Microstructure and Properties of AC TIG Welded AZ31B Magnesium Alloy Thin Plate

Literature Overview

This 2013 study by Wu Xiaojun, Wang Huaijian, and Bai Li, published in the Journal of Ordnance Materials and Science and Engineering, investigates the microstructure and mechanical properties of AC TIG welded joints of 2.0 mm thick AZ31B wrought magnesium alloy thin plate. Using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), shear testing, and fractographic analysis, the authors characterize the weld microstructure, elemental distribution, fracture morphology, and joint strength. The results reveal significant magnesium oxidation loss in the weld zone, a shear strength of only 21 MPa, and brittle quasi-cleavage fracture characteristics. This study highlights the significant challenges of welding magnesium alloys and provides valuable data for process optimization.

Core Technical Points

AZ31B is a widely used wrought magnesium alloy containing approximately 3% aluminum and 1% zinc, with the remainder being magnesium. Its low density and good strength-to-weight ratio make it attractive for lightweight applications, but its high reactivity and low melting point (650°C) make welding extremely challenging.

The key findings from this study include:

• Magnesium oxidation: Significant magnesium burn-off occurs during AC TIG welding, leading to MgO formation and altered weld composition.
• Low shear strength: The weld joint shear strength of 21 MPa is substantially lower than the base metal, indicating poor joint integrity.
• Brittle fracture: The fracture morphology shows river patterns characteristic of quasi-cleavage fracture, indicating limited ductility in the weld zone.
• Elemental redistribution: EDS analysis reveals non-uniform distribution of Al and Zn in the weld metal, with potential segregation at grain boundaries.

Microstructural Analysis

The microstructure of the weld joint can be divided into three distinct zones:

The AC TIG welding process produces a wider and shallower weld pool compared to DCEN welding, which may contribute to increased oxidation. The alternating polarity causes periodic cathodic and anodic effects, which can help clean oxide layers on the base metal but also increases the overall heat input and oxidation potential.

Mechanical Properties and Fracture Analysis

The shear strength of 21 MPa is notably low for a magnesium alloy joint. For comparison, the base metal shear strength of AZ31B is typically in the range of 120-150 MPa. This dramatic reduction in strength is attributed to several factors:

1. Porosity: Gas porosity from hydrogen and oxygen dissolved in the molten pool reduces the effective load-bearing area.
2. Oxide inclusions: MgO particles formed during welding act as crack initiation sites.
3. Brittle phases: Possible formation of brittle intermetallic compounds at grain boundaries.
4. Residual stress: High residual stresses from the welding thermal cycle promote crack propagation.

The river patterns observed on the fracture surface indicate that cracks initiated at multiple sites and propagated rapidly through the weld metal, consistent with a brittle fracture mechanism. This is particularly concerning for structural applications where ductile fracture behavior is required for damage tolerance.

Engineering Practice and Process Optimization

The results of this study underscore the need for significant process improvements when welding magnesium alloys. Based on my experience and the literature, the following optimization strategies should be considered:

• Shielding gas: Use high-purity argon with helium addition to reduce arc voltage and heat input. Flow rates of 15-25 L/min are typically required for adequate protection.
• Current type: DCEN welding may produce a narrower, deeper weld with less oxidation compared to AC welding. However, AC cleaning action may be beneficial for oxide removal.
• Filler metal: Use matching or slightly over-alloyed filler wire to compensate for magnesium burn-off.
• Preheating: Moderate preheating (100-150°C) can reduce thermal gradients and residual stresses but must be carefully controlled to avoid excessive grain growth.
• Post-weld treatment: Solution heat treatment followed by aging can improve weld metal properties and reduce residual stresses.

Key Reflections and Study Insights

This study provides a sobering assessment of the current state of magnesium alloy welding technology. The low shear strength and brittle fracture behavior observed in the AC TIG welded joints highlight the significant metallurgical challenges that remain to be overcome. While magnesium alloys offer tremendous potential for lightweight applications, their weldability remains a major barrier to widespread structural use.

The findings of this paper should serve as a reminder that welding process development must be guided by rigorous metallurgical analysis and mechanical testing. The combination of SEM, EDS, and fractographic analysis used in this study is the gold standard for weld characterization, and every welding engineer should be familiar with these techniques. The low shear strength result should not be interpreted as a fundamental limitation of magnesium alloy welding, but rather as an indication that the process parameters and consumables used in this study require optimization.