CLADDING TECHNOLOGY SHANXI CO., LTDInterface Microstructure and Joint Properties in Aluminum to Galvanized Steel Dissimilar Metal MