CLADDING TECHNOLOGY SHANXI CO., LTDDissimilar TIG Brazing of AZ61 Magnesium Alloy and DP590 Galvanized Steel
Literature Overview
The study by Jiang Li and Liu Hui, conducted under the Xinjiang Uygur Autonomous Region University Research Program (XJEDU2014S074) and published in 2018, addresses a critical engineering challenge in lightweight structural fabrication: joining AZ61 magnesium alloy to DP590 galvanized steel via TIG brazing. This work falls squarely within the domain of dissimilar metal joining, a field that has gained enormous significance as the automotive and aerospace industries pursue weight reduction without sacrificing structural integrity. The authors investigated thermal processing parameters and joint formation mechanisms, providing valuable data for engineers dealing with lightweight dissimilar metal assemblies.
Core Technical Analysis
AZ61 magnesium alloy possesses a density of approximately 1.81 g/cm³, while DP590 galvanized steel has a density of 7.85 g/cm³, creating a density mismatch ratio of roughly 4.3:1. This dramatic difference introduces significant challenges during the joining process, including differential thermal expansion, intermetallic compound formation, and galvanic corrosion susceptibility. The melting point of AZ61 is approximately 650°C, whereas DP590 steel melts at around 1495°C, necessitating a brazing approach where the joint temperature remains below the melting point of both base metals.
The TIG brazing process was selected because it offers excellent heat input control, minimal contamination, and the ability to achieve narrow heat-affected zones. The brazing alloy selection is critical; zinc-based or aluminum-based filler materials are typically employed for magnesium-steel joints. The zinc-rich coating on DP590 steel introduces additional complexity, as zinc has a low boiling point of 907°C and can vaporize during welding, causing porosity and spatter.
Process Parameter Window and Defect Analysis
| Parameter | Typical Range | Rationale |
|---|---|---|
| Arc Current | 80-150 A | Low current to minimize thermal distortion |
| Travel Speed | 150-350 mm/min | Balance between wetting and heat input |
| Shielding Gas | Ar or Ar/He mix | Prevent oxidation of Mg and Zn coating |
| Nozzle Distance | 8-12 mm | Adequate shielding with minimal arc interference |
| Preheat Temperature | 100-200°C | Reduce thermal gradient across dissimilar metals |
| Brazing Temperature | 350-450°C | Below Mg melting point, sufficient for Zn/Al wetting |
The primary defects observed in such joints include intermetallic compound overgrowth at the Mg/steel interface, zinc porosity from vaporization, incomplete wetting of the steel surface, and microcracking in the AZ61 near the heat-affected zone. The formation of brittle Mg-Zn and Mg-Fe intermetallic phases is a common concern, and these phases can significantly reduce joint ductility.
Engineering Practice Integration
In practical applications, such as automotive battery enclosure assemblies or aerospace secondary structures, the joint strength requirement typically falls in the range of 150-250 MPa shear strength. The authors' findings suggest that controlling the brazing temperature within a narrow window of 380-420°C, combined with appropriate filler alloy composition, can achieve acceptable joint properties. Surface preparation of the DP590 steel—specifically the controlled removal of the zinc coating in the joint area—is essential to prevent zinc vaporization defects.
A critical insight from this work is that the galvanic coupling between AZ61 and steel creates a significant corrosion risk in humid environments. The potential difference between magnesium and steel in chloride-containing solutions can exceed 1.5 V, making cathodic protection or barrier coatings mandatory for outdoor applications. This study provides a solid foundation for further development of dissimilar metal joining technologies in lightweight vehicle design, though the long-term durability data remains an area requiring additional investigation.
Study Insights and Implications
The research demonstrates that TIG brazing is a viable technique for joining AZ61 to DP590 galvanized steel, but the process window is narrow and demands careful parameter control. The interplay between zinc coating behavior, intermetallic formation, and joint mechanics represents a complex multi-physics problem. Engineers working on similar dissimilar metal applications should pay particular attention to post-weld heat treatment to relieve residual stresses and to comprehensive surface protection strategies to mitigate galvanic corrosion. The work underscores the importance of material compatibility assessment before selecting joining methods for lightweight structural assemblies.