Interface Microstructure and Joint Properties of Aluminum-Galvanized Steel Dissimilar Metal Bypass Shunt MIG Arc Brazing

Literature Overview

This research, published in the Welding Journal in 2014 by researchers from Harbin Engineering University's Key Laboratory of Underwater Vehicle Technology and the College of Ship Engineering, addresses a critical challenge in shipbuilding and marine engineering: the reliable joining of aluminum to galvanized steel through MIG arc brazing with a bypass shunt configuration. The work was supported by the National Natural Science Foundation of China (51005049) and the Harbin Municipal High-Tech Industry Special Fund (ZX2012ZC006015). The study investigates the interfacial microstructure evolution and mechanical performance of the brazed joint, which is essential for lightweight marine structures where aluminum superstructures are bonded to steel hulls.

Core Technical Content

The bypass shunt MIG arc brazing technique represents a specialized approach to dissimilar metal joining that avoids full penetration welding between aluminum and steel. In this method, the welding arc is shunted through a bypass path, allowing controlled heat input to melt a brazing filler alloy without fully melting either base metal. This is particularly significant because direct fusion welding of aluminum to steel produces brittle intermetallic compounds (such as FeAl, Fe2Al5, and FeAl3) that severely degrade joint integrity.

Interface Microstructure Analysis

The interfacial region in aluminum-galvanized steel brazed joints exhibits a complex layered structure. The galvanized coating (typically Zn-Fe alloy layers) interacts with the aluminum brazing filler during the brazing process, forming intermediate phases. Key observations typically include:

• A thin Fe-Zn intermetallic layer at the steel side interface
• A transition zone containing Zn-Al compounds (such as Al2Zn3 and Al6Zn3)
• The brazing alloy matrix (typically Al-Si or Al-Cu based)
• A diffusion zone at the aluminum base metal side

The thickness of the intermetallic layer is directly correlated with the heat input during brazing. Excessive heat input leads to thick intermetallic layers (>20 μm), which become brittle fracture sites. Optimal conditions typically maintain the intermetallic layer thickness below 10-15 μm.

Joint Performance Characteristics

The mechanical performance of these joints is primarily governed by the interfacial bond quality rather than the bulk properties of the brazing filler. Fracture analysis typically reveals that failure occurs at the interface when intermetallic layers are excessive, while good joints exhibit fracture within the brazing alloy itself.

Process Parameters and Their Influence

The bypass shunt configuration allows for independent control of the primary welding arc and the shunt current, which melts the brazing filler. This dual-parameter control is advantageous because:

1. The primary arc provides the necessary heat for wetting and spreading of the brazing alloy
2. The shunt current controls the filler melting rate and joint fill
3. The separation of functions allows optimization of each parameter independently

Key process variables include welding current (typically 120-200 A for the primary arc), shunt current (30-80 A), travel speed (300-600 mm/min), wire feed speed, and gas shielding conditions (usually Ar or Ar/CO2 mixtures).

Engineering Practice Implications

For shipbuilding applications, the aluminum-steel joint must withstand fatigue loading from wave action, potential corrosion in marine environments, and occasional impact loads. The galvanized steel substrate introduces additional complexity because the Zn coating composition varies with the galvanizing process, affecting the interfacial reaction kinetics. In practice, surface preparation of the galvanized steel (removal of excess zinc spangles, cleaning) is critical for achieving consistent joint quality.

The findings of this research directly inform the development of welding procedure specifications (WPS) for aluminum-to-steel brazed joints in marine structures. Engineers should note that the bypass shunt technique offers a practical alternative to explosive bonding or mechanical fastening when moderate joint strengths are acceptable and full penetration welding is not feasible.

Key Reflections

The fundamental insight from this work is that controlled interfacial reaction—rather than elimination of intermetallic formation—is the key to achieving acceptable joint performance in aluminum-steel brazed connections. The bypass shunt MIG technique provides sufficient process flexibility to manage this reaction within acceptable bounds. However, the narrow process window and sensitivity to base metal surface condition remain challenges for production environments. Future work should focus on real-time monitoring of interfacial reactions during brazing to enable adaptive process control.