Engineering Review: Multi-pass Welding 6-Axis Collaborative Welder – Budapest, Hungary

Field Report: Implementing Multi-pass 6-Axis Collaborative Welding in Structural Steel Fabrication

1. Project Overview and Site Conditions

This report details the field deployment and performance evaluation of a 6-Axis Collaborative Welder integrated into a high-output structural steel welding facility located in Budapest, Hungary. The objective was to transition heavy-gauge multi-pass fillet and butt joints from manual Metal Active Gas (MAG) welding to a semi-autonomous Automated Welding workflow.

The facility specializes in S355JR structural sections for European infrastructure projects. The primary challenge identified was the inconsistency of manual welds on 20mm to 30mm thick plates, where welder fatigue during the 4th, 5th, and 6th passes often led to slag inclusions or lack of fusion at the toes. The deployment of the 6-axis cobot aimed to standardize the heat input and bead geometry across extended shifts.

2. Technical Specifications of the 6-Axis Collaborative Welder

In the context of structural steel welding, the “collaborative” aspect of the hardware is often misunderstood as merely a safety feature. In this Budapest field application, the 6-Axis Collaborative Welder served as a precision tool-head capable of mimicking the complex wrist movements of a veteran welder while maintaining a duty cycle impossible for a human operator.

4.1 Degree of Freedom and Torch Manipulation

The 6-axis configuration allowed for precise Work Angle and Travel Angle adjustments mid-weld. This is critical for multi-pass applications where the torch must be offset by specific increments (e.g., 2.5mm laterally and 2.0mm vertically) for each subsequent bead. The integration of the 6th axis enables the system to maintain a constant contact-tip-to-work distance (CTWD) even when navigating the undulating surfaces of heavy-duty structural beams.

4.2 Synergy with Automated Welding Software

The true “automated welding” capability was realized through the software’s multi-pass logic. Rather than programming each bead manually, we utilized a parametric offset library. By defining the root pass, the software automatically calculated the trajectories for the subsequent filler and cap passes, accounting for the cumulative cross-sectional area of the weld deposit. In the Budapest workshop, this reduced programming time by 70% compared to traditional industrial robots.

3. Structural Steel Welding: Material and Joint Configuration

The test subjects were 20mm S355JR steel plates prepared with a 60-degree V-groove. For structural steel welding, adherence to EN ISO 15614-1 is non-negotiable. Our goal was to achieve a mechanical property profile that met or exceeded the base metal requirements, specifically regarding Charpy V-notch toughness at -20°C.

3.1 Weld Procedure Specification (WPS) Parameters

• Process: 135 (MAG – Metal Active Gas)
• Wire: 1.2mm G3Si1 / ER70S-6
• Gas: M21 (82% Argon / 18% CO2) at 16 L/min
• Root Pass: 180A, 19V, Travel Speed 25 cm/min
• Fill Passes (3-5): 240A, 26V, Travel Speed 32 cm/min
• Cap Pass (6-8): 210A, 24V, Travel Speed 30 cm/min (with slight weave)

The 6-axis collaborative welder maintained a consistent weave frequency of 2.5Hz for the cap pass, ensuring a smooth transition at the weld toes—a common failure point in manual structural inspections in the Budapest sector.

4. Automated Welding Integration: The Budapest Implementation

The implementation in Budapest highlighted the synergy between human expertise and automated welding. The “Collaborative” nature allowed the Hungarian welding technicians to “lead-through” the initial path teaching. This is a significant shift from traditional G-code programming.

4.1 Surface Preparation and Tack Welding

Lessons learned in the first week emphasized that automated welding is only as good as the preparation. Because the cobot follows a pre-programmed path, any deviation in the joint gap (due to poor tacking or thermal distortion) can result in a missed root. We implemented a “Touch Sensing” routine where the 6-Axis Collaborative Welder uses the wire tip to find the plate edges before striking the arc. This compensates for variations in the structural steel assembly.

4.2 Thermal Management and Interpass Temperature

In structural steel welding, excessive heat input can enlarge the Heat Affected Zone (HAZ), reducing the yield strength of the S355 material. The automated system was programmed with an interpass temperature limit of 250°C. Using an integrated infrared sensor, the cobot would pause the automated welding cycle until the temperature dropped to the setpoint, ensuring metallurgical integrity without requiring constant technician monitoring.

5. Lessons Learned: Challenges and Field Fixes

No field deployment is without friction. Our experience in Budapest provided three critical takeaways for senior engineers looking to adopt 6-axis collaborative welder technology in heavy industry.

5.1 The “Sway” Factor in Long Reach Scenarios

While the 6-axis arm is versatile, at its maximum reach (1300mm), we observed a minor harmonic vibration during high-speed oscillations (weaving). This was mitigated by adjusting the acceleration/deceleration curves in the controller and ensuring the mounting pedestal was chemically anchored to the shop floor’s 200mm concrete slab. For structural steel welding, stability is paramount.

5.2 Spatter Management and Torch Maintenance

Automated systems run at much higher duty cycles than manual operators. In Budapest, we found that the standard gas nozzle would clog every 45 minutes of arc-on time. We integrated an automatic reaming station. The 6-Axis Collaborative Welder was programmed to visit the reamer every three passes. This maintained gas coverage and eliminated the porosity issues we saw in the initial trial runs.

5.3 Wire Feeding Consistency

Because the cobot moves in 6 axes, the wire conduit (liner) is subject to constant twisting. This can lead to erratic wire feed speeds (WFS). We switched to a high-flexibility “marathon pack” conduit system. This ensured that even when the arm was at an acute angle inside a box girder, the WFS remained within +/- 2% of the setpoint.

6. Quantitative Performance Analysis

After 30 days of operation in the Budapest facility, we compared the automated welding data against historical manual benchmarks for the same structural steel components.

The reduction in the repair rate is the most significant financial driver. In structural steel welding, the cost of gouging out a failed multi-pass weld and re-welding is roughly 4x the cost of the initial weld. The consistency of the 6-axis collaborative welder essentially paid for the unit’s lease within the first quarter of operation.

7. Conclusion and Recommendations

The deployment in Budapest confirms that the 6-Axis Collaborative Welder is no longer just for thin-gauge sheet metal. When paired with robust automated welding software and a deep understanding of structural steel welding metallurgy, it becomes a force multiplier for heavy fabrication.

For future installations, I recommend:

1. Standardizing Fixturing: Move toward modular welding tables to ensure the cobot’s “home” position relative to the workpiece is consistent within 1mm.
2. Advanced TAST: Implement Through-Arc Seam Tracking for joints exceeding 2 meters in length to account for real-time thermal warping.
3. Skill Upgrading: Train existing manual welders to become “Cobot Operators.” The best results come when the person who understands the puddle is the one supervising the automated welding parameters.

Budapest’s industrial landscape is ripe for this technology. The synergy of local engineering talent and collaborative robotics is the only viable path to maintaining competitiveness in the European structural steel market.

Report Prepared By: Senior Welding Engineer
Location: District XXI, Budapest, Hungary
Status: Deployment Successful / Operational Phase Alpha