CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of TA22/TA2 Dissimilar Titanium Alloy Thick Plate TIG Welds
Literature Overview
This 2021 publication from the Luoyang Ship Materials Research Institute addresses a technically demanding challenge in titanium alloy fabrication: the TIG welding of dissimilar thick plate joints between TA22 and TA2 grades. The research team includes seven authors from the institute, reflecting the collaborative nature of this technically complex investigation. The work was published in Hot Working Technology (热加工工艺), indicating its practical orientation toward manufacturing process development. Titanium alloy welding remains one of the most challenging areas in joining technology due to the extreme chemical reactivity of titanium at elevated temperatures, the sensitivity of titanium microstructures to thermal cycling, and the stringent requirements for weld quality in aerospace and marine applications.
Material Background and Technical Challenges
TA2 (commercially pure titanium, Grade 2 equivalent) and TA22 (a near-alpha titanium alloy containing approximately 5% vanadium) represent fundamentally different material classes with distinct microstructures and mechanical properties. TA2 exhibits a fully alpha microstructure with relatively low strength but excellent ductility and corrosion resistance. TA22, with its near-alpha microstructure, offers significantly higher strength at elevated temperatures but reduced ductility and more complex microstructural evolution during welding.
| Property | TA2 | TA22 |
|---|---|---|
| Microstructure | Fully alpha | Near-alpha (alpha + beta) |
| Tensile Strength | ~240 MPa | ~550 MPa |
| Yield Strength | ~170 MPa | ~345 MPa |
| Elongation | ~20% | ~10% |
| Density (g/cm³) | 4.51 | 4.43 |
| Melting Point | ~1660°C | ~1600°C |
| Thermal Conductivity | ~22 W/(m·K) | ~7 W/(m·K) |
The dissimilar joint presents unique challenges: differential thermal expansion during cooling, mismatched thermal conductivity affecting heat flow distribution, potential for microstructural incompatibility at the joint interface, and the risk of property degradation in the weld and HAZ regions.
Welding Process Development
TIG welding of titanium alloys requires extremely rigorous protection against atmospheric contamination. Titanium readily absorbs oxygen, nitrogen, and hydrogen at temperatures above 400°C, leading to severe embrittlement. The welding process must therefore include:
- Pre-weld cleaning with mechanical and chemical methods to remove surface oxides
- Inert gas pre-flow before arc strike to purge the joint area
- Post-flow of inert gas after arc extinction until the weld cools below 400°C
- Back-gas protection to prevent oxidation of the weld root
- Use of high-purity argon or helium shielding gas (99.99% minimum)
For thick plate welding, multi-pass welding is required, with careful control of interpass temperature (typically below 150-200°C) to prevent excessive grain growth and maintain microstructural integrity.
Microstructural Analysis
The weld metal microstructure depends on the filler metal composition and solidification conditions. For dissimilar TA22/TA2 joints, the weld metal composition represents a blend of the two base metals and the filler metal, creating a gradient composition across the weld width. The solidification microstructure may exhibit:
- Alpha-beta microstructure in the weld center (from TA22-like composition)
- Predominantly alpha microstructure near the TA2 side
- Widmanstätten alpha laths in the HAZ of TA22 due to beta phase dissolution and subsequent transformation
- Coarse alpha grain structure in the HAZ of TA2 due to grain growth during welding
Mechanical Property Evaluation
Mechanical testing of dissimilar titanium alloy welds reveals complex property distributions:
- Tensile properties: The weld joint strength is typically governed by the weaker base metal (TA2), with the weld metal and HAZ showing intermediate properties
- Hardness profiles: A hardness gradient exists across the joint, with TA22 showing higher hardness than TA2, and the HAZ regions showing hardness variations due to microstructural changes
- Impact properties: The TA22 HAZ may show reduced impact toughness due to Widmanstätten microstructure formation
Engineering Practice Considerations
For pressure vessel fabrication involving dissimilar titanium alloy joints, the welding procedure must be qualified in accordance with applicable standards such as ASME IX or NB/T 47014. The qualification testing must include:
- Full-scale mechanical testing of the weld joint
- Non-destructive examination (RT, UT, PT)
- Microstructural examination of representative sections
- Corrosion resistance testing if applicable to the service environment
The differential thermal expansion between TA2 and TA22 creates residual stresses at the joint that must be considered in the design and fabrication process. Post-weld heat treatment (PWHT) may be employed to relieve residual stresses, but the treatment parameters must be carefully selected to avoid adverse effects on either material.
Study Insights and Reflections
This research addresses a practically important but technically challenging problem in titanium alloy fabrication. The systematic approach to dissimilar joint welding, encompassing process development, microstructural analysis, and mechanical property evaluation, provides valuable guidance for engineers working on titanium alloy pressure vessels and structural components. The findings reinforce the importance of careful process control and thorough qualification testing for dissimilar material joints, where the interaction between different microstructures and properties creates complex failure modes that must be understood and mitigated. The work contributes to the expanding knowledge base for titanium alloy fabrication in demanding industrial applications.