Welding procedures in aluminum processing

Electric cars, hybrid vehicles, modern transport systems for freight traffic – the future belongs to electromobility. More product functions mean more electronics, which in turn mean more weight for the designs. On the other hand, there are the legal regulations for reducing emissions, which can often only be complied with or fulfilled by reducing weight. One solution is the use of lightweight materials for constructions of modern transport systems, such as aluminum. But aluminum tends to be a bit of a "diva". You need very specific prerequisites and welding equipment to weld aluminum alloys properly. Process reliability is a top priority in aluminum processing.

Aluminum alloys are the most commonly used light metals due to their low density and high strength. Especially in vehicle construction and plant engineering. However, the processing of aluminum is complicated by its physical properties.

With the right material preparation and welding equipment, aluminum can be welded well. This article is all about:

• the potential of aluminum as a material
• the suitable welding processes in aluminum processing
• the challenges in processing
• approaches to solutions for more process reliability
• and a look into the future of aluminum and aluminum alloys

Aluminum: The great potential

We can find aluminum everywhere in our daily lives. Its versatility and durability make aluminum an integral part of, for example, windows, doors, roof and wall systems, railings, photovoltaic systems, facades and supporting structures. Aluminum is also used in heat exchangers, solar and wind energy systems as well as the entire construction sector, to name but a few. A strongly growing sector is the transport and traffic industry. There, aluminum alloys are used in heat exchangers, wheels, engines, bonnets and car bodies.

Aluminum belongs to the base metals and is rarely found in its pure form. Pure aluminum has a low strength only. However, in order to obtain the desired strength for light metal construction, such as vehicle construction, aluminum requires additional alloying components. High strengths of aluminum (Al) are achieved by using zinc (Zn), magnesium (Mg) and copper (Cu). Medium strengths are achieved with manganese (Mn) and silicon (Si) as alloying elements.

Compared to iron (Fe), aluminum has a lower density, a higher heat capacity and a higher thermal conductivity:

Their low density and high strength combined with low weight make aluminum alloys an interesting quality material for many industries. However, the processing of aluminum, such as welding, requires extensive process knowledge.

Challenges in aluminum welding

If you have ever welded aluminum manually yourself, you probably remember your first attempt. Maybe your weld was riddled with pores or cracks or there was a hole in your sheet metal.

The reasons for this are:

• the different melting points of aluminum and aluminum oxide
• the high solubility of hydrogen in liquid aluminum and the low solubility of hydrogen in solid aluminum

Aluminum processing

When processing aluminum, knowledge of materials science is a basic requirement.

One property of aluminum is that this metal immediately forms an aluminum oxide layer on contact with oxygen. At 2050 °C, aluminum oxide has a much higher melting temperature than aluminum at 660 °C. This property leads to problems with energy coupling and unstable welding processes: The arc couples energy into the material. The aluminum base material below the oxide layer is already liquid, while the oxide layer is still solid. The oxide layer then breaks up sporadically and floats as small "islands" on the liquid aluminum. This leads to a partial jumping back and forth of the arc.

The high solubility of hydrogen in liquid and the low solubility in solid hydrogen lead to pore formation and hot cracks: During the welding process, existing hydrogen (e. g. from contaminated surfaces) is dissolved in the liquid aluminum. As soon as the weld pool solidifies, the solubility of the hydrogen drops sharply. This leads to the hydrogen being excreted and literally balling up into pores and pore nests.

In addition, due to the high conductivity of aluminum, a high heat input must be generated during welding, which in turn places a heavy load on the welding equipment used.

The following solutions are available:

Welding preparation of aluminum

Good preparation is half the battle when it comes to welding aluminum. It is essential that all aluminum components as well as the filler metals used are absolutely clean and dry. Components and welding wire should therefore also be stored for the shortest possible time before use to prevent oxide growth.

In automotive construction, aluminum sheets are even given special coatings that prevent the uncontrolled growth of oxides. In aerospace, aluminum sheets are even washed with acid before welding. In other industries, mechanical cleaning by brushing is used to remove the oxide layer. To reduce the oxygen content, larger wire diameters are recommended for the filler metal, which minimises the surface area.

Welding procedures and equipment

Aluminum can be processed with all common thermal welding methods. These are:

• MIG
• TIG
• Plasma
• Laser
• MIG laser hybrid

The first aluminum welds were carried out using the TIG process. In this process, the oxide layer is "blasted" or removed with the positive half-wave and the fusion penetration is achieved with the negative half-wave.

Process principle and penetration conditions for TIG welding

TIG welding of aluminum sheets close up

The different ways of removing or breaking up the oxide layer on aluminum are described in more detail in the article "How to properly weld aluminum".

Thicker sheets of aluminum can also be processed with plasma welding. This works with both direct current and alternating current. Compared to TIG welding, plasma welding achieves higher welding speeds. Another advantage is the even better surface quality of the weld seam.

Principle of plasma welding

Besides plasma welding, you can also achieve a higher welding speed with MIG welding. The reason: In MIG welding, the metal melting rate is higher than in TIG welding. However, if the speed is too high, there is a possibility that hydrogen in the liquid metal cannot escape before solidification. The result is pore formation and thus a reduced quality of the product.

Pulse welding, AC welding and modified short arc processes are also used for welding thin sheets.

Principle of the MIG welding process with double cooling circuit

Due to the high heat input, which is also caused by reflections on the workpiece surface, welding torches with a double cooling circuit are highly recommended. They additionally cool the torch and also ensure a longer service life for the wear parts.

Examples of thin sheet welding of AlMg alloys with alternating current in the MIG process

Manual MIG welding

As the aluminum wire is very sensitive as a filler material, wire feeding systems that generate as little friction as possible and ensure a uniform wire feed are important. The same applies to welding wires made of other materials, which are more difficult to feed the thinner they are. The MFS-V3.1 Master-Feeder-System from ABICOR BINZEL is such a system because it guarantees uniform and constant wire feeding both for gas-shielded welding and for laser joining.

The complete Master-Feeder-System MFS-V3.1 for laser welding and laser brazing

The modular Master Feeder System MFS-V3.1. has two precisely matched wire feeders as its core components. This wire feeding system feeds wires of different materials. Its field of application includes the welding processes MIG, TIG, plasma and laser.

Laser welding has become an indispensable process, especially in the automotive industry, where speed and precision count. In the MIG laser hybrid process, laser and arc welding processes are combined. This means that a focussed laser beam hits the seam to be welded at the same time as the arc from the MIG welding process. This is a very stable process that produces a high deposition rate and high thermal efficiency.

Process principle MIG laser hybrid

Does aluminum have a future?

In the automotive industry alone, the demand for aluminum is constantly increasing. In vehicle construction, aluminum solves the all-round demand for weight reduction with a simultaneous increase in efficiency. According to media reports, Volvo and Tesla, for example, are to produce complete body components for the next generation of electric vehicles using the aluminum die-casting process.

Use of aluminum in vehicle bodies until 2030 (Source: Statista)

Aluminum processing presents some welding challenges. However, with process knowledge, good weld preparation and the use of suitable equipment such as welding torches, wire feeders and welding robots, this material can be mastered well. Aluminum alloys offer enormous potential for lightweight construction.

If you would like to learn more about aluminum welding, the article "How to properly weld aluminum" is highly recommended. Please also feel free to contact me with your questions if you would like to know more about the different welding processes.

Happy Welding!