Engineering Review: Multi-pass Welding Laser Welding Cobot – Milan, Italy

Field Report: Multi-pass Integration for Structural Steel Welding

Location: Milan, Italy – Industrial Fabrication Sector

This report outlines the technical findings and operational deployment of a 3kW **Laser Welding Cobot** system within a heavy fabrication facility in Milan. The objective was to replace traditional Gas Metal Arc Welding (GMAW) for thick-section Structural Steel welding (S355JR) using advanced **Laser Technology**. The transition to multi-pass laser welding represents a significant shift in Lombardy’s manufacturing landscape, prioritizing precision and reduced thermal distortion over bulk heat input.

In this specific field application, we focused on 12mm and 15mm plate thicknesses—dimensions typically considered the “ceiling” for single-pass laser applications. By utilizing a multi-pass strategy, we expanded the system’s capability, ensuring full penetration and high-quality fillet profiles that meet Eurocode 3 standards.

The Synergy of Laser Technology and Motion Control

Hardware Interfacing in the Milan Workshop

The core of the system is the integration of a 3kW Ytterbium fiber laser source with a high-precision 6-axis collaborative robot. In the Milan facility, the Laser Welding Cobot was tasked with maintaining a constant TCP (Tool Center Point) speed while modulating power output based on real-time joint feedback.

The synergy here is critical: Laser Technology provides a high power density beam (approximately 150-micron spot size), but without the repeatable pathing of the cobot, this energy is impossible to manage on Structural Steel welding joints where fit-up variance exists. Unlike manual laser welding, where human tremor can cause inconsistencies in the keyhole, the cobot ensures a steady “wobble” pattern. We utilized a circular wobble of 4.0mm width at 150Hz for the fill passes to ensure sidewall fusion—a parameter set that would be physically exhausting and inconsistent if performed manually.

Thermal Management and Heat Affected Zone (HAZ)

One of the primary “lessons learned” during the Milan deployment was the behavior of S355JR steel under concentrated laser energy. While Laser Technology is lauded for its narrow HAZ, multi-pass sequences on structural sections can lead to heat accumulation. We implemented a strictly monitored inter-pass temperature limit of 150°C. The Laser Welding Cobot was programmed with dwell times between passes, allowing the structural integrity of the steel to remain uncompromised, preventing the grain coarsening often seen in multi-pass MAG welding.

Technical Execution: Multi-pass Strategy

The Root Pass: Establishing the Keyhole

For the 12mm V-groove joints (60-degree included angle), the first pass is the most critical. We utilized the Laser Welding Cobot in keyhole mode.
* Power: 2800W
* Speed: 18mm/s
* Shielding Gas: 100% Argon at 20L/min (Side-jet configuration)

The high-intensity beam of the Laser Technology achieved 5mm of root penetration with minimal spatter. The challenge in Structural Steel welding is the mill scale; even with light grinding, the laser reacts to impurities. We found that a slightly defocused beam (+2mm) helped stabilize the plasma plume during this initial pass.

Filling and Capping: The Wobble Parameter

The subsequent two fill passes required a shift from keyhole to conduction-limited welding. This is where the Laser Welding Cobot earns its keep. By adjusting the “wobble” frequency, we were able to bridge the widening gap of the V-groove.
* Second Pass: 2500W, 12mm/s, 3.5mm linear wobble.
* Cap Pass: 2200W, 10mm/s, 5.0mm figure-eight wobble.

The use of a figure-eight pattern for the cap pass provided a ripple aesthetic similar to high-end TIG welding but at four times the travel speed. This level of control is a direct result of the software integration between the laser source and the cobot’s motion controller.

Material Feed Integration

For Structural Steel welding, volume is required to fill the joint. We integrated a synchronous wire feeder using 1.2mm ER70S-6 wire. The Laser Welding Cobot logic was set to “Wire-Lead,” where the wire enters the leading edge of the melt pool. This cools the pool slightly, allowing for better bead reinforcement and preventing the “underfill” common in autogenous laser welding.

Operational Challenges and Lessons Learned

Fit-up Tolerances: The “Hard Truth” of Laser Tech

The most significant hurdle in the Milan workshop was not the Laser Technology itself, but the upstream preparation of the Structural Steel welding components. Traditional structural fab allows for gaps of 1.5mm to 2.0mm. A Laser Welding Cobot is far less forgiving.

Lesson Learned: If the gap exceeds 0.5mm, the laser beam tends to “blow through” rather than bridge, even with a wire feeder. We had to implement a mandatory CNC-plasma or cold-sawing requirement for all joint edges. In Milan, we revamped the shop’s prep-station to include edge milling for the 15mm plates, ensuring a zero-gap fit-up. This increased prep time by 15% but reduced weld-time by 60% and eliminated post-weld grinding entirely.

Safety and Infrastructure

Deploying a Laser Welding Cobot in an open Milanese workshop requires a shift in safety culture. We installed a Class 4 laser enclosure with interlocked doors. Unlike traditional welding, the “scatter” from Laser Technology is invisible but catastrophic to eyesight.
* Observation: The cobot’s “collaborative” nature is often misunderstood. While the arm is safe to be around, the laser process is not. We treated the cell as a “hybrid” zone—collaborative during setup, but fully isolated during emission.

Comparative Analysis: Laser Cobot vs. Manual MAG

Productivity Metrics

Based on the three weeks of data collection in Milan, the metrics for Structural Steel welding on a 1-meter seam (12mm thick) are as follows:
1. Manual MAG: 45 minutes (including inter-pass cleaning and 4 passes).
2. Laser Welding Cobot: 12 minutes (3 passes, zero slag, minimal cleaning).

The reduction in total heat input resulted in a 70% decrease in plate bowing. This is a massive cost-saver for the Milan facility, as it removes the need for hydraulic flame-straightening post-welding.

Quality and NDT Results

All multi-pass samples underwent Ultrasonic Testing (UT) and Radiographic Testing (RT). The high-density energy of the Laser Technology produced a very “clean” weld metal. However, we did notice occasional “root sagging” if the cobot speed dropped below 15mm/s on the first pass. By fine-tuning the cobot’s acceleration/deceleration curves at the start and end of the seam, we eliminated these craters.

Structural Integrity of S355 Steel

Hardness testing (Vickers) showed a slight increase in the fusion zone compared to MAG, but remained within the acceptable limits for structural applications in Italy. The multi-pass approach actually “tempered” the previous layers, resulting in a more uniform hardness profile than a single-pass high-power laser weld would have provided.

Final Technical Summary

The deployment of the Laser Welding Cobot in Milan demonstrates that Laser Technology is no longer reserved for thin-gauge automotive or medical parts. When applied to Structural Steel welding, the key to success lies in the multi-pass strategy and rigorous joint preparation.

The synergy between the cobot’s precision and the fiber laser’s power allows for a level of efficiency that manual processes cannot match. Moving forward, the industry must focus on “design for laser manufacturing,” where the tolerances of the parts match the precision of the tools. For the Milanese structural sector, this marks the end of “rough-in” fabrication and the beginning of high-precision structural engineering.

Engineering Note: Always check the fiber optic cable’s bend radius on the cobot’s 5th axis. In high-repetition multi-pass cycles, we noticed slight cladding temperature increases if the cable management was too restrictive. Ensure 200mm+ loops for longevity.