CLADDING TECHNOLOGY SHANXI CO., LTDWeldability of AZ91D Magnesium Alloy with Pure Aluminum Wire MIG Welding
Literature Overview
This 2018 study from North University of China (School of Materials Science and Engineering) investigates the MIG welding processability of AZ91D magnesium alloy using pure aluminum welding wire. Published in Casting Technology, the research addresses a challenging metallurgical combination where significant differences in melting point, thermal conductivity, and metallurgical compatibility create numerous processing difficulties.
Core Technical Content
AZ91D magnesium alloy (9% Al, 1% Zn, with Mn and rare earth additions) is a widely used die-cast magnesium alloy. The selection of pure aluminum wire as the filler material creates an interesting metallurgical challenge:
| Property | AZ91D (Base Metal) | Pure Aluminum (Filler) | Implication |
|---|---|---|---|
| Melting Point | 481°C | 660°C | Filler melts after base |
| Thermal Conductivity | 72 W/m·K | 237 W/m·K | Rapid heat dissipation |
| Density | 1.81 g/cm³ | 2.70 g/cm³ | Pool flow dynamics |
| Oxide Film | MgO (high melting) | Al₂O₃ (very high melting) | Both require arc disruption |
| Coefficient of Thermal Expansion | 26 × 10⁻⁶/K | 23.1 × 10⁻⁶/K | Moderate mismatch |
The welding process parameters investigated typically include:
- Welding current: 80–150 A
- Voltage: 14–20 V
- Travel speed: 200–500 mm/min
- Shielding gas: Ar or Ar/CO₂ mixtures
- Wire diameter: 1.0–1.2 mm
Interpretation of Technical Points
The primary metallurgical concern is the formation of brittle intermetallic compounds at the weld zone. The aluminum-magnesium system forms several intermetallic phases including Al₃Mg₂, Al₈Mg₅, and Al₁₂Mg₁₇, each with varying degrees of brittleness. The cooling rate from welding determines which phases form and their morphology within the solidified weld metal.
The oxide film challenge is compounded by the use of aluminum filler wire on a magnesium substrate. Both materials form tenacious oxide films, but the aluminum oxide (Al₂O₃, melting point 2054°C) is particularly refractory. The welding arc must simultaneously disrupt both oxide films to achieve proper fusion.
The high thermal conductivity of the aluminum wire, combined with the moderate conductivity of the magnesium substrate, creates a complex heat flow pattern that affects weld pool shape, penetration profile, and solidification structure.
Application to Lightweight Bimetal Structures
While this specific research focuses on magnesium alloy welding rather than traditional cladding applications, the principles are directly relevant to lightweight bimetal component fabrication. In aerospace and automotive applications, magnesium-aluminum bimetal joints are increasingly important for weight reduction.
The understanding of intermetallic formation and growth kinetics developed in this research can inform the design of dissimilar metal joints where controlled interfacial reactions are acceptable or even beneficial. For pressure vessel applications involving lightweight construction, similar metallurgical considerations apply when joining dissimilar materials.
Key Reflections and Engineering Practice
The research highlights that filler material selection is not merely about matching base metal composition but involves a complex optimization of metallurgical compatibility, processability, and final joint properties. In cladding applications, this principle is equally important: the selection of overlay material must consider not only corrosion resistance but also the metallurgical interaction with the substrate.
The process parameter windows identified for this challenging combination provide valuable insights for similar difficult-to-weld material pairs. The methodology of systematic parameter variation with comprehensive microstructural and mechanical characterization remains the gold standard for welding process development.
The study reinforces that successful welding of dissimilar materials requires understanding of both the process physics and the materials science, with neither discipline alone providing sufficient guidance for process optimization.