Modern transport systems have the challenge to integrate more and more functions. This increases the weight of the structures. On the other hand demands and the legal regulations for emissions can only be fulfilled if the weight is reduced. This results in an ongoing increase of the usage of lightweight materials. Due to its low density and high strength Aluminium Alloys are the most used lightweight metals. However, some other physical properties hamper the processing of these alloys. The publication shows ways to overcome these challenges applying appropriate material preparation and handling in combination with specialized welding equipment for Aluminium welding. Application examples demonstrate the state of the art in Aluminium welding.
The low density and the high specific strength of Aluminium alloys are the main reasons for the increased usage.
The graph clearly shows that Aluminium use in cars will continuously increase over the next years. Other sources also show the material distribution for different materials in automotive industry since 1975. The usage of Aluminium was rising from 3% to 13% in the last 40 years.
The challenges to apply Aluminium are created by the high thermal conductivity, the melting points of the oxydes and the solubility for Hydrogen. In alloys additionally so-called hot cracking occurs due to the solidification kinetics.
| Al | Fe | |
|---|---|---|
| Density r [kg/dm³] | 2.7 | 7.87 |
| Cristal lattice | cfo | cro |
| Young’s modulus [N/mm²] | ≈71*10³ | ≈210*10³ |
| Tensile strength Rm [N/mm²] | ≈50 | ≈200 |
| Melting point [˚C] | 660 | 1539 |
| Specific heat [J/kg˚C] | ≈896 | ≈460 |
| Electrical conductivity λth [m/Ωmm²] | 36 | 8.3 |
| Thermal expansion coefficient [1/˚C] | 24*10−6 | 12*10−6 |
| Oxides | Al2O3 | FeO Fe2O3 Fe3O4 |
| Melting point of the oxides [˚C] | ≈2050 | ≈1400 ≈1455 ≈1600 |
| Thermal conductivity [W/mK] | 235 | 78 |
The high thermal conductivity requires a high heat-input, which in turn requires high rating of the welding equipment used.
The high melting point of Aluminium Oxyde of 2050˚C compared to that of Aluminium of 660˚C results in problems with energy coupling and instable welding processes.
The high solubility of hydrogen in liquid and the low solubility in solid hydrogen results in pore formation and hot cracking.
These problems require special attention during weld preparation, the weld itself and the welding equipment used [
In order to achieve a proper preparation both the Aluminium components and the used filler materials (mainly wires) have to be dry and clean and should be stored only for a minimum time to prevent oxide growth.
In automotive applications aluminium sheets get special coatings to prevent the uncontrolled growth of oxydes. In aerospace environments Aluminium sheets are even acid washed before welding. Other industries just use mechanical cleaning by brushing.
To lower the oxygen contents larger wire diameters for the filler wires are recommended in order to minimize the surface.
Using appropriate equipment all generally used thermal welding processes can also be used for Aluminium alloys e.g. MIG, TIG, Plasma, Laser and Laser-MIG hybride processes.
In the beginning mainly TIG processes were used for Aluminium welding. Applying AC welding the oxide layer could be removed with the positive wave and the penetration was achieved with the negative wave [
For higher sheet thickness also Plasma welding can be used. Here welding with both DC or AC current is possible. In plasma welding higher welding speeds and even better surface quality is possible compared to TIG welding [
To increase the welding speed also MIG welding is possible, because the metal transfer rate of MIG is higher as compared to TIG welding [
speed is too high, hydrogen in the liquid metal may not be able to escape before solidification and then form pores. For thin sheet applications also pulsed welding, AC welding or modified short arc processes are used.
In order to optimize wire feeding for Aluminium Welding ABICOR Binzel developed a special wire feeding concept called Master Feeder System (MFS).
Manual applications for Aluminium Welding are well-known.
The TIG torch that is applied here, has a very high rating, combined with a small volume. The small volume allows welding also in very restricted areas. These ABITIG torches have only three spare parts, compared to the so-called Linde style TIG torches with 5 parts that have to be replaced regularly.
The first industrial robotic applications were developed in the 2000 nds. ABICOR Binzel introduced robotic welding of Aluminium in the first Audi A8 series. Several other models followed.
Today also the Audi A6 axles are welded using the ABICOR Binzel MFS system and ABIROB A torches.
The challenges that Aluminium alloys show for thermal joining processes can be overcome by process knowledge, correct weld preparation and by using appropriate equipment. This allows making use of the huge lightweight potential of Aluminium alloys especially in lightweight design.
The use of Aluminium applications is still increasing and thermal joining processes will have to be further elaborated to increase reliability and compatibility with surrounding components e.g. the different types of welding robots.
Also welding of material combinations e.g. Aluminium to steel or to Titanium has to be treated.
Schubert, E. (2018) Challenges in Thermal Welding of Aluminium Alloys. World Journal of Engineering and Technology, 6, 296-303. https://doi.org/10.4236/wjet.2018.62018