Fiber lasers: why is it better to tackle welding for e-mobility with finesse, not brute force?

Success in the toughest welding tasks in automotive and e-mobility requires precise control over how the laser power is delivered to the workpiece.

The use of fiber lasers in automotive manufacturing has been very successful - they have been used for a variety of welding and cutting applications, including welding bodywork, suspension parts, drivetrain components and more. This is no surprise. Fiber lasers offer several advantages over most previously used technologies, both laser and non-laser.

However, the automotive industry continues to be a major source of innovation. Although high-power fiber lasers have been successfully used in automotive manufacturing for some time, the most complex welding processes now coming into use to support e-mobility and lightweighting require more than just raw power and brute force. While there are in fact quite a number of different individual applications, most of them typically involve:

very thin or heat sensitive material
"difficult" to weld materials such as aluminum, copper and high strength steel.
Joining of heterogeneous materials

In order to accomplish these more challenging types of tasks, the laser must offer two basic capabilities. The first is sufficient power to support the required production capacity. High power is also required to achieve sufficient material penetration when working with thicker parts. The second is the ability to precisely control how the laser power is distributed over the work area - both spatially and temporally.

Power with precision

Coherent developed the Adjustable Ring Mode (ARM) fiber laser specifically to provide both performance and accuracy. To achieve this, the ARM uses dual beam output - creating a central point that is surrounded by another concentric ring of laser light. The output at both the central and ring points can be independently controlled and even modulated.

Coherent HighLight FL-ARM series fiber lasers are available with a total power of up to 10 kW. This is more than sufficient to perform virtually all of the more critical welding tasks at usable throughput. In fact, most of the most delicate and demanding applications typically use less than half this value. Coherent ARM lasers therefore provide the ability to direct sufficient laser power exactly where and when it is needed.

One example of how this works can be seen when welding copper. Some manufacturers have switched to using a green laser for copper welding because this beam color is better absorbed by copper than the infrared output of a fiber laser. However, this is only true at room temperature. Once the copper gets hot, it absorbs infrared light well, and once the keyhole is formed, it's even better. And then the lower absorption actually becomes an advantage because it allows deeper penetration of the laser light, making it easier to weld thicker substrates.

So the ARM copper laser welding scenario starts with the power being only in the ring beam, which heats the material until it melts. Then the high power center beam is turned on to create the keyhole. However, during welding, some power is kept in the ring beam because it stabilizes the keyhole, making it less turbulent and chaotic. This reduces spatter and achieves more consistent results. When the beam reaches the end of the weld joint, the power to the ring shuts off completely and the core power is continuously reduced to produce a clean, uniform end.

The same ability - to adjust the material's heating profile to maximize keyhole stability and consistency, and to increase and decrease power at the end of the weld - brings similar benefits when welding other problematic materials such as aluminum and galvanized steel. It also enables very precise welding of thin, delicate or heat-sensitive materials.

The logic of excessive power

Some fibre laser manufacturers claim that their products, like Coherent ARM, allow them to convert 100% of the total power into core or circular beams as if this were an advantage. But it's not - the whole magic of the ARM laser is that it splits the power between the core and the ring, thereby distributing the heat input to the part in a way that produces better results than a single beam - just as in the copper welding example described earlier. Otherwise, why not use a standard (and cheaper) single beam fiber laser?

They also raised the concern that the Coherent ARM architecture is not "flexible". To understand this claim, you need to know that ARM lasers are actually constructed using two or more fiber laser modules, each of which is connected to either a core or a ring to achieve different maximum power ratios. (In operation, the power in each can then be varied continuously from 0% to 100% of this maximum.)

The number of modules to be plugged into the core and ring is set when the system is manufactured. Thus, an 8 kW ARM laser constructed from four 2 kW modules can be configured with three different maximum core-to-ring power ratios. These are 6 kW/2 kW, 4 W/4 kW or 2 kW/6 kW. And these overall ratios cannot be changed afterwards, hence the alleged "inflexibility".

However, the configuration used for a particular customer's laser is based on process tests performed long before the laser was purchased. These determine the power levels and power ratios required to successfully perform the target operation in production. And they provide a large enough process window to make any changes needed to accommodate process changes (for example, changes in raw materials between batches). In addition, a given laser typically allows very wide latitude for subsequent changes to the process itself. Crucially, determining the correct power and power ratio at the outset eliminates the need to make large changes to the laser power later.

Competitors may claim that Coherent's product is not a "real" 8 kW fiber laser. And they are right - it is an 8 kW ARM fiber laser. That means it does a better job than anything else at applying laser power in exactly the way that produces the best results for a particular task. And that same 8kW fiber laser will continue to deliver better results even as your needs change or evolve.