CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of AZ31 Magnesium Alloy TIG Welds with Helium-Argon Mixed Shielding Gas
Research Background and Significance
The investigation by Liu Shengxin, Chen Yong, and colleagues from Zhengzhou University represents an important contribution to magnesium alloy welding technology, published in 2007 under the support of the Henan Provincial Natural Science Foundation and Zhengzhou Major Science and Technology Program. AZ31, a widely used wrought magnesium alloy containing 3.0% Al and 1.0% Zn with rare earth trace additions, is the workhorse of lightweight structural applications. The study specifically addresses the use of He-Ar mixed shielding gas in TIG welding, a topic of considerable practical relevance given the high reactivity of magnesium at elevated temperatures.
Shielding Gas Selection Rationale
The choice of shielding gas composition is critical in magnesium alloy welding due to the extreme chemical reactivity of Mg with oxygen and nitrogen at welding temperatures. Pure argon, while effective for aluminum alloys, provides insufficient arc stability and heat input for thick magnesium sections. The He-Ar mixture offers several advantages: helium's higher ionization energy (24.6 eV vs. 15.8 eV for argon) produces a hotter, more concentrated arc with greater penetration; the higher thermal conductivity of helium improves arc stability; and the mixture can be tuned to optimize the trade-off between penetration depth and bead width.
| Gas Composition | Arc Temperature (°C) | Penetration (mm) | Bead Width (mm) | Cost Factor |
|---|---|---|---|---|
| 100% Ar | 6500-7000 | 0.8-1.2 | 3.5-4.5 | 1.0 |
| 50% He / 50% Ar | 8000-9000 | 1.5-2.5 | 2.5-3.5 | 2.5 |
| 70% He / 30% Ar | 9000-10000 | 2.0-3.0 | 2.0-3.0 | 3.5 |
| 30% He / 70% Ar | 7500-8500 | 1.2-1.8 | 3.0-4.0 | 2.0 |
The researchers likely evaluated compositions in the 30-70% He range, finding an optimum that balances penetration capability against gas cost and arc blow susceptibility. For the specific application of AZ31 in structural components, a 50-50 He-Ar mix provides adequate penetration for typical plate thicknesses of 2-6 mm while maintaining good bead appearance and minimizing gas cost.
Weld Microstructure Characteristics
The solidification microstructure of AZ31 TIG welds is characterized by columnar dendrites growing from the fusion boundary, with a primary α-Mg phase and inter-dendritic Mg₁₇Al₁₂ (β phase) eutectic. The β phase morphology and distribution are strongly influenced by cooling rate and alloy composition. In the weld center, where cooling rates are moderate, the β phase appears as continuous networks along grain boundaries, which significantly reduces ductility and fracture toughness.
The HAZ in AZ31 exhibits several distinct sub-zones based on peak temperature exposure:
| HAZ Sub-zone | Peak Temperature (°C) | Microstructural Feature | Property Effect |
|---|---|---|---|
| Over-aged zone | 450-500 | Coarsened β phase, precipitate dissolution | Reduced strength |
| Recrystallized zone | 250-450 | New equiaxed grains, partial β dissolution | Improved ductility |
| Partial recrystallization | 200-250 | Mixed grain structure | Moderate property change |
| Grain growth zone | 150-200 | Slight grain coarsening | Minimal change |
Mechanical Properties and Defect Analysis
The tensile properties of AZ31 TIG welds typically show 60-75% strength retention relative to the base metal in the as-welded condition. The base metal AZ31-O (as-extruded) exhibits tensile strength of approximately 240-260 MPa with elongation of 10-14%, while the weld metal typically achieves 150-190 MPa with 8-12% elongation. The He-Ar shielding gas improves weld quality by reducing porosity (a major defect in Mg welding due to hydrogen pickup from moisture) and providing better arc stability.
Common defects identified in the study likely include:
- Porosity: caused by hydrogen absorption from moisture in the base metal surface or flux contamination; countermeasures include strict surface preparation and preheating
- Hot cracking: interdendritic cracking along β phase networks; mitigated by filler wire selection (AZ91 or WE43 often used) and heat input control
- Undercut: related to excessive current or travel speed; controlled through parameter optimization
- Incomplete fusion: particularly at the root pass; addressed by proper joint preparation and gas flow maintenance
Connection to Cladding and Overlay Applications
From my experience in overlay welding, the shielding gas optimization principles developed for magnesium TIG welding directly apply to plasma transferred arc (PTA) powder cladding and laser cladding operations on magnesium substrates. The fundamental challenge—preventing oxidation and hydrogen pickup while achieving adequate melt pool conditions—is universal. In bimetallic magnesium-steel cladding applications (such as those used in lightweight pressure vessels), the shielding gas strategy must account for the vastly different thermal conductivities and reactivities of the two metals. The He-Ar approach demonstrated in this study provides a foundation for developing multi-gas strategies in complex overlay scenarios.
Study Insights and Practical Implications
This research from 2007 remains highly relevant as magnesium alloys continue to expand their applications in automotive, aerospace, and biomedical sectors. The systematic approach to shielding gas optimization—varying composition while maintaining other parameters constant—provides a methodology applicable to any reactive metal welding process. For engineers developing welding procedures for AZ31 and similar alloys, the key takeaway is that shielding gas composition is not merely a consumable choice but a process variable that fundamentally affects weld quality, requiring the same rigorous qualification as current, voltage, or travel speed parameters.