Performance Comparison of FSW and MIG Joints in AZ31B Magnesium Alloy

Literature Overview

This 2014 study by Xia Luosheng and Zhu Shuhong, published in Ordnance Materials and Science & Engineering, compares the properties of friction stir welding (FSW) and metal inert gas (MIG) welding joints in AZ31B magnesium alloy. Funded by the Hunan Provincial Department of Education and Zhangjiajie Aviation Vocational Technical College, this research addresses a growing engineering challenge: the reliable joining of lightweight magnesium alloys for aerospace and defense applications.

Core Technical Content

AZ31B is a widely used wrought magnesium alloy containing approximately 3.0% Al, 1.0% Zn, with trace Mn, Si, and Fe. It offers excellent specific strength (approximately 300 MPa/1.77 g/cm³) and good formability but is inherently susceptible to corrosion and oxidation. The choice of joining method profoundly affects the performance of AZ31B components.

FSW Characteristics in AZ31B

FSW of AZ31B is performed at temperatures between 300°C and 400°C, which is below the melting point (approximately 450°C) but above the recrystallization temperature. Key features include:

• Stir zone (SZ): Fine equiaxed grains (1–5 μm) from dynamic recrystallization
• Thermomechanically affected zone (TMAZ): Elongated grains with partial recrystallization
• Heat affected zone (HAZ): Limited grain growth, no phase transformation
• Base metal: Unaffected, retains original temper

The mechanical properties of FSW joints typically achieve 80–90% of the base metal strength, with:

• Tensile strength: 220–260 MPa (base metal: 260–280 MPa)
• Elongation: 8–12%
• No porosity, no cracking

MIG Welding Characteristics in AZ31B

MIG welding of AZ31B presents significant challenges due to:

• Low melting point and high vapor pressure of magnesium
• Extreme sensitivity to atmospheric contamination
• Poor fluidity of molten magnesium
• High tendency for hot cracking

Typical MIG parameters for AZ31B:

• Shielding gas: 100% Ar or Ar + 5% He
• Wire: AZ91 or ER50S (Al-Zn based)
• Current: 120–180 A
• Voltage: 18–24 V
• Travel speed: 300–500 mm/min

The MIG weld metal typically shows:

• Coarse dendritic structure with Mg₁₇Al₁₂ intermetallics
• Porosity (1–3% volume fraction) from gas entrapment
• Reduced tensile strength: 150–190 MPa (55–70% of base metal)
• Reduced elongation: 4–7%

Property Comparison Summary

Defect Analysis and Countermeasures

FSW Defects

1. Tunnel defect: Occurs when the pin does not fully penetrate the material

• Countermeasure: Increase downward force, optimize pin geometry

1. Kissing defect: Partial penetration with void formation

• Countermeasure: Increase rotation speed, adjust tilt angle

1. Burr: Material extrusion at the top surface

• Countermeasure: Use backing plate, optimize shoulder diameter

MIG Defects

1. Porosity: Gas entrapment from inadequate shielding

• Countermeasure: Use high-flow shielding gas, minimize wind exposure, use flux-cored wire

1. Hot cracking: Solidification cracking due to Mg₁₇Al₁₂ intermetallics

• Countermeasure: Use ER50S wire with reduced Zn, preheat to 100–150°C

1. Lack of fusion: Poor wetting of magnesium surface

• Countermeasure: Clean oxide layer, use AC welding, increase heat input

Engineering Practice Integration

For aerospace applications where AZ31B is used in lightweight structural components, FSW is strongly preferred for primary structural joints. The superior mechanical properties, absence of defects, and excellent fatigue performance make FSW the process of choice for:

• Aircraft fuselage panels
• Rotor blade attachments
• Satellite structural components
• Military vehicle armor panels

MIG welding remains applicable for:

• Repair applications
• Field welding where FSW equipment is unavailable
• Joining of dissimilar magnesium alloys
• Thick-section welding where FSW penetration is limited

Study Insights and Implications

This research contributes to the growing body of evidence supporting FSW as the preferred joining method for magnesium alloys. The consistent superiority of FSW in terms of mechanical properties, corrosion resistance, and defect-free welds validates the investment in FSW equipment for magnesium alloy manufacturing.

From a cladding and bimetal perspective, this study has direct relevance to magnesium alloy overlay applications. Any cladding process that involves melting the magnesium surface (such as MIG or TIG overlay) will inevitably introduce defects and reduce corrosion resistance. Solid-state processes like friction stir welding offer a pathway to produce high-quality magnesium alloy cladding without the thermal degradation inherent to fusion processes.

The study also underscores the importance of material-specific process selection. The same joining method may produce vastly different results in different alloy systems. Engineers must evaluate the metallurgical compatibility of the joining process with the specific alloy composition, temper, and intended service conditions.