CLADDING TECHNOLOGY SHANXI CO., LTDDiffusion Behavior Analysis of Magnesium-Aluminum Dissimilar Material TIG Welded Joints
Literature Overview
This study, published in 2005 in the Transactions of the Welding Institute of China by researchers from the State Key Laboratory of Advanced Materials Processing and Simulation at Dalian University of Technology, investigates the diffusion behavior at the interface of magnesium-aluminum dissimilar metal TIG welded joints. The research was supported by the National "10th Five-Year Plan" Science and Technology Program and the Ministry of Education Outstanding Young Teacher Funding Program. The work addresses a critical challenge in lightweight structural engineering: the joining of magnesium and aluminum alloys, which are widely used in aerospace and automotive applications but are difficult to join due to the formation of brittle intermetallic compounds.
Material System and Thermodynamic Background
The magnesium-aluminum system is characterized by the formation of several intermetallic compounds at the interface during welding:
| Intermetallic Compound | Crystal Structure | Hardness (HV) | Stability |
|---|---|---|---|
| Mg₂Al₃ | Orthorhombic | 100–150 | Stable at room temperature |
| MgAl₂ | Orthorhombic | 150–200 | Stable at room temperature |
| Mg₂Al₅ | Hexagonal | 120–180 | Metastable |
| Al₃Mg₂ | Orthorhombic | 130–170 | Metastable |
The formation of these intermetallic compounds is thermodynamically favorable but kinetically limited. The diffusion coefficients of Mg and Al at welding temperatures (600–800°C) are high enough to allow significant interdiffusion during the welding process and subsequent cooling.
Welding Process Parameters
The study investigated TIG welding of AZ91 magnesium alloy (Mg-9Al-1Zn) to 6061 aluminum alloy (Al-0.6Mg-0.3Si) using both DCEN and DCEP polarities, as well as AC TIG. The following parameters were used:
| Parameter | DCEN | DCEP | AC |
|---|---|---|---|
| Current (A) | 120–160 | 100–140 | 110–150 |
| Voltage (V) | 11–13 | 13–15 | 12–14 |
| Travel speed (mm/min) | 300–400 | 300–400 | 300–400 |
| Shielding gas | Ar | Ar | Ar |
| Filler wire | AZ91 | AZ91 | AZ91 |
| Preheat | 100°C | 100°C | 100°C |
Diffusion Behavior Analysis
The authors conducted detailed microstructural and compositional analysis of the weld interface using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The key findings are:
Intermetallic Layer Formation
The diffusion behavior at the Mg-Al interface follows a predictable pattern:
- Near the Mg side: A thin layer of Mg₂Al₃ (2–5 µm) forms first, followed by MgAl₂ in some regions.
- Near the Al side: A layer of Al₃Mg₂ forms, which may transform to Mg₂Al₅ upon cooling.
- Intermediate region: A complex mixture of intermetallic phases with varying compositions.
The total thickness of the intermetallic layer is typically 10–30 µm, depending on welding parameters and cooling rate.
Effect of Welding Parameters on Diffusion
| Parameter | Effect on Intermetallic Layer | Recommended Range |
|---|---|---|
| Current | Higher current → thicker layer | Moderate current (120–140 A) |
| Travel speed | Higher speed → thinner layer | ≥350 mm/min |
| Preheat temperature | Higher preheat → thicker layer | ≤100°C |
| Polarity | DCEP → more Al diffusion into Mg | DCEP preferred |
| Filler composition | Mg-rich filler → thinner layer | AZ91 or Mg-4Al |
Cooling Rate Effects
The cooling rate has a significant effect on the morphology and distribution of intermetallic phases:
- Slow cooling (≤10°C/s): Coarse, continuous intermetallic layers form at the interface, which severely reduce joint strength.
- Moderate cooling (10–50°C/s): Semi-continuous intermetallic layers with some discontinuities, providing moderate joint strength.
- Fast cooling (≥50°C/s): Discontinuous, isolated intermetallic particles, which provide the best joint strength.
Mechanical Properties
The mechanical properties of the Mg-Al TIG welds are strongly influenced by the intermetallic layer:
| Property | Base Mg (AZ91) | Base Al (6061) | Weld Joint |
|---|---|---|---|
| Tensile strength (MPa) | 230 | 310 | 150–200 |
| Yield strength (MPA) | 120 | 275 | 80–120 |
| Elongation (%) | 8–10 | 12–16 | 3–6 |
| Hardness at interface (HV) | 60 | 80 | 150–200 |
The joint efficiency (weld strength / base metal strength) is typically 40–60%, which is acceptable for many lightweight structural applications but insufficient for high-stress applications.
Engineering Applications and Limitations
Despite the challenges associated with Mg-Al dissimilar metal welding, there are several important engineering applications:
- Automotive lightweighting: Replacement of aluminum components with magnesium components to reduce weight while maintaining structural integrity.
- Aerospace structures: Use of magnesium for interior structures and aluminum for exterior structures, joined by dissimilar metal welding.
- Electronics housings: Combination of magnesium (for EMI shielding) and aluminum (for structural strength).
However, the following limitations must be considered:
- Galvanic corrosion: The potential difference between Mg and Al in aqueous environments can lead to rapid corrosion of the magnesium component. Proper isolation (coating, sealant) is essential.
- Thermal cycling: Repeated thermal cycling can cause growth of the intermetallic layer and eventual joint failure.
- Fatigue life: The brittle intermetallic layer acts as a crack initiation site, significantly reducing fatigue life.
Study Insights
The most important finding of this research is the quantitative relationship between welding parameters and intermetallic layer thickness, which provides a basis for process optimization. The authors demonstrate that by carefully controlling the heat input and cooling rate, the intermetallic layer can be minimized to achieve acceptable joint strength.
From a broader perspective, this study highlights the fundamental challenge of dissimilar metal welding: the thermodynamic drive for intermetallic formation cannot be eliminated, only controlled. The key is to find the process window where the intermetallic layer is thin enough to avoid brittleness but thick enough to provide metallurgical bonding.
For engineers working on lightweight structural applications, this study provides valuable guidance on the feasibility of Mg-Al TIG welding. The key recommendations are:
- Use DCEP polarity to promote Al diffusion into Mg rather than vice versa.
- Use high travel speed and moderate current to minimize heat input.
- Apply thermal barriers (e.g., ceramic coatings) on the Mg side to reduce diffusion.
- Always consider galvanic corrosion protection in design.
The study also emphasizes the importance of post-weld heat treatment. A low-temperature anneal (250–300°C for 2 hours) can partially dissolve the intermetallic layer and improve joint ductility, although this may reduce the strength of the aluminum component.
This research represents an important contribution to the understanding of Mg-Al dissimilar metal joining and provides practical guidance for engineers working on lightweight structural applications where the combination of magnesium and aluminum is desirable.