Engineering Review: Multi-pass Welding 6-Axis Collaborative Welder – Munich, Germany

Field Report: Implementation of 6-Axis Collaborative Welding in Heavy Structural Steel (Munich Site)

Date: October 14, 2024
Location: Facility 4, Munich, Germany
Subject: Multi-pass optimization for S355J2+N Structural Steel using 6-Axis Collaborative Systems

1.0 Introduction and Site Objectives

This report details the field implementation of a 6-Axis Collaborative Welder within our Munich structural steel fabrication line. The primary objective was to transition high-volume, multi-pass V-groove joints from manual GMAW (Gas Metal Arc Welding) to a semi-autonomous state using Automated Welding protocols. In the Munich facility, the labor shortage for high-certified pressure-vessel grade welders has necessitated a shift where the “welder” acts as a cell supervisor, while the “6-Axis Collaborative Welder” executes the physically demanding, repetitive torch paths required for heavy-section structural steel welding.

2.0 The Technical Role of the 6-Axis Collaborative Welder

The 6-Axis Collaborative Welder differs from traditional industrial robotics primarily in its kinesthetic teaching capability and safety-rated monitored stop functions. In the context of our 25mm thick S355 structural plates, the six degrees of freedom are not a luxury but a requirement.

4.1 Torch Orientation and Work Angle

In multi-pass Automated Welding, the work angle must shift progressively as the groove fills. A 4-axis or 5-axis system often struggles with torch-to-workpiece clearance when the nozzle is deep within a 60-degree included angle V-groove. The 6th axis allows for intricate “wrist” rotations, maintaining a consistent 15-degree push angle while simultaneously adjusting the work angle to ensure sidewall fusion on the second and third fill passes. Without this 6th axis, we observed a 12% increase in cold lap defects during initial trials due to the torch body interfering with the jigging.

3.0 Automated Welding Logic for Multi-pass Sequences

Transitioning to Automated Welding for structural steel is not a “set and forget” operation. It requires a rigid adherence to the Weld Procedure Specification (WPS), translated into digital offsets. For the Munich project, we utilized a pulsed GMAW process (GMAW-P) to manage the heat-affected zone (HAZ) in the S355J2+N material.

3.1 Root Pass Dynamics

The root pass remains the most critical phase. We utilized the 6-Axis Collaborative Welder’s “lead-through” programming to map the root gap. Despite the precision of the Munich facility’s plasma-cut edges, thermal expansion during the root pass causes the gap to wander. We found that the synergy between the automated path and the collaborative sensors allowed the operator to “nudge” the path in real-time without halting the arc, a feature that reduced our rework rate on root passes by 22% compared to non-collaborative automated systems.

3.2 Fill and Cap Strategy

For a 25mm plate, we standardized an 8-pass sequence:

• Pass 1: Root (160A, 19V, 35cm/min)
• Pass 2-3: Hot pass and First Fill (210A, 24V, 40cm/min)
• Pass 4-7: Main Fill with 2.5mm weave (240A, 26V, 32cm/min)
• Pass 8: Cap (190A, 22V, 28cm/min)

The 6-Axis Collaborative Welder handled the weaving patterns with a precision that manual welders could not sustain over an 8-hour shift. The consistency in the “dwell time” at the edges of the weave ensured zero undercut, which is a frequent failure point in manual structural steel welding under Eurocode 3 standards.

4.0 Structural Steel Welding: Material Specifics and Heat Management

Structural steel welding in the German market requires strict adherence to DIN EN ISO 15614-1. The S355J2+N used in Munich has a carbon equivalent (CEV) that makes it susceptible to hydrogen-induced cracking if interpass temperatures are not monitored.

4.1 Interpass Control in Automated Cells

One “lesson learned” during the first week was the accumulation of heat. Unlike manual welding, where the welder naturally pauses to clean slag or change positions, Automated Welding is relentless. The 6-Axis Collaborative Welder can maintain an arc-on time of 85%. This led to interpass temperatures exceeding 250°C, which risked grain coarsening in the HAZ. We had to program “cool-down” dwell periods into the automation logic, triggered by an integrated infrared pyrometer. This synergy between sensor data and the 6-axis movement ensured that the structural integrity of the steel was never compromised for the sake of speed.

5.0 The Munich Workshop Synergy: Human-Machine Interface

The term “Collaborative” is often misunderstood in the field. In our Munich workshop, the synergy is defined by the division of labor. The human welder handles the fit-up, tack welding, and the critical visual inspection of the root. The 6-Axis Collaborative Welder handles the high-deposition fill passes where the arc radiation and fume levels are highest.

5.1 Handling Fit-up Inconsistencies

In structural steel welding, the fit-up is rarely perfect. Large-scale beams often have a +/- 2mm variance. Traditional Automated Welding fails here because the path is fixed. By using a 6-Axis Collaborative Welder, our engineers in Munich used “Touch-Sensing” routines. The robot uses the welding wire itself to touch the left and right plates, recalculating the center of the groove before every pass. This adaptability is the bridge between “dumb” automation and true collaborative engineering.

6.0 Lessons Learned and Operational Data

After 90 days of operation in the Munich facility, the following data points have been finalized:

6.1 Consumable Efficiency

We observed a 15% reduction in wire waste. In manual structural steel welding, over-welding (depositing a 10mm fillet where an 8mm is specified) is common to “be safe.” The 6-Axis system deposits exactly the volume required by the WPS, leading to significant cost savings across 1,000 meters of weldment.

6.2 Shielding Gas Laminar Flow

A technical hurdle we encountered was gas turbulence at the 6-axis wrist during rapid weave movements. We had to switch to a larger gas lens and increase the flow of the M21 (82% Ar/18% CO2) mixture to 18 L/min. The 6-axis movement is faster than a human hand, and the resulting “wind” can pull atmospheric oxygen into the pool if the shielding setup isn’t tuned for automated speeds.

6.3 Software Offsets vs. Physical Reality

The most significant lesson was that “the program is a lie.” Even the best 6-axis path needs a “Live Offset” capability. We implemented a joystick-based override that allows the senior welder to adjust the torch height (Z-axis) by 0.5mm increments during the weld without stopping the program. This solved the issue of plate warping during the fill passes.

7.0 Conclusion

The integration of the 6-Axis Collaborative Welder into the Munich structural steel workflow has successfully validated that Automated Welding is no longer restricted to thin-gauge automotive parts. For multi-pass, heavy-section S355 steel, the collaborative model provides the necessary precision and repeatability while allowing the human operator to manage the metallurgical variables that a computer cannot yet sense. Future implementations will focus on integrating laser-vision seam tracking to further reduce the need for manual path offsets.

Prepared by:
Senior Welding Engineer, Munich Field Office