Engineering Review: Single Pulse Collaborative Arc Welding System – Ulsan, South Korea

Field Technical Report: Implementation of Single Pulse Collaborative Arc Welding Systems (Ulsan Facility)

This report details the operational deployment and performance evaluation of a Single Pulse Collaborative Arc Welding System within the heavy industrial manufacturing sector of Ulsan, South Korea. The project focused on high-specification fabrication of Grade 5 (Ti-6Al-4V) titanium components for maritime pressure vessels. The objective was to bridge the gap between traditional fixed Automated Welding and manual precision welding through the introduction of collaborative robotics.

Synergy: Collaborative Arc Welding System and Automated Welding

In the Ulsan workshop environment, the distinction between a Collaborative Arc Welding System and traditional Automated Welding is not merely a matter of hardware, but of operational philosophy. Traditional automated welding excels in high-volume, repetitive longitudinal seams where part geometry is consistent. However, the complex curvatures of titanium pressure vessel nozzles require a level of adaptability that hard automation lacks.

The Collaborative Advantage in High-Mix Production

The Collaborative Arc Welding System (cobot) was integrated to handle the intricate geometries of the nozzle-to-shell welds. Unlike fixed-track automated welding, the cobot allows for rapid reconfiguration. In Ulsan, we observed that the primary synergy lies in the “Human-in-the-loop” workflow. A senior welder sets the initial torch angle and lead-in, while the system’s automated sensors maintain the voltage-to-distance ratio through the arc sensing protocol.

Operational Interfacing

The synergy was most evident during the multi-pass filling stage. While the hard automated welding gantry handled the circumferential girth welds of the main 20mm titanium shell, the collaborative system was deployed simultaneously on the internal fittings. This parallel processing reduced the total fabrication lead time by 35%. The technical challenge was ensuring that the electromagnetic interference (EMI) from the high-frequency starts of the automated gantry did not disrupt the collaborative system’s sensitive force-torque sensors. We resolved this through localized shielding and independent grounding circuits for the cobot base.

Technical Deep-Dive: Titanium Welding in Ulsan

Titanium welding remains one of the most demanding processes in the Ulsan maritime sector due to the material’s extreme reactivity to atmospheric gases at temperatures exceeding 400°C. The use of a Collaborative Arc Welding System in this context required a radical redesign of the standard gas delivery system.

Managing the Heat-Affected Zone (HAZ)

Titanium welding requires precise control over heat input to prevent grain growth and the formation of brittle “alpha case” layers. We utilized a Single Pulse waveform rather than a standard CV (Constant Voltage) or Double Pulse. The Single Pulse regime allows for a controlled droplet transfer at lower average currents. This is critical for titanium, where maintaining a small, manageable molten pool is the only way to ensure the trailing gas shield remains effective.

Trailing Shield Integration on Collaborative Arms

One of the “lessons learned” during the first week in Ulsan was the mechanical drag of the trailing shield. Titanium requires a primary shield (the torch), a trailing shield (to protect the cooling weld bead), and often a backing shield. Traditional automated welding uses heavy, rigid trailing shields. For a Collaborative Arc Welding System, these shields add significant mass and torque to the robot’s wrist. We had to engineer a lightweight, 3D-printed titanium trailing shield housing that used sintered stainless steel diffusers to ensure laminar argon flow without exceeding the cobot’s 10kg payload limit.

Argon Purity and Coastal Humidity

The Ulsan facility is located near the coast. High ambient humidity can lead to hydrogen porosity in titanium welds. We implemented a dual-stage gas desiccant system and switched to 99.999% (5.0 grade) Argon. The collaborative system was programmed to perform a 10-second pre-purge and a 30-second post-purge—longer than standard steel protocols—to ensure the weld remained silver and free of straw-colored or blue oxidation, which indicates contamination.

Parametric Analysis of Single Pulse Performance

The success of automated welding on titanium hinges on the pulse parameters. During the Ulsan trials, we focused on three primary variables: Pulse Frequency, Peak Current, and Background Current.

Pulse Frequency and Arc Stiffness

We found that a pulse frequency of 120Hz provided the optimal “arc stiffness” required for out-of-position welding on the vessel’s hemispherical heads. Lower frequencies resulted in a globular transfer that was too turbulent, causing the argon shield to break. The higher frequency stabilized the arc, allowing the Collaborative Arc Welding System to maintain a consistent 2mm standoff distance, even as the thermal expansion shifted the workpiece slightly during the 45-minute weld cycle.

The Peak-to-Background Ratio

For 6mm Ti-6Al-4V plate, we established a peak current of 220A and a background current of 85A. This 2.5:1 ratio ensured sufficient penetration while allowing the weld pool to solidify rapidly between pulses. In a manual environment, maintaining this consistency over a two-meter weld path is impossible. The automated welding logic of the cobot maintained this ratio with a variance of less than 0.5%, resulting in a weld crown with exceptional ripple consistency and zero recorded tungsten inclusions.

Field Observations and Lessons Learned

Transitioning from a pure manual or pure automated welding setup to a collaborative one in a high-pressure environment like Ulsan provided several critical insights into “real-world” engineering.

Lesson 1: Cable Management is a Failure Point

In automated welding, cables are often managed in fixed energy chains. In a Collaborative Arc Welding System, the arm moves in six degrees of freedom. The stiff, gas-cooled liners required for titanium welding (to prevent wire shaving and contamination) created unexpected torque on the cobot’s joints. We had to implement a “zero-gravity” overhead spring balancer to support the torch lead, preventing the robot from triggering a “safety stop” due to external force detection.

Lesson 2: Surface Preparation is Non-Negotiable

While the Single Pulse system is forgiving in steel, it is ruthless in titanium. Any residual oils from the Ulsan shipyard’s machining center caused immediate arc instability. We moved to a strict “welding-within-four-hours” rule after stainless steel wire brushing and acetone cleaning. The Collaborative Arc Welding System was then used to “pre-heat” the joint slightly with a non-transferred arc to drive off any surface moisture before the wire feed began.

Lesson 3: The “Tiptoe” Calibration

The synergy between the operator and the machine requires a calibration we called the “Tiptoe.” Before any titanium welding, the operator uses the cobot’s lead-through teaching mode to trace the joint. However, we learned that the thermal expansion of the titanium vessel during welding (up to 4mm over the diameter) necessitated the use of “Through-Arc Seam Tracking” (TAST). Integrating TAST into a collaborative platform required custom software patches to ensure the robot’s motion remained smooth while making real-time path corrections.

Concluding Technical Assessment

The deployment in Ulsan confirms that a Collaborative Arc Welding System is not a replacement for traditional automated welding, but a necessary evolution for high-value materials like titanium. The Single Pulse process, when coupled with the precision of a collaborative arm, produces metallurgical results that exceed AWS D17.1 Class A requirements.

For future installations, the focus must remain on the rigidity of the gas shielding apparatus and the refinement of the “handshake” between the human operator and the automated pathing logic. The Ulsan project proves that in the realm of titanium welding, the consistency of the machine and the intuition of the engineer are no longer mutually exclusive; they are synergistic requirements for the next generation of maritime fabrication.

Engineering Summary of Key Metrics:

• Material: Ti-6Al-4V (Grade 5)
• Process: Single Pulse GMAW-P
• Shielding: 99.999% Ar (Laminar Flow)
• Duty Cycle achieved: 75% (vs. 25% Manual)
• Rejection Rate: <1.2% (Visual and X-ray)