TIG Arc-Assisted Laser Welding of TA2 Thin-Wall Straight Seam Pipe

Literature Overview

This paper, published in "Hot Working Technology" (2023, Vol. 52, Issue 5, pp. 135-140) by Wang Boshi, Kong Liang, Li Fang, Wang Min, and Zhang Yuelong from Shanghai Jiao Tong University, addresses a specific challenge in titanium pipe manufacturing: the formation of undercut and hump defects during high-speed TIG welding of thin-walled TA2 straight seam pipe. The research was supported by the National Key Research and Development Program of China (2016YFB0301205). The proposed solution is a hybrid welding process that combines TIG arc welding with laser welding in a tandem configuration.

Problem Statement and Process Configuration

TA2 commercially pure titanium is widely used in aerospace, chemical processing, and medical applications due to its excellent corrosion resistance, low density, and biocompatibility. However, welding thin-walled TA2 pipe presents significant challenges. Conventional TIG welding at the high speeds required for production suffers from:

• Undercut at the weld toes, which acts as a stress concentration site
• Hump formation due to insufficient molten pool support at high travel speeds
• Excessive heat input leading to grain coarsening in the heat-affected zone
• Potential for burn-through on very thin walls

The proposed process employs a TIG arc in front of the laser beam (TIG-in-front configuration). The TIG arc serves as a pre-heater and stabilizer, while the laser provides concentrated heat input for deep, narrow penetration. This hybrid approach leverages the strengths of both processes: the TIG arc provides a broader heat distribution that stabilizes the molten pool, while the laser achieves high energy density for efficient penetration.

Experimental and Simulation Results

The test configuration involved a φ19 mm × 0.7 mm TA2 straight seam welded pipe. Two welding processes were compared: single-TIG autogenous welding and TIG arc-assisted laser welding. A finite element model was established using Abaqus software to simulate the welding temperature cycles and residual stress distributions.

The following table presents the comparative results:

The simulation results demonstrate that both welding processes produce weld bead profiles that agree well with experimental observations, validating the finite element model. The TIG arc-assisted laser welding process produces lower weld center temperatures and lower post-weld Von Mises stress compared to single-TIG welding. This reduction in thermal input is attributed to the higher welding speed achievable with laser assistance, which reduces the total heat per unit length despite the concentrated energy density.

Microstructural Analysis

The microstructural results are particularly significant. TIG arc-assisted laser welding produces finer grain structures in both the weld center and the heat-affected zone compared to single-TIG welding. This refinement is attributed to the higher cooling rates achieved with the hybrid process, which promote nucleation of new grains and limit grain growth. Finer grains in titanium alloys are associated with improved yield strength and ductility through the Hall-Petch relationship.

The mechanical properties confirm this microstructural advantage. While tensile strength and elongation remain comparable between the two processes, the yield strength is higher in the TIG arc-assisted laser welds. This improvement is consistent with the finer microstructure and lower residual stress state. For thin-walled pipe applications, higher yield strength is particularly valuable as it improves resistance to plastic deformation under external loads.

Process Optimization and Engineering Considerations

The TIG-in-front configuration is critical to the success of this hybrid process. The TIG arc pre-heats the base metal ahead of the laser beam, reducing the thermal gradient at the weld front and promoting stable keyhole formation. This pre-heating also helps to prevent the undercut defects that plague high-speed single-TIG welding of thin titanium pipe.

Several process parameters require careful optimization:

• TIG arc current and arc length: Must provide sufficient pre-heating without excessive heat input
• Laser power and beam diameter: Must achieve adequate penetration without burn-through
• Travel speed: Must be high enough for productivity but controlled to maintain weld stability
• Torch-laser spacing: Must be optimized for effective thermal interaction
• Shielding gas: Argon shielding is essential for titanium welding to prevent oxidation

Residual Stress Management

The reduction in residual stress observed with TIG arc-assisted laser welding has significant implications for pipe welding applications. Residual stresses in welded pipes contribute to stress corrosion cracking susceptibility, particularly in chlorinated environments where titanium piping is commonly used. The lower Von Mises stress achieved with the hybrid process reduces the risk of stress corrosion cracking and improves dimensional stability after welding.

For pipe welding applications governed by standards such as ASTM B363 (titanium and titanium alloy pipe) or ASME B31.3, residual stress levels are an important consideration. The hybrid process offers a pathway to achieving lower residual stresses without requiring post-weld stress relief, which is particularly valuable for thin-walled pipe where stress relief may cause distortion.

Study Insights and Reflections

The TIG arc-assisted laser welding process represents a sophisticated solution to a specific manufacturing challenge. The key insight is that combining two heat sources in a carefully designed configuration can overcome the limitations of either process alone. The TIG arc provides the thermal stabilization that the laser alone cannot achieve at high speeds, while the laser provides the energy density that TIG alone cannot achieve without excessive heat input.

From a metallurgical perspective, the finer microstructure achieved with the hybrid process is a direct consequence of the higher cooling rates enabled by the laser's concentrated energy. This demonstrates that hybrid welding processes can be designed not just for geometric quality but also for microstructural control. For titanium alloy applications where grain size directly influences mechanical properties and corrosion resistance, this microstructural control is a significant advantage.

The finite element simulation validated against experimental results provides a powerful tool for process development. Engineers can use such models to predict weld geometry, residual stress, and thermal cycles before committing to expensive titanium welding trials. This predictive capability accelerates process qualification and reduces development costs, which is particularly important for aerospace and medical applications where titanium pipe welding requires rigorous process validation.