instructions here.
With regard to welding with aluminium wire: SG-AlMg4.5MnZr is often used due to its very low susceptibility to welding cracks, as are SG-AlMg4.5Mn and SG-AlMg5 due to their increased strength.
Common problems when TIG welding aluminium and how to avoid them
- To avoid porosity caused by contamination when welding aluminium, it is important to clean the weld seams thoroughly with degreasing agents, remove all paint and oxide layers and ensure that only dry workpieces are welded.
- Problems can also be caused by the electrode. A dirty, bent or defective TIG electrode can lead to pores and bonding defects and must be replaced, as must an electrode whose diameter does not match the thickness of the workpiece or the welding position.
- When MIG welding aluminium, it is also important to use the correct gas protection consisting of argon, helium or their mixtures in sufficient quantities.
- The welding equipment can also influence porosity. Faults can occur if air gets into the shielding gas flow, the shielding gas flows too short upstream or downstream or the welding wire is fed unevenly. Leaks in the cooling water system can also lead to problems. Welding torches with a closed cooling system are therefore preferable and should be checked regularly together with the hosepacks and cooling water lines and replaced if necessary.
- To prevent the ingress of air, the inclination of the torch should be adjusted and the distance between the gas nozzles reduced. Uniform wire feeding can be achieved by checking the contact pressure of the feed rollers, adjusting the roller axles and the wire inlet nozzle and by using a shorter hosepack.
- It is also important to ensure a good ground connection and to protect the work area from draughts. The workpiece temperature should always be higher than the ambient temperature and it may be necessary to preheat the workpiece. For tack welds, bevel grinding with suitable grinding discs or tools may be advantageous.