Zhuojin Pipe Fitting Co., LtdEffect of Pulse Current Intensity on Microstructure and Tensile Properties of TIG Additively Manufactured 2219 Aluminum Alloy
Literature Overview
This paper, published in Materials in Mechanical Engineering (2023, Vol. 47, No. 9, pp. 6–13) by Deng Wei, Wu Yong, Wu Rubo, and Xiao Jianye from Nanjing University of Aeronautics and Astronautics and the 725th Research Institute of China Shipbuilding Group Corporation, investigates the influence of pulse current intensity on the microstructure and tensile properties of 2219 aluminum alloy fabricated via TIG arc additive manufacturing. The research was funded by the Aviation Science Fund (Project No. 2018ZE52058). The study examines three current modes: non-pulse, weak pulse, and strong pulse AC-TIG, providing a systematic comparison of how pulse parameters affect grain morphology, texture, and mechanical anisotropy after T6 heat treatment.
Core Technical Findings
Microstructural Characteristics
The microstructure of all three specimens (non-pulse, weak pulse, and strong pulse) consists primarily of the α(Al) matrix phase and the second-phase θ(Al₂Cu) particles. The central region of the build-up exhibits equiaxed grains in all cases. However, the pulse current significantly influences grain size and uniformity:
| Parameter | Non-Pulse | Weak Pulse | Strong Pulse |
|---|---|---|---|
| Grain size | Larger, less uniform | Smaller, more uniform | Smaller, more uniform |
| Grain morphology | Equiaxed (central region) | Equiaxed (central region) | Equiaxed (central region) |
| Matrix phase | α(Al) | α(Al) | α(Al) |
| Second phase | θ(Al₂Cu) | θ(Al₂Cu) | θ(Al₂Cu) |
The refinement mechanism is attributed to the periodic variation of heat input during pulsed operation. During the base current phase, the molten pool cools and partially solidifies; during the peak current phase, the pool re-melts. This cyclic thermal cycling promotes nucleation and suppresses grain growth, resulting in finer and more homogeneous grain structures.
Texture Analysis
All specimens exhibit weak texture characteristics, with a maximum orientation density of only 3.42. This is a notably low value, indicating that the crystallographic orientation is nearly random. The pulse current intensity has no significant effect on grain orientation, which is an important finding for applications where anisotropic mechanical behavior is undesirable.
Mechanical Properties After T6 Heat Treatment
The T6 temper (solution treatment followed by aging) was applied to all specimens. The results reveal an interesting directional dependence:
- Under strong pulse current, the tensile strength in the scan direction decreases compared to the non-pulse condition.
- Under strong pulse current, the tensile strength in the build direction increases compared to the non-pulse condition.
- Overall anisotropy is reduced with strong pulse current application.
This suggests that the grain refinement achieved through pulsing partially eliminates the directional solidification effects that typically cause pronounced mechanical anisotropy in arc additive manufacturing.
Technical Interpretation and Engineering Relevance
Mechanism of Grain Refinement
The grain refinement observed with pulsed current can be understood through the thermal cycling mechanism. In non-pulse AC-TIG, the heat input is continuous, and the solidification front advances steadily, promoting columnar grain growth. In pulsed AC-TIG, the periodic reduction of current causes the solidification front to pause or even partially resolidify, creating new nucleation sites. The weak pulse mode provides moderate thermal cycling, while the strong pulse mode provides more aggressive cycling, both resulting in finer grains than the non-pulse baseline.
Anisotropy Reduction
In conventional TIG additive manufacturing, mechanical anisotropy arises from the columnar-to-equiaxed transition (CET) behavior and the directional heat flow. The scan direction typically exhibits lower strength due to the alignment of grains and the presence of interlayer boundaries. The strong pulse current promotes a more isotropic grain structure, which reduces the directional dependence of tensile properties. This is particularly valuable for structural components where load paths are unpredictable or multi-axial.
Engineering Practice Considerations
From an engineering practice perspective, the following observations are critical:
- Process selection for 2219 alloy: The 2219 alloy is widely used in aerospace applications (aircraft fuselage, pressure vessels) due to its excellent strength-to-weight ratio and fatigue resistance. TIG additive manufacturing offers the potential for rapid prototyping and repair of 2219 components, and the pulse current parameter provides a lever to optimize both microstructure and mechanical properties.
- T6 heat treatment necessity: The study confirms that T6 aging is essential for developing the θ(Al₂Cu) precipitate strengthening in 2219 alloy. Without proper aging, the as-built microstructure would not achieve the desired strength levels.
- Pulse parameter optimization: While both weak and strong pulse modes improve grain refinement, the strong pulse mode is more effective in reducing anisotropy. However, the trade-off is a reduction in scan-direction tensile strength. Engineers must balance these competing objectives based on the specific loading conditions of the application.
Key Questions and Reflections
A significant question arises from the findings: why does strong pulse current reduce scan-direction tensile strength while improving build-direction strength? One plausible explanation is that the more aggressive thermal cycling in strong pulse mode promotes a higher density of grain boundaries and precipitate-free zones in the scan direction, which may act as preferential crack initiation sites under tensile loading. This is consistent with observations in other arc additive manufacturing studies where excessive grain refinement can reduce ductility and fracture toughness.
Another reflection concerns the weak texture observed in all specimens. The maximum density of 3.42 is remarkably low, suggesting that the AC-TIG process inherently produces near-random orientations. This is advantageous for applications requiring isotropic mechanical behavior but may limit opportunities for texture engineering, where specific crystallographic orientations are deliberately developed to enhance particular properties.
Study Insights and Implications
The study provides valuable evidence that pulse current intensity is a critical process parameter in TIG additive manufacturing of 2219 aluminum alloy. The ability to tailor grain size, uniformity, and anisotropy through pulse parameter selection opens new avenues for process optimization in aerospace manufacturing. The finding that strong pulse current reduces overall anisotropy is particularly significant for complex-shaped components where load paths vary significantly.
For engineering practice, the recommendation is to employ strong pulse AC-TIG for 2219 additive manufacturing when isotropic mechanical properties are required, followed by proper T6 heat treatment. The process window should be established through systematic parameter studies, considering the trade-offs between grain refinement, strength, and ductility. Future work should investigate the fatigue and fracture behavior of pulsed TIG additively manufactured 2219 alloy, as these properties are critical for aerospace structural applications.