Zhuojin Pipe Fitting Co., LtdEffect of Filler Material on Microstructure and Mechanical Properties of AZ31B Magnesium Alloy TIG Welds
Literature Overview
Published in Armaments Material Science and Engineering (2008, Vol. 31, No. 6, pp. 41-44), this paper by Tan Bing, Chen Donggao, Ming Zhu, and Wang Youqi from the Ningbo Branch of the China Ordnance Science and Technology Institute investigates the effect of filler material on the microstructure and mechanical properties of TIG welded AZ31B magnesium alloy joints. The study uses two filler wires, AZ31 and AZ61, to weld 10 mm thick AZ31B magnesium alloy plates and compares the resulting weld joint characteristics.
Material Background
The AZ31B magnesium alloy is a widely used wrought magnesium alloy with approximately 3.0 wt% aluminum and 1.0 wt% zinc. It is valued for its excellent formability, good corrosion resistance, and moderate strength, making it suitable for automotive, aerospace, and consumer electronics applications. The AZ61 alloy contains approximately 6.0 wt% aluminum and 1.0 wt% zinc, providing higher strength but reduced formability.
Microstructure Analysis
The study employs optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to characterize the weld joint microstructure. The key findings are summarized below:
| Characteristic | AZ31 Filler Wire | AZ61 Filler Wire |
|---|---|---|
| Weld surface appearance | Excellent, no visible defects | Good, slightly less uniform |
| Weld metal microstructure | Fine equiaxed grains | Coarser grains with Al12Mg17 intermetallic phase at grain boundaries |
| HAZ microstructure | Recrystallized grains, no significant phase change | Recrystallized grains with possible Al12Mg17 precipitation |
| Phase composition | Primarily α-Mg solid solution | α-Mg solid solution with Al12Mg17 intermetallic phase |
| Hardness distribution | Maximum hardness in base metal region | Maximum hardness in weld metal region |
Mechanical Property Comparison
The mechanical properties of the weld joints are significantly influenced by the filler material choice:
| Property | AZ31 Filler Wire | AZ61 Filler Wire |
|---|---|---|
| Tensile strength | Baseline value | Approximately 20% higher than AZ31 filler |
| Elongation | Higher elongation | Lower elongation due to harder, more brittle weld metal |
| Hardness | Maximum in base metal | Maximum in weld metal |
| Corrosion resistance | Better due to absence of intermetallic phases | Potentially reduced due to Al12Mg17 phase |
Engineering Practice Implications
The choice of filler material for magnesium alloy welding is a critical decision that affects the mechanical properties, corrosion resistance, and service life of the weld joint. The study demonstrates that the AZ61 filler wire provides higher tensile strength due to the formation of the Al12Mg17 intermetallic phase, which acts as a strengthening phase in the weld metal. However, this comes at the cost of reduced ductility and potentially reduced corrosion resistance.
For applications where high strength is the primary requirement, the AZ61 filler wire may be preferred. However, for applications where corrosion resistance and ductility are more important, the AZ31 filler wire is the better choice. The absence of the Al12Mg17 intermetallic phase in the AZ31 filler weld metal reduces the risk of galvanic corrosion between the intermetallic phase and the matrix, which is a common failure mode in magnesium alloy welds.
Welding Process Considerations
The TIG welding of magnesium alloy requires careful control of the welding parameters to minimize porosity and oxidation. The key process parameters include:
- Welding current: 150-250 A for 10 mm thick plates, depending on the number of passes
- Arc voltage: 16-20 V, controlled by torch angle and travel speed
- Travel speed: 200-400 mm/min, adjusted based on current and desired penetration
- Shielding gas: Argon or helium, with a flow rate of 15-25 L/min
- Interpass temperature: Controlled below 100°C to prevent excessive grain growth
The study's emphasis on microstructure characterization is important because the microstructure directly influences the mechanical properties and corrosion behavior of the weld joint. The formation of the Al12Mg17 intermetallic phase in the AZ61 filler weld metal is a key finding, as this phase can act as a site for corrosion initiation and crack propagation.
Key Reflections
The study highlights the trade-off between strength and ductility in magnesium alloy welding. The AZ61 filler wire provides higher strength due to the formation of the Al12Mg17 intermetallic phase, but this comes at the cost of reduced ductility and potentially reduced corrosion resistance. The AZ31 filler wire provides a more balanced combination of properties, with good strength, ductility, and corrosion resistance.
The study also emphasizes the importance of microstructure characterization in understanding the mechanical properties of weld joints. The use of OM, SEM, and XRD provides a comprehensive view of the weld joint microstructure, allowing for the identification of phases, grain boundaries, and other microstructural features that influence mechanical behavior.
Conclusion
This paper provides valuable insights into the effect of filler material on the microstructure and mechanical properties of AZ31B magnesium alloy TIG welds. The study demonstrates that the choice of filler material is a critical decision that affects the weld joint properties, with the AZ61 filler providing higher strength but reduced ductility and potentially reduced corrosion resistance. The AZ31 filler provides a more balanced combination of properties, making it the preferred choice for most applications. The study's emphasis on microstructure characterization and the identification of the Al12Mg17 intermetallic phase provides valuable guidance for engineers working on magnesium alloy welding applications.