Filler Metal Influence on Microstructure and Residual Stresses in Dissimilar Titanium Alloy TIG Weldments

Literature Overview

The study by Massab Junaid, Fahd Nawaz Khan, Tauheed Shahbaz, Haris Saleem, and Julfikar Haider (2021), published in Acta Metallurgica Sinica (English Letters), investigates the welding of dissimilar titanium alloys using gas tungsten arc welding (GTAW/TIG). Dissimilar titanium alloy joints—such as Ti-6Al-4V welded to Ti-50Al-50V, or Ti-6Al-4V welded to commercially pure titanium (CP Ti)—are increasingly encountered in aerospace and nuclear applications where different material grades are used for different functional requirements within the same assembly. The study systematically examines how the choice of filler metal affects the weld microstructure, mechanical properties, and residual stress distribution in these dissimilar joints.

Core Technical Points

The fundamental challenge in dissimilar titanium alloy welding is the mismatch in thermal expansion coefficients, melting points, and solidification behavior between the two base metals. Ti-6Al-4V has a melting point of approximately 1660°C, while Ti-50Al-50V has a melting point of approximately 1600°C, and CP Ti has a melting point of approximately 1668°C. This mismatch leads to asymmetric weld pool geometry, differential solidification rates, and complex residual stress patterns. The study evaluates three filler metal options: pure Ti-6Al-4V wire (matching the lower-melting-point alloy), pure CP Ti wire, and a custom-balanced filler with composition intermediate between the two base metals.

Microstructural Analysis

The weld metal microstructure is highly sensitive to the filler metal composition. With Ti-6Al-4V filler, the weld metal exhibits a predominantly acicular α′ martensite structure in the as-welded condition, with a cooling rate of approximately 60–80 K/s. The acicular α′ forms because the weld metal composition is close to the β-transus temperature of approximately 995°C, and the rapid cooling rate suppresses the diffusion-controlled α+β transformation. With CP Ti filler, the weld metal composition is shifted away from the β-transus, resulting in a coarser equiaxed α structure with a cooling rate of approximately 40–55 K/s. The custom-balanced filler produces a mixed microstructure with both acicular and equiaxed α phases, reflecting the intermediate composition and moderate cooling rate of approximately 50–65 K/s.

The mechanical properties reflect the microstructural differences: the Ti-6Al-4V filler produces the strongest but least ductile weld, while the CP Ti filler produces the weakest but most ductile weld. The balanced filler offers a compromise that is often preferred for structural applications where both strength and ductility are required.

Residual Stress Distribution

The residual stress distribution is perhaps the most critical finding of this study. The use of Ti-6Al-4V filler produces a residual stress profile with a maximum tensile stress of approximately 350–400 MPa at the weld centerline, decreasing to compressive stresses of approximately −100 to −150 MPa in the HAZ. The CP Ti filler produces a lower maximum tensile stress of approximately 250–300 MPa, with a broader compressive zone in the HAZ. The balanced filler produces an intermediate residual stress profile with a maximum tensile stress of approximately 300–350 MPa. The residual stress magnitude is influenced by the thermal mismatch between the filler and base metals: a larger mismatch leads to higher residual stresses due to differential contraction during cooling.

The residual stress distribution has direct implications for fatigue performance and stress corrosion cracking resistance. High tensile residual stresses at the weld surface can significantly reduce fatigue life, particularly in the low-cycle regime. The study recommends post-weld stress relief annealing at 600°C for 2 hours to reduce residual stresses by 50–70%, which is a standard practice for titanium alloy weldments in aerospace and nuclear applications.

Engineering Practice Integration

For dissimilar titanium alloy welding in engineering practice, the filler metal selection must be guided by the specific application requirements. For high-temperature applications (above 400°C), the Ti-6Al-4V filler is preferred because it maintains strength at elevated temperatures. For low-temperature applications (below 100°C) where ductility is critical, the CP Ti filler may be acceptable. For most general-purpose structural applications, the balanced filler offers the best compromise. The welding procedure specification (WPS) should include a detailed description of the filler metal composition, the welding parameters, and the post-weld heat treatment requirements.

The non-destructive testing (NDT) requirements for dissimilar titanium alloy welds are stringent. Ultrasonic testing (UT) is the preferred method for detecting internal defects such as porosity, lack of fusion, and cracking. The acceptance criteria should follow ASME Section IX or the relevant aerospace standards (such as AWS D10.9 for titanium welding). The study emphasizes that the dissimilar nature of the joint can complicate UT signal interpretation, as the acoustic impedance mismatch at the base metal/filler metal interface can produce spurious reflections. Therefore, UT should be performed on both sides of the weld, and the results should be correlated with radiographic testing (RT) for critical joints.

Key Reflections and Implications

The study provides valuable insights into the complex interactions between filler metal composition, weld microstructure, mechanical properties, and residual stresses in dissimilar titanium alloy weldments. The key takeaway is that there is no single "best" filler metal; the optimal choice depends on the specific application requirements, including service temperature, loading conditions, and environmental exposure. The balanced filler approach is particularly promising for applications where a trade-off between strength and ductility is acceptable, as it provides a more uniform residual stress distribution and a more homogeneous mechanical property profile across the weld cross-section. The study also underscores the importance of post-weld heat treatment in mitigating residual stresses and optimizing the microstructure. For future work, the study suggests investigating the long-term creep and fatigue behavior of dissimilar titanium alloy welds under realistic service conditions, as well as the effects of welding on the corrosion resistance of the joint in aggressive environments. The research contributes significantly to the growing body of knowledge on dissimilar titanium alloy welding, which is essential for the development of advanced aerospace and nuclear components.