Researchers used powerful X-rays to obtain high-resolution images of the ARM laser welding process, delving deeper into the welding process to develop better processes.
Engineers at Coherent's Applications Lab, with the help of X-rays, have taken a closer look at the fiber laser welding process.
What happens beneath the surface of laser welding?
In the past, researchers have studied the fiber laser welding process extensively using conventional high-speed video. High-speed video provides a reference for studying the dynamics of the molten metal and vapor pools (called "vias") created during the welding process.
Typically, a camera is placed over the part to record what is happening on the weld surface from a top-down perspective. However, there is much more going on inside a small hole than can be seen from the top.
How can you really understand the changes occurring inside the part? People have done this in the past with X-ray videos. But these videos never provided enough detail because the X-ray source was not powerful enough.
In a research collaboration between the production technology team at the Technical University of Ilmenau, Germany, and the Coherent Application Laboratory in Hamburg, they envisioned using an X-ray source that is more powerful than ever before to see the changes that occur inside a part by passing it directly through solid metal. This allows high-resolution images of the welding process to be viewed from the side, making it possible to see the exact shape and evolution of small holes during the welding process.
Improving ARM Fiber Laser Welding Performance
The team used this approach to study the operation of the Coherent Tunable Annular Mode Fiber Laser (FL-ARM), which provided amazing results - crack-free welding of high-strength steels, aluminum welding without filler wire, and successful welding of copper. This was largely due to the ARM laser's ability to precisely control the heating and cooling of the part during the welding process; however, we didn't fully understand how this happened and the nuances of each step.
The team studied the welding process of fiber lasers in welding copper, aluminum, and other very thin, thermally sensitive plates that are more difficult to weld. They took a deeper look at how all of these processes work by visualizing the welding process, revealing small hole dynamics, and understanding the effect of different ARM laser power distributions on spatter formation during the welding of copper materials. The ultimate goal of the research is to improve welding results and develop more reliable production methods.
European Synchrotron Radiation Devices
There are only a handful of devices in the world that can produce X-rays powerful enough to perform the kind of imaging the team needs. One of the best-known sources of powerful X-rays is the European Synchrotron Radiation Facility Extremely Bright Source (ESRF-EBS) in Grenoble, France, which specializes in serving researchers in fields as diverse as health, clean energy, materials science, the arts, and anthropology, and which has even been used to study beehives and 119-million-year-old fish fossils.
The synchrotron itself is a tube with a circumference of 844 meters and a very high internal vacuum. Electrons surround it and are accelerated to near the speed of light. Magnets around the ring are used to cause the electrons to rapidly change their direction of travel, and when this happens, the electrons emit unusually high-energy X-rays.
These X-rays are then directed downward into one or more of 44 different "beamlines". These beamlines are located in the laboratories and instruments where the actual research takes place.
Image 1: Engineers at Coherent Applications Laboratory used very powerful X-rays to obtain a high-resolution cross-sectional view of the ARM laser welding process at the European Synchrotron Radiation Facility.
The Coherent Applications Lab team assembled a welding setup that included an 8kW HighLight FL-ARM fiber laser. A small group of researchers from the Production Technology team at the Technical University of Ilmenau built a mechanism to automatically hold and move parts during the welding process, as well as focusing optics and an auxiliary gas delivery system.
All of this equipment was brought to the ESRF and placed in one of the beamlines in an "experimental chamber" (a room completely encased in 75 mm thick solid lead shielding). The researchers sat safely in another room some distance away and performed computer-controlled soldering while exposing the device to X-rays. A camera system that converts the X-rays to visible light records the welding action at 50,000 frames per second. The team performed hundreds of individual weld tests on a variety of metals, including stainless steel, copper and aluminum.
What information did this process provide? That's 14 terabytes of data to analyze, and it will take some time to fully answer that question. But as we have seen, in the copper busbar welding tests, the video clearly shows that: with the proper power distribution (roughly equal power in the center beam and ring beam), the small holes behave stably and there is no constriction at the bottom of the small holes; in contrast, when the center spot power is too high, the capillaries constrict at the bottom, which leads to spattering and the formation of air holes; and if the ring power is too high, the molten material overflows into the small holes, suddenly evaporates, and lead to material ejection.
In addition, the team investigated the effect of shielding gas on capillary tube formation, and these findings provide a deeper insight into profile welding.
Further analysis of the data will help provide a better and more accurate understanding of how the power ratios between the center and ring beams, affect the results of various welding processes. This knowledge will help Coherent Application Labs develop more robust and consistent weld process formulations, providing customers with better and even faster welds.
