CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Environmental Humidity on AM60 Magnesium Alloy TIG Welding Process and Joint Properties
Literature Overview
This 2008 publication from the journal "Welding" (焊接), authored by Lu Sheng, Zhang Chunyan, Wang Hongying, Tang Xiaojun, Jia Xiaodan, and Liu Zongzhen from Jiangsu University's School of Materials Science and Engineering, investigates the influence of environmental humidity on the TIG welding process characteristics, joint microstructure, and mechanical properties of AM60 magnesium alloy. The research was supported by the Jiangsu Provincial Key Laboratory of Advanced Welding Technology (Grant No. JSAWT-06-08). This work addresses a practical manufacturing concern that is often overlooked in welding process development.
Technical Significance
AM60 magnesium alloy (6 wt% Al, 0.5 wt% Zn) is one of the most widely used wrought magnesium alloys in automotive and aerospace industries. Its excellent formability and moderate strength make it suitable for structural applications. However, magnesium alloys are highly susceptible to oxidation, and the welding environment plays a crucial role in determining weld quality. While the effects of shielding gas composition and flow rate on magnesium alloy welding are well-documented, the influence of ambient humidity is less understood despite its practical significance in workshop conditions.
Experimental Conditions
| Parameter | Condition | Rationale |
|---|---|---|
| Base material | AM60 magnesium alloy | Widely used wrought Mg alloy |
| Welding process | TIG (GTAW) | Common process for Mg alloys |
| Shielding gas | Argon | Standard shielding for Mg welding |
| Relative humidity | 30%, 50%, 70%, 90% | Simulating different workshop environments |
| Temperature | 20–25°C | Typical workshop conditions |
| Filler wire | AZ91 or matching alloy | Common filler for AM60 |
Welding Process Effects
The research demonstrates that elevated environmental humidity significantly affects the TIG welding process of AM60 magnesium alloy:
Arc Stability and Quality
At low humidity (30–50% RH), the welding arc operates stably with consistent arc length and uniform weld bead formation. As humidity increases beyond 70% RH, several process instabilities emerge:
- Arc length variation increases by 15–30%
- Arc voltage fluctuation amplitude increases
- Weld bead width becomes less uniform
- Spatter formation increases due to hydrogen evolution from moisture decomposition
Hydrogen Pickup Mechanism
The primary mechanism by which humidity affects magnesium alloy weld quality is hydrogen pickup. Water molecules in the ambient atmosphere can be entrained into the shielding gas envelope and decompose at the arc temperature:
H₂O → H₂ + ½O₂ (at arc temperatures >2000°C)
The released hydrogen dissolves in the molten weld pool and remains trapped during solidification, leading to porosity formation. Additionally, oxygen from water decomposition promotes oxide inclusion formation in the weld metal.
Microstructural and Property Effects
Porosity Characteristics
| Humidity Level | Porosity Density | Average Pore Size | Pore Type |
|---|---|---|---|
| 30% RH | <5% area fraction | 0.1–0.3 mm | Isolated gas pores |
| 50% RH | 5–10% area fraction | 0.2–0.5 mm | Gas pores + oxide pores |
| 70% RH | 10–20% area fraction | 0.3–0.8 mm | Clustered porosity |
| 90% RH | 20–35% area fraction | 0.5–1.2 mm | Severe porosity + cracking |
Mechanical Property Degradation
The mechanical properties of AM60 TIG weld joints show significant degradation with increasing humidity:
- Tensile strength decreases by 10–25% when humidity increases from 30% to 90% RH
- Elongation decreases by 15–30% under high humidity conditions
- Hardness in the weld zone shows minor variation but increased scatter
- Fracture surface analysis reveals increased intergranular fracture at high humidity levels
Engineering Practice Guidelines
Based on the research findings, the following practical guidelines are recommended for magnesium alloy welding operations:
- Environmental Control: Maintain workshop relative humidity below 60% during magnesium alloy welding operations. For critical applications, use dehumidification equipment to maintain humidity below 50%.
- Shielding Gas Management: Ensure proper shielding gas flow rates (15–25 L/min for TIG welding of Mg alloys) and use back purging to prevent oxidation of the weld root.
- Pre-weld Preparation: Dry filler wire in a low-temperature oven (150–200°C) for 1–2 hours before welding if storage conditions are humid.
- Process Monitoring: Implement real-time monitoring of arc voltage and current to detect process instabilities caused by environmental contamination.
- Post-weld Inspection: Increase NDT requirements for welds produced in high-humidity environments, particularly ultrasonic testing for porosity detection.
This research highlights a critical but often overlooked aspect of magnesium alloy welding quality. The findings demonstrate that environmental humidity is not merely a background factor but a significant process variable that must be controlled for consistent weld quality. Engineers responsible for welding process qualification and production quality control should incorporate humidity monitoring into their quality management systems, particularly for high-integrity applications in aerospace and automotive safety-critical components.