Multi-laser systems: challenges with overlap zones
Stitching, plume and heat build-up: How 12 lasers control a building board at the same time.
1. Introduction: The Race for Productivity
A Laser Powder Bed Fusion (PBF-LB) system with a single laser only builds a few cubic centimeters of metal per hour. A laser is not enough to produce large components (such as engine blocks or huge rocket engines) economically in a reasonable time. The industry has therefore developed machines with two, four, eight and now up to twelve lasers (multi-laser systems) that work on one component at the same time.
But adding lasers is not simple mathematics. When several high-power lasers operate on the same powder layer, completely new physical challenges arise.
Why can't all lasers print everywhere?
Each laser is mounted at a specific position above the construction space via mirrors (galvanometers). The further the laser beam deflects, the worse the focus quality becomes and the more oval the laser spot becomes at the edge. Therefore, every laser has a defined field (scan field) in which it works optimally. These fields overlap in the middle (stitching zone).
2. The problem of the overlapping zone (stitching zone)
If one laser prints the left part of a large component and the other laser prints the right part, both halves must join perfectly in the middle.
- Seams (stitching): If the two lasers meet exactly on a line, a weld seam is created. This seam is metallurgically extremely susceptible to cracks or lack of fusion. If this seam line is drawn through all 5,000 layers at exactly the same X/Y position, a massive predetermined breaking point is created in the component.
- Solution: Shifting. Modern software shifts this overlapping zone (stitching zone) by a few millimeters per layer. The seam does not run straight up through the component, but rather in a zigzag shape (shifted stitching). This eliminates the weak points.
3. Smoke and smoke (Spatters and Plume)
When a laser melts powder, welding fumes (plumes) are created and metal spatters fly through the chamber. The protective gas (argon flow) blows off this smoke.
If four lasers work at the same time, four times as much smoke is produced. When Laser A operates in the headwind of Laser B's gas flow, the laser beam from A shoots through the smoke cloud from B. The smoke absorbs the laser energy, Laser A loses massive power, and the powder underneath does not melt properly (shadow formation). Complex algorithms assign dynamic ranges to the lasers so that no laser ever works in the "smoke shadow" of another.
4. Thermal management for 12 lasers
A 1 kilowatt laser brings gigantic heat into the powder bed. Twelve lasers create a real build-up of heat.
If 12 lasers scan a small space at the same time, the component heats up uncontrollably. The melt pool explodes, the powder evaporates and the component becomes unusable. The software must calculate the laser paths so that the lasers always work at the maximum distance from each other and give the local zones enough time to cool down without reducing productivity.
5. Conclusion: Software as a bottleneck
Multi-laser machines (like the Nikon SLM NXG XII 600) are physical masterpieces, but their success depends on the software that conducts the orchestra. When the calibration system (laser alignment with micrometer precision) and the gas flow simulation work together perfectly, multi-laser systems achieve build rates that finally catapult powder bed 3D printing into the age of large-scale automotive production.