CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of AZ71 Magnesium Alloy TIG Welded Joints
Literature Overview and Research Background
This study by Wu Jingtang, You Guoqiang, Guo Qiang, and Long Siyuan from Chongqing University (2011), supported by the Chongqing Science and Technology Key Project (Cstc2009AB4010), investigates the microstructural evolution and mechanical performance of AZ71 magnesium alloy welded joints fabricated by gas tungsten arc welding (GTAW/TIG). AZ71, belonging to the Mg-Al-Zn system, is an advanced die-cast magnesium alloy with elevated strength compared to conventional AZ91D, making it increasingly attractive for lightweight structural applications in automotive and aerospace sectors. The research addresses a critical gap in understanding how TIG welding parameters influence the thermally affected zone (TAZ), weld nugget, and overall joint integrity of high-aluminum magnesium alloys, which are notoriously susceptible to hot cracking and porosity during fusion welding.
Core Technical Points and Microstructural Analysis
The AZ71 alloy contains approximately 7 wt% Al and 1 wt% Zn, which significantly influences solidification behavior and phase formation during welding. During TIG welding, the rapid heating and cooling rates produce a complex microstructural gradient across the weld cross-section. The weld nugget typically exhibits a coarse dendritic structure composed of primary alpha-Mg phase and eutectic Mg17Al12 intermetallic compounds at the interdendritic regions. The TAZ undergoes partial recrystallization, grain coarsening, and dissolution or precipitation of secondary phases depending on peak temperature exposure.
A critical finding from this research is that the TIG welding process, with its relatively low heat input compared to MIG or plasma arc welding, produces a narrower TAZ but still induces significant grain coarsening in the heat-affected regions. The cooling rate in the TAZ of AZ71 can range from 5 to 50 K/s depending on heat input, which directly governs the morphology and distribution of Mg17Al12 phases. When cooling rates are insufficient, excessive intermetallic precipitation occurs at grain boundaries, severely degrading ductility and impact toughness.
| Parameter | Typical Range for AZ71 TIG Welding | Effect on Joint Quality |
|---|---|---|
| Welding current | 80–150 A | Controls heat input and penetration depth |
| Welding speed | 200–600 mm/min | Governs cooling rate and TAZ width |
| Shielding gas flow | 10–20 L/min (Ar) | Prevents oxidation and Mg vaporization |
| Heat input | 0.5–2.5 kJ/mm | Determines microstructural gradient severity |
| Interpass temperature | <150°C | Prevents over-aging and grain growth |
| Backing gas | Ar or He (10–15 L/min) | Protects root from oxidation |
Mechanical Property Assessment and Engineering Implications
The mechanical properties of AZ71 TIG welded joints reveal a characteristic strength-ductility trade-off. The weld nugget typically exhibits tensile strength 10–20% lower than the base metal due to coarse grain structure and increased volume fraction of brittle Mg17Al12 phases. The TAZ often represents the weakest region, with ductility reductions of 30–50% compared to the parent material. Impact energy in the TAZ can drop to 5–15 J (Charpy V-notch) versus 40–60 J for the base metal, indicating severe embrittlement.
From a pressure vessel and cladding engineering perspective, this research carries important implications for magnesium alloy composite structures. While magnesium alloys are not commonly used for pressure vessel applications due to their limited elevated-temperature performance and corrosion susceptibility, the microstructural analysis methodology and understanding of Mg-Al intermetallic phase behavior are transferable to understanding similar intermetallic formation in steel-stainless steel clad plate welds. The Mg17Al12 intermetallic, being a brittle intermetallic compound, serves as an analog for understanding the role of Fe-Cr intermetallics (such as FeCr, Fe2Cr, and sigma phase) at the clad-base metal interface in stainless steel/carbon steel clad plate welds.
Defect Analysis and Process Optimization
Common defects identified in AZ71 TIG welded joints include:
- Hot cracking: Caused by the low melting point of Mg17Al12 eutectic (449°C), which remains liquid at the grain boundaries during solidification, creating crack paths under tensile stresses.
- Porosity: Magnesium has extremely low hydrogen solubility and high vapor pressure, leading to gas porosity when shielding is inadequate.
- Undercut: Results from excessive heat input or inappropriate torch angle, particularly problematic in thin-section AZ71 components.
- Incomplete fusion: Occurs at low heat input settings, especially in multi-pass welds where interpass temperature control is critical.
The study demonstrates that optimizing the heat input to the range of 0.8–1.5 kJ/mm provides the best compromise between penetration quality and microstructural refinement. Higher heat inputs promote grain coarsening and excessive intermetallic precipitation, while lower heat inputs result in incomplete fusion and reduced weld bead continuity.
Study Insights and Connections to Cladding Practice
Reflecting on this research from the perspective of bimetal product manufacturing, several transferable lessons emerge. First, the concept of microstructural gradient control through heat input management is directly applicable to weld overlay cladding operations. In stainless steel overlay cladding on carbon steel pressure vessels, controlling the heat input per pass is equally critical to prevent excessive dilution and undesirable phase formation at the clad-base metal interface. Second, the importance of shielding gas integrity cannot be overstated — just as magnesium oxidizes rapidly in air, nickel-based overlay cladding materials (Inconel 625, Hastelloy C276) require meticulous gas protection to prevent contamination and oxidation that degrades corrosion resistance.
The research also highlights the significance of understanding intermetallic phase formation kinetics. In cladding applications, the formation of brittle intermetallic layers at the dissimilar metal interface (such as Fe-Ni intermetallics in steel-Inconel 625 weld overlay joints) follows analogous thermodynamic and kinetic principles to the Mg17Al12 formation in AZ71 welds. Understanding these mechanisms enables better prediction and control of bond strength and long-term service reliability in bimetal pressure vessel components.
This study, while focused on lightweight structural alloys, provides valuable methodological frameworks and conceptual understanding that enrich the broader welding metallurgy knowledge base applicable to clad plate fabrication, weld overlay engineering, and bimetal pressure vessel manufacturing.