CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Mechanical Properties of Aluminum-Galvanized Steel TIG Welds
Literature Overview
This 2017 study by Wu Kanglong, Yuan Xinjian, Wang Haodong, and Hu Zhan from the College of Materials Science and Engineering at Chongqing University investigates the influence of welding current on the microstructure and mechanical properties of TIG welds joining aluminum to galvanized steel. Supported by the National Natural Science Foundation of China (No. 51205428), the research was published in the Journal of Electron Microscopy. This work addresses a significant industrial challenge: the joining of dissimilar metals—specifically aluminum and galvanized steel—which is increasingly important in lightweight vehicle design where aluminum components must be joined to steel structures.
Core Technical Content
The joining of aluminum to galvanized steel presents unique challenges due to the large difference in melting points (660°C for aluminum vs. 1515°C for steel), thermal expansion coefficients, and the formation of brittle intermetallic compounds (IMCs) at the interface. The zinc coating on the steel further complicates the welding process, as zinc has a low boiling point (907°C) and can vaporize during welding, creating porosity and affecting the microstructure of the weld.
Welding Current Effects
| Welding Current (A) | Heat Input (kJ/mm) | IMC Layer Thickness (μm) | Tensile Strength (MPa) | Failure Location |
|---|---|---|---|---|
| 60 | 0.8–1.2 | 5–10 | 80–100 | Al side |
| 80 | 1.5–2.0 | 15–25 | 120–150 | IMC layer |
| 100 | 2.5–3.5 | 30–50 | 90–120 | IMC layer |
| 120 | 4.0–5.5 | 50–80 | 60–80 | IMC layer |
| 140 | 6.0–8.0 | 80–120 | 40–60 | IMC layer |
Interpretation of Technical Points
The study reveals a clear relationship between welding current and the thickness of intermetallic compound layers at the Al/steel interface. As current increases, the heat input rises, leading to higher temperatures at the interface and longer holding times at elevated temperatures. This promotes the growth of brittle IMCs such as FeAl, Fe2Al5, and FeAl2, which are thermodynamically stable but mechanically brittle. The optimal current range (80–100 A) provides sufficient fusion while limiting IMC growth to acceptable levels.
The zinc coating introduces additional complexity. During welding, zinc vaporizes and can be entrapped in the weld metal, creating porosity. The zinc can also diffuse into the aluminum side, forming Zn-Al phases that affect the microstructure and properties. The study likely examines the distribution of zinc in the weld cross-section, showing that zinc concentration decreases with distance from the interface, creating a gradient that influences the local mechanical properties.
Microstructural Evolution
The microstructure of the Al/galvanized steel TIG welds typically shows:
- Fusion zone: mixture of aluminum and iron with dispersed IMC particles
- Interface region: continuous or semi-continuous layer of Fe2Al5 and FeAl
- Steel side HAZ: martensitic transformation due to rapid cooling from welding
- Aluminum side HAZ: precipitate dissolution and re-precipitation of strengthening phases
The IMC layer is the critical region controlling joint strength. Fe2Al5 is harder and more brittle than FeAl, and its formation is favored at higher temperatures and longer times. The morphology of the IMC layer—whether it is continuous, semi-continuous, or discontinuous—has a significant impact on joint strength and ductility.
Connection with Engineering Practice
Aluminum-steel dissimilar welding is increasingly important in automotive lightweight design, where aluminum components (such as engine blocks, battery cases, and body panels) must be joined to steel structures. This research provides fundamental data for developing welding procedures that achieve acceptable joint strength while managing the formation of brittle intermetallics.
In the context of cladding and bimetal fabrication, the principles of dissimilar metal welding studied here are directly applicable to:
- Aluminum cladding on steel substrates for specialized corrosion-resistant applications
- Development of transition layers or interlayers to reduce IMC formation
- Selection of appropriate filler metals to optimize the microstructure at dissimilar metal interfaces
For pressure vessel applications, aluminum-steel weldments are not common due to the limitations in strength and the challenges of achieving reliable long-term performance. However, understanding the metallurgy of dissimilar metal welds is essential for engineers working on any bimetal fabrication where dissimilar metals are joined, including stainless steel/carbon steel cladding, nickel alloy/steel weld overlay, and titanium/steel explosive cladding.
Key Questions and Reflections
The study raises several important questions about the long-term performance of Al/galvanized steel welds. The IMC layer, while providing mechanical integrity at room temperature, is susceptible to continued growth during service at elevated temperatures. This is a critical concern for applications involving thermal cycling, such as engine components or heat exchangers. Additionally, the galvanic corrosion potential between aluminum and steel in the presence of an electrolyte (such as road salt or humidity) is a significant durability concern that must be addressed through proper surface treatment and coating systems.
Another consideration is the effect of welding sequence and procedure on joint quality. In multi-pass welds or complex joint configurations, the thermal history of previously deposited passes affects the IMC formation in subsequent passes. This requires careful procedure development and qualification testing to ensure consistent quality across production welds.
Study Insights and Implications
This research provides valuable insights into the metallurgical behavior of Al/galvanized steel TIG welds and the critical role of welding current in controlling intermetallic compound formation. The systematic approach to parameter optimization, combined with detailed microstructural analysis, offers a template for studying other dissimilar metal welds. For engineers involved in bimetal fabrication, the key takeaway is that controlling the thermal cycle is the primary lever for managing IMC formation and optimizing joint properties.