Modern transportation manufacturers are increasingly faced with the challenge of integrating more and more functions into their vehicles. This, in turn, increases the weight of the vehicles that still must adhere to legal regulations for emissions and fuel efficiency – regulations that often can only be fulfilled if the weight of the vehicle is reduced.
To achieve these weight reductions while also injecting more functions into today’s vehicles requires an ongoing increase of lightweight materials in their construction. Low-density, high-strength aluminum alloys are among the most used lightweight metals today.
Welding aluminum alloys, however, can be challenging. Some of the physical properties hamper the processing of these materials, but there are ways to overcome these challenges. From applying appropriate material preparation and handling to specialized welding equipment, proper welding of aluminum alloys can be better achieved.
Potentials and problems
Aluminum has been used in cars for decades, but since the 1970s, its use has increased year over year. As shown in Figure 1, aluminum usage has risen significantly in the last 40 years. And as the forecast shows, the trend will be a lasting one.
Compared to steel, the challenges with aluminum are created by the high thermal conductivity, melting points of the oxides and solubility for hydrogen.
To better understand the differences in the two materials, Table 1 compares typical properties of aluminum to iron. In aluminum alloys, so-called hot cracking occurs due to the solidification kinetics. Furthermore, the high thermal conductivity of aluminum alloys requires a high heat input that, in turn, requires a high rating of the welding equipment used.
The challenges can vary, depending on the alloy. For example, the high melting point of aluminum oxide (2,050 degrees C) compared to that of aluminum (660 degrees C) results in problems with energy coupling and instable welding processes. There is also the high solubility of liquid hydrogen and the low solubility in solid hydrogen that results in pore formation and hot cracking.
Unsurprisingly, these problems require special attention to weld preparation, the weld itself and the welding equipment used.
Prep, process, equipment
To achieve a proper preparation, the aluminum components and the selected filler materials (mainly wires) must be dry and clean and should be stored for a minimum time to prevent oxide growth.
In automotive applications, aluminum sheets often receive special coatings to prevent the uncontrolled growth of oxides. In aerospace environments, aluminum sheets are even acid washed before welding. Other industries just use mechanical cleaning by brushing. To lower the oxygen contents, larger diameters for the filler wires are recommended to minimize the wire surface exposed to the environment.
Equipment that is generally used for thermal welding processes can also be used for aluminum alloys, including equipment for MIG, TIG and laser-MIG hybrid processes.
Traditionally, mainly TIG processes were used for aluminum welding. However, applying AC welding to the oxide layer of aluminum can also be utilized.
In this approach, the oxide layer can be removed with the positive wave and the penetration can be achieved with the negative wave [2]. The process principle is shown in Figure 2 as is an example for a robotic TIG torch with integrated wire feeding.
For higher sheet thickness, plasma welding can also 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 [3], but it will be up to the user to decide whether those benefits outweigh the higher equipment costs.
Another method for increasing the welding speed can also be found by leveraging MIG welding [4]. However, if the speed is too high, hydrogen in the liquid metal may not be able to escape before solidification, which can result in pores. For thin sheet applications, pulsed welding, AC welding and modified short arc processes can also be used.
In Figure 3, the schematics of the MIG welding process and a typical MIG welding torch are shown. Due to the high heat input (also from reflections from the surface), torches with double cooling circuit are recommended.
To optimize wire feeding for aluminum welding, a special wire feeding concept was developed by Abicor Binzel called the Master Feeder System (MFS). Figure 4 shows Abicor’s MFS, including its core components: two synchronized wire feeders that allow the use of various wire packages and various welding processes.
Application examples
Manual applications for aluminum welding are well-known and widely used. Figure 5 illustrates TIG welding of aluminum ladders and bicycle frames, featuring a TIG torch that boasts a high rating coupled with a smaller designed space. The small volume allows welding in hard-to-reach areas. When using Abicor TIG torches, there are only three spare parts to be concerned about, compared to the so-called Linde-style TIG torches, which have five parts that have to be replaced regularly.
When it comes to automated aluminum welding, the first industrial robotic applications were developed in the 2000s. For Abicor, its first aluminum robotic welding application was delivered for production of Audi’s A8 series.
Shortly thereafter, several other models followed. Examples include Audi A6 axles that are welded using Abicor’s MFS system and its AbiRob A torches. Furthermore, Figure 6 shows aluminum welding operations for the Audi A2 model.
The challenges that aluminum alloys show for thermal joining processes can be overcome by process knowledge, correct weld preparation and using the appropriate equipment. This allows industry to make use of the huge lightweight potential of aluminum alloys.
Use of aluminum applications continues to increase and thermal joining processes will have to be further studied to increase reliability and compatibility with surrounding components, such as the different types of welding robots. Also, welding of material combinations, such as aluminum to steel or to titanium, has to be further developed.
