Zhuojin Pipe Fitting Co., LtdTIG Welding Process Research on High-Strength Ti-V-Mo Titanium Alloy
Literature Overview
This paper by Feng Jing and colleagues from Tianjin University, in collaboration with Angang Group Beijing Research Institute and Chengdu Advanced Metal Materials Industry Technology Research Institute, was published in Iron and Steel Vanadium and Titanium in 2021 (Volume 42, Issue 6, pages 158-163). The study investigates TIG welding of a novel high-strength Ti-V-Mo titanium alloy using both automatic wire feeding and manual filler rod techniques.
Material Characteristics
The Ti-V-Mo alloy investigated is a high-strength titanium alloy designed for demanding structural applications. The base metal microstructure is described as a dual-phase (α+β) structure with a relatively high equiaxed α phase fraction. This microstructural configuration is significant because it influences both the welding behavior and the post-weld microstructural evolution.
The parent material properties are characterized by high strength, which is achieved through the combined solid solution strengthening of vanadium and molybdenum, along with the precipitation and phase transformation effects during heat treatment.
Welding Process Comparison
Two TIG welding approaches were evaluated:
| Parameter | Automatic Wire Feeding TIG | Manual Filler Rod TIG |
|---|---|---|
| Wire feed method | Continuous automatic | Manual intermittent |
| Heat input control | More consistent | Operator dependent |
| Deposition rate | Higher | Lower |
| Weld bead appearance | Smooth, no spatter or undercut | Smooth, no spatter or undercut |
| Bead surface color | Silvery white | Silvery white |
| Tensile strength | 822 MPa | 612 MPa |
| HAZ impact energy | 72.2 J | 84.9 J |
Microstructural Analysis
Base Metal: The parent material exhibits a dual-phase microstructure with a high volume fraction of equiaxed α phase embedded in a β matrix. The equiaxed α phase provides a balance between strength and ductility.
Heat-Affected Zone (HAZ): The HAZ in both welding configurations shows significantly coarsened grains compared to the base metal. The internal structure is dominated by needle-like α' martensite. This is a critical observation because the formation of α' martensite in the HAZ indicates that the cooling rate exceeded the critical cooling rate for diffusive α transformation. The coarse grain size in the HAZ is a consequence of the high thermal conductivity of titanium alloys combined with the high heat input of TIG welding.
Weld Metal: The weld metal also exhibits coarse grains, primarily consisting of lamellar α phase transformation products with a small amount of needle-like α' martensite. The lamellar α structure is characteristic of β-phase solidification followed by α transformation during cooling. The presence of both lamellar α and α' martensite suggests a complex solidification and transformation sequence influenced by the cooling rate and alloy composition.
Mechanical Property Discussion
The tensile strength results reveal an interesting inverse relationship with the welding method. The automatic wire feeding TIG produces a significantly higher tensile strength of 822 MPa compared to 612 MPa for the manual filler rod TIG. This difference can be attributed to several factors:
- The automatic wire feeding provides a more consistent and controlled heat input, resulting in a more uniform weld metal microstructure with finer and more uniformly distributed lamellar α.
- The higher deposition rate of automatic feeding may produce a more homogeneous weld composition, reducing local segregation effects.
- The manual technique may introduce more interpass heat accumulation variability, affecting the transformation sequence.
However, the HAZ impact energy shows the opposite trend: the manual filler rod TIG achieves 84.9 J compared to 72.2 J for the automatic method. This suggests that the lower heat input of manual welding produces a slightly less severe HAZ, with marginally better toughness retention. The difference in HAZ impact energy, while notable, is not dramatic—both values indicate good toughness for a titanium alloy weld HAZ.
Engineering Considerations
For practical application of this Ti-V-Mo alloy in welded structures, several factors must be considered:
- The HAZ grain coarsening is unavoidable with TIG welding and represents a potential weakness in fatigue and creep service. Post-weld heat treatment to refine the HAZ microstructure may be necessary for critical applications.
- The formation of α' martensite in both the HAZ and weld metal introduces a potential for stress-induced transformation and associated ductility loss under certain loading conditions.
- The surface quality of the weld beads is excellent in both configurations, with no spatter or undercut, which is essential for maintaining the corrosion resistance of titanium alloys.
- The choice between automatic and manual TIG should be guided by the specific property requirements: automatic feeding for maximum strength, manual feeding for marginally better HAZ toughness.
Reflections
This study provides valuable baseline data for the welding of a high-strength Ti-V-Mo alloy, but several aspects warrant further investigation. The absence of post-weld heat treatment in the study means that the reported properties represent the as-welded condition, which may not be representative of the final service condition. Additionally, the study does not address long-term property stability, such as creep behavior or resistance to hydrogen embrittlement, which are critical for high-temperature titanium alloy applications.
The microstructural description, while informative, could benefit from quantitative characterization such as phase fraction measurements, grain size distributions, and α lamellar spacing quantification. These parameters directly correlate with mechanical properties and would enable more precise process-microstructure-property relationships.