Welding with high laser power: how beam shaping can overcome process bottlenecks

Introduction: Laser welding with high laser power

Laser welding is rapidly penetrating both manufacturing processes in many industries, including fast growing markets such as E-Mobilty.

This increasing demand for laser welding is supported by increasing high power laser availability and decreasing cost per watt, allowing laser systems to compete and replace traditional welding solutions.

The high laser power increasingly used in welding is posing new challenges to machine integrators attempting to increase welding speed. Increase in welding speed or thickness of welded material does not scale linearly with the laser power used, once laser power levels ago above certain levels. This creates a process bottleneck that must be overcome by various beam shaping techniques. In this article we review some common bottlenecks in high laser power welding and show how Holo/Or’s beam shaping solutions can help overcome these bottlenecks.

Typical high power laser welding system architecture

Typical high output power lasers used in welding are fiber lasers, mostly in the IR ~1um wavelength range. The fiber output power in welding applications is in the 100’sW to multi kW range. The fiber laser output is then collimated, typically by large lenses of at least 1’’, and the laser beam is focused onto the workpiece by a focusing lens (F-theta, Objective of just a standard lens, depending on NA and desired work field).
The laser spot is scanned on the work piece by mirror deflection, stage movement, or a combination of both. Various mirror deflectors are used, with the typical case being deflection by Galvo mirror. Many high-power laser welding systems are mounted on robot arms, eliminating need for stages.
These high laser power systems often need to be water cooled, especially for kW scale systems, and optics used must be often the highest grade to prevent laser damage.

Welding bottlenecks when using high laser power to achieve high speed

In many high-power laser welding processes, the need is for speedy welding. This means that the laser spot spends a short time on target, thus high laser power must be used to reach the same total thermal laser energy transferred to the metal. When laser power is increased, concurrent with movement speed, the following effects tend to arise

• Humping- The weld seam tends to have discrete ripples of “humps” due to the melted metal cooling too fast and beading.
• Incomplete weld – When laser power density is high on the melt pool, but exposure time is short, the welding keyhole can be disrupted resulting in only the top of the weld having a good connection.
• Brittle weld- High laser power densities can result in weld pool instability leading to turbulence and trapping of gas bubbles in the weld, reducing weld strength and increasing brittleness.

Overcoming these bottlenecks often requires the use of beam shaping.

What is laser beam shaping? Beam shaping methods suitable for high laser power

Beam shaping is a modification of the laser beam so that the focused spot shape and intensity profile changes as desired. This includes the splitting of a single laser beam into an array of beams, the changing of laser intensity profile into a top hat, and many other types of beam manipulations.
Diffractive Optical Elements (DOEs) are a type of beam shaper that is especially suitable for high power laser welding, due to the following reasons:

• Holo/OR DOEs are monolithic fused silica components, with high LDT
• Our DOEs are flat, thin windows that can be integrated into existing welding systems easily
• DOEs have a high level of design flexibility- they can be designed to accommodate any laser wavelength, multi mode or single mode laser beam inputs, and can generate output power distributions that cannot be generated by refractive optics.

Beam shaping solution to overcome high laser power welding bottlenecks

Various beam shaping solutions are used for laser welding using high power lasers, including:

• Ring +Spot shapers, such as the Flexishaper Module. These solutions generate a ring around the high power laser spot, with an adjustable ratio of power between the ring and the spot. With some power in the ring, the melt pool cools slower once the laser spot moves past the point, resulting in less humping and a better weld even at high speed. This approach has been studied in cooperation with our sister company Blackbird Robotersysteme and showed significant improvement in welding speed.
• Double spot splitters, such as our high efficiency HEDs series, can improve weld speed by simultaneously heating both sides of the weld in butt joint welding. By correctly adjusting the beams power ratios, the melt pool can be kept stable and the weld strong, even at high weld speeds. This was show in cooperation with TWI during the TaylorWeld project.
• Customized shapes such as C shape and Brazing shape illumination can redistribute the intensity of the laser in a way that is tailored to the specific welding process and material, enabling better melt pool stability and improved weld strength at high speeds.

Conclusion

Laser welding speeds are constantly increasing , driven by the increasing availability of powerful lasers at lower costs per watt. To truly utilize the high power laser for welding speed increase , machine builders should use process- tailored beam shaping to bypass bottlenecks such as humping, incomplete welds and gas bubble cracking . Diffractive beam shapers are especially suitable for high laser power welding, due to their great shaping flexibility and high LDT.