Zhuojin Pipe Fitting Co., LtdFine Microstructure Characterization of Marine Titanium Alloy TIG Welded Joints
Literature Overview
This paper, published in the Acta Metallurgica Sinica (Welding) in 2016 by Gao Fuyang, Liao Zhiqian, Xiong Jinhui, and Li Shikai from the 725th Research Institute of China Shipbuilding Industry Corporation, presents a detailed microstructure characterization of a 12 mm thick Ti-6Al-3Mo-2.5Cr-1.5Fe (Ti6321) marine titanium alloy TIG welded joint. The study employed optical microscopy (OM) and transmission electron microscopy (TEM) to analyze the microstructure of the weld joint, dividing it into distinct zones and examining the microstructural evolution in each zone. The work addresses the critical need for understanding the microstructural features of titanium alloy welds in marine applications, where the joints are subjected to cyclic loading, corrosion, and potential hydrogen embrittlement.
Core Technical Findings
The study established a comprehensive microstructural map of the Ti6321 TIG welded joint, identifying distinct zones within the weld metal and the HAZ based on the thermal cycle experienced. In the weld metal, the microstructure transitioned from columnar grains at the surface to equiaxed grains at the center, reflecting the change in solidification conditions from the surface to the interior of the weld. Within the grains, the alpha phase exhibited a trend toward coarsening and equiaxing as the high-temperature dwell time increased and the temperature gradient decreased.
The HAZ was divided into three distinct sub-zones: a transition zone, a fine-grained zone, and a coarse-grained zone. The transition zone contained equiaxed alpha, rod-like alpha, and residual beta phases. The fine-grained zone contained rod-like alpha and residual beta phases. The coarse-grained zone contained needle-like alpha and residual beta phases. In the HAZ, as the high-temperature dwell time decreased, the intragranular alpha became finer and the dislocation density decreased.
Microstructural Zones and Features
| Zone | Location | Microstructure | Key Features |
|---|---|---|---|
| Weld surface | Surface of weld | Columnar grains | High temperature gradient, rapid cooling |
| Weld center | Center of weld | Equiaxed grains | Lower temperature gradient, longer dwell time |
| Transition zone | HAZ, near weld | Equiaxed alpha + rod-like alpha + residual beta | Moderate thermal cycle |
| Fine-grained zone | HAZ, intermediate | Rod-like alpha + residual beta | Lower thermal cycle |
| Coarse-grained zone | HAZ, far from weld | Needle-like alpha + residual beta | Highest peak temperature, longest dwell time |
Dislocation Density and Alpha Morphology
| Zone | Alpha Morphology | Dislocation Density | Thermal Cycle Characteristic |
|---|---|---|---|
| Weld surface | Columnar, fine | Higher | High gradient, short dwell |
| Weld center | Equiaxed, coarser | Lower | Lower gradient, longer dwell |
| Transition zone | Equiaxed + rod-like | Moderate | Moderate peak temperature |
| Fine-grained zone | Rod-like, fine | Moderate | Lower peak temperature |
| Coarse-grained zone | Needle-like, coarse | Lower | Highest peak temperature, longest dwell |
Interpretation of Technical Points
Weld Metal Microstructural Evolution
The transition from columnar to equiaxed grains in the weld metal is a direct consequence of the solidification conditions. At the surface of the weld, where the temperature gradient is high and the solidification rate is rapid, columnar grains grow preferentially in the direction of heat extraction. As one moves toward the center of the weld, the temperature gradient decreases, and the solidification rate slows, allowing for the nucleation and growth of equiaxed grains. This microstructural transition has implications for the mechanical properties of the weld, as columnar grains can exhibit anisotropic mechanical behavior, while equiaxed grains provide more uniform properties.
The observation that the intragranular alpha phase exhibits a coarsening and equiaxing trend with increasing high-temperature dwell time is consistent with the Ostwald ripening mechanism. During the welding thermal cycle, the beta phase is formed at the peak temperature, and upon cooling, the alpha phase precipitates within the beta matrix. The morphology and size of the precipitated alpha depend on the cooling rate and the time spent at elevated temperatures. Longer dwell times at high temperatures allow for more complete diffusion and coarsening of the alpha phase, resulting in larger, more equiaxed alpha particles.
HAZ Microstructural Zones
The division of the HAZ into three sub-zones based on the peak temperature experienced is a standard approach in welding metallurgy. The transition zone, adjacent to the weld, experiences a peak temperature that is high enough to cause partial melting or complete dissolution of the alpha phase, followed by rapid cooling that produces a mixture of equiaxed and rod-like alpha phases. The fine-grained zone, further from the weld, experiences a lower peak temperature that is sufficient to cause grain refinement through recrystallization but not complete dissolution of the alpha phase. The coarse-grained zone, the farthest from the weld, experiences the highest peak temperature and the longest dwell time, leading to significant grain growth and the formation of needle-like alpha phases within the coarse beta grains.
The decrease in dislocation density with decreasing high-temperature dwell time in the HAZ is an interesting finding. In the coarse-grained zone, the high temperature causes recrystallization and grain growth, which reduces the dislocation density. In the fine-grained zone, the lower peak temperature is insufficient for complete recrystallization, but the thermal cycle is still sufficient to cause some recovery and dislocation annihilation. The transition zone, with its rapid cooling, retains a higher dislocation density due to the limited time for recovery processes.
