Effect of Welding Current on Microstructure and Properties of 5052 Aluminum Alloy TIG Welds

Literature Overview

The study by Liu Min, Zhao Yanjun, Liu Yahong, Dai Qingsong, Lv You, and Liang An, published in the Journal of Guangxi University (Natural Science Edition) (2019, Vol. 44, No. 3, pp. 891-898), investigates the influence of welding current (100-120 A range) on the microstructure, mechanical properties, and corrosion resistance of TIG welds in 5052 aluminum alloy. Funded by Guangxi provincial research programs, this work addresses a practically important question: how to optimize welding current to achieve the best balance of strength, ductility, and corrosion performance in a widely used aluminum alloy.

Material Background and Welding Challenges

5052 aluminum alloy is an Al-Mg alloy containing approximately 2.2-2.8% magnesium, widely used in aerospace, automotive, shipbuilding, and general fabrication applications due to its excellent combination of formability, corrosion resistance, and moderate strength. The alloy is typically supplied in the H32 or H34 temper (strain-hardened and partially annealed), and welding represents a significant challenge because:

• The weld zone loses strain-hardening temper through thermal exposure
• Mg-rich phases dissolve during welding and reprecipitate during cooling, potentially forming brittle intermetallics
• Hot cracking susceptibility increases with Mg content and impurity levels
• The oxide film (Al₂O₃) has a melting point far above the base metal, requiring proper arc initiation and wetting

Welding Current as a Process Control Variable

Welding current directly determines heat input in TIG welding (along with travel speed and arc voltage). Higher currents produce deeper penetration, wider fusion zones, and higher peak temperatures, all of which influence the resulting microstructure and properties. The study examines a current range of 100-120 A, which is appropriate for thin-section 5052 alloy applications (typically 1.5-3 mm thickness).

Key Findings and Technical Analysis

Mechanical Property Trends

The study reveals a clear trend: tensile strength increases monotonically with welding current, while ductility (measured by reduction of area) decreases. At 115 A, the weld achieves a tensile strength of 212.1 MPa, representing 90.8% of the base metal strength. However, the reduction of area at this current is only 28.9%, approximately half of the base metal value of 50.3%. This significant ductility loss is characteristic of aluminum alloy welds and is attributed to:

1. Grain coarsening in the weld zone: Higher currents produce larger weld pools with slower cooling rates in the fusion zone, promoting grain growth
2. Precipitate-free zones at grain boundaries: Dissolution of strengthening precipitates (Mg₂Al₃ and Al₃Mg₂) during welding, with incomplete reprecipitation during cooling
3. Microsegregation and grain boundary segregation: Mg enrichment at grain boundaries during solidification creates weak interfaces susceptible to intergranular failure

Corrosion Behavior Evolution

A particularly interesting finding is the transition in corrosion mechanism with increasing welding current. At lower currents (100-110 A), pitting corrosion dominates, while at higher currents (115-120 A), intergranular corrosion becomes the primary attack mechanism. This transition correlates with microstructural changes:

• Lower currents produce finer grain structures with less pronounced grain boundary segregation
• Higher currents promote coarser grains with more Mg-rich grain boundaries that are susceptible to selective intergranular attack
• The corrosion current does not show a clear monotonic trend with welding current, suggesting complex electrochemical behavior influenced by multiple factors

Optimal Current Selection (115 A)

The identification of 115 A as the optimal welding current represents a practical engineering decision based on the principle of balanced properties rather than maximization of any single parameter. At this current:

• Strength reaches 90.8% of base metal (excellent for aluminum welds)
• Ductility, while reduced, remains acceptable for most structural applications
• Corrosion behavior is in transition, not yet dominated by the more detrimental intergranular mechanism
• The weld pool geometry likely provides adequate penetration without excessive heat input

Engineering Practice Applications

Process Parameter Optimization Methodology

This study exemplifies a systematic approach to welding process optimization that can be applied broadly:

1. Define the parameter range: Based on material thickness and minimum penetration requirements
2. Select the response variables: Mechanical properties (strength, ductility) and corrosion resistance
3. Conduct systematic experiments: Vary one parameter while controlling others
4. Analyze trends and identify optimal values: Balance competing requirements
5. Validate through additional testing: Confirm findings under service-relevant conditions

Practical Recommendations for 5052 Alloy TIG Welding

Based on the findings of this study and general engineering knowledge:

• For thin sections (1.5-2.5 mm): welding current of 110-120 A with appropriate travel speed
• Shielding gas: high-purity argon (99.99% minimum) to prevent oxide inclusion
• Travel speed: optimized to achieve desired bead width-to-depth ratio
• Preheating: generally not required but may be beneficial for thicker sections or cold ambient conditions
• Post-weld treatment: solution treatment and aging may be considered for applications requiring full base metal properties restoration

Comparison with Other Aluminum Alloy Welding

The 90.8% strength ratio achieved at optimal current is comparable to or better than typical results reported for 5052 alloy welds in the literature. However, the significant ductility loss (reduction of area dropping from 50.3% to 28.9%) remains a concern for applications requiring high toughness or forming capability in the weld zone. This limitation is inherent to the welding process for strain-hardened aluminum alloys and cannot be fully eliminated by parameter optimization alone.

Study Insights and Independent Reflection

This research provides valuable quantitative data for welding process specification of 5052 aluminum alloy. The systematic investigation of current effects on both mechanical and corrosion properties demonstrates the interconnected nature of weld quality factors—optimizing for one property often compromises another.

The corrosion behavior transition from pitting to intergranular attack with increasing current is particularly significant for applications in marine or atmospheric environments. Intergranular corrosion is generally more detrimental than pitting because it can lead to loss of structural integrity without significant weight loss, making it harder to detect during inspection. This finding suggests that for corrosion-critical applications, welding current should be selected not only based on mechanical properties but also on the resulting corrosion mechanism.

One limitation of the study is the relatively narrow current range examined (100-120 A). For thicker sections or different joint configurations, the optimal current range would shift, and the relationship between current and properties might differ. Additionally, the study does not address the effects of travel speed, which in practice is always coupled with current to determine heat input. Future work should consider multi-parameter optimization using statistical design of experiments approaches to capture interaction effects between current, travel speed, and other process variables.

The practical significance of this work lies in its clear recommendation: for 5052 alloy TIG welding in the examined thickness range, 115 A represents a well-balanced choice that provides high strength while avoiding the most detrimental corrosion mechanisms. This finding can directly inform welding procedure specifications and quality assurance requirements for manufacturing operations involving 5052 alloy components.