Zhuojin Pipe Fitting Co., LtdDC-PMIG Welding Characteristics of AZ31B Magnesium Alloy
Literature Overview
This paper, published in the Transactions of Nonferrous Metals Society of China (Vol. 23, Issue 2, 2013, pp. 315–322) by Zhang Zhaodong, Liu Liming, and Song Gang from the School of Materials Science and Engineering at Dalian University of Technology and the Liaoning Key Laboratory of Advanced Joining Technology, investigates the welding characteristics of AZ31B magnesium alloy using direct current pulsed metal inert gas (DC-PMIG) welding. The study examines metal transfer behavior, stable welding parameter ranges, microstructure, and mechanical properties of welds produced on 3 mm and 8 mm thick plates using 1.6 mm diameter filler wire. The research is supported by the Changjiang Scholars and Innovative Research Team Program (IRT1008) and the National Natural Science Foundation of China (Grant Nos. 51005035, 51025520).
Material Background and Welding Challenges
AZ31B is one of the most widely used wrought magnesium alloys, containing approximately 3 wt% aluminum and 1 wt% zinc. Magnesium alloys offer excellent specific strength, good corrosion resistance, and excellent electromagnetic shielding properties, making them attractive for lightweight structural applications. However, magnesium alloys present significant welding challenges:
- Low melting point (650°C) and high vapor pressure lead to significant magnesium evaporation during welding.
- High thermal conductivity causes rapid heat dissipation from the weld zone.
- Strong affinity for oxygen and nitrogen leads to oxidation and nitride formation.
- Hydrogen porosity is a common defect due to the high solubility of hydrogen in liquid magnesium.
- Low ductility at elevated temperatures increases the risk of hot cracking.
These challenges make conventional MIG welding of magnesium alloys difficult, and pulsed MIG welding offers advantages through controlled heat input and stable metal transfer.
Metal Transfer Behavior
The study identifies three distinct metal transfer modes in DC-PMIG welding of AZ31B magnesium alloy:
| Transfer Mode | Droplet Diameter | Linear Energy | Spatter | Weld Quality |
|---|---|---|---|---|
| Globular transfer | Large (>1.6 mm) | Low | Large spatter | Poor |
| Projected transfer | 1.6–0.9 mm | 242–271 J/cm | Minimal | Good |
| Spray transfer | 0.9 mm or less | 242–271 J/cm | Minimal | Good |
The transition frequency for stable metal transfer ranges from 30 to 69 Hz. The projected transfer and spray transfer modes produce high-quality weld joints with few spatters, while the globular transfer mode produces large spatter and is unsuitable for quality welding. The optimal linear energy range of 242–271 J/cm corresponds to the transition between projected and spray transfer modes, which provides the best combination of heat input control and metal transfer stability.
Microstructure and Mechanical Properties
The microstructure of the DC-PMIG welds consists of the weld zone, heat-affected zone (HAZ), and base metal. The weld zone typically exhibits a fine-grained dendritic structure, while the HAZ shows grain coarsening due to the thermal cycle. The mechanical properties of the weld beads are reported as 94.2% of the base metal tensile strength, which is an excellent result for magnesium alloy welding.
| Property | Weld Bead | Base Metal | Ratio |
|---|---|---|---|
| Tensile strength | ~94.2% of base metal | Reference | 94.2% |
The high weld strength ratio indicates that the DC-PMIG welding process preserves most of the base metal's mechanical properties. This is significant because magnesium alloys are typically strengthened by solid solution and precipitation hardening, and welding can cause precipitation dissolution and grain coarsening in the HAZ, which reduces strength.
Stable Welding Parameter Range
The study defines the stable welding parameter range for DC-PMIG welding of AZ31B magnesium alloy:
- Linear energy: 242–271 J/cm
- Transition frequency: 30–69 Hz
- Droplet diameter: 0.9–1.6 mm (projected and spray transfer)
- Filler wire diameter: 1.6 mm
- Plate thickness: 3 mm and 8 mm
These parameters provide a practical starting point for welding AZ31B magnesium alloy in industrial applications. The wide frequency range (30–69 Hz) allows flexibility in adjusting the welding process for different joint configurations and production requirements.
Engineering Practice Analysis
For welding engineers working with magnesium alloys, this study provides several practical insights:
- DC-PMIG welding is a viable process for producing high-quality welds in AZ31B magnesium alloy, with weld strength reaching 94.2% of base metal.
- The projected and spray transfer modes should be targeted for production welding, as they produce minimal spatter and good weld quality.
- The linear energy range of 242–271 J/cm provides a narrow but well-defined window for stable welding. Operators must carefully control welding current and speed to remain within this range.
- The transition frequency of 30–69 Hz is relatively low compared to aluminum alloy pulsed MIG welding, which typically operates at higher frequencies. This is consistent with the lower thermal conductivity and different metal transfer dynamics of magnesium alloys.
From a quality control perspective, the following NDT methods should be considered for magnesium alloy welds:
- Visual inspection (VT) for surface defects and spatter
- Dye penetrant testing (PT) for surface-breaking defects
- Ultrasonic testing (UT) for internal porosity and lack of fusion
- X-ray radiography (RT) for volumetric defect detection
The high porosity risk in magnesium alloy welds makes UT and RT particularly important for critical applications.
Key Reflections and Recommendations
The study demonstrates that DC-PMIG welding can produce high-quality welds in AZ31B magnesium alloy, but the stable welding window is relatively narrow and must be carefully controlled. The metal transfer behavior is the key factor determining weld quality, and operators must ensure that the welding parameters are set to achieve projected or spray transfer modes.
For future work, the following areas deserve investigation:
- The effect of shielding gas composition (Ar, He, Ar/He mixtures) on metal transfer and weld quality.
- The effect of welding position (flat, horizontal, vertical, overhead) on metal transfer stability and weld quality.
- Long-term corrosion resistance of the DC-PMIG welds, particularly in chloride-containing environments.
- Fatigue performance of the welds under cyclic loading, which is critical for structural applications.
- The effect of welding parameters on residual stress distribution and distortion.
This study is a valuable contribution to the welding of magnesium alloys, providing quantitative data on metal transfer behavior, stable welding parameters, and mechanical properties. The 94.2% weld-to-base-metal strength ratio is particularly encouraging for structural applications, as it indicates that DC-PMIG welding can preserve most of the base metal's mechanical properties. For engineers working on lightweight magnesium alloy structures, this work provides a solid foundation for process development and quality assurance.