Laser welding in a vacuum optimises copper welding for e-mobility
The technological development in the automotive industry towards electrification offers many opportunities. But it also comes with some challenges, such as the increasing processing of copper materials.
The growing consumption of copper places new demands on the joining process in terms of automation capability, quality of the joint and process speed.
Here, laser welding offers enormous potential due to its high precision, high intensities and ease of automation. In particular, laser welding in a vacuum (LaVa welding) delivers high productivity and high quality using low energy input – even when welding difficult materials such as electrolytic tough pitch (ETP) copper.
Due to their properties, copper materials are now a fundamental component of batteries, electric motors and high-performance electronics in electric cars. ETP copper in particular has established itself as a material in electrical engineering due to its favorable manufacturability as well as its good conductivity. The problem: these copper materials – such as ETP copper – are only suitable for welding to a very limited extent. Laser welding is particularly problematic for processing.
Challenges in laser beam welding ETP copper
Until now, there have been various challenges when welding copper with infrared lasers, for example with regard to the reproducibility of consistent quality as well as increased pore and spatter formation.
The formation of pores is a typical defect that occurs more frequently with Cu-ETP materials. Pore formation is also intensified by residues from the insulation layer, which evaporates due to the heat generated by the laser and dissolves in the melt pool. The bigger problem, however, is the strong spattering that occurs due to the low viscosity of the melt. In addition to the loss of mass, adhering spatter can further destroy the insulation layer and cause short circuits. Furthermore, due to its residual oxygen content, ETP copper tends to absorb hydrogen during welding, leading to embrittlement or even cracking.
Heat generation due to the high reflection properties of copper when processing with infrared wavelengths is also a problem. With classic infrared laser welding, enormous amounts of energy are lost – as well as due to the high thermal conductivity of copper. As a result, many companies have been looking for alternative processes in recent years – and are focusing on green and blue lasers as the new solution for copper welding. What brings advantages in material processing also means high investment costs for expensive lasers for the companies themselves.
Laser welding in vacuum as an alternative
Laser welding in a vacuum, for example, is an alternative that combines high quality with increased productivity – and at significantly lower costs. It uses an infrared laser, but as the name suggests, this process works in a vacuum. Due to the special environmental properties of LaVa welding with infrared laser sources, it is no longer necessary to use lasers with wavelengths from the visible range – blue and green – for copper welding.
Ambient pressure and evaporation temperature are reduced, thus stabilising the welding process and avoiding spatter, cracks and pore formation. While conventional copper welding with infrared lasers requires high laser powers for deep welding, LaVa welding can even achieve welding depths >1mm per 1,000W due to the increase in process efficiency.
Cross-section of an aluminium weld performed in a vacuum on a 5mm thick busbar with 1,800W laser power
For example, copper sheets with a thickness of 6mm can be welded with a power of approximately 6kW with free root formation and without pores. At the same time, the weld scaling is very fine and also free of ejections and adherent spatter. According to current studies, the maximum welding depth that can be achieved with 12kW beam power on ETP copper is up to 11mm.
The following application examples show the advantages of laser beam welding in a vacuum when joining components for electromobility:
Copper busbars for battery packs: In order to manufacture lithium-ion battery packs, the individual cells are welded with so-called cell connectors, also known as busbars. When joining individual lithium-ion batteries, care must be taken to ensure that the integrity of the battery cells is not compromised and that the battery bodies are not heated to more than 80°C. When laser welding in a vacuum (with pressure of 20mbar) with a 2kW single-mode fibre laser, Cu-ETP busbars can already be welded almost pore-free with a power of only 1kW at a welding speed of 50mm/s.
Bipolar plates for fuel cells: In addition to lithium-ion batteries, fuel cells are considered a key technology in the energy transition, with tens of millions expected to be produced annually. Therefore, high demands are placed on the cost-effectiveness of the welding process. When using conventional single-mode fibre lasers, however, humping occurs from a welding speed of approximately 700mm/s. The quality of the weld seam becomes insufficient due to this effect. LaVa welding has the advantage over conventional laser beam welding that the energy required for welding can be reduced by up to 60 per cent. This enables practically distortion-free welding, which is a major advantage, especially with the 80μm thick stainless steel foils usually used for bipolar plates.
Weld seam top of a bipolar plate made of 1.4301 stainless steel with a wall thickness of 75µm - without post-treament
The defined atmosphere also enables oxide-free welding as well as targeted influence on the melt by varying the composition of the shielding gas composition. With LaVa welding, a welding speed of 900mm/s can be achieved with a beam power of only 150W. A rough vacuum of approximately 100mbar enables rapid evacuation within approximately 10s. The use of a pressure stage system also allows a continuous flow of components, so that the bipolar plates can be welded ‘on the fly’. Evacuation as a non-productive time is therefore completely eliminated.
Copper hairpins for electric motors: In stators in electric motors, the ends of the individual conductors are welded as so-called hairpins. However, conventional laser beam welding of Cu-ETP hairpins with an insulating coating produces many pores and spatters. For Cu-ETP hairpins, LaVa welding has the significant advantage that the energy required for the laser beam can be reduced by up to 50 per cent. By reducing the evaporation temperature, the temperature of the melt pool decreases, which leads to a stabilisation and thus to a significant reduction of spatter.
Cross-section of a copper hairpin welded in a vacuum
Furthermore, the formation of pores is reduced, since outgassing from the melt is facilitated in a vacuum. For example, LaVa welding can be performed with a 2kW single-mode fibre laser at a pressure of 20mbar – with an evacuation time of just three seconds.
In addition to all the qualitative features, the reduction in energy consumption in particular also plays an important role in the LaVa welding process. In addition to the cost reduction and the increased quality, this is another important plus point for laser welding in a vacuum – and can secure a competitive advantage in the long term.
Dr Christian Otten is CEO at LaVa-X