CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Preheat on TIG Welding of AZ61 Magnesium Alloy
Literature Overview
Published in 2012 in the International Journal of Minerals, Metallurgy and Materials, this study by Shen and Xu from Chongqing University examines the influence of preheating on the TIG welding of AZ61 magnesium alloy. The research was supported by Chongqing's Key Scientific and Technological Project and the Natural Science Foundation of Chongqing, indicating strong regional industrial motivation—Chongqing being a major center for magnesium alloy production in China. AZ61 is a widely used wrought magnesium alloy with 6% aluminum and 1% zinc, valued for its excellent castability, corrosion resistance among Mg alloys, and good mechanical properties.
Fundamental Challenges in Magnesium Alloy Welding
Magnesium alloys present unique welding challenges that distinguish them from aluminum and steel:
- Extreme oxidation: MgO forms immediately at ambient temperature with a melting point of 2852°C, creating a continuous oxide barrier.
- Low thermal conductivity: Despite being a lightweight metal, Mg alloys have lower thermal conductivity than Al, leading to rapid heat concentration.
- High vapor pressure: At welding temperatures, magnesium evaporates readily, causing arc instability and porosity.
- Hydrogen absorption: Mg readily absorbs hydrogen from the atmosphere and flux, leading to hydrogen-induced porosity and delayed cracking.
- Hot cracking susceptibility: The eutectic temperature of the Mg-Al system is 450°C, creating a wide freezing range that promotes solidification cracking.
Preheating addresses several of these challenges simultaneously. By raising the base metal temperature before welding, preheat reduces thermal gradients, decreases cooling rates, and modifies the weld metal solidification behavior.
Preheat Parameter Analysis
The study systematically varied preheat temperatures and evaluated their effects on weld quality, microstructure, and mechanical properties.
| Preheat Temperature | Weld Penetration | Cracking Tendency | Microhardness | Grain Structure |
|---|---|---|---|---|
| 100°C | Shallow, incomplete | High | 85–110 HV | Fine, dendritic |
| 150°C | Moderate | Moderate | 75–95 HV | Coarsening begins |
| 200°C | Deep, full penetration | Low | 65–80 HV | Coarse, equiaxed |
| 250°C | Excessive | Very low | 55–70 HV | Overheated, coarse |
The optimal preheat range for AZ61 TIG welding was identified as 150–200°C, with 180°C providing the best balance between penetration, cracking resistance, and mechanical properties.
Microstructural Evolution
Preheating fundamentally alters the solidification behavior of the AZ61 weld metal. At low preheat temperatures, the high cooling rate produces fine dendritic structures with interdendritic eutectic phases (β-phase Mg₁₇Al₁₂ and α-Al). While fine structures provide high strength, they also create significant residual stresses and promote solidification cracking.
At optimal preheat (150–200°C), the reduced cooling rate allows:
- More complete liquid film healing at crack tips, reducing hot cracking susceptibility
- Transition from dendritic to more equiaxed grain structure
- Modified eutectic morphology—reduced volume fraction of brittle β-phase
- Improved ductility without excessive strength loss
Engineering Implications for Pressure Vessel Fabrication
Magnesium alloy pressure vessels are emerging in aerospace applications where weight reduction is critical. The preheat requirement has significant implications for fabrication methodology:
- Fixture design: Preheating to 150–200°C requires reliable fixture heating elements or induction heating systems that can maintain temperature during welding.
- Heat input control: Preheat combined with TIG welding creates a specific heat input regime that must be documented in the welding procedure specification (WPS).
- Post-weld treatment: The preheated weld zone may require solution treatment and aging (typically 420°C for 4 hours, followed by 175°C for 8 hours) to achieve optimal mechanical properties.
- Inspection considerations: Preheated welds may show different UT signal characteristics due to altered grain structure, requiring updated acceptance criteria.
Study Insights and Practical Recommendations
The most significant finding is that preheat is not merely a process aid but a metallurgical control parameter that fundamentally determines weldability. The study demonstrates that without adequate preheat, AZ61 cannot be reliably welded by TIG regardless of other parameter optimization. This has profound implications for welding procedure qualification—preheat temperature must be specified as a mandatory variable in the WPS, not as an optional parameter.
For engineering practice, I emphasize that magnesium alloy welding requires a holistic approach: preheat, filler metal selection (typically AZ91 or AZ92 for AZ61), shielding gas composition (high-purity argon, optionally with helium), and post-weld heat treatment must all be coordinated. The study provides the foundational understanding necessary for developing such integrated procedures.