Weldability Study of AM50 Magnesium Alloy Using TIG Welding

Literature Overview

This study by Liu Zhengjun and colleagues from Shenyang University of Technology, published in Journal of Shenyang University of Technology (2007, Vol. 29, No. 5, pp. 492-496), investigates the weldability of AM50 magnesium alloy using TIG welding technology. AM50 is a wrought magnesium alloy with approximately 5% aluminum and 0.5% zinc, known for its excellent combination of strength, corrosion resistance, and formability. However, magnesium alloys present unique welding challenges due to their low melting point, high reactivity, and susceptibility to hot cracking.

AM50 Magnesium Alloy Characteristics

AM50 magnesium alloy possesses the following characteristics relevant to welding:

• Low melting point: approximately 450°C
• High thermal conductivity: approximately 120 W/m·K
• High chemical reactivity with oxygen and nitrogen
• Susceptibility to hot cracking due to low melting point eutectic phases
• Limited solid solubility of aluminum in magnesium

These characteristics make AM50 challenging to weld using conventional processes, requiring careful process parameter optimization to achieve sound welds.

TIG Welding Process Parameters

The study systematically investigated the effects of welding current, travel speed, and shielding gas flow rate on weld appearance and mechanical properties. The following parameter ranges were evaluated:

The optimal parameter combination produced welds with good surface formation, minimal spatter, and acceptable mechanical properties. The study developed a complete welding procedure specification (WPS) for AM50 TIG welding based on these findings.

Microstructural Analysis

The microstructural analysis revealed significant changes in the weld zone compared to the base metal:

The weld metal consisted of an alpha-Mg matrix with beta-Al₂Mg₁₇ phases concentrated at grain boundaries. The presence of these intermetallic phases at grain boundaries is significant because they can act as crack initiation sites and reduce ductility.

The base metal microstructure showed beta-Al₂Mg₁₇ phases at grain boundaries, which are typical for AM50 in the as-supplied condition. During welding, these phases partially dissolve in the heat-affected zone and reprecipitate during cooling, potentially in a different morphology or distribution.

Mechanical Properties

The mechanical properties of the weld joints were found to be lower than the base metal:

The reduction in tensile strength and hardness in the weld joint is attributed to:

1. Changes in the distribution and morphology of second phases
2. Grain structure changes in the weld and HAZ
3. Potential porosity or lack of fusion defects
4. Residual stress effects

The beta-Al₂Mg₁₇ phase at grain boundaries in the weld metal may reduce ductility by providing preferential crack paths. The reduction in hardness suggests that the weld metal has a softer, more ductile microstructure compared to the base metal.

Welding Challenges and Countermeasures

Magnesium alloy welding presents several unique challenges that require specific countermeasures:

The study emphasized the importance of shielding gas coverage, particularly for the back side of the weld, to prevent oxidation of the molten pool. Back-side shielding with argon or helium is essential for achieving sound welds in magnesium alloys.

Engineering Applications

AM50 magnesium alloy is used in applications where lightweight, high-strength materials are required:

• Automotive components (engine blocks, transmission housings)
• Aerospace structures
• Consumer electronics housings
• Sports equipment

For pipe and fitting applications, magnesium alloys are less common but may be used in specialized applications such as:

• Lightweight pressure vessels
• Chemical processing equipment (where compatible)
• Aerospace fuel systems

The TIG welding process developed in this study provides a baseline for joining AM50 components in such applications, though additional qualification testing would be required for pressure-containing applications.

Critical Reflections

The study provides a foundational understanding of AM50 TIG weldability, but several limitations should be acknowledged. The relatively low current range tested (100-250 A) may not be sufficient for thicker sections or higher production rates. Additionally, the study does not address fatigue performance, which may be critical for cyclic loading applications.

The reduction in mechanical properties in the weld joint raises questions about the feasibility of AM50 for structural applications requiring full-strength welds. Post-weld heat treatment or advanced welding processes (e.g., laser welding, electron beam welding) may be necessary to achieve acceptable joint properties.

The presence of beta-Al₂Mg₁₇ phases at grain boundaries is a concern for long-term durability, as these phases can promote intergranular cracking under certain loading conditions. Further investigation into the stability of these phases under thermal cycling and mechanical loading would be beneficial.

Summary

This research establishes a baseline TIG welding procedure for AM50 magnesium alloy, identifying optimal parameter ranges for welding current, travel speed, and shielding gas flow. The study reveals that weld joint mechanical properties are lower than base metal values due to microstructural changes, particularly the formation of beta-Al₂Mg₁₇ phases at grain boundaries. While the developed procedure provides a starting point for AM50 welding applications, further optimization through advanced processes and post-weld treatments may be necessary for demanding structural applications. The findings highlight the unique challenges of magnesium alloy welding and the importance of careful process parameter control to achieve acceptable joint quality.