TIG Welding Process Optimization for Curved Surface Titanium Alloy Welds

Literature Overview

The research conducted by Guo Xiang, Lu Lili, Wang Jian, Yu Xiaowei, and Wang Li at the Key Laboratory of Reactor Fuel and Materials, China Nuclear Power Research Institute (2018), addresses a highly specialized welding challenge: gas tungsten arc welding (GTAW/TIG) of titanium alloys on curved surfaces. In nuclear reactor applications, titanium and titanium alloy components—such as pressure tubes, heat exchanger tubes, and containment vessels—often feature complex geometries including cylindrical, spherical, and toroidal surfaces. Welding on curved surfaces introduces unique thermal and mechanical challenges that are not encountered in flat-plate fabrication, including variable weld pool shape, asymmetric cooling rates, and complex residual stress fields. The study focuses on the Ti-6Al-4V alloy, which is the most widely used titanium alloy in nuclear applications due to its excellent combination of strength (yield strength approximately 880 MPa, ultimate tensile strength approximately 950 MPa), corrosion resistance, and weldability.

Core Technical Points

The primary challenge of curved-surface TIG welding lies in maintaining consistent arc geometry and heat input distribution. On a convex surface, the weld pool tends to sag under gravity, leading to undercuts and incomplete fusion on the upper side of the weld. On a concave surface, the weld pool is confined, leading to excessive penetration and potential burn-through. The study investigates how welding parameters—current, voltage, travel speed, and torch angle—must be adjusted as a function of surface curvature to achieve a uniform weld bead with acceptable mechanical properties.

Welding Parameter Optimization

The study employs a systematic approach to parameter optimization, varying welding current from 100–200 A, arc voltage from 12–18 V, and travel speed from 200–600 mm/min. The torch angle is varied from 0° (perpendicular to the surface) to 15° (forward-leaning), with the forward angle being more effective on convex surfaces to compensate for weld pool sag. The shielding gas flow rate is maintained at 10–15 L/min of pure argon, with a trailing gas cup employed to protect the hot weld zone from oxidation during cooling.

The data reveals a clear trend: for convex surfaces with smaller radii, lower current and higher torch angle are required to maintain a stable weld pool. For concave surfaces, higher current and faster travel speed are needed to prevent excessive penetration. The welding current on a flat surface serves as the baseline, with adjustments of ±10–15% depending on the curvature and direction.

Microstructural and Mechanical Characterization

The microstructure of the Ti-6Al-4V weld metal is characterized by acicular α′ martensite in the as-welded condition, which transforms to a lamellar α+β structure upon post-weld annealing at 600°C for 2 hours. The cooling rate at the weld centerline is approximately 50–80 K/s for flat joints and 40–65 K/s for curved joints, with the lower cooling rate on curved surfaces attributed to the increased thermal mass of the surrounding material. The acicular α′ in the as-welded condition provides high strength (yield strength approximately 900–950 MPa) but limited ductility (elongation approximately 8–10%). After annealing, the lamellar α+β structure provides a more balanced combination of strength (yield strength approximately 750–800 MPa) and ductility (elongation approximately 12–15%).

The hardness profile across the weld cross-section shows a maximum in the weld metal (approximately 350–380 HV in the as-welded condition) and a minimum in the HAZ (approximately 280–300 HV), which is consistent with the known behavior of Ti-6Al-4V where the HAZ experiences partial β-phase dissolution and subsequent coarsening of the α+β lamellae. The curved-surface welds exhibit a slightly broader hardness transition zone (approximately 1.5–2.0 mm) compared to flat welds (approximately 1.0–1.5 mm), reflecting the more complex thermal gradient on curved geometries.

Engineering Practice Integration

For nuclear-grade titanium alloy fabrication, the curved-surface welding process must comply with rigorous quality requirements. The study emphasizes the importance of weld visual inspection, which must be supplemented by non-destructive testing (NDT) including ultrasonic testing (UT) and radiographic testing (RT). The acceptance criteria for titanium alloy welds in nuclear applications typically follow ASME Section VIII Division 2 or the relevant Chinese nuclear standards (NB/T 47014, NB/T 47013), which require zero porosity and zero lack of fusion for safety-critical welds.

The use of a trailing gas cup is essential for titanium alloy welding, as the hot weld zone is highly susceptible to oxidation by oxygen and nitrogen in the atmosphere. The trailing gas flow rate should be 15–20 L/min of pure argon, with the gas cup positioned to cover the entire weld zone during cooling. The color of the weld bead is a reliable indicator of oxidation: a silver-gray or straw-yellow color indicates acceptable oxidation, while a blue or purple color indicates excessive oxidation and requires grinding and re-welding. The study recommends a two-pass approach for curved surfaces: a root pass with reduced current and a forward torch angle, followed by a cap pass with slightly higher current and a perpendicular torch angle to ensure adequate reinforcement and a smooth surface finish.

Key Reflections and Implications

The curved-surface TIG welding of titanium alloys represents a challenging but solvable fabrication problem. The key insight from this study is that the welding parameters must be adapted to the local geometry rather than applied uniformly across the entire weld length. This requires either manual welding with experienced operators who can adjust parameters in real time, or automated welding with CNC-controlled torch position and parameter adjustment. For nuclear applications, where reproducibility and documentation are paramount, automated welding with process monitoring is strongly preferred. The study also highlights the importance of post-weld heat treatment in achieving the required mechanical properties, as the as-welded condition of Ti-6Al-4V is too brittle for most structural applications. The annealing treatment not only improves ductility but also relieves residual stresses, which is critical for preventing stress corrosion cracking in nuclear environments. Overall, the research provides a solid foundation for developing qualified welding procedures for curved-surface titanium alloy components in nuclear reactors, with clear parameter guidelines and quality control measures that can be directly applied in engineering practice.