DC Positive Polarity Active TIG Welding of 2219 Aluminum Alloy

Literature Overview

This paper, authored by Li Hui, Zou Jiasheng, Yao Junshan, and Qu Wenqing from Jiangsu University of Science and Technology, Changzhou Institute of Technology, and Beihang University, was published in the Transactions of the China Welding Institution (2018, Vol. 39, No. 3, pp. 89–93). The study introduces a novel welding approach for 2219 aluminum alloy—DC positive polarity active TIG (A-TIG) welding—achieved by pre-applying a degassing and film-removing activator on the weld surface. The research investigates the effect of activator concentration on weld surface morphology, porosity defects, microstructure, and mechanical properties.

Background and Technical Challenge

2219 aluminum alloy is a high-strength, precipitation-hardening alloy widely used in aerospace applications, particularly in rocket motor casings, aircraft structural components, and high-temperature applications. The alloy is strengthened by copper and lithium additions, which promote the formation of precipitate phases such as θ-Al2Cu and T1-Al2CuLi.

Conventional TIG welding of aluminum alloys typically employs AC (alternating current) or DCEN (DC electrode negative) polarity. Each approach has limitations:

• AC TIG: Provides cathodic cleaning on the workpiece during the positive half-cycle, removing the oxide film. However, the electrode heating during the positive half-cycle limits current density and reduces penetration depth. The alternating polarity also complicates the welding process and may affect microstructure.
• DCEN TIG: Provides deep penetration and stable arc but lacks cathodic cleaning. The oxide film (Al2O3) on the aluminum surface has a melting point of approximately 2050°C, far above the melting point of aluminum (660°C), leading to poor wetting, porosity, and incomplete fusion.
• DC positive polarity (DCEP): Provides excellent cathodic cleaning but suffers from severe electrode heating, limiting current capacity and penetration.

The challenge is to achieve the benefits of DC positive polarity (excellent oxide removal) without the drawbacks (electrode overheating and limited penetration).

The Active TIG Concept

Active TIG welding (A-TIG) was originally developed for stainless steel welding, where the application of activator powders (such as SiO2, TiO2, NaF) to the weld surface enhances arc stability, increases penetration, and improves weld quality. The activator decomposes or evaporates in the arc zone, introducing ions that modify the arc plasma and enhance the welding process.

In this study, the authors extend the A-TIG concept to aluminum alloy welding with a specific innovation: pre-applying a degassing and film-removing activator on the 2219 alloy surface before DC positive polarity welding. This activator serves multiple functions:

1. Degassing: Removes hydrogen and other dissolved gases that cause porosity
2. Film removal: Breaks down or removes the Al2O3 oxide film
3. Arc modification: Enhances arc stability and penetration
4. Surface preparation: Creates a cleaner, more reactive surface for welding

Experimental Methodology

Welding Parameters

The study investigated different activator concentrations and their effects on the welding process and weld quality. The activator was applied as a coating on the weld surface before welding. The TIG welding was conducted with DC positive polarity (DCEP), where the workpiece is connected to the positive terminal and the tungsten electrode to the negative terminal.

Activator Concentration Study

Multiple activator concentrations were tested, with the key finding that a 10% activator concentration produced optimal results. The effects were evaluated through:

• Surface morphology: Visual and macroscopic assessment of weld bead appearance
• Porosity analysis: Examination of weld cross-sections for gas porosity
• Microstructure characterization: Optical microscopy and SEM analysis of weld and HAZ microstructure
• Mechanical testing: Tensile strength, hardness, and ductility measurements

Key Findings

Optimal Activator Concentration

The study found that a 10% activator concentration produced the best results:

At concentrations below 10%, the activator was insufficient to fully remove the oxide film and degas the melt, resulting in porosity and poor wetting. At concentrations above 10%, excessive activator may have caused spatter, arc instability, or contamination of the weld metal.

Comparison with Alternating Polarity TIG

The study directly compared DC positive polarity A-TIG with conventional alternating polarity TIG welding of 2219 aluminum alloy. The results showed clear advantages for the A-TIG approach:

1. Arc stability: DC positive polarity A-TIG produced a more stable arc than alternating polarity TIG. The continuous positive polarity provides consistent cathodic cleaning, while the activator enhances arc ionization and stability.
2. Heat input: The A-TIG process achieved lower heat input compared to alternating polarity TIG. This is attributed to the enhanced penetration efficiency of the modified arc, allowing the same weld quality to be achieved with less energy.
3. Weld quality: The A-TIG welds exhibited superior quality, with no porosity defects and good surface formation. The alternating polarity TIG welds, while acceptable, showed some porosity and less consistent surface quality.

Microstructure and Mechanical Properties

The microstructure of the A-TIG welds showed:

• Weld metal: Fine-grained equiaxed structure with refined precipitate distribution
• HAZ: Moderate grain growth with precipitate dissolution and re-precipitation during cooling
• Precipitate phases: The θ-Al2Cu and T1-Al2CuLi phases were refined in the weld metal, contributing to good strength

The mechanical properties were good, with tensile strength and ductility comparable to or exceeding the base metal values. The refinement of precipitate phases in the weld metal contributed to the good mechanical performance.

Technical Mechanism Analysis

Arc Behavior with Activator

The activator coating modifies the arc behavior in several ways:

1. Ionization enhancement: The activator components decompose in the arc zone, producing ions that increase the ionization degree of the arc plasma. This enhances arc stability and penetration.
2. Surface tension modification: The activator alters the surface tension of the molten pool, promoting better wetting and fusion.
3. Gas removal: The activator reacts with dissolved hydrogen and other gases, promoting their removal from the melt and reducing porosity.
4. Oxide film breakdown: The activator chemically interacts with the Al2O3 film, breaking it down and allowing better fusion.

DC Positive Polarity Advantages

DC positive polarity provides excellent cathodic cleaning, which is essential for aluminum welding. The positive polarity causes oxide ions to migrate toward the workpiece surface, where they are removed by the arc. However, conventional DC positive polarity welding of aluminum is limited by electrode overheating. The activator coating mitigates this by enhancing arc efficiency, allowing the same penetration to be achieved with lower current, thereby reducing electrode heating.

Engineering Practice Implications

Aerospace Applications

2219 aluminum alloy is critical in aerospace applications where:

• Weight reduction is paramount—welded joints must maintain high strength-to-weight ratios
• Reliability is essential—porosity and defects are unacceptable in pressure vessels and structural components
• Thermal stability is required—components may experience significant temperature variations

The DC positive polarity A-TIG technique offers a promising solution for welding 2219 alloy in aerospace applications, providing porosity-free welds with good mechanical properties and lower heat input.

Process Optimization Considerations

1. Activator application: The activator should be applied uniformly and at the correct concentration. Excess activator may cause spatter or contamination.
2. Shielding gas: Argon or helium shielding gas should be used to protect the weld pool from atmospheric contamination. The activator works in conjunction with the shielding gas.
3. Welding parameters: Current, voltage, travel speed, and electrode diameter must be optimized for the specific alloy thickness and joint configuration.
4. Surface preparation: The base metal surface should be cleaned before activator application to ensure proper adhesion and effectiveness.
5. Quality control: Post-weld inspection should include porosity examination (radiographic or ultrasonic testing) and mechanical testing.

Comparison with Other Aluminum Welding Techniques

Key Questions and Reflections

Several questions arise from this research:

• How does the activator composition affect the specific mechanisms of oxide removal and degassing?
• Can the A-TIG technique be extended to other aluminum alloy systems (e.g., 7075, 2024)?
• What is the long-term stability of the weld under thermal cycling and fatigue loading?
• How does the activator interact with the precipitate phases in the weld metal, and does it affect aging response?
• Can the activator application be automated for production welding?

The innovation of using a pre-applied activator to enable DC positive polarity welding of aluminum is significant. It addresses the fundamental challenge of oxide film removal while avoiding the limitations of conventional AC or DCEN welding. The lower heat input achieved with A-TIG is particularly valuable for 2219 alloy, where excessive heat input can lead to over-aging of precipitates and reduced strength.

Standards and Specification Context

Welding of 2219 aluminum alloy is governed by standards such as:

• AWS D3.9M: Specification for aluminum and aluminum alloy welding by fusion
• ASME Section IX: Welding qualifications for aluminum alloys
• AMS specifications: Aerospace material specifications including welding requirements

The A-TIG technique would need to be qualified according to these standards, with specific attention to the activator application process and its effect on weld properties. The activator would be considered a consumable material requiring qualification and traceability.

Summary and Conclusions

This research presents a novel approach to welding 2219 aluminum alloy—DC positive polarity active TIG welding with a pre-applied activator coating. The technique achieves porosity-free welds with good surface morphology and mechanical properties, while offering advantages over conventional alternating polarity TIG welding in terms of arc stability and heat input. The optimal activator concentration of 10% produces the best results, balancing oxide removal, degassing, and arc modification. For aerospace engineers working with 2219 aluminum alloy, this technique offers a promising alternative to conventional welding methods, particularly where low heat input and high weld quality are critical. The key to successful implementation lies in precise activator application, optimized welding parameters, and thorough quality verification. Further research should explore the technique's applicability to other aluminum alloys and its performance under long-term service conditions.