What really matters when welding aluminum
Frames of bicycles or motorcycles, trailers for trucks, profiles of rail vehicles, materials in space travel – aluminum is THE material when it comes to reducing weight and still achieving stability. In addition, a beautifully welded aluminum seam is a real eye-catcher.
Thanks to its low density and good strength, aluminum has become an integral part of modern production. In addition to all the advantages, there are also some tricky aspects when processing this metal. Anyone who has ever accidentally burned a hole in an aluminum sheet knows what we are talking about. Welding aluminum requires special knowledge and skills. Read more in this blog about what is important for aluminum welding and how to properly weld aluminum.
What makes aluminum welding so difficult
Aluminum has one property that makes welding this metal so difficult: as soon as aluminum is exposed to the ambient air, it forms a wafer-thin layer of aluminum oxide. And it is this layer that gives the metal its unmistakable silver-gray appearance. But it is also makes the aluminum corrosion-resistant to water, oxygen and even many chemicals. It protects the aluminum, so to speak. This protection must first of all be literally »cracked« because, like a solid armor, the oxide layer prevents the arc and the weld pool from forming a connection.
The oxide layer has a melting temperature of 2050 ° C, aluminum itself melts at approx. 660 ° C. So you would have to apply a temperature three times higher to the surface to crack the oxide layer by welding alone. With such a high energy input, there is a great risk that the aluminum will melt away as soon as the oxide layer is broken. For this reason, it is essential that aluminum has to be prepared for a welding process: the oxide layer has to be taken off.
If you observe and implement the following 5 points, you are doing everything correctly when welding aluminum
1. Prepare properlyBefore getting to the oxide layer, any kind of dirt such as grease or oil must be removed from the workpiece. For welding aluminum, a clean surface of the workpiece is a basic requirement for a good welding result. It is best to use a microfiber cloth that you have previously soaked with solvents such as butanol, acetone or paint thinner. Make sure that no dirt remains, as this would burn in easily during the welding process and would be more difficult to remove afterwards.
Once all grease lubrication and grease residues have been removed, you can start removing the aluminum oxide layer. On the one hand, this works well with a brush or a fleece, i. e. mechanically. A brush leaves scratches in the soft aluminum, which are often not wanted and also do not look nice. Instead, we recommend using a synthetic fleece that contains particles bound in the synthetic, with which the layer can be removed comparatively gently. Since aluminum oxidizes again after a few minutes, depending on the ambient conditions, you may have to go through the entire preparation procedure several times. Black, sooty residues after welding on the metal indicate impurities from either the gas, the base material or the welding wire. Cleanliness when welding aluminum is very important.
On the other hand, the oxide layer can also be broken up in the welding process by welding with alternating current, where there is a change between positive and negative half-waves. The oxide layer is broken up with the positive half-wave. The welding depth, the so-called penetration, is achieved with the negative half-wave. The electrode also cools down again. This results in an ideal combination of the two half-waves in AC welding.
Especially when welding with alternating current, another thing must be taken into account: With the electrode you weld a ball on the front, the so-called calotte. With this you can push the melted oxide layer forward like clods so that they do not get into the weld seam.
If you want to weld thicker sheets of aluminum– slightly more than 10 mm –, we recommend preheating the workpiece. Without preheating, too much heat would go out of the process into the workpiece during welding, which would make the formation of the weld seam considerably more difficult.
2. The selection of the welding torch
Aluminum can be welded using different processes, namely TIG, MIG and plasma. The TIG welding process with alternating current is mostly used for thinner sheets. Butt joints in particular can be welded well with a TIG torch. For example, if you want to connect two 2 mm thick sheets of aluminum, you have to make sure that the edge on the back is broken. Only then there will be a nice root, proper wetting of the weld metal and a high quality weld. Thicker sheets are preferably processed using the MIG welding process, because these torches can be used to achieve a higher melting rate through the wire. MIG torches are particularly suitable for fillet welds in order to properly introduce the welding filler. The plasma welding process with direct current has the advantage that the heat can be introduced into the workpiece in a targeted manner, but it is a very challenging process.
Another tip: If you are welding aluminum with a MIG welding torch, it is advantageous to use a curved torch neck with a bend of at least 22 degrees. This means that you always have a forced contact that allows the wire to run better and ensures good current transfer.
Aluminum can also be welded using a laser and laser-MIG hybrid process. However, the most widely used process for welding aluminum is the TIG process with alternating current.
3. Special equipment for aluminum welding
With the MIG / MAG welding process:
Equipping the torch with the correct wear parts for aluminum welding should not be forgotten either. A common mistake is that a liner made of steel is used. However, if the wire were is fed, it would rub against the steel of the liner and inadvertently bring steel particles into the weld pool. For welding with aluminum wire, we recommend a special liner made of carbon PTFE, which, thanks to its graphite content, enables better lubricity. The contact tip used must have a larger diameter compared to welding other materials because aluminum, as a very good heat conductor, expands more than, for example, steel wire. On the other hand, it could lead to wire feeding difficulties or burnback. In the case of aluminum welding, such a burnback is constantly present, which is due to the oxides on the surface. This is because they act like an insulator that interrupts the flow of current from the wire to the workpiece. If the wire is fed continuously through the torch head, it will destroy the wearing parts. Another reason why the oxide layer must be removed.
With the TIG welding process:
It is important when welding aluminum in alternating current with the TIG process that the tungsten electrode is selected correctly, because no oxides may be included. For this reason, you should use the undoped, green electrode made of pure tungsten, especially for welding aluminum alloys, which ensures good arc stability. However, the tungsten components of the electrode can emit, which in turn can contaminate the base material by causing small amounts of residues to accumulate in the weld. You can see this through small white dots, which is an indication of a binding error. This is where the previously mentioned welding of a spherical cap comes into play in order to drive the oxides like clods forward. The purple E3® made of rare earths or mixed oxides is an alternative to pure tungsten electrodes. It consists of tungsten as a carrier material and also lanthanum as a doping element. In addition there are the rare earths, such as ytrium, which give the E3® its outstanding stability.
4. Wire and wire feedingSince the metal aluminum is very soft, not only special wear parts for aluminum welding have to be used, but special transport rolls for the wire feed from the welding machine. These should have a U-groove to prevent a risk of deformation. It is difficult to convey an aluminum wire over three and more meters. Push-pull systems are very suitable for mechanized welding, in which the welding torch has a drive and pulls the wire and a feed motor in the welding machine transports the wire to the front.
Depending on the base material and the desired properties of the joint, the additional material is selected. AlMg alloys have a higher strength than, for example, AlSi filler wires.
5. The right shielding gas
Inert gases – argon or argon mixtures – are used as protective gases for welding aluminum. Pure argon is used up to a sheet thickness of approx. 12.5 mm. In the case of thicker material, the argon is mixed with helium in order to get a higher temperature coupling in the welding process. The proportion of helium in the protective gas varies from 25 % to 75 % depending on the recommendation.
Another note about the gas:
What you should always have in mind is the dew point of your shielding gas. Argon 4.6 is mostly used for welding, which has a purity of 99.996 % and thus a dew point of -62 ° C. As long as your gas supplier meets these specifications, you have no problems with the gas that comes straight from the bottle. However, moisture can always find a way through the gas line, for example, into the welding process, which then increases the dew point. So check the gas pipes regularly for condensation. If the humidity is too high, hydrocarbon can get into the weld seam and make it porous or otherwise faulty.
Our recommendation for welding aluminum
Once a welding station has been set up to weld aluminum, we recommend leaving it set up exclusively for aluminum welding and setting up another station for welding steel. Of course, only where that is possible. So you can quickly switch from steel to aluminum without having to change everything and readjust it. You simply change the workplace.
And after welding aluminum
After welding, some aluminum alloys have to go through a hardening process whose degree of hardening can be influenced. So you can directly influence the hardenability when choosing the aluminum alloy. The hardenable alloys include AlZnMgCu, AlZnMg, AlMgSi and AlCuMg, while AlMn, AlMgMn, AlMg, AlSi and AlSiCu fall under the non-hardenable aluminum alloys. Depending on the composition of the aluminum alloy, the temperature introduced and the temperature during the aging process, the lattice structure of this metal changes and thus the mechanical properties. To prevent the dissolved alloy elements from precipitating too early and to control their distribution, the correct aging temperature after welding hardenable aluminum has to be ensured. Since the aging temperature also affects the level of strength, the following applies in principle: the achievable strength decreases with increasing temperature. This means that the highest solidness is achieved with cold curing – which usually takes place at room temperature. Any diffusion is suppressed by sudden cooling. At this point, this is only a little background information on the subject of outsourcing aluminum after welding. If you want to read more, you can find good specialist literature on the Internet such as www.hochschule-technik.de or www.maschinenbau-wissen.de.
Admittedly, welding aluminum is a bit more complicated than welding steel. But if you implement our tips and recommendations, you will soon become a real specialist in aluminum welding.