Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of TIG Welded TA22-TA2 Dissimilar Titanium Alloy Thick Plates
Literature Overview
This study by Ma Zhaowei and colleagues from the Luoyang Ship Material Research Institute, published in Hot Working Technology in 2021, investigates the microstructure and mechanical properties of multi-pass TIG welded joints in 20 mm thick TA22/TA2 dissimilar titanium alloy plates. The research addresses a practical engineering challenge in shipbuilding and aerospace manufacturing: the joining of different titanium alloy grades in thick-section components where TIG welding is the preferred process due to the high reactivity of titanium at elevated temperatures.
Core Findings and Technical Analysis
The study successfully joined 20 mm thick TA22 and TA2 plates using multi-layer, multi-pass TIG welding. The weld bead exhibited a bright silver color with visible fish-scale patterns and good formation. Low-magnification examination of the weld revealed no porosity, lack of fusion, or cracking defects. The microstructure varies significantly across the joint:
| Zone | Microstructure | Grain Morphology |
|---|---|---|
| TA22 HAZ | Grain boundary α + lamellar α (Widmanstätten) | Coarse lamellar |
| Weld near fusion line | Coarse plate-like α | Coarse lath |
| Weld center (filler) | Blocky α with serrated grain boundaries | Blocky, medium |
| TA2 HAZ | Blocky α with serrated grain boundaries | Blocky, decreasing size toward base metal |
| Base metal (TA22) | Widmanstätten structure | Fine lamellar |
| Base metal (TA2) | Equiaxed α + β | Fine equiaxed |
The hardness distribution shows that the root pass of the weld has the highest hardness, filler passes have lower hardness, and the weld surface shows a slight hardness increase. From the TA22 base metal to the weld zone, hardness first increases and then drops sharply; from the TA2 base metal to the weld zone, hardness decreases slightly. The average tensile strength of the joint is 501.5 MPa, with fracture occurring in the TA2 base metal. Side bend testing revealed no cracking.
Engineering Practice Implications
The successful fabrication of a defect-free weld in 20 mm thick dissimilar titanium alloy plates demonstrates that multi-pass TIG welding can achieve reliable joints when proper shielding and process parameters are maintained. The microstructural gradient from Widmanstätten structure in the TA22 HAZ to blocky α in the TA2 HAZ reflects the different transformation temperatures and cooling rates experienced by the two alloy grades. The fracture in the TA2 base metal rather than the weld or HAZ indicates that the joint strength is adequate for structural applications, though the strength mismatch between TA22 (higher strength) and TA2 (lower strength) should be considered in design calculations.
| Property | TA22 Base Metal | TA2 Base Metal | Weld Zone |
|---|---|---|---|
| Tensile Strength (MPa) | ~690 (typical) | ~450 (typical) | 501.5 (average) |
| Hardness Trend | Higher | Lower | Highest at root, decreasing |
| Fracture Location | Not applicable | Fracture occurred here | Not applicable |
For shipbuilding and marine applications governed by standards such as ASTM B348, ASTM B381, and DNV standards, the dissimilar TA22/TA2 joint must be qualified through comprehensive mechanical and corrosion testing. The Widmanstätten microstructure in the TA22 HAZ, while strong, may exhibit reduced ductility and fatigue resistance compared to the equiaxed structure of the TA2 HAZ, which is a concern for cyclic loading applications.
Key Questions and Reflections
The paper does not report the specific welding parameters used, including current, voltage, travel speed, and shielding gas flow rate, which limits the reproducibility of the results. The absence of detailed hardness profiles across the full joint width prevents a quantitative assessment of the hardness gradient and its implications for residual stress distribution. Furthermore, the study does not address corrosion resistance, which is a critical concern for titanium alloy applications in marine environments. The fracture in the TA2 base metal, while indicating adequate joint strength, raises questions about the overall structural integrity of the joint under multiaxial loading conditions. Engineers should also consider the potential for β-phase transformation during welding, which could influence the phase distribution and mechanical properties in the HAZ of both alloy grades.
Study Insights and Implications for Dissimilar Alloy Welding
The successful TIG welding of 20 mm thick TA22/TA2 dissimilar titanium alloy plates provides valuable engineering data for applications where different titanium grades must be joined in thick-section components. The microstructural analysis reveals that the welding process creates a complex gradient of α-phase morphologies, from Widmanstätten in the TA22 HAZ to blocky α in the TA2 HAZ, with the weld metal exhibiting intermediate characteristics. The absence of cracking and the satisfactory tensile strength of 501.5 MPa indicate that the joint is structurally sound, though further qualification testing including fatigue, corrosion, and elevated temperature performance is necessary for critical applications. Engineers working on titanium alloy fabrication should view this study as evidence that multi-pass TIG welding can produce acceptable dissimilar joints when proper process control is maintained, while recognizing that the microstructural complexity introduced by dissimilar joining requires careful consideration in design and qualification procedures.