AC TIG Welding of AZ31B Magnesium Alloy Thin Sheet: Microstructure and Mechanical Properties

Literature Overview

The paper by Wu Xiaojun, Wang Huaijian, and Bai Li, published in Ordnance Material and Engineering in 2013, investigates the welding of 2.0 mm thick AZ31B magnesium alloy thin sheet using AC TIG welding. The research was supported by a university-level research project at Chongqing Vocational and Technical College of Industry. AZ31B is a widely used wrought magnesium alloy containing approximately 3% aluminum and 0.8% zinc, known for its excellent combination of strength, formability, and corrosion resistance. The study employs scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and shear testing to characterize the weld microstructure, elemental distribution, fracture morphology, and mechanical properties.

Magnesium Alloy Welding Challenges

Magnesium alloys present unique welding challenges that distinguish them from conventional ferrous and aluminum alloys:

• High reactivity: Magnesium is highly reactive with oxygen and nitrogen at elevated temperatures, forming magnesium oxide (MgO) and magnesium nitride (Mg₃N₂) that are difficult to remove and can lead to porosity and inclusions in the weld metal.
• Low melting point: The melting point of magnesium (650°C) is significantly lower than that of aluminum (660°C) and iron (1538°C), which increases the risk of burn-through in thin sheet applications.
• High thermal conductivity: Magnesium alloys have relatively high thermal conductivity, which can lead to rapid heat dissipation and incomplete penetration if the heat input is insufficient.
• Hydrogen absorption: Magnesium readily absorbs hydrogen from the atmosphere, which can lead to hydrogen porosity in the weld metal.
• Limited alloying elements: The limited range of alloying elements available for magnesium alloys constrains the options for filler metal selection and weld composition optimization.

AC TIG Welding Process

AC TIG welding is particularly suitable for magnesium alloy welding because of the cathodic cleaning effect of the AC cycle. During the negative half-cycle (electrode negative), the electron bombardment of the workpiece surface removes oxide layers, providing a self-cleaning effect that is essential for producing clean, oxide-free welds.

The key process parameters for AC TIG welding of AZ31B thin sheet include:

The AC balance is a critical parameter that must be optimized to achieve a balance between cathodic cleaning (electrode positive) and penetration (electrode negative). A balance of 60-70% electrode positive provides sufficient cleaning action while maintaining adequate penetration depth.

Microstructural Analysis

Weld Zone Microstructure

SEM examination reveals a fine-grained equiaxed structure in the weld metal, with grain sizes in the range of 10-30 μm. The fine grain structure is attributed to the rapid solidification rate resulting from the high thermal conductivity of the magnesium alloy and the relatively low heat input of the TIG process.

The weld metal microstructure consists primarily of α-Mg solid solution with a minor amount of β-Mg₁₇Al₁₂ intermetallic phase. The β-phase appears as fine particles dispersed along the grain boundaries, with sizes typically less than 5 μm. The distribution and morphology of the β-phase significantly influence the mechanical properties of the weld joint.

HAZ Microstructure

The heat-affected zone (HAZ) exhibits a gradient of microstructural changes from the weld metal to the base metal. Adjacent to the weld metal, the HAZ shows a partially recrystallized structure with grain sizes of 20-50 μm. Further from the weld, the HAZ microstructure transitions to the original wrought microstructure of the AZ31B base metal.

The elemental distribution analysis using EDS reveals that magnesium exhibits some degree of oxidation and burn-off in the weld zone. The magnesium content in the weld metal is slightly lower than in the base metal, indicating that some magnesium has been lost to the atmosphere during welding. This magnesium loss can affect the composition and properties of the weld metal.

Mechanical Properties

Shear Strength

The shear strength of the weld joint is reported as 21 MPa, which is relatively low compared to the base metal shear strength of AZ31B (typically 100-150 MPa). This low shear strength is attributed to several factors:

1. Thin sheet thickness: The 2.0 mm thickness provides limited material for load transfer across the weld joint.
2. Weld geometry: The weld bead profile and penetration depth may not provide adequate load-bearing cross-section.
3. Microstructural factors: The presence of brittle β-Mg₁₇Al₁₂ phase along grain boundaries can reduce the shear strength.
4. Oxidation and porosity: The oxidation of magnesium and potential porosity in the weld metal can create weak interfaces that reduce shear strength.

Fracture Morphology

The fracture surface of the weld joint exhibits a river pattern morphology, which is characteristic of brittle quasi-cleavage fracture. This fracture mode indicates that the weld joint failed in a brittle manner, with limited plastic deformation prior to fracture. The river pattern consists of parallel ridges that flow from the crack initiation site toward the crack propagation direction, indicating a rapid, unstable crack growth.

The brittle fracture mode is concerning from a structural integrity perspective, as it indicates that the weld joint has limited capacity for plastic deformation and energy absorption. This is particularly problematic for applications involving impact loading or cyclic fatigue, where ductility and toughness are critical.

Defect Analysis and Countermeasures

The low shear strength and brittle fracture mode identified in this study point to several areas for process improvement:

Study Insights and Implications

This study provides valuable insights into the challenges of welding thin magnesium alloy sheet. The low shear strength and brittle fracture mode highlight the need for careful process optimization and quality control in magnesium alloy welding applications.

The key finding of magnesium oxidation and burn-off in the weld zone is particularly significant, as it indicates that even with adequate shielding gas coverage, some degree of magnesium loss is inevitable during TIG welding. This finding has implications for filler metal selection and weld composition control.

The brittle fracture mode observed in this study is a concern for structural applications, as it indicates that the weld joint may not provide adequate energy absorption capacity. Future work should focus on optimizing the welding process to produce weld joints with improved ductility and toughness.

The implications for engineering practice are clear: AC TIG welding of thin magnesium alloy sheet requires careful attention to process parameters, shielding gas coverage, and post-weld quality verification. The low shear strength observed in this study suggests that additional process development is needed to achieve weld joints with mechanical properties comparable to the base metal.

Future research should investigate the effects of filler metal selection, welding speed, and heat input on the mechanical properties of AZ31B weld joints. The development of welding procedure specifications (WPS) that ensure adequate shear strength and ductility would facilitate the wider adoption of magnesium alloy welding in structural applications.

In conclusion, this study demonstrates that AC TIG welding of 2.0 mm AZ31B magnesium alloy thin sheet is technically feasible but requires careful process optimization to achieve acceptable mechanical properties. The low shear strength and brittle fracture mode identified in this study highlight the need for further process development and quality improvement.