Engineering Review: Multi-pass Welding Laser Welding Cobot – Busan, South Korea

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

Location: Industrial Zone, Gangseo-gu, Busan, South Korea

1. Executive Summary and Scope

This report details the field deployment and optimization of a 3kW Fiber Laser Welding Cobot system at a medium-scale maritime component fabrication facility in Busan. The primary objective was to transition from traditional Gas Metal Arc Welding (GMAW) to Laser Technology for thick-section Mild Steel welding (specifically ASTM A36 and SS400 grades). The focus of this evaluation is the transition from single-pass thin-gauge joining to multi-pass structural welding on 12mm to 15mm plate thickness.

2. The Industrial Context: Busan’s Manufacturing Shift

Busan remains the heartbeat of South Korea’s shipbuilding and heavy equipment sectors. However, the local industry is currently grappling with a skilled welder shortage and rising labor costs. During my time on-site, it became evident that traditional manual welding for heavy mild steel structures is no longer sustainable at current throughput requirements. The introduction of the Laser Welding Cobot represents a strategic pivot. Unlike high-cost, fixed-cell robotics, the cobot provides the flexibility required for the high-mix, low-volume production typical of Busan’s secondary tier suppliers.

3. Technical Synergy: Laser Technology and Cobot Kinematics

The core of this installation is the synergy between high-density Laser Technology and the collaborative robot’s motion control. In Mild Steel welding, heat management is critical. A standard 6-axis cobot allows for “Lead-Through Programming,” where the head welder can physically move the laser torch to define the path.

However, the “synergy” is more than just motion. The laser power source communicates via EtherCAT with the cobot controller, allowing for real-time adjustment of the “wobble” parameters. For multi-pass applications, we utilized a figure-eight wobble pattern with a frequency of 150Hz. This specific marriage of Laser Technology and cobot precision allows the beam to bridge fit-up gaps that would traditionally cause a “blow-through” in a standard laser setup.

4. Multi-pass Strategy for Heavy Mild Steel

Transitioning to 12mm+ Mild Steel welding requires a strategic multi-layer approach. Laser welding has historically been viewed as a thin-sheet solution, but with the current 3kW to 6kW power density available in Busan’s workshops, multi-pass is now a viable reality.

Pass 1: The Root Run
The root pass was executed using a “Keyhole” mode. By focusing the Laser Technology exactly at the interface, we achieved full penetration on a 4mm land. The cobot’s ability to maintain a consistent 0.5mm standoff distance is superior to manual laser handling, which is critical because focal point variance of even 1mm can lead to lack of fusion at the root.

Pass 2 & 3: Fill and Cap
For the subsequent filler passes, we integrated a synchronized wire feeder (0.8mm ER70S-6 wire). Here, the Laser Welding Cobot switched from keyhole mode to conduction-limited welding. We widened the wobble width to 4.5mm. The primary challenge in Mild Steel welding at this stage is the accumulation of surface oxides. We found that a 95% Argon / 5% CO2 gas mix provided the best balance between arc stability (for the wire) and plume suppression for the laser.

5. Material Considerations: Mild Steel Characteristics

Working in the humid coastal environment of Busan presents specific challenges for Mild Steel welding. Surface oxidation (rust) and mill scale are prevalent.
* Lesson Learned: Laser Technology is significantly less forgiving than GMAW regarding surface contaminants. We implemented a mandatory mechanical grinding step to remove mill scale 15mm back from the joint prep. Failure to do so resulted in significant porosity and “spitting,” which contaminated the laser’s protective window.
* Thermal Conductivity: Mild steel’s thermal conductivity meant that by the third pass, the base material temperature exceeded 200°C. We had to program “interpass cooling delays” into the cobot’s logic to prevent the Heat Affected Zone (HAZ) from expanding beyond acceptable limits, which would negate the grain-refinement benefits of using a laser.

6. Comparative Analysis: Manual vs. Cobot Laser Integration

In the Busan workshop, we ran a side-by-side trial. A manual laser operator vs. the Laser Welding Cobot.
1. Consistency: The manual operator suffered from “torch shake” during the 800mm long seams, leading to inconsistent bead profiles. The cobot maintained a path accuracy of ±0.05mm.
2. Duty Cycle: The manual operator required a break every 10 minutes due to the weight of the water-cooled laser head. The cobot operated at a 100% duty cycle, only stopping for part change-outs.
3. Quality: Radiographic testing (RT) showed zero slag inclusions in the cobot-welded Mild Steel samples, a common issue in manual multi-pass GMAW.

7. Optimizing Laser Technology Parameters

To achieve the results required for South Korean maritime standards (KR Class), we dialed in the following parameters for the Laser Welding Cobot:
* Laser Power: 2800W (Root), 2400W (Fill/Cap)
* Wobble Type: Sine wave for root; Circle for fill.
* Travel Speed: 12mm/sec for root; 8mm/sec for wire-fed fill.
* Wire Feed Speed: 3.5 m/min.

The integration of “Sensing” technology was the final piece of the puzzle. We used a laser line tracker mounted to the cobot’s sixth axis. In Busan’s heavy fabrication environment, part fit-up is rarely perfect. The line tracker adjusted the cobot’s path in real-time to compensate for warped Mild Steel plates, ensuring the Laser Technology stayed centered in the V-groove.

8. Safety and Infrastructure Lessons

One cannot discuss Laser Technology in a field report without addressing safety. Transitioning a Busan workshop from GMAW to Laser requires a complete overhaul of safety protocols.
* Class 4 Enclosures: We had to install specialized laser-rated curtains. Unlike the “flash” of arc welding, the 1064nm wavelength of fiber lasers is invisible and can cause permanent retinal damage from reflections off the shiny Mild Steel weld pool.
* Fume Extraction: Laser welding of mild steel produces a finer, more concentrated particulate than GMAW. We upgraded the localized extraction systems to ensure the cobot’s optical sensors remained clear of soot.

9. Economic Impact and Throughput

The data from the Busan site indicates that the Laser Welding Cobot reduced total welding time by 40% compared to traditional multi-pass GMAW. While the initial investment in Laser Technology is higher, the reduction in post-weld grinding and straightening (due to low heat input) saved an additional 15 man-hours per assembly. In the context of Mild Steel welding, where distortion usually requires heavy hydraulic pressing to correct, the laser’s “clean” finish is a significant cost-saver.

10. Conclusion and Forward Outlook

The Busan deployment confirms that the Laser Welding Cobot is no longer restricted to laboratory environments or thin-gauge electronics. When properly calibrated for multi-pass schedules, it is a workhorse for structural Mild Steel welding.

The success of this implementation hinges on three factors: rigorous surface preparation, precise interpass temperature control, and the utilization of “wobble” geometry to overcome fit-up tolerances. As Laser Technology continues to scale in power and decrease in cost, the sight of cobots working alongside human welders in Busan’s shipyards will become the industry standard, not the exception.

Final Note: Engineers must respect the sensitivity of the optics. A single fingerprint on the protective lens can terminate a 3kW beam with catastrophic results for the torch head. Precision is the price of productivity.

End of Report.
Signed,
Senior Welding Engineer, Field Operations (Busan Sector)