Mechanical Properties and Microstructure Comparison of AZ31B Magnesium Alloy FSW and MIG Welds

Literature Overview

This research by Xia Luosheng and Zhu Shuhong from Zhangjiajie Aviation Industrial Vocational and Technical College, published in Ordnance Materials Science and Engineering (2014, Vol. 37, No. 1, pp. 43-46), compares the mechanical properties, microstructural characteristics, and fracture morphology of AZ31B deformed magnesium alloy joints produced by friction stir welding (FSW) and MIG welding. The study was supported by the Hunan Provincial Department of Education (Grant 11C1290) and a university research project (Grant ZHKT2013-006). Magnesium alloys are increasingly used in aerospace, automotive, and defense applications due to their exceptional specific strength, making reliable joining technologies critical.

Core Technical Content

Microstructural Comparison

The study revealed fundamentally different microstructural characteristics between the two welding processes:

Mechanical Properties Comparison

The mechanical performance comparison demonstrated clear superiority of FSW joints:

The FSW joint achieving 96.1% of base metal tensile strength is remarkable for a magnesium alloy weld, which typically suffers significant strength loss due to coarse grain formation and precipitate coarsening in the weld zone.

Fracture Morphology Analysis

The fracture surface analysis revealed characteristic differences:

FSW fracture characteristics:

• Fracture angle approximately 45° to tensile axis
• Fracture location in the HAZ (not the stir zone)
• Fracture surface shows dimples of widely varying sizes
• Indicates ductile fracture with some microstructural heterogeneity

MIG fracture characteristics:

• Fracture angle approximately 45° to tensile axis
• Fracture location in the weld zone
• Fracture surface shows dimples and tear ridges
• Indicates mixed ductile-brittle fracture mode

Technical Analysis

Why FSW Outperforms MIG in Magnesium Alloys

The superior performance of FSW for magnesium alloys can be explained through several mechanisms:

1. Solid-state process advantage: FSW does not involve melting, eliminating the risk of hot cracking, porosity, and non-equilibrium solidification that plague arc welding of magnesium alloys.
2. Dynamic recrystallization: The combined action of severe plastic deformation and elevated temperature produces fine, equiaxed recrystallized grains that provide high strength through grain boundary strengthening.
3. Precipitate preservation: The base metal precipitates (β-phase Mg₁₇Al₁₂) are partially preserved during FSW, maintaining precipitation strengthening that would be lost during melting and solidification.
4. Reduced thermal distortion: Lower heat input compared to MIG welding minimizes residual stresses and distortion, contributing to better mechanical performance.

Fracture Location Significance

The observation that FSW fractures occur in the HAZ rather than the stir zone is significant. It indicates that:

• The stir zone (weld zone) is stronger than the HAZ
• The HAZ represents the weakest link, likely due to partial precipitate dissolution and grain growth
• The overall joint strength is limited by the HAZ, not the weld itself

For MIG welds, fracture in the weld zone indicates that the fusion zone is the weakest region, consistent with coarse grain structure and non-equilibrium precipitate distribution.

Engineering Practice Implications

Application Guidelines for Magnesium Alloy Welding

Based on this study, the following guidelines are recommended:

1. Primary recommendation: FSW should be the preferred joining method for AZ31B and similar magnesium alloys where mechanical properties are critical
2. MIG welding limitations: MIG welding of magnesium alloys should be restricted to non-critical applications or where FSW is geometrically impractical
3. Post-weld treatment: For MIG welds, post-weld heat treatment may be necessary to refine the microstructure and improve mechanical properties
4. Design consideration: FSW joints should be designed with the HAZ as the critical region for strength calculations

Quality Control Considerations

For production implementation of FSW on magnesium alloys:

• Monitor welding parameters (rotation speed, travel speed, tilt angle) for consistency
• Perform periodic tensile testing to verify mechanical properties remain within specification
• Conduct metallographic examination to verify stir zone grain refinement
• Inspect fracture surfaces of failed joints to identify failure mechanisms
• Implement torque and thrust force monitoring to detect process anomalies

Key Insights and Reflections

This study provides clear evidence that FSW is superior to MIG welding for magnesium alloy applications where mechanical properties are a primary concern. The 96.1% strength retention achieved by FSW is exceptional and demonstrates the potential of solid-state joining for lightweight structural alloys.

The microstructural analysis reveals the fundamental mechanisms behind the performance difference: fine recrystallized grains in FSW versus coarse solidification grains in MIG. This understanding enables engineers to predict and optimize weld performance for specific applications. The fracture analysis further demonstrates that the HAZ, not the weld zone itself, governs the mechanical performance of FSW joints—a critical insight for design and qualification purposes.

For the growing applications of magnesium alloys in aerospace, automotive, and defense sectors, this study reinforces the importance of selecting appropriate joining technologies that can maintain the excellent mechanical properties of the base material. The relatively modest equipment requirements for FSW compared to other advanced joining methods make it a practical choice for industrial implementation.