TIG Welding Microstructure and Mechanical Properties of TA22/TA2 Dissimilar Titanium Alloy Thick Plate

Literature Overview

Published in Hot Working Technology in 2021 (Vol. 50, No. 21, pp. 42-45), this study reports on the successful TIG welding of 20 mm thick TA22/TA2 dissimilar titanium alloy plates using a multi-layer, multi-pass approach. Conducted by researchers at the Luoyang Ship Materials Research Institute, the work addresses a significant engineering challenge: joining dissimilar titanium alloys with different compositions and properties in thick-section applications. The study provides valuable data for engineers working on titanium alloy fabrication in aerospace, marine, and petrochemical applications.

Weld Configuration and Appearance

The welded joint exhibited the following characteristics:

The bright silver coloration indicates good gas shielding throughout the welding process, preventing oxidation of the hot weld metal. The fish-scale pattern (overlap ripples) is characteristic of well-controlled TIG welding with appropriate travel speed and arc stability.

Microstructural Analysis

The microstructure varies significantly across the joint, reflecting the different thermal histories and compositional influences:

The TA22 heat-affected zone exhibits a Widmanstätten structure composed of grain boundary α phase and lamellar α phase, which is typical for near-α titanium alloys subjected to high-temperature thermal cycles. The weld zone shows a gradient in α phase morphology, with coarse lamellar structures near the TA22 fusion line transitioning to finer blocky structures toward the TA2 side.

Hardness Distribution

The hardness profile reveals important information about microstructural variations:

• Weld root layer: Highest hardness values
• Filler layers: Lower hardness values
• Weld surface: Slight hardness increase
• TA22 base to weld zone: Initial increase followed by sharp decrease
• TA2 base to weld zone: Slight decrease

The root layer hardness maximum is attributed to the highest cooling rate during the first pass, which promotes finer microstructural features. The subsequent passes experience slower cooling due to thermal accumulation from previously deposited layers, resulting in coarser microstructures and lower hardness in the filler passes.

Mechanical Properties

The tensile strength of 501.5 MPa with fracture occurring in the TA2 base material indicates that the weld joint strength exceeds the weaker base material (TA2), which is a desirable outcome for dissimilar welds. The side bend test confirming crack-free behavior demonstrates adequate ductility and toughness in the joint.

Engineering Practice Integration

For engineers working with dissimilar titanium alloy weldments, several practical considerations emerge:

1. Process qualification: Multi-layer, multi-pass TIG welding of 20 mm thick titanium plates requires careful control of inter-pass temperature (typically below 250°C for titanium alloys) and consistent gas shielding throughout the entire welding sequence.
2. Microstructural management: The Widmanstätten structure in the TA22 HAZ may affect fatigue performance. Engineers should consider post-weld heat treatment to refine the microstructure if fatigue-critical applications are anticipated.
3. Joint design: The fracture occurring in the TA2 side suggests that TA2 is the weaker component. Joint design should account for stress concentrations at the dissimilar interface, and consideration should be given to using TA2 as the base material for the more highly stressed component.
4. Inspection requirements: Despite the absence of macroscopic defects, detailed non-destructive testing (UT, RT, or PAUT) should be performed on production welds to detect subsurface defects that may not be visible on the macrosection.
5. Heat input control: The hardness gradient from root to surface layers indicates that heat input management is critical for achieving uniform properties across the weld thickness. Engineers should document and control the heat input per pass to ensure consistent microstructural development.

Key Questions and Reflections

The successful welding of 20 mm thick dissimilar titanium alloy plates demonstrates that TIG welding can produce acceptable joints for thick-section dissimilar combinations. However, the microstructural heterogeneity across the joint raises questions about long-term service performance, particularly regarding stress corrosion cracking resistance and fatigue life. The Widmanstätten structure in the TA22 HAZ, while not immediately problematic for static loading, may be susceptible to cracking under cyclic loading conditions.

The absence of detected defects in the macrosection examination is encouraging but does not guarantee the absence of microstructural discontinuities at the grain boundary level. Advanced characterization techniques such as EBSD (Electron Backscatter Diffraction) and SEM-EDS mapping would provide more detailed information about grain boundary characteristics and elemental segregation at the dissimilar interface.

Study Insights and Implications

This research provides valuable engineering data for the TIG welding of dissimilar titanium alloy thick plates. The combination of acceptable mechanical properties, good weld appearance, and absence of macroscopic defects demonstrates the feasibility of this approach for industrial applications. Engineers should use these findings as a starting point for process development while recognizing that comprehensive qualification testing, including fatigue, creep, and environmental exposure testing, is necessary before deploying dissimilar titanium alloy weldments in critical service applications.