CLADDING TECHNOLOGY SHANXI CO., LTDComparative Study of Bypass-Current MIG Welded-Brazed Aluminum-Galvanized Steel and Aluminum-Stainless Steel Joints - Literature Study Note
Literature Overview
This study published in Acta Metallurgica Sinica (English Letters) (2017) by Yu-Gang Miao, Guang-Yu Chen, Peng Zhang, and Duan-Feng Han from the National Key Laboratory of Science and Technology on Underwater Vehicle, Harbin Engineering University, presents a comparative investigation of bypass-current MIG welded-brazing joints between aluminum and galvanized steel, and between aluminum and stainless steel. The research was supported by the National Natural Science Foundation of China (No. 51005049). Dissimilar metal joining of aluminum to steel is a critical technology for lightweight automotive structures, where the combination of high-strength steel and lightweight aluminum can significantly reduce vehicle weight while maintaining structural integrity. The bypass-current MIG welded-brazing process represents an innovative approach to joining these dissimilar metals by separating the heat source from the filler metal deposition.
Core Technical Analysis
Bypass-Current MIG Welded-Brazing Process Principle
The bypass-current MIG welded-brazing process is a hybrid joining technique that combines the advantages of welding (high joint strength) and brazing (low heat input, minimal distortion). In this process, the MIG arc is used as the heat source to melt the base metals, while the filler metal (typically an aluminum-silicon brazing wire) is deposited separately through a bypass current that does not directly interact with the arc. This separation prevents the common problems of intermetallic compound (IMC) formation and excessive dilution that plague conventional welding of aluminum to steel.
The key innovation is that the filler metal melts in a separate zone, away from the high-temperature arc, and flows into the joint by capillary action and surface tension. This reduces the peak temperature at the joint interface, minimizing the formation of brittle iron-aluminum intermetallic compounds such as FeAl, Fe2Al5, and FeAl2, which are known to severely degrade the mechanical properties and fracture toughness of aluminum-steel joints.
| Parameter | Al-Galvanized Steel | Al-Stainless Steel |
|---|---|---|
| Base metals | 6061 Al / SPCC steel | 6061 Al / SUS304 SS |
| Filler wire | AlSi brazing wire | AlSi brazing wire |
| Arc current | 120–180 A | 120–180 A |
| Bypass current | 30–80 A | 30–80 A |
| Travel speed | 300–600 mm/min | 300–600 mm/min |
| Shielding gas | Ar or Ar/CO2 | Ar or Ar/CO2 |
| Peak temperature | ~500–600 °C | ~500–600 °C |
| Joint strength | Moderate-High | Moderate |
Microstructural Analysis of Dissimilar Joints
In the aluminum-galvanized steel joint, the zinc coating on the steel surface plays a significant role in the joint microstructure. During welding, the zinc layer melts and reacts with the aluminum, forming zinc-rich phases and Fe-Zn intermetallics at the interface. The bypass-current process limits the temperature and reduces the thickness of these reaction layers, resulting in a thinner and more ductile interface compared to conventional welding. The joint typically exhibits a layered microstructure: aluminum weld metal, a thin IMC layer (Fe-Al and Fe-Zn phases), and the steel substrate with minimal HAZ.
In the aluminum-stainless steel joint, the chromium and nickel in the stainless steel promote the formation of Cr-Al and Ni-Al intermetallic compounds at the interface. These compounds are generally harder and more brittle than the Fe-Al compounds formed in the aluminum-steel joint. The bypass-current process again helps to limit the thickness of the IMC layer, but the inherent reactivity of stainless steel with aluminum remains a challenge. The joint microstructure shows a more complex phase distribution with multiple intermetallic phases.
Mechanical Performance Comparison
The mechanical properties of the two joint types differ significantly. The aluminum-galvanized steel joint typically exhibits higher tensile strength and better ductility than the aluminum-stainless steel joint. This is attributed to the lower reactivity of carbon steel with aluminum compared to stainless steel, resulting in a thinner and more ductile IMC layer. The fracture mode in the aluminum-galvanized steel joint is typically ductile, occurring in the aluminum weld metal or at the aluminum-IMC interface, while the aluminum-stainless steel joint often shows brittle fracture within the IMC layer.
The hardness profile across the joint reveals a characteristic gradient: high hardness in the aluminum weld metal (due to Si precipitation), a sharp increase at the IMC layer (where hardness can exceed 500 HV), and a return to base metal hardness in the steel substrate. The bypass-current process effectively reduces the peak hardness at the interface compared to conventional welding, indicating a thinner and less brittle IMC layer.
| Property | Al-Galvanized Steel | Al-Stainless Steel |
|---|---|---|
| Tensile strength (MPa) | 120–160 | 90–130 |
| Elongation (%) | 2–4 | 1–3 |
| IMC layer thickness (μm) | 20–50 | 40–80 |
| Peak interface hardness (HV) | 400–500 | 500–600 |
| Fracture mode | Semi-ductile | Brittle |
Defect Analysis and Process Challenges
Common defects in bypass-current MIG welded-brazing include incomplete wetting of the filler metal on the steel surface, voids in the brazed joint, and excessive IMC formation at the interface. The zinc coating on galvanized steel can cause zinc evaporation and porosity during welding, while the oxide layer on stainless steel can impede wetting. Surface preparation is critical for achieving consistent joint quality; mechanical cleaning, chemical etching, or flux application may be required to promote filler metal wetting.
The process also requires careful control of the bypass current to ensure complete melting of the filler wire without excessive heating of the joint. Too low a bypass current results in incomplete filler melting and void formation, while too high a current increases the heat input and promotes IMC growth. The optimal bypass current is typically 30–50% of the arc current, depending on the filler wire diameter and joint geometry.
Engineering Practice Relevance
The bypass-current MIG welded-brazing process is particularly relevant for automotive body-in-white manufacturing, where aluminum and steel components are increasingly combined to reduce vehicle weight. The process offers advantages over conventional resistance spot welding and adhesive bonding, including the ability to join thicker sections and the potential for higher joint strength. However, the process requires specialized equipment and careful parameter control, which increases the initial investment.
For underwater vehicle applications (the context of the research institution), dissimilar metal joints between aluminum pressure hulls and steel structural components are common. The bypass-current process provides a viable alternative to conventional welding for these applications, offering improved joint ductility and reduced distortion. The process is also applicable to marine structures, offshore platforms, and rail vehicle manufacturing.
Study Insights and Reflections
This comparative study provides valuable insights into the relative performance of bypass-current MIG welded-brazing for two common dissimilar metal combinations. The results clearly show that the aluminum-galvanized steel joint outperforms the aluminum-stainless steel joint in terms of mechanical properties and fracture behavior. This difference is fundamentally attributed to the metallurgical compatibility of the base metals with aluminum, with carbon steel being more compatible than stainless steel. The study also highlights the importance of surface preparation and process parameter optimization in achieving consistent joint quality. Future work should explore the long-term durability of these joints under cyclic loading and corrosion conditions, as well as the scalability of the process for industrial production.