Keyhole TIG Welding on Medium and Thick TA2 Titanium Plates

Literature Overview

The paper by Yin Yayun and colleagues from the Luoyang Ship Materials Research Institute, published in Hot Working Technology (2023, Vol. 52, No. 9, pp. 136-139), presents experimental investigations on keyhole TIG welding of TA2 titanium plates at thicknesses of 16 mm and 30 mm. This work is particularly timely given the increasing demand for titanium alloy components in aerospace, marine, and nuclear applications where thick-section fabrication is required. The study examines process parameters, weld geometry, internal quality, microstructure, and mechanical properties under different welding configurations.

Technical Significance of Keyhole TIG for Thick Titanium

TA2 (Grade 2) titanium is the most widely used commercially pure titanium grade, characterized by excellent corrosion resistance, good formability, and moderate strength (yield strength approximately 240 MPa, tensile strength 340-450 MPa). Traditional multi-pass TIG welding of thick TA2 plates requires extensive backing gas arrangements, multiple fill passes, and significant operator time, making it economically challenging for large components.

Keyhole TIG welding exploits the plasma jet effect where the arc concentrates into a narrow, deep-penetrating channel. When applied to thick plates, this technique can achieve full penetration in a single pass from one side, dramatically simplifying the welding sequence. The study investigates two configurations:

• 16 mm plate: No-groove single-sided welding with double-sided forming
• 30 mm plate: Y-groove preparation with keyhole TIG root pass followed by conventional TIG fill and cap passes

Experimental Results and Microstructural Analysis

16 mm No-Groove Configuration

The optimized parameters for 16 mm full-penetration single-sided welding achieved sound weld formation with acceptable surface quality on both sides. However, the study notes that high welding currents lead to weld metal microstructure coarsening relative to the base metal, with slight hardening observed in the joint region. This is expected given the elevated solidification temperatures and slower cooling rates at the centerline of thicker sections.

30 mm Y-Groove Configuration

For the 30 mm plate with Y-groove preparation:

• Back-side keyhole TIG root pass achieved full penetration through a 14 mm land (unmilled root)
• Front-side keyhole TIG root pass achieved full penetration through a 10 mm land
• The root pass exhibited excellent surface quality and internal soundness
• Conventional TIG fill and cap passes could be directly applied without additional preparation

The mechanical properties of both configurations were found to be comparable to the base metal, confirming that the keyhole TIG process does not introduce significant property degradation when properly controlled.

Process Control Considerations

Keyhole TIG welding of titanium alloys requires exceptional attention to several process variables:

1. Shielding gas purity: Oxygen contamination above 200 ppm significantly degrades titanium weld properties. Helium or high-purity argon with dedicated gas delivery systems is mandatory.
2. Arc stability: The keyhole mode is inherently unstable and requires precise control of current, voltage, and travel speed. Any disturbance can cause keyhole collapse, leading to incomplete penetration or excessive burn-through.
3. Fit-up tolerance: For no-groove welding, gap tolerance must be maintained within ±0.5 mm. For Y-groove configurations, the land thickness must be precisely controlled to ensure reliable keyhole formation.
4. Welding position: The technique is most reliable in the flat (1G) position. Vertical and overhead applications require additional process development and are not yet well established for thick titanium sections.

Engineering Practice Integration

This technology has direct applications in:

• Nuclear reactor pressure vessel internals where thick titanium alloy cladding is required
• Marine propeller and rudder fabrication requiring thick-section titanium alloy forgings
• Aerospace structural components where weight reduction is critical
• Chemical processing equipment requiring corrosion-resistant thick titanium alloys

The combination of keyhole TIG root pass with conventional TIG fill/cap represents a practical production strategy that balances penetration efficiency with weld quality. The ability to achieve 14 mm land penetration from the back side is particularly significant for repair welding applications where access is limited to one side.

Study Insights and Reflections

This research confirms that keyhole TIG welding is a viable technology for medium and thick TA2 titanium plates, with the 16 mm no-groove single-sided welding representing a particularly attractive option for production efficiency. The slight microstructural coarsening observed at high currents is a trade-off that must be carefully managed through parameter optimization.

The finding that 14 mm land penetration is achievable from the back side with keyhole TIG opens new possibilities for repair and maintenance welding of existing titanium components. In nuclear and marine applications where titanium alloy components are installed in inaccessible locations, this capability could eliminate the need for costly disassembly and replacement.

The study also implicitly validates the concept of hybrid process combinations—using advanced techniques (keyhole TIG) for critical passes (root) and conventional methods (standard TIG) for bulk deposition. This philosophy of applying the right process to each pass, rather than forcing a single technique throughout, represents sound engineering practice that maximizes both quality and productivity.