Zhuojin Pipe Fitting Co., LtdEffect of Electromagnetic Stirring on the Microstructure and Mechanical Properties of TIG Welding Rapid Prototyping of GH4169 Nickel-Based Superalloy
Literature Overview
This paper by Wang Lin, Liu Fencheng, Chen Peng, Wang Weixin, Zhou Baosheng, and Huang Chunping, published in the Journal of Nanchang Hangkong University (Natural Science Edition) (Volume 30, Issue 2, 2016, pp. 60–66), investigates the influence of electromagnetic stirring on the microstructure and mechanical properties of GH4169 nickel-based superalloy fabricated by TIG welding rapid prototyping. The research was supported by the National Natural Science Foundation of China (Grants 51565041 and 51201087) and the Jiangxi Provincial Department of Education Science Fund (Grant GJJ150710). GH4169 is a precipitation-hardened nickel-based superalloy widely used in aerospace and power generation applications due to its excellent high-temperature strength, fatigue resistance, and corrosion resistance. The alloy is known to be susceptible to the formation of brittle Laves phase (Mo-rich) in the interdendritic regions during solidification, which can severely degrade the mechanical properties and fracture resistance.
Background and Motivation
Welding rapid prototyping (WRP) is a metal additive manufacturing technique that builds three-dimensional components layer by layer using a welding process. The TIG welding variant of WRP offers the advantage of using inert gas shielding and a stable arc, which produces welds with low porosity and good surface finish. However, the layer-by-layer deposition process introduces several metallurgical challenges:
- Columnar grain growth: The solidification front tends to grow epitaxially along the build direction, resulting in a fully columnar microstructure with pronounced anisotropy.
- Interdendritic segregation: The rapid solidification rates and low cooling rates typical of WRP promote the formation of interdendritic segregation of alloying elements, particularly Mo and Ti, which leads to the formation of brittle Laves phase.
- Layer interface defects: Incomplete fusion or lack of bonding between layers can occur if the process parameters are not properly controlled.
The introduction of electromagnetic stirring is intended to address these issues by applying a controlled magnetic field to the molten pool during deposition. The Lorentz force generated by the interaction of the magnetic field with the electric current in the molten pool induces fluid flow, which enhances heat and mass transfer, refines the grain structure, and reduces interdendritic segregation.
Experimental Design
The authors conducted TIG welding rapid prototyping of GH4169 alloy under different magnetic field current levels. The experimental matrix included:
| Magnetic Field Current | Description |
|---|---|
| 0 A | Baseline (no electromagnetic stirring) |
| 0.5 A | Low electromagnetic stirring |
| 1.0 A | Moderate electromagnetic stirring |
| 9.0 A | High electromagnetic stirring |
The deposited specimens were examined using metallographic analysis, microhardness testing, and tensile testing at room temperature.
Microstructural Analysis
The metallographic examination revealed the following key findings:
Grain Structure
All specimens, regardless of magnetic field current, exhibited continuous epitaxial growth along the build direction, with no visible layer interfaces. This indicates that the TIG welding rapid prototyping process achieved full fusion between layers, which is critical for the mechanical integrity of the fabricated component.
Laves Phase Distribution
The volume fraction of interdendritic Laves phase was found to decrease with increasing magnetic field current up to a certain point. The Laves phase, which is a Mo-rich intermetallic compound with the formula MoNi₄, is brittle and acts as a preferential site for crack initiation. The reduction in Laves phase volume fraction is attributed to the enhanced fluid flow in the molten pool, which promotes solute mixing and reduces local segregation.
Microhardness
The microhardness of the deposited material increased from HV 232 (no magnetic field) to HV 253 (1.0 A magnetic field current). This improvement in microhardness is attributed to the refinement of the grain structure and the reduction of brittle Laves phase, which results in a more homogeneous and stronger microstructure.
Mechanical Properties
The tensile test results at room temperature are summarized as follows:
| Magnetic Field Current | Tensile Strength (MPa) |
|---|---|
| 0 A | 550 |
| 0.5 A | 660 |
| 1.0 A | Data not explicitly stated (implied similar to 0.5 A) |
| 9.0 A | 520 |
The tensile strength increased significantly from 550 MPa to 660 MPa when the magnetic field current was increased from 0 A to 0.5 A. However, when the magnetic field current was increased further to 9.0 A, the tensile strength decreased to 520 MPa, which is below the baseline value. This non-monotonic behavior suggests that there exists an optimal magnetic field current range for electromagnetic stirring, beyond which the benefits are outweighed by adverse effects.
Study Insights and Engineering Reflections
This work provides valuable insights into the use of electromagnetic stirring to improve the metallurgical quality of additively manufactured nickel-based superalloys. The key findings can be interpreted as follows:
- Optimal stirring intensity: The existence of an optimal magnetic field current range (approximately 0.5–1.0 A) indicates that electromagnetic stirring is a delicate process that must be carefully controlled. Excessive stirring can introduce turbulence, entrain gas, and cause spatter, which can degrade the weld quality.
- Laves phase suppression: The reduction of Laves phase is a significant achievement, as this phase is the primary contributor to the poor fracture resistance and creep properties of GH4169 alloy. In engineering practice, the suppression of Laves phase is often achieved through solution heat treatment followed by double aging, but this is not feasible for in-situ fabricated components. The ability to suppress Laves phase during fabrication is therefore a major advantage.
- Epitaxial growth: The continuous epitaxial growth observed in all specimens is a favorable microstructural feature, as it indicates good interlayer bonding and a coherent grain structure. However, the fully columnar grain structure is anisotropic, which can be a disadvantage for components that are subjected to multi-axial loading. Future work should investigate the possibility of introducing grain refinement through other means, such as grain refiner additions or oscillating arc techniques.
- Comparison with wrought material: The tensile strength of 660 MPa achieved with electromagnetic stirring is still below the typical tensile strength of solution-treated and double-aged GH4169 wrought material (approximately 1100–1200 MPa). However, the as-deposited condition of the fabricated material is not directly comparable to the heat-treated condition of wrought material. The mechanical properties of additively manufactured GH4169 can be significantly improved through post-fabrication heat treatment, and the reduced Laves phase content achieved by electromagnetic stirring should result in better heat treatment response.
Practical Implications
This research has important implications for the development of additively manufactured components for aerospace and power generation applications. The ability to fabricate GH4169 components with improved microstructural homogeneity and reduced brittle phase content is a significant step toward the qualification of additively manufactured superalloy components for critical service. However, before any such components can be used in service, a comprehensive qualification program must be conducted, including mechanical property testing, fatigue testing, creep testing, and non-destructive examination. The process parameters and magnetic field current levels identified in this study should serve as a starting point for such qualification efforts.