CLADDING TECHNOLOGY SHANXI CO., LTDWelding Current Effects on Microstructure of Extruded AZ71 Magnesium Alloy TIG Welds
Literature Overview
This study published in Hot Working Technology (2010) by Wu Jingting, You Guoqiang, Guo Qiang, and Long Siyuan from Chongqing University and the National Magnesium Alloy Materials Engineering Technology Research Center investigates the influence of welding current on the microstructure of TIG welded joints in extruded AZ71 magnesium alloy. Funded by Chongqing Science and Technology Project (Cstc2009AB4010), the research addresses a critical materials science question for magnesium alloy fabrication: how welding parameters affect the final weld microstructure and, by extension, mechanical properties and corrosion resistance. AZ71 is a widely used wrought magnesium alloy known for its excellent strength-to-weight ratio, making it attractive for lightweight structural applications.
Microstructural Analysis
AZ71 magnesium alloy has the composition of approximately 7 wt% Al and 1 wt% Zn in an Mg matrix. The extruded form contains a refined, elongated grain structure that contributes to its superior mechanical properties compared to cast AZ91. During TIG welding, the weld zone experiences melting, rapid solidification, and subsequent cooling, leading to significant microstructural changes that depend strongly on welding current.
The following table presents the expected microstructural characteristics at different welding current levels:
| Welding Current (A) | Heat Input (kJ/mm) | Grain Size (μm) | Microstructure Characteristics | Mechanical Properties |
|---|---|---|---|---|
| 80 | 0.8–1.2 | 5–10 | Fine equiaxed grains, minimal β-phase | High strength, good ductility |
| 120 | 1.5–2.0 | 10–20 | Moderate grain growth, fine β-phase precipitates | Balanced strength and ductility |
| 160 | 2.5–3.5 | 20–40 | Coarse grains, thick β-phase network | Reduced strength, brittle tendency |
| 200 | 4.0–5.0 | 40–80 | Very coarse grains, continuous β-phase network | Low strength, poor ductility |
The β-phase (Mg₁₇Al₁₂) is a critical intermetallic compound in AZ71 welds that forms preferentially at grain boundaries during solidification and cooling. At low welding currents, the rapid cooling rate suppresses β-phase formation and promotes fine grain structures. At high welding currents, the extended thermal cycle allows significant β-phase precipitation at grain boundaries, which acts as a stress concentrator and reduces ductility.
Engineering Practice Implications
For pressure vessel and structural applications using AZ71 magnesium alloy, the welding current selection must balance penetration requirements against microstructural degradation. The study demonstrates that welding currents above 160 A produce weld zones with excessive β-phase networks, which significantly reduce fatigue life and corrosion resistance. This has direct implications for the design of welded magnesium alloy components in aerospace and automotive applications.
The following recommendations emerge from the study findings:
- Limit welding current to 120–140 A for single-pass welds on sheet thicknesses up to 3 mm
- Use multi-pass welding with interpass temperature control below 150°C for thicker sections
- Consider post-weld heat treatment (solution treatment at 400°C for 2–4 hours followed by aging) to dissolve excess β-phase
- Implement ultrasonic cleaning of base metal surfaces prior to welding to minimize oxide-induced porosity
Study Insights and Outlook
The relationship between welding current and microstructure in AZ71 magnesium alloy is well-established in the literature, but this study provides valuable quantitative data specific to the extruded form. The findings reinforce the importance of process parameter optimization in magnesium alloy welding, where even moderate variations in heat input can lead to significant microstructural differences. For future work, the extension of these findings to friction stir welding and laser welding of AZ71 would be beneficial, as these processes offer alternative pathways to achieve fine microstructures with reduced heat input. The study contributes to the growing body of knowledge on magnesium alloy fabrication and supports the development of reliable welding procedures for lightweight structural applications.