Standards and Process Analysis
The welding of titanium alloys is governed by standards such as AWS D14.1 for welding of titanium and titanium alloys, ASME Section IX for qualification of welding procedures, and ISO 10736 for welding of titanium and titanium alloys. These standards emphasize the importance of shielding gas purity, as titanium is highly reactive at elevated temperatures and can pick up oxygen, nitrogen, and hydrogen from the atmosphere, leading to embrittlement.
TIG Welding Parameters for Titanium Alloys
| Parameter | Typical Range | Rationale |
|---|---|---|
| Shielding gas | High-purity argon or helium | Prevent oxidation and nitrogen pickup |
| Gas flow rate | 15-30 L/min | Ensure complete protection of weld pool and HAZ |
| Back purge | Argon or helium, 5-15 L/min | Protect the back side of the weld |
| Current type | DCEN (DC electrode negative) | Concentrated heat input, deep penetration |
| Travel speed | Moderate | Balance penetration and heat input |
| Electrode | Ceriated tungsten or lanthanated tungsten | Stable arc, reduced electron emission |
The use of TEM in this study provides unique insights into the dislocation structures and precipitate morphologies that are not accessible through conventional OM analysis. The dislocation density and arrangement within the alpha and beta phases influence the mechanical properties, particularly the yield strength and ductility, of the weld joint. In marine applications, where the joints are subjected to cyclic loading and corrosion, the microstructural features characterized by TEM are directly relevant to the assessment of fatigue and corrosion resistance.
Integration with Engineering Practice
In marine engineering, titanium alloys are used for components exposed to seawater, such as propeller shafts, hull penetrations, and heat exchanger tubes. The welding of these components must produce joints that are resistant to both mechanical fatigue and corrosion. The microstructural characterization presented in this study provides the foundation for understanding the corrosion resistance of the weld joint, as the alpha and beta phases have different corrosion resistance in seawater.
The coarse-grained zone of the HAZ, with its needle-like alpha phases and residual beta grains, is often the region of lowest corrosion resistance in titanium alloy welds. The beta phase, which contains the alloying elements such as Al, Mo, Cr, and Fe in solid solution, is more susceptible to intergranular corrosion than the alpha phase. In the coarse-grained zone, where the beta grains are large and the alpha precipitates are needle-like, the corrosion attack can proceed along the grain boundaries, leading to intergranular corrosion.
A practical engineering approach is to apply a post-weld heat treatment (PWHT) to refine the microstructure of the HAZ and improve the corrosion resistance. A solution treatment followed by aging can homogenize the microstructure and reduce the size of the beta grains in the coarse-grained zone. However, PWHT must be carefully controlled to avoid excessive grain growth and to maintain the required mechanical properties.
Key Questions and Reflections
One question that arises from this study is the correlation between the microstructural features characterized by TEM and the mechanical and corrosion properties of the weld joint. While the study provides a detailed microstructural map, it does not explicitly correlate the dislocation density and alpha morphology with mechanical property data. A comprehensive study that combines TEM microstructural characterization with mechanical testing and corrosion testing would provide a more complete understanding of the weld joint performance.
Another important consideration is the effect of welding parameters on the microstructural zones identified in this study. The width and characteristics of the HAZ sub-zones depend on the welding heat input, which is a function of the current, voltage, and travel speed. A parametric study varying the welding parameters and correlating the changes in HAZ microstructure with the welding parameters would provide valuable guidance for welding procedure optimization.
Furthermore, the study focuses on a single pass weld of a 12 mm thick plate. In practice, thicker plates require multi-pass welding, and the thermal cycles experienced by the earlier passes due to subsequent passes can significantly alter the microstructure. The effect of multi-pass welding on the microstructural zones identified in this study is not addressed but is relevant for the welding of thick-section marine components.
Study Insights and Implications
This study provides a detailed microstructural characterization of Ti6321 TIG welded joints that is essential for understanding the performance of titanium alloy welds in marine applications. The key insight is that the microstructure of the weld joint is highly heterogeneous, with distinct zones exhibiting different alpha morphologies, dislocation densities, and thermal cycle histories. This heterogeneity has direct implications for the mechanical properties, corrosion resistance, and fatigue performance of the weld joint.
For engineers designing and qualifying titanium alloy welds for marine service, the microstructural map provided by this study serves as a reference for evaluating the quality of the weld joint. The presence of a coarse-grained HAZ zone with needle-like alpha phases and residual beta grains is expected, but the width and characteristics of this zone should be controlled through welding procedure optimization. The use of TEM for microstructural characterization, while not routine in industrial practice, provides valuable insights into the dislocation structures that influence mechanical and corrosion properties.
The study also highlights the importance of the welding thermal cycle in determining the microstructure of titanium alloy welds. The peak temperature, cooling rate, and dwell time at elevated temperatures all influence the alpha morphology, dislocation density, and overall microstructural characteristics. In engineering practice, this means that the welding procedure specification must be carefully developed and qualified to ensure that the thermal cycle produces a microstructure that meets the required performance criteria for marine service.