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.
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.
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.
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.
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.
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.
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.
Once a welding station has been set up to weld aluminum, we recommend leaving it set up exclusively for welding on aluminum 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.
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.
Fume extraction is also an important issue when welding aluminum. Find out more about this topic in our free e-book: