Microstructure and Properties of TIG Welded Joints in Ti2AlNb-Based Alloy

Literature Overview

This paper, published in Welding Journal (2014, No. 2, pp. 47-50), authored by Zhao Haitao, Wan Xiaohui, Guo Delun, Wu Sujun, Liu Xueli, and Ji Yingping from the Beijing Institute of Aeronautical Manufacturing Technology and Beihang University, investigates the microstructure and mechanical properties of TIG welded joints in a Ti2AlNb-based alloy. The study examines both no-fill and filler-wire-assisted pulsed TIG welding of 1.5 mm thick sheets, providing insights into the weldability of this emerging lightweight structural material.

Core Technical Content and Findings

Ti2AlNb-based alloys are a class of ordered intermetallic compounds with the L10 crystal structure, offering a combination of high strength, good creep resistance, and low density at elevated temperatures. These properties make them attractive for high-temperature structural applications in aerospace and energy sectors. However, their weldability is limited by the tendency to form brittle phases during the welding thermal cycle and the sensitivity of the ordered microstructure to thermal disruption.

Weld Microstructure Characteristics

The weld metal in both no-fill and filler-wire-assisted welds exhibited a dendritic solidification pattern, consistent with the high cooling rates achieved in thin-sheet TIG welding. A notable observation is the presence of a layered or stratified microstructure within the weld metal, which is attributed to the oscillation of the molten pool surface during the pulsed TIG process. The pulse cycle causes periodic variations in heat input, leading to alternating regions of coarser and finer dendrite arm spacing.

The addition of pure titanium filler wire significantly altered the weld metal microstructure. The titanium addition diluted the aluminum and niobium concentrations in the weld pool, shifting the solidification path and promoting a larger equiaxed grain region. This is beneficial because equiaxed grains are generally associated with improved ductility and reduced anisotropy compared to columnar grains.

Phase Distribution and Hardness Profile

A systematic analysis of the phase composition across the weld cross-section revealed a gradient in O-phase content. The base metal, with its fully ordered L10 structure, exhibited the highest O-phase fraction. Moving toward the heat-affected zone (HAZ) and then into the weld metal, the O-phase content progressively decreased due to the thermal disruption of the ordered structure and the dilution effects of the welding process.

The hardness distribution followed a pattern directly correlated with the phase composition. The HAZ exhibited the highest hardness values, exceeding those of both the base metal and the weld metal. This is attributed to the formation of fine-scale precipitates and the partial disruption of the ordered structure, which increases the density of dislocation obstacles. The weld metal, with the lowest O-phase content and the most disordered microstructure, exhibited the lowest hardness values.

Mechanical Properties and Fracture Behavior

Tensile testing revealed that the no-fill and Ti-filler welds exhibited comparable tensile strengths, indicating that the addition of titanium filler wire did not significantly compromise the strength of the joint. However, the elongation after fracture was markedly higher for the Ti-filler welds, demonstrating a substantial improvement in ductility.

Fractographic analysis provided clear evidence of the microstructural influence on fracture behavior. The no-fill welds exhibited a mixed ductile-brittle fracture mode, with features of both dimpled rupture and cleavage. The Ti-filler welds, on the other hand, displayed predominantly ductile fracture characteristics, with extensive dimple formation and minimal cleavage. This improvement in fracture toughness is directly attributable to the more equiaxed grain structure and reduced phase anisotropy achieved with the titanium filler wire.

Engineering Practice Considerations

The use of pure titanium filler wire in welding Ti2AlNb-based alloys represents a practical strategy for improving joint ductility without sacrificing strength. In aerospace applications, where fatigue resistance and damage tolerance are critical, the improved ductility of Ti-filler welds could significantly enhance the service life of welded components. However, the dilution of alloying elements must be carefully controlled to avoid excessive reduction in the ordered phase fraction, which could compromise high-temperature strength and creep resistance.

For production welding, pulsed TIG parameters should be optimized to minimize the layered microstructure effect. Pulse frequencies in the range of 50-100 Hz with appropriate pulse widths can be used to achieve a more homogeneous weld metal microstructure. Back-purging with high-purity argon is essential to prevent oxidation of the titanium alloy during welding.

Key Questions and Reflections

The long-term high-temperature stability of the welded joints remains an open question. The disordered weld metal microstructure may be susceptible to phase transformation during prolonged exposure to elevated temperatures, potentially leading to embrittlement or dimensional instability. Accelerated aging tests at service-relevant temperatures are necessary to establish the durability of these welded joints.

Furthermore, the effect of welding on the fatigue performance of Ti2AlNb-based alloys is not addressed in this study. Given the aerospace application context, fatigue resistance is a critical design parameter, and dedicated fatigue testing of welded specimens would be valuable for future work.

Study Insights and Implications

This study provides valuable baseline data on the TIG weldability of Ti2AlNb-based alloys and demonstrates the effectiveness of pure titanium filler wire as a microstructure-modifying agent. The findings suggest that with appropriate filler selection and process parameter optimization, acceptable welded joints can be produced from this challenging alloy system. For engineering practice, the results support the development of welding procedures for Ti2AlNb-based components, particularly for thin-section applications where TIG welding is feasible. The study also highlights the importance of filler metal selection in controlling the microstructure and mechanical properties of intermetallic alloy welds, a principle that can be extended to other ordered intermetallic systems.