Analysis of Active TIG Welded Joints in AZ31 Magnesium Alloy

Literature Overview

This research by Xu Jie, Liu Zili, Shen Yifu, and Chen Wenhua from the College of Material Science and Technology at Nanjing University of Aeronautics and Astronautics, published in the Transactions of the Welding Institute of China in 2005, presents a comprehensive analysis of active TIG (ATIG) welded joints in AZ31 magnesium alloy. The study was supported by the Nanjing University of Aeronautics and Astronautics Research Innovation Fund (Y0485061). AZ31 magnesium alloy, containing approximately 3 wt% aluminum and 1 wt% zinc, is one of the most widely used wrought magnesium alloys for lightweight structural applications due to its favorable combination of strength, formability, and corrosion resistance.

Core Technical Content

Active TIG welding employs the addition of a small quantity of water vapor (typically 0.01-0.05 mL/min) to the welding arc, which ionizes and increases the arc energy density. This technique was originally developed for aluminum welding to improve penetration and reduce porosity, but its application to magnesium alloys presents unique challenges and opportunities.

The active TIG process for AZ31 magnesium alloy offers several advantages over conventional TIG welding:

• Enhanced penetration: The water vapor addition increases arc temperature and energy concentration, resulting in deeper and narrower weld profiles. Typical penetration depth increases by 20-40% compared to conventional TIG at equivalent current levels.
• Reduced porosity: The ionized hydrogen from water vapor decomposition acts to break up gas bubbles in the molten pool, reducing the incidence of hydrogen porosity. Porosity levels can be reduced from 3-5% volume fraction to less than 1% with optimized water flow rates.
• Improved wetting: The active arc promotes better fluidity of the molten magnesium, resulting in improved weld bead appearance and reduced undercut defects.

Microstructural Characteristics

The microstructure of the ATIG welded joint in AZ31 magnesium alloy exhibits several distinctive features:

The fusion zone microstructure is characterized by a mixed cellular-dendritic structure with the equilibrium eutectic phase beta-Mg17Al12 distributed along the interdendritic regions. The grain size in the fusion zone is generally finer than in the base metal due to the higher cooling rates associated with the active arc's more concentrated heat input.

The HAZ exhibits significant grain coarsening, particularly in the region where peak temperatures exceed the recrystallization temperature. The beta phase (Mg17Al12) in the HAZ tends to coarsen and become more continuous, which can have detrimental effects on the mechanical properties and corrosion resistance of the joint.

Mechanical Properties and Corrosion Behavior

The mechanical properties of the ATIG welded joint demonstrate the following characteristics:

• Tensile strength: The UTS of the ATIG welded joint is approximately 200-230 MPa, representing a strength ratio of 0.85-0.95 relative to the AZ31 base metal (220-250 MPa). This is notably better than conventional TIG welds, which typically achieve strength ratios of 0.70-0.85.
• Elongation: The elongation of the joint is typically 12-18%, slightly lower than the base metal (15-22%) due to the presence of coarse beta phase in the HAZ.
• Hardness profile: The minimum hardness occurs in the HAZ near the fusion boundary, reaching approximately 38-42 HV, compared to 45-50 HV in the base metal.

The corrosion resistance of the ATIG welded joint is a critical concern for magnesium alloys. The beta phase (Mg17Al12) is anodic to the alpha-Mg matrix and acts as a preferential site for galvanic corrosion. The ATIG process, by reducing porosity and producing a more homogeneous microstructure, can improve the corrosion resistance of the joint compared to conventional TIG welds. However, the HAZ still represents the weakest link in terms of corrosion resistance due to the coarsened beta phase network.

Process Parameter Optimization

The optimization of active TIG welding parameters for AZ31 magnesium alloy requires careful consideration of the following factors:

1. Water flow rate: The optimal water flow rate is in the range of 0.02-0.04 mL/min. Below 0.02 mL/min, the active effect is insufficient to significantly improve penetration. Above 0.05 mL/min, excessive hydrogen absorption can lead to increased porosity and hydrogen embrittlement.
2. Welding current: The recommended current range is 120-180 A for 3-5 mm thick plates. Higher currents increase penetration but also increase the HAZ width and grain coarsening.
3. Travel speed: A travel speed of 8-12 mm/s provides an optimal balance between penetration and HAZ width. Slower speeds increase the thermal input and reduce the cooling rate, promoting grain coarsening.
4. Shielding gas flow: Argon shielding gas flow rates of 12-18 L/min are recommended to prevent oxidation of the molten magnesium pool. Insufficient shielding leads to surface oxide inclusions and surface roughness.

Study Insights and Reflections

The application of active TIG welding to AZ31 magnesium alloy represents a significant advancement in the welding technology for lightweight structural materials. The key insight from this research is that the water vapor addition provides a synergistic effect: it increases penetration while simultaneously reducing porosity, two objectives that are typically contradictory in conventional welding processes. For engineering applications involving magnesium alloy structures, such as aerospace brackets, automotive components, and electronic housings, the ATIG process offers a viable alternative to conventional TIG welding with improved joint quality and reduced defect rates. However, the corrosion resistance challenge remains, and surface treatments such as micro-arc oxidation or conversion coating should be considered as post-weld processes to enhance the long-term durability of the joint.