Polarity-Reversed TIG Welding of Scandium-Containing 5B70 Aluminum Alloy

Literature Overview

This study by Su Zhiqiang and colleagues from Capital Aerospace Machinery Co., Harbin Institute of Technology, and the Hubei Provincial Key Laboratory of Advanced Technology, published in Welding (2014, Issue 9, pp. 45-48), examines the microstructure and mechanical properties of 6 mm thick 5B70 Sc-containing aluminum alloy plates welded using polarity-reversed TIG welding. The 5B70 alloy belongs to the Al-Mg-Sc system, which has gained increasing attention in aerospace applications due to its excellent combination of strength, corrosion resistance, and weldability compared to conventional Al-Mg alloys.

Core Technical Analysis

Polarity-Reversed TIG Welding Mechanism

Polarity-reversed TIG welding, also known as alternating current TIG (AC TIG) or polarity-reversed arc welding, is a specialized process variant that alternates between direct current electrode negative (DCEN) and direct current electrode positive (DCEP) polarity during the welding cycle. In the DCEN phase, the majority of arc heat is concentrated at the workpiece, providing deep penetration and high deposition rate. In the DCEP phase, the cathodic cleaning effect removes the protective Al2O3 film from the workpiece surface, ensuring clean fusion.

Effect of Scandium on Weld Microstructure

The study reveals that scandium plays a multifaceted role in the weld joint microstructure:

1. Grain refinement in the weld zone: Sc forms ScAl3 dispersoids that act as potent heterogeneous nucleation sites during solidification. These nano-scale particles reduce the grain size significantly compared to conventional Al-Mg alloys without Sc.
2. Equiaxed grain layer at the fusion boundary: The formation of an equiaxed grain layer at the fusion line is a critical finding. This layer acts as a transition zone between the columnar weld grain structure and the base metal grain structure, reducing the crystallographic mismatch and improving the metallurgical compatibility between the weld and base metal.
3. Suppression of recrystallization in the HAZ: Sc dispersoids pin grain boundaries and impede dislocation movement, effectively suppressing dynamic and static recrystallization in the HAZ. This preserves the precipitate-free zone (PFZ) width and maintains the strength of the HAZ.

Mechanical Performance

The strength ratio of 0.89 is notably high for aluminum alloy welding, where typical values range from 0.5 to 0.75. This exceptional performance is attributed to the combined effects of Sc-induced grain refinement and HAZ recrystallization suppression.

Microhardness Distribution

The hardness profile shows the lowest values at both sides of the weld zone, with slightly higher hardness at the weld center. This distribution is characteristic of Al-Mg alloys where the PFZ forms at the fusion boundary due to the dissolution of Mg-rich precipitates (such as β-phase Mg5Al8) during welding. The PFZ is inherently soft due to the lack of precipitate strengthening. However, the Sc dispersoids provide some resistance to softening, which is why the strength ratio remains high despite the presence of a PFZ.

Engineering Practice Integration

Aerospace Application Context

The 5B70 alloy is used in aerospace structural components such as fuselage frames, wing skins, and pressure vessel shells. The Capital Aerospace Machinery Company's involvement in this study directly connects the research to practical aerospace manufacturing needs. For these applications, the weld joint must satisfy:

• Static strength requirements: The 368 MPa tensile strength meets most aerospace structural requirements for Al-Mg alloy components.
• Fatigue resistance: The equiaxed grain layer at the fusion boundary and the suppressed HAZ recrystallization both contribute to improved fatigue performance by reducing stress concentration at the weld interface.
• Corrosion resistance: The uniform microstructure with fine grain size reduces the susceptibility to intergranular corrosion and stress corrosion cracking.

Process Parameter Considerations

The study does not explicitly report the specific welding parameters used, which limits the direct applicability of the findings. However, for 6 mm thick 5B70 plates, typical polarity-reversed TIG parameters would include:

• Current range: 150-250 A depending on joint configuration
• Polarity ratio: DCEP time typically 20-40% of the total cycle
• Shielding gas: High-purity Ar (99.99%+) or Ar/He mixtures
• Travel speed: 200-400 mm/min
• Filler wire: ER5183 or matching 5B70 composition

The polarity ratio is a critical parameter that requires optimization. A higher DCEP ratio provides better cathodic cleaning but reduces penetration depth. For 6 mm thickness, a balanced ratio is necessary to achieve full penetration while maintaining adequate cleaning.

FMEA Analysis of Weld Defects

Study Insights and Implications

This study demonstrates that scandium is not merely a strengthening element in aluminum alloys but also a weldability-enhancing element. The dual role of Sc dispersoids in grain refinement and recrystallization suppression provides a metallurgical basis for the high strength ratio achieved. For engineers designing welding processes for Sc-containing aluminum alloys, the key insight is that the polarity-reversed TIG process offers a practical solution that leverages both the metallurgical benefits of Sc and the process advantages of cathodic cleaning. The equiaxed grain layer at the fusion boundary is particularly noteworthy as it represents a microstructural feature that naturally develops during welding, requiring no additional process intervention. This finding has broader implications for the design of other aluminum alloy systems where fusion boundary compatibility is a critical concern.