Fiberlaser welding has been studied in detail in the past using high-speed video. The video makes it possible to study the dynamics of the molten metal and the steam (called the "keyhole") that is produced during welding. Typically, the camera is positioned over the product and observes what is happening on the surface. But what happens inside the keyhole has been a mystery until now.
How can we see inside? In the past, X-ray videos were used for this purpose, but they never provided sufficient detail because the X-ray sources were not powerful enough. A research collaboration between the Production Technology Group at the Technical University of Ilmenau and the Coherent Applications Lab in Hamburg imagined using a much more powerful X-ray source than ever before. The idea was to use X-rays strong enough to pass through solid metal.
This would make it possible to obtain high-resolution images of the welding process from a side view. The side view would reveal much more useful information - specifically, the exact shape and evolution of the keyhole during welding.
Improvements in ARM fiber laser welding
The research group wanted to use this approach to specifically investigate the precise operation of the Coherent Adjustable Ring Mode (FL-ARM) laser. We already know that the FL-ARM laser produces amazing results in welding high strength steel without cracks, welding aluminum without additional wire, and successfully welding copper. And we know that this comes from the ARM laser's ability to precisely control the heating and cooling of the material during the welding process. But we don't always understand every detail of how it all happens. So the team focused in particular on some of the most critical and challenging applications of fiber lasers in various welding tasks in electromobility. Specifically, welding copper, aluminum and other traditionally "difficult" materials to weld - and often with very thin, heat-sensitive sheet metal. They were also interested in the study of so-called "profile welding". This process is often used to make tubes.
The aim of this research was to gain a greater understanding of how these processes work, by visualising the processes and keyhole dynamics and observing the effect of different distributions of ARM laser power on material spatter formation when welding copper materials. The aim was, of course, to improve the results and develop more reliable production methods.
European Synchrotron Radiation Facility
There are only a few facilities in the world that can generate sufficiently powerful X-rays to perform the type of imaging required. One of these sites is the European Synchrotron Radiation Facility Extremely Brilliant Source (ESRF-EBS) in Grenoble, France. It was built specifically to meet the needs of researchers in the fields of health, clean energy, materials science, art and anthropology.
The synchrotron itself is a tube 844 metres in circumference with a vacuum inside. Electrons orbit inside and are accelerated to nearly the speed of light. Magnets around the circuit are used to rapidly change the direction of the electrons. During this change, the electrons emit extremely powerful X-rays. This X-ray is then directed into one or more of 44 different "light lines". The light lines contain the laboratories and associated instruments used to carry out the actual research.
Experimenting outside the normal boundaries
A team of researchers from Coherent Labs assembled a welding kit that included an 8 kW HighLight FL-ARM fiber laser. A research group from the Technical University of Ilmenau designed and assembled the equipment for holding and moving the parts during welding, as well as the optics and auxiliary gas delivery system. All this equipment was placed in an "experimental cabin" (a room completely surrounded by a 75 mm thick lead sheath) on one of the light lines. Welding was carried out under computer control while being exposed to X-rays. The camera system, which converts the X-rays into visible light, recorded the action at up to 50,000 frames per second. A team of 14 people worked in four shifts over seven days and carried out several hundred individual welding tests on a variety of metals, including stainless steel, copper and aluminium.
The testing resulted in 14 TB of data for analysis. Thus, their evaluation will take some time. However, the first results have already shown that with appropriate power distribution (approximately equal power in the center and edge beams), the keyhole is stabilized and there is no taper at the base. Conversely, when the power in the central beam is too high, the capillary constricts at the base. This results in material spattering and pore formation. If the power is too high in the edge beam, the molten material rolls into the keyhole, here it evaporates rapidly and causes material spatter.
The effect of shielding gases on capillary formation was also investigated. These findings provide a better understanding of the processes involved in welding, which could lead to improvements in fiber laser welding and expand its applications in industrial manufacturing.
Overall, this collaboration between researchers at the Technical University of Ilmenau and the Coherent Applications Lab in Hamburg has provided a better understanding of the processes involved in fiber laser welding, particularly when using the Coherent Adjustable Ring Mode (FL-ARM) laser. The results of this research could lead to improved welding methods and expanded applications of fiber lasers in industrial manufacturing.
