Engineering Review: Multi-pass Welding Fiber Laser Cobot – Eindhoven, Netherlands

Field Engineering Report: Implementation of Multi-pass Fiber Laser Cobot Systems

Location: Brainport Region, Eindhoven, Netherlands

Project Overview and Objective

This report details the operational deployment and parameter optimization of a 6kW Fiber Laser Cobot system within a heavy-industrial workshop in Eindhoven. The primary objective was to transition from conventional Submerged Arc Welding (SAW) and Gas Metal Arc Welding (GMAW) to advanced **Laser Technology for Thick Plate Steel welding** (specifically S355J2+N structural steel, thicknesses ranging from 20mm to 40mm).

The Eindhoven facility serves as a pilot site for integrating collaborative robotics into heavy fabrication. Unlike traditional high-power laser cells that require massive fixed infrastructure, the Fiber Laser Cobot offers a flexible footprint, allowing engineers to bring the laser to the large-scale workpiece. The technical challenge addressed here is the management of multi-pass strategies to ensure full penetration and structural integrity in thick-section joints.

1. Synergy of Fiber Laser Cobot and Laser Technology

Precision Motion Control and Beam Delivery

In the Eindhoven workshop, the synergy between the Fiber Laser Cobot and modern Laser Technology is the foundation of our high-speed production. A standard 6kW fiber source provides high power density, but on thick-section steel, manual application is impossible due to the precision required for beam-to-joint alignment.

The cobot acts as the precision delivery mechanism. By utilizing a 6-axis collaborative arm integrated with a laser welding head (equipped with a wobble module), we achieve a degree of path repeatability within ±0.05mm. This is critical because Laser Technology, while efficient, is notoriously unforgiving regarding joint tracking. The fiber laser’s high brightness allows for a concentrated heat source, but the cobot’s software enables the complex “wobble” patterns—circular, figure-eight, or transverse—that are necessary to bridge fit-up gaps in Thick Plate Steel welding.

The Collaborative Advantage in a Dutch Engineering Context

The Eindhoven industrial sector emphasizes “Smart Industry” or Industry 4.0. The Fiber Laser Cobot fits this paradigm by allowing human operators to remain in proximity to the welding process (following strict Class 4 laser safety protocols and enclosure standards) to make real-time adjustments. During our field tests, we found that the cobot’s force-sensing capabilities allowed for faster “lead-through” programming of complex multi-pass paths compared to traditional industrial robots.

2. Technical Challenges in Thick Plate Steel Welding

Managing the Aspect Ratio and Keyhole Stability

When dealing with Thick Plate Steel welding, the primary hurdle is achieving the required depth-to-width ratio without inducing porosity or solidification cracking. In Eindhoven, we worked with 25mm thick S355 steel plates. A single-pass fiber laser weld can typically penetrate 8mm to 10mm efficiently, but for 25mm sections, a multi-pass strategy is mandatory.

The Laser Technology employed utilizes a “Keyhole Mode” for the root pass. We observed that maintaining keyhole stability in a deep groove requires meticulous control of the shielding gas flow. Too much pressure can collapse the keyhole; too little results in oxidation. For the fill passes, the strategy shifts toward a “Conduction Mode” or a “Hybrid-like” approach using integrated wire feeding.

Edge Preparation and Fit-up Tolerances

Thick Plate Steel welding with lasers demands higher quality edge preparation than GMAW. We utilized a 60-degree V-groove with a 2mm root face. Our field findings indicated that any gap exceeding 0.5mm resulted in significant underfill or drop-through. To mitigate this, we integrated a laser line scanner on the cobot’s head to perform pre-weld joint mapping, allowing the Fiber Laser Cobot to dynamically adjust the wire feed speed and weave amplitude to compensate for minor fit-up variations.

3. Multi-Pass Execution Strategy

Root Pass Optimization

The root pass was executed at 5.5kW with a travel speed of 1.2 m/min. We utilized a 1.2mm S316L filler wire to improve the toughness of the root, though S355-equivalent wire is standard. The Fiber Laser Cobot was programmed for a 2mm linear wobble to ensure side-wall fusion. The lesson learned here: the focal point must be buried approximately 2mm below the surface of the root face to ensure consistent penetration.

Fill and Cap Passes

For the subsequent five fill passes, power was modulated to 4.5kW to manage the heat-affected zone (HAZ). One of the significant advantages of Laser Technology in this multi-pass context is the drastic reduction in total heat input compared to GMAW. In our Eindhoven tests, the interpass temperature remained below 150°C without external cooling, which is essential for maintaining the fine-grained microstructure of the S355 steel.

1. Pass 2-3 (Intermediate): Focus shifted to 0mm (surface). Wire feed increased to 6 m/min.
2. Pass 4-5 (Fill): Focus shifted to +2mm (above surface). Increased wobble width to 4mm to bridge the widening V-groove.
3. Cap Pass: Focus at +4mm. Reduced power to 3.5kW to ensure a smooth aesthetic finish and avoid undercut.

4. Field Observations and Lessons Learned

Lesson 1: Thermal Management and Distortion

The most striking result from the Eindhoven site was the lack of angular distortion. In traditional Thick Plate Steel welding, a 25mm plate would typically require significant pre-bending or post-weld straightening. Because the Fiber Laser Cobot concentrates energy so precisely, the global heat input was roughly 30% of what we previously recorded with GMAW. The internal stresses were lower, and the plate remained within a 1mm flatness tolerance over a 2-meter weldment.

Lesson 2: Gas Shielding Dynamics

We initially struggled with “internal oxidation” during the third pass. We discovered that the narrow groove of the thick plate created a chimney effect, drawing in atmospheric oxygen despite the primary gas nozzle. We solved this by designing a custom trailing shield for the cobot, which flooded the groove with Argon/CO2 (80/20) for 50mm behind the melt pool. This is a critical adjustment for anyone using Laser Technology on deep-groove steels.

Lesson 3: The “Cobot” Factor in Tooling

Unlike a fixed robot, the Fiber Laser Cobot in our Eindhoven workshop was moved between three different workstations. This mobility introduced calibration drift. We learned that a “Zero-Point” calibration routine must be performed every time the cobot base is locked into a new position. Without this, the multi-pass layers would slowly drift off-center, leading to lack of side-wall fusion in the deep groove.

5. Metallurgical and Quality Analysis

Post-weld inspections conducted at our Eindhoven lab included X-ray radiography and Charpy V-notch impact testing. The results for the Thick Plate Steel welding were exceptional:
* Radiography: Zero detected porosity in the root; minor inclusions in the cap pass (remedied by cleaning wire between passes).
* Hardness: The HAZ showed a peak hardness of 280 HV10, well within the acceptable limits to prevent hydrogen-induced cracking.
* Tensile Strength: All specimens failed in the base metal, confirming that the fiber laser multi-pass weld is overmatched in terms of strength.

The integration of Laser Technology through a collaborative platform has proven that high-precision, high-power welding is no longer confined to the automotive assembly line. In the context of Dutch heavy manufacturing, this approach reduces labor hours per joint by approximately 60% while significantly increasing the fatigue life of the welded components.

Conclusion

The deployment of the Fiber Laser Cobot for Thick Plate Steel welding in Eindhoven marks a significant shift in fabrication methodology. By leveraging the precision of the cobot and the concentrated energy of fiber lasers, we have successfully addressed the challenges of multi-pass welding on heavy sections. Future iterations will focus on integrating real-time melt-pool monitoring to further automate the compensation for fit-up inconsistencies. The data confirms: the synergy of these technologies provides a robust, repeatable, and highly efficient solution for modern structural steel challenges.