Grain Refinement Characteristics of TIG Weld Pool in Thin 5B70 Aluminum Alloy

Literature Overview

This paper by Tian Zhijie, Liu Yan, Shang Aoshuang, Du Han, and Hao Shuangxi from Capital Aerospace Machinery Co., Ltd., published in Missile and Space Vehicle Technology (2025, Issue 1, pp. 85–90), investigates the microstructure evolution and grain refinement characteristics in TIG welded joints of 1.5 mm thick 5B70 aluminum alloy plates using 5B71 alloy wire as filler material. The research is supported by the National Defense Basic Research Program and Hunan Province Major Science and Technology Special Project, indicating its relevance to aerospace structural applications. The work is classified under TG444.

Material System and Welding Challenges

The 5B70 aluminum alloy belongs to the Al-Zn-Mg-Cu family, widely used in aerospace structural components due to its excellent combination of strength, fatigue resistance, and formability. The "5" designation indicates a Zn-Mg-Cu system, while the specific composition includes critical additions of Sc and Zr for grain refinement and dispersoid strengthening. The 5B71 filler wire is compositionally matched to provide adequate strength and corrosion resistance in the weld metal.

Welding 1.5 mm thick plates presents specific challenges:

• Thin section effects: Rapid heat dissipation through the thin plate leads to high cooling rates, which can promote fine grain formation but also increase the risk of solidification cracking.
• Heat input sensitivity: Excessive heat input causes excessive grain growth and potential burn-through, while insufficient heat input results in incomplete penetration and lack of fusion.
• Grain refinement potential: The high cooling rates inherent to thin plate welding, combined with the Sc and Zr additions, create favorable conditions for grain refinement.
• Distortion control: Thin plates are highly susceptible to welding distortion, requiring careful heat input management.

Microstructural Observations

The TIG welding process produced welds with excellent external appearance and sound internal quality under optimized parameters. The internal microstructure exhibited distinct characteristics:

Fusion Zone (FZ) Microstructure:

• Predominantly equiaxed cast structure composed of relatively uniform grains.
• Grain size distribution is non-uniform, with the majority of grains measuring 40–50 μm in diameter.
• Ultrafine grain regions (approximately 20 μm diameter) are present as thin strips and small patches.
• Ultrafine grain regions occupy a significantly smaller area fraction compared to the 40–50 μm grain regions.
• Ultrafine grain regions are distributed primarily parallel to the fusion line, with a small fraction appearing at the weld surface.

Grain Size Distribution Pattern:

The non-uniform grain size distribution reflects the complex solidification dynamics within the weld pool. The fusion line region experiences different thermal conditions than the weld center, leading to variations in nucleation and growth rates.

Role of Sc and Zr in Grain Refinement

The study provides clear evidence that Sc and Zr elements play a decisive role in grain refinement during weld pool solidification. The mechanism operates through two complementary pathways:

Pathway 1: Heterogeneous Nucleation

At the fusion line and weld surface, Sc and Zr act as modification elements, precipitating as second-phase particles during solidification. These particles serve as heterogeneous nucleation sites, reducing the nucleation energy barrier and increasing the number of nucleation centers. The result is a higher grain count and consequently smaller grain size. Regions where Sc is enriched exhibit more pronounced grain refinement, indicating that Sc is the more effective of the two elements for this purpose.

Pathway 2: Undercooling Zone Formation

A "undercooling zone" forms ahead of the solid-liquid interface during solidification. This zone, where the liquid is below its equilibrium liquidus temperature but above the solidus, promotes equiaxed grain formation. The undercooling increases the nucleation rate and reduces the critical nucleation radius, leading to further grain refinement. The grain size in the undercooling zone is approximately half that of the surrounding region, representing a significant refinement effect.

The synergistic action of these two mechanisms produces the observed microstructural pattern. The Sc and Zr particles provide the nucleation sites, while the undercooling zone provides the thermodynamic driving force for rapid nucleation and equiaxed grain growth.

Engineering Implications for Aerospace Applications

The grain refinement achieved through TIG welding of 5B70 alloy has direct implications for aerospace structural performance:

1. Fatigue resistance: Smaller grains improve fatigue crack initiation resistance through the Hall-Petch mechanism. The ultrafine grain regions, particularly near the fusion line where stress concentrations develop, contribute significantly to fatigue life.
2. Fracture toughness: Fine equiaxed grains provide better fracture toughness than coarse columnar grains by promoting crack deflection and increasing the crack propagation resistance.
3. Corrosion resistance: Uniform fine grain structure reduces the potential for intergranular corrosion, which is a concern for Al-Zn-Mg-Cu alloys in marine and atmospheric environments.
4. Weld quality consistency: The non-uniform grain size distribution suggests that process parameter control is critical. Variations in heat input, welding speed, or gas flow could shift the balance between the 40–50 μm and 20 μm grain populations, affecting mechanical property consistency.

Process Optimization Recommendations

Based on the findings, the following process optimization strategies are suggested for production welding of thin 5B70 alloy plates:

• Heat input control: Maintain heat input within a narrow window that promotes undercooling zone formation without causing excessive melting or burn-through. Lower current and higher speed favor finer grains but must ensure full penetration.
• Gas flow optimization: Adequate shielding gas flow is essential to prevent oxidation of the molten pool, which would consume Sc and Zr at the surface and reduce their refinement effectiveness.
• Electrode condition: Tungsten electrode wear and contamination can affect arc stability and heat distribution, indirectly influencing grain refinement. Regular electrode dressing or replacement is recommended.
• Joint preparation: Tight fit-up and proper joint geometry minimize gas porosity and ensure consistent heat flow patterns that support uniform grain refinement.

Study Insights and Reflections

This research provides valuable insight into the solidification microstructure of thin-section aerospace aluminum alloy welds. The identification of the undercooling zone as a critical region for grain refinement is a significant metallurgical finding. The dual mechanism of heterogeneous nucleation by Sc/Zr particles and undercooling-driven equiaxed grain formation explains the observed microstructural pattern and provides a framework for predicting grain structure under different process conditions.

The finding that Sc enrichment correlates with greater grain refinement is particularly important for filler material selection. The 5B71 wire, with its specific Sc and Zr content, appears well-suited for 5B70 base metal welding. However, the study does not investigate the effect of varying Sc and Zr content on refinement, which would be valuable for optimizing filler composition for specific applications.

A limitation of the study is the absence of mechanical property data. While the microstructural analysis is thorough, the relationship between the observed grain refinement and actual mechanical performance (tensile strength, fatigue life, fracture toughness) remains to be established. Future work should correlate the microstructural features with mechanical properties to provide a complete process-performance map.

Reference Value and Outlook

This work contributes essential knowledge for the TIG welding of thin aerospace aluminum alloy plates. The understanding of Sc and Zr-mediated grain refinement mechanisms provides a basis for process optimization aimed at maximizing grain refinement and associated mechanical property benefits. For production welding of 5B70 alloy components, the findings suggest that careful control of heat input and welding parameters can exploit the inherent refinement potential of the alloy system. Future research should extend to thicker sections, different welding processes (such as friction stir welding or laser welding), and comprehensive mechanical property characterization to fully establish the process-microstructure-property relationships for this important aerospace alloy system.