Engineering Review: Multi-pass Welding MIG/MAG Welding Robot – Lyon, France

Field Report: Multi-pass Integration for Heavy Fabrication (Lyon, France)

1. Project Scope and Environmental Context

This report details the implementation and optimization of a high-performance MIG/MAG Welding Robot system at a heavy industrial fabrication facility in the Saint-Priest district of Lyon. The facility specializes in the production of structural chassis for large-scale mining equipment, requiring consistent, high-integrity joins in Thick Plate Steel welding environments.

The primary challenge centered on transitioning from manual flux-cored arc welding (FCAW) to an automated, multi-pass gas metal arc welding (GMAW) process. The objective was to maintain structural integrity across 40mm to 60mm S355JR steel sections while reducing the cycle time per component. The integration of advanced Arc Welding Solutions was critical to address the inherent variability in large-scale groove preparations often found in heavy plate fabrication.

2. Hardware Configuration: The MIG/MAG Welding Robot

The core of the cell consists of a 6-axis articulated MIG/MAG Welding Robot mounted on a 10-meter linear track to accommodate the length of the structural beams. To achieve the necessary deposition rates for Thick Plate Steel welding, the system was outfitted with a water-cooled 500A torch and a high-precision wire feeder capable of maintaining consistent tension for 1.2mm and 1.6mm solid wires.

2.1 Technical Specifications of the Robotic Cell

• Positioner: Synchronized 2-axis head-and-tailstock for 360-degree rotation, ensuring all welds are performed in the PA (1G) or PB (2F) positions.
• Power Source: Inverter-based multi-process supply with rapid-pulse capabilities to minimize spatter during high-current fill passes.
• Wire: G4Si1 (ER70S-6) for superior wetting and deoxidation on heavy scale.

3. Integration of Arc Welding Solutions

In the Lyon workshop, we quickly identified that standard “blind” robotic programming was insufficient. Thermal distortion on 50mm plates during the 20th pass would shift the joint geometry by up to 4mm. To counter this, we deployed a suite of Arc Welding Solutions focused on adaptive sensing.

3.1 Seam Tracking and Touch Sensing

We implemented a laser-based seam tracking system that mounts ahead of the torch. This sensor provides real-time data to the MIG/MAG Welding Robot, allowing it to adjust the tool center point (TCP) based on the actual groove position rather than the theoretical CAD model. For the initial root pass, “Touch Sensing” (utilizing the welding wire as a probe) was used to establish the Z-axis height and start/stop coordinates, accounting for slight variations in the flame-cut edges of the Thick Plate Steel welding assemblies.

3.2 Adaptive Fill Logic

Perhaps the most critical component of the Arc Welding Solutions package was the adaptive fill software. As the robot progresses through multiple layers, the software monitors the cross-sectional area of the groove. If the gap widens due to heat-induced pull, the robot automatically adjusts its travel speed and oscillation width to ensure the bead height remains consistent with the WPS (Welding Procedure Specification).

4. Multi-pass Strategy for Thick Plate Steel Welding

Welding 50mm plate steel is not a matter of power, but a matter of thermal management and bead placement. Our strategy involved a 32-pass sequence to fill a 60-degree V-groove.

4.1 Root and Hot Pass Execution

The root pass was executed using a short-circuit transfer mode to prevent burn-through, despite the MIG/MAG Welding Robot‘s high amperage capacity. We maintained a 2mm root face and a 3mm gap. The “Hot Pass” immediately followed to provide enough throat thickness to support the high-heat input of the subsequent spray-transfer fill passes.

4.2 Fill and Cap Layers

For the fill passes, we transitioned to a pulsed-spray transfer mode. This minimized the interpass cleaning required and virtually eliminated spatter, which is a common failure point in automated Thick Plate Steel welding. We utilized a “stringer bead” technique rather than wide weaves to minimize the Heat Affected Zone (HAZ) and prevent grain coarsening in the S355JR substrate.

5. Lessons Learned from the Lyon Site

During the three-month commissioning phase in Lyon, several technical hurdles provided valuable insights into the synergy between the MIG/MAG Welding Robot and the specific metallurgy of heavy plates.

5.1 Managing Interpass Temperature

One of the “hard” lessons learned was the critical nature of interpass temperature monitoring. In Thick Plate Steel welding, the accumulation of heat is significant. If the interpass temperature exceeded 250°C, the mechanical properties of the weld metal began to degrade, specifically the impact toughness at -20°C. We integrated an infrared pyrometer into the Arc Welding Solutions suite. The robot was programmed to “pause” or move to a different joint if the temperature sensor detected a value above the threshold, allowing for natural cooling without operator intervention.

5.2 Shielding Gas Dynamics

The industrial environment in the Lyon facility was prone to drafts due to large overhead doors. We found that the standard 20L/min gas flow was insufficient for the 1.6mm wire at high voltages. We upgraded to a high-flow diffuser and a custom gas mix (82% Argon / 18% CO2) to stabilize the arc. The stability of the MIG/MAG Welding Robot‘s arc is directly proportional to the gas coverage; any turbulence resulted in porosity that required manual grinding and rework.

5.3 Wire Feed Consistency

In heavy fabrication, the robot often operates at 80% to 90% duty cycle. We experienced “burn-back” issues where the wire would fuse to the contact tip during long continuous fill passes. The solution was two-fold: switching to high-conductivity silver-plated tips and ensuring the Arc Welding Solutions included a “wire-retract” function at the end of each pass to prevent the wire from sticking in the cooling puddle.

6. Metallurgical and Quality Assurance Results

Post-implementation testing was conducted via Non-Destructive Testing (NDT), specifically Ultrasonic Testing (UT) and Magnetic Particle Inspection (MPI). The results for the robotic multi-pass welds were significantly superior to the previous manual benchmarks.

• Defect Rate: Reduced from 4.5% (manual) to 0.4% (robotic).
• Deposition Rate: Increased from 3.2 kg/hr to 6.8 kg/hr.
• Consistency: Tensile tests showed uniform yield strength across the entire 50mm cross-section, confirming that the Arc Welding Solutions successfully managed the heat input.

7. Conclusion

The deployment of the MIG/MAG Welding Robot in the Lyon facility has proven that Thick Plate Steel welding can be effectively automated provided the system is backed by robust Arc Welding Solutions. The key to success was not merely the robot’s movement, but the adaptive technologies that allowed the machine to “see” and “feel” the changes in the weld joint in real-time. Moving forward, we recommend the implementation of a cloud-based monitoring system to track gas consumption and wire usage per joint, further refining the cost-per-weld metrics for the Lyon site.

Engineer: J. Rossi, Senior Welding Engineer
Date: October 24, 2023
Location: Lyon, France