CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Filler Materials on Microstructure and Properties of Magnesium Alloy TIG Welds
Literature Overview
This 2008 paper by Tan Bing, Chen Donggao, Ming Zhu, and Wang Youqi from the Ningbo Branch of the China Ordnance Science Academy, published in Ordnance Materials and Materials Science and Engineering, investigates the influence of different filler wire compositions on the microstructural evolution and mechanical performance of gas tungsten arc welds in magnesium alloys. The work is conducted within the context of ordnance applications where lightweight, high-strength structural components are essential, making magnesium alloys an attractive but technically challenging material system.
Core Technical Content
Magnesium alloys, particularly AZ-series and AZ91-type alloys, are susceptible to several welding-specific degradation mechanisms including oxidation, hydrogen-induced porosity, and microstructural coarsening in the heat-affected zone. The study examines how filler metal selection modulates these phenomena.
Filler Material Comparison
| Filler Wire | Composition | Weld Strength (MPa) | Elongation (%) | Porosity Level |
|---|---|---|---|---|
| ER50A | Mg-0.5Al-0.5Mn | 165–175 | 4–6 | Moderate |
| AZ91 matching wire | Mg-9Al-1Zn | 150–160 | 3–5 | Low |
| ZK60 wire | Mg-6Zn-0.5Zr | 170–180 | 5–7 | Low |
| Pure Mg wire | Mg-99.9% | 140–150 | 6–8 | High |
Microstructural Observations
The authors report distinct microstructural differences across the weld cross-section:
- Weld metal: The AZ91-matching filler produces a dendritic Mg + β (Mg₁₇Al₁₂) eutectic structure similar to the base metal, while the ZK60 filler yields a finer Mg + MgZn₂ phase distribution with reduced intermetallic volume fraction.
- Heat-affected zone: A narrow recrystallized zone adjacent to the fusion line exhibits grain coarsening of 50–80% relative to the base metal, regardless of filler selection. However, the ZK60 filler reduces the HAZ width by approximately 15% due to lower overall heat input requirements.
- Interface region: The base metal filler interface shows a diffusion zone of 20–50 μm where aluminum and zinc redistribute, creating a composition gradient that influences local mechanical properties.
Mechanical Property Analysis
Tensile tests conducted on both transverse and longitudinal sections reveal that the weld joint efficiency ranges from 78% to 92% depending on filler selection. The ZK60 filler achieves the highest joint efficiency (91–92%), while pure magnesium wire produces the lowest (78–82%). The improved performance with ZK60 is attributed to its higher melting point, which promotes more stable arc characteristics and reduced oxidation, and its zinc-rich composition, which suppresses the formation of brittle Mg-Al intermetallics at the fusion boundary.
Defect Mechanisms and Countermeasures
The study identifies three primary defect categories:
- Hydrogen porosity: Magnesium readily absorbs hydrogen from moisture in shielding gas or on the base metal surface. Countermeasures include strict gas drying (dew point below −60 °C), thorough surface cleaning, and the use of rare earth elements in the filler to promote hydrogen recombination.
- Oxidation and surface roughness: The MgO layer formed during welding degrades surface quality. Application of flux-free welding with high-purity argon at elevated flow rates (15–20 L/min) mitigates this issue.
- Hot cracking: Occurs in the weld metal when the solidification range is too wide. The ZK60 filler's narrower solidification range (approximately 180 °C) compared to AZ91 (approximately 220 °C) significantly reduces hot crack susceptibility.
Study Insights and Engineering Implications
This work provides a clear demonstration that filler metal selection is not merely a metallurgical exercise but a decisive factor in determining weldability, joint strength, and service reliability of magnesium alloy structures. For pressure vessel and containment applications involving magnesium alloy components, the findings suggest that zinc-containing fillers offer superior performance despite the potential long-term corrosion concerns associated with zinc-rich microstructures. The methodology employed — systematic variation of one parameter (filler composition) while maintaining constant welding conditions — exemplifies the controlled experimental approach necessary for process qualification. The study's emphasis on correlating microstructural features with macroscopic mechanical properties establishes a foundation for future work on post-weld heat treatment optimization and long-term creep performance evaluation.