Engineering Review: Multi-pass Welding Cobot Welding Machine – Curitiba, Brazil

Field Report: Deployment of Multi-pass Cobot Welding Systems in Curitiba Industrial Sector

This report details the technical implementation, operational challenges, and performance metrics of a high-payload Cobot Welding Machine deployed at a heavy machinery fabrication facility in the Cidade Industrial de Curitiba (CIC), Brazil. The primary objective was the automation of multi-pass GMAW (Gas Metal Arc Welding) processes on ASTM A572 Grade 50 Thick Plate Steel welding applications. Unlike traditional industrial robots, the integration of Collaborative Robotics in this specific workshop environment allowed for a hybrid workflow where human intuition in weld-path adjustment met the repeatability of automated systems.

1. Site Conditions and Infrastructure Integration

Curitiba’s industrial climate presents specific challenges for high-precision welding. During the deployment phase in Q3, ambient humidity levels frequently exceeded 75%, necessitating stringent controls on consumable storage and pre-heating protocols. The Cobot Welding Machine was integrated into a bay previously used for manual stick and semi-automatic MIG welding.

The transition to Collaborative Robotics allowed us to bypass the installation of extensive light curtains and physical fencing, which would have been impossible given the tight floor plan of the Curitiba facility. However, the open-cell nature meant we had to implement local flash-shielding solutions to protect adjacent manual welders. The power grid stability at the site was found to be sufficient, though we installed dedicated line conditioners to ensure the cobot’s control electronics didn’t suffer from the voltage drops common when the facility’s large plasma tables initiated their arcs.

2. Technical Specifications for Thick Plate Steel Welding

The project focused on structural box girders with wall thicknesses ranging from 35mm to 60mm. For Thick Plate Steel welding, the heat input management is critical to prevent grain growth in the Heat Affected Zone (HAZ) and to ensure full root penetration.

Joint Geometry and Preparation

We utilized a single-V butt joint with a 60-degree included angle. Given the 50mm thickness, the Cobot Welding Machine was programmed to execute a 12-pass sequence. The first three passes (root and hot passes) were the most critical. We utilized a ceramic backing strip to allow the cobot to run a high-amperage root pass without burn-through, a technique that significantly reduced the need for back-gouging.

Parameter Settings

• Wire: 1.2mm ER70S-6 Solid Wire.
• Gas: 82% Argon / 18% CO2 at 20 L/min.
• Current/Voltage: 280A / 28V for fill passes; 240A / 24V for the root.
• Travel Speed: Maintained at 350mm/min for fill passes to ensure adequate wetting.

3. The Synergy of Collaborative Robotics in Multi-pass Logic

The true value of Collaborative Robotics in this Curitiba application was the “Lead-Through” programming capability. In Thick Plate Steel welding, part fit-up is rarely perfect. Variations in the root gap are common in large-scale Brazilian fabrications.

The Cobot Welding Machine utilized an integrated “Touch-Sensing” protocol. Before each pass, the cobot uses the welding wire to touch the side walls of the groove, recalibrating its center-line based on the actual physical location of the steel rather than the theoretical CAD model. This synergy allows the operator—a skilled welder—to guide the arm to the start point and then let the software handle the complex weaving and step-over logic for the subsequent 11 passes. This reduces the cognitive load on the welder while maintaining the precision required for UT (Ultrasonic Testing) compliance.

4. Lessons Learned: Challenges with Multi-pass Heat Management

One of the primary ‘lessons learned’ during the first month in Curitiba was the impact of inter-pass temperature. When using a Cobot Welding Machine, the duty cycle is significantly higher than that of a manual welder. We initially observed that the Thick Plate Steel welding was retaining too much heat, leading to a fluid puddle that was difficult to control in the 2G (horizontal) position.

Corrective Actions

We programmed “cooling offsets” into the Collaborative Robotics control loop. If the infrared pyrometer detected an inter-pass temperature exceeding 250°C, the cobot would automatically pause and move to a “park” position, allowing the plate to cool. This prevented the loss of mechanical properties in the Grade 50 steel and ensured the slag from the previous pass remained brittle enough for easy removal—an essential factor for automated multi-pass success.

5. Programming for Weaving and Step-Over

For the cap passes on the 50mm plates, we moved away from simple stringer beads. The Cobot Welding Machine was programmed with a trapezoidal weave pattern. In Collaborative Robotics, the smoothness of the motion is paramount to avoid undercut at the toes of the weld.

We found that a weave frequency of 1.5Hz with a 0.2-second dwell at the side walls provided the best aesthetic and structural results. The step-over (the distance the robot moves up for the next pass in a multi-layer sequence) was set at 2.5mm. This overlap ensured a flat profile, which is required by the local Curitiba inspectors to minimize stress concentrations.

6. Ergonomics and Workforce Reception

A notable observation in the Curitiba workshop was the shift in the welder’s role. Initially, there was skepticism regarding the Cobot Welding Machine. However, once the team realized that the Collaborative Robotics system was handling the “dirty” work—specifically the long-duration, high-heat fill passes—the reception improved.

The manual welders became “Cobot Technicians.” They focused on joint preparation, tacking, and the critical final visual inspection. This division of labor reduced physical fatigue and respiratory exposure to welding fumes, as the operator could stand three meters away from the arc while still being “collaboratively” involved in the process via the tablet interface.

7. Technical Performance Metrics

After three months of operation, the data indicates the following:

• Arc-on Time: Increased from 25% (manual) to 65% (cobot).
• Defect Rate: Repair rates on Thick Plate Steel welding dropped from 8% to less than 1.5% (primarily due to the elimination of stop/start defects).
• Consumable Efficiency: 12% reduction in wire waste due to optimized pathing.

8. Final Engineering Summary

The deployment in Curitiba confirms that a Cobot Welding Machine is not merely a replacement for a manual welder but a sophisticated tool that enhances the capability of a skilled technician. In the context of Thick Plate Steel welding, the precision of Collaborative Robotics in managing multi-pass layers is superior to manual application, provided that the initial programming accounts for real-world variables like thermal expansion and humidity-driven porosity.

Moving forward, we recommend the integration of through-arc seam tracking to further automate the compensation for plate warping during the welding of 60mm+ sections. The Curitiba site now serves as the benchmark for our South American operations regarding the automation of heavy structural joints.

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Report Prepared By: Senior Welding Engineer, Heavy Fabrication Division
Location: Curitiba, PR, Brazil
Status: Operational / Process Optimized