Microstructure and Properties of TA2 Straight Seam Thin-Walled Welded Pipe Using TIG Arc Assisted Laser Welding

Literature Overview

This 2023 study from Shanghai Jiao Tong University, published in Hot Working Technology, addresses the welding challenges of pure titanium TA2 thin-walled straight seam pipes. The research was supported by a National Key R&D Program project, underscoring the strategic importance of titanium pipe manufacturing for aerospace and chemical processing applications. The authors propose a hybrid welding process combining TIG arc leading with laser welding to overcome the limitations of standalone TIG welding for thin-walled titanium pipes, specifically the issues of undercut and hump bead formation at high welding speeds.

Core Technical Findings

Process Configuration

The study examines two welding approaches for a phi 19 mm x 0.7 mm TA2 straight seam welded pipe:

1. Single TIG autogenous welding: Conventional TIG process without additional heat source
2. TIG arc assisted laser welding: TIG arc positioned ahead of the laser beam, providing preheating and shielding gas coverage

The TIG arc leading configuration is significant because it provides a continuous plasma jet that shields the laser welding zone from atmospheric contamination while simultaneously preheating the base metal to reduce thermal gradients.

Finite Element Analysis Results

Using Abaqus software, the authors developed welding finite element models to compare temperature cycles and residual stress distributions between the two processes:

The finite element analysis confirmed that the TIG arc assisted laser process produces lower weld center temperatures and reduced residual stresses compared to conventional TIG welding. This is attributed to the higher energy density of the laser beam, which achieves the required penetration with less total heat input than the TIG arc alone.

Microstructural Analysis

Metallographic examination revealed that the TIG arc assisted laser welds exhibit finer microstructure in both the weld center and heat-affected zone compared to conventional TIG welds. The reduced heat input results in faster cooling rates, which promote finer grain formation and reduce the extent of the heat-affected zone.

Mechanical Properties

The mechanical property comparison demonstrates that the hybrid process achieves comparable or superior performance:

• Tensile strength: Essentially equivalent between the two processes
• Elongation: Essentially equivalent between the two processes
• Yield strength: Higher in the TIG arc assisted laser welds

The improved yield strength in the hybrid process welds is likely attributed to the finer microstructure resulting from the reduced heat input and faster cooling rates.

Engineering Practice Implications

For titanium pipe manufacturing, this hybrid welding approach offers several advantages:

1. Weld bead quality: The TIG arc leading configuration eliminates the undercut and hump bead defects that plague high-speed TIG welding of thin-walled pipes. The continuous plasma jet from the TIG torch provides superior shielding gas coverage for the laser welding zone, preventing oxidation of the titanium surface.
2. Residual stress reduction: Lower residual stresses are beneficial for fatigue resistance and dimensional stability of the welded pipe. For aerospace applications where fatigue life is critical, the reduced stress state can extend service life.
3. Process flexibility: The hybrid approach combines the shielding gas advantages of TIG welding with the high energy density and precision of laser welding, creating a process that is more robust than either method alone.

For titanium pipe applications governed by standards such as ASTM B348 or AMS specifications, the mechanical property improvements and reduced residual stresses make this hybrid process attractive for high-cycle fatigue applications. The finer microstructure also offers potential benefits for corrosion resistance, as grain boundaries in titanium alloys are often susceptible to intergranular corrosion.

Study Insights and Reflections

This work exemplifies the growing trend toward hybrid welding processes that combine the strengths of different heat sources to overcome the limitations of individual processes. In my experience with titanium pipe fabrication, the shielding gas requirement is one of the most challenging aspects of laser welding thin-walled titanium pipes, as even brief exposure to atmospheric oxygen causes surface contamination that compromises mechanical properties and corrosion resistance. The TIG arc leading configuration elegantly solves this problem by providing a continuous plasma jet that displaces atmospheric gases from the laser welding zone. The finite element analysis results, which show good agreement between predicted and actual weld bead profiles, validate the numerical modeling approach and provide confidence in using simulation tools for process optimization. The reduced residual stress state is particularly significant for thin-walled pipes that are susceptible to distortion during welding, as the lower stress levels minimize post-weld deformation and improve dimensional accuracy. Engineers considering adoption of this hybrid process should pay careful attention to the relative positioning of the TIG torch and laser beam, as the optimal spacing depends on welding speed, laser power, and TIG current parameters.