Aluminum stud welding has been tried and tested for many years in different industries. However, this requires careful preparation of the surface and suitable welding parameters to receive results that are comparable to those achieved by the welding of steels.
The load-bearing properties of aluminum stud welds are above all compromised by porosity and incomplete fusion. To avoid adverse effects of this kind, a thorough surface preparation and a precise adjustment of the diverse welding and framework conditions are necessary. On the one hand, the welding energy has to be adjusted to the stud diameter, the sheet thickness, and the welding process. On the other hand, a high plunging speed is necessary to prevent a cold plunge.
The treatment of the stud also has a decisive influence on the welding result. To prevent the welding elements from oxidizing and to ensure clean stud end faces as well as optimal conditions of contact, it is essential to store the studs in a dry place. Therefore, the wrapping of the studs should only be opened immediately prior to the stud welding process and unused studs should be promptly sealed again in an airtight manner. Mixing different lots can also have a negative impact. At the same time, the welding studs should be promptly used so as to prevent corrosion due to long storage times.
As a basic principle, the workpiece surfaces should be dry and bare and not have any coatings unsuitable for welding. Dust and fingerprints have just as negative effects on the welding result as oil and deep drawing agents. Paint and anodization may even prevent the lighting of the drawn arc. After the welding process, an insufficiently prepared surface can be recognized by a dark circle around the welded stud. As a rule of thumb, the workpiece must be cleaned the more thoroughly the faster the welding process is conducted.
The greatest challenge when welding aluminum is the natural oxide layer that forms on the light metal under the influence of oxygen. Oxides in the welding zone may have the following effects:
- Decreased hardness/increased brittleness;
- Higher risk of incomplete fusion;
- Increase of the arc blow effect.
The latter occurs when the electrical arc is deflected from its center axis due to changes in the surrounding electromagnetic field. Arc blow can cause asymmetrical melting, or an unfavorable burn through, as well as welding mistakes, such as increased porosity, an uneven weld collar or undercut bases, which have a negative effect on quality.
To remove the oxide layer, it is possible to use mechanical methods (e.g., chip removal) as well as chemical treatments. Manual grinding and unsuitable abrasives, on the contrary, can have negative effects. This may result in scratches on the surface of the oxide layer, which make it harder to remove impurities. Insofar as other methods do not qualify, it is recommended to leave the aluminum oxide on the surface of the workpiece.
During drawn-arc stud welding, a white residue appears around the welded stud. This should be removed prior to welding in the same area to avoid scattering results.
For thin sheets, i.e., sheets with a thickness of about 1 to 4 mm, capacitor-discharge stud welding with tip ignition is primarily used. When using tip ignition, the heat input is very small, because the welding process lasts only one to three milliseconds, and the melting zone is at the most 0.5 mm deep. That is why there is usually no thermal or geometric marking on the reverse side of the workpiece. A shielding gas is not required.
For aluminum welding, the gap method is recommended, which is characterized by particularly shorter welding times and higher plunging speeds than the contact method. Stud diameters from M3 to M6 (dia. 3 to 6) are very suitable, whereas M8 is considered insecure.
For drawn-arc stud welding, the short-cycle process with shielding gas is primarily recommended. This has similar benefits (short welding times, small thermal load of the component, very good automation capacity) as stud welding with tip ignition and, furthermore, has a very high level of process reliability.
Short-cycle drawn-arc stud welding must be carried out with sufficient gas protection so as to prevent a poor weld or decreased load-bearing capacity of the welded joint due to macropores. Ideal shielding gases are inert gases like Argon or a mixture of Argon and Helium.
Also important in drawn-arc stud welding of aluminum is the correct polarity of the welding elements in combination to the sheet metal. In case of smaller sheet thicknesses of up to 2 mm, negatively polarized studs may have advantages; in case of larger sheet thicknesses or stud diameters positively polarized studs have proven successful.
When it comes to aluminum stud welding, welding machines with inverter technology are state of the art. These comparably inexpensive welding power sources have proven successful, for example, in the metalworking industry, in the window, door and facade-wall fabrication industry, in vehicle manufacture and shipbuilding, as well as in the welding of thin sheets. As it is possible to reverse the polarity and set the parameters in accordance with the respective application, the desired quality requirements are relatively easy to achieve.
Since more stringent requirements apply to the moving apparatus than to the welding of steel, consideration must be paid not only to the electrical parameters of the power source, but also to the parameters of the welding gun. Thus, precisely running, mechanically acting magnet-spring systems can map highly dynamic motion profiles, especially with short welding times of less than 20 ms.
When welding vehicle parts (e.g., brackets, bodies), AC inverters have been special used in automotive industry effective for years. Thanks to this AC-technology, the preparation of the component surface requires less effort, since the heat of the alternating current has a cleaning effect. Reversing the polarity is possible in this case as well. This, in conjunction with programming tailored to the respective stud and motor-controlled welding guns or welding heads, makes it possible to meet special requirements for high-quality welded joints for the market-specific special studs used in the automobile industry. Since welding with alternating current and engine-driven welding guns or welding heads, respectively, is more complex in terms of the equipment, this technology is more expensive compared to direct current.
The welding elements or studs for aluminum stud welding with tip ignition are specified in DIN EN ISO 13918. Problem: At the DIN EN ISO 13918 there is no specification for aluminum studs for drawn-arc or short-cycle drawn-arc studs. Currently there is no international standard for welding elements for the drawn-arc processes.
The automotive and shipbuilding industries are regulatory exceptions. In the automobile industry, the technical rules (factory standards) of the manufacturers apply. The welding elements used are generally manufacturer-specific drawing parts. These studs are usually made of AlMg5 due to the high strength requirements. There are similar solutions in shipbuilding.
It is technically not possible to evaluate the welding result of aluminum stud welding by means of completely non-destructive testing methods. To ensure the quality of the weld, measures to control the relevant process parameters and associated pre-defined random checks are carried out.
The assessment of the quality requirements can basically be carried out using DIN EN ISO 14555, which defines, besides the stud welding processes, materials and process parameters, the permitted assessment and quality criteria of stud welds. Further information regarding quality criteria can be found in DIN EN 1999-1-1/NA as well as in the manufacturer-specific technical rules and standards of the automobile industry.
Aluminum stud welding can be used in any aluminum-processing metal construction company. For an optimal welding result, it is however necessary that the user has, in addition to certain manual skills, also knowledge of welding technologies. This knowledge enables them to assess changes and abnormalities, if need be, take countermeasures and thereby achieve a high quality comparable to that of the welding of steel studs.