Microstructure and Mechanical Properties of TIG Welded AZ31 Magnesium Alloy Joints

Literature Overview

The paper by Zhang Xin'en, Zhou Jixue, Zhan Chengwei, Zhao Dongqing, Li Weihong, Tang Shouqiu, and Yang Yuansheng, published in Shandong Science (Vol. 25, No. 3, 2012, pp. 92–94), investigates the microstructure and mechanical performance of DC-TIG welded joints in 6.0 mm thick AZ31 magnesium alloy extruded plate. AZ31 is the most widely used wrought magnesium alloy, valued for its excellent combination of strength, formability, and corrosion resistance in automotive and aerospace lightweighting applications. The study was supported by the Shandong Provincial Natural Science Foundation and the National Science and Technology Support Program, reflecting the strategic importance of magnesium alloy welding in China's lightweight materials development.

Core Technical Findings

Microstructural Characterization

The welded joint consists of three distinct regions, as is typical for fusion-welded joints:

1. Base metal (BM): The as-extruded AZ31 plate exhibits a typical extruded microstructure with elongated α-Mg matrix grains and dispersed β-Mg₁₇Al₁₂ intermetallic particles along grain boundaries.
2. Heat-affected zone (HAZ): The HAZ shows grain coarsening due to the elevated temperatures experienced during welding. The β-phase particles at grain boundaries may undergo partial dissolution and subsequent re-precipitation, depending on the peak temperature reached.
3. Weld metal (WM): The weld zone exhibits a columnar dendritic solidification structure characteristic of rapid solidification under TIG welding conditions. The columnar grains grow epitaxially from the partially melted base metal grains at the fusion boundary.

A notable finding is the formation of a transition zone at the groove interface between the HAZ and the weld metal, characterized by fine-grained microstructure. This zone represents the mixing region where base metal and filler wire melts intermix, resulting in a locally modified composition and solidification behavior that produces finer grains.

Mechanical Properties

The mechanical performance of the welded joint is summarized as follows:

The tensile strength retention of 86.0% is acceptable for most structural applications of AZ31, though the significant reduction in ductility (to 63.6% of base metal) is a concern for applications requiring substantial post-weld forming or strain tolerance. The fracture consistently occurs in the HAZ, exhibiting a mixed ductile-brittle fracture morphology. This indicates that the HAZ is the weakest link in the joint, which is a common finding in magnesium alloy welding where grain coarsening and intermetallic particle redistribution reduce local ductility.

Welding Process Parameters

DC-TIG welding was employed, which is the most common and reliable method for magnesium alloy welding due to:

• Stable arc with good penetration for the relatively thin (6.0 mm) plate thickness
• Effective inert gas shielding that prevents the severe oxidation of magnesium
• Controllable heat input suitable for the narrow weldability window of AZ31

Key process considerations include:

• Shielding gas: High-purity argon (≥99.99%) is essential; even trace oxygen or nitrogen can cause severe oxidation of the molten pool.
• Polarity: Direct current with electrode negative (DCEN) provides stable arc and adequate penetration.
• Current range: Typically 100–200 A for 6 mm thickness, depending on the specific joint geometry and travel speed.
• Travel speed: Moderate to fast to minimize heat input and reduce HAZ grain coarsening.
• Preheating: Generally avoided for AZ31 to prevent excessive grain growth; however, for thicker sections, mild preheating (up to 100–150°C) may be necessary to prevent cold cracking.

Engineering Practice Implications

The study highlights several practical considerations for magnesium alloy welding in production environments:

• Surface preparation: Thorough removal of oxide layers (MgO, Mg(OH)₂) is critical before welding, as these oxides have much higher melting points than the base metal and can cause porosity and lack of fusion.
• Joint design: Double-sided welding is preferred for thicker sections to ensure complete penetration and reduce residual stress. The groove geometry should be optimized to balance penetration with minimal heat input.
• Filler selection: Matching filler wire (AZ31 or AZ91) is recommended; however, the choice of filler composition can influence the weld metal microstructure and, consequently, the joint strength.
• Post-weld treatment: Stress-relief annealing at 200–250°C can reduce residual stresses without significantly compromising strength. However, solution treatment followed by aging may be necessary for applications requiring higher strength, though this can be challenging for large welded assemblies.

Key Questions and Reflections

The mixed ductile-brittle fracture in the HAZ suggests that the β-phase distribution in this region may be suboptimal. A more uniform distribution of fine β-particles, perhaps achieved through a carefully designed heat treatment cycle, could improve the HAZ ductility and shift the fracture location to the weld metal, where the solidification structure may be more forgiving. Additionally, the study does not address long-term corrosion behavior of the welded joint, which is a critical concern for AZ31 in humid or chloride-containing environments. The HAZ, with its modified microstructure, may exhibit different corrosion resistance compared to the base metal.

Summary

This study provides a solid foundation for understanding the weldability of AZ31 magnesium alloy under DC-TIG conditions, confirming that acceptable strength retention (86%) can be achieved with proper process control. The identification of the HAZ as the fracture-initiating region and the mixed fracture morphology point to the need for further optimization of the HAZ microstructure through heat treatment or modified welding parameters. For engineers designing magnesium alloy welded structures, this work underscores the importance of balancing strength and ductility through careful selection of welding parameters, filler material, and post-weld heat treatment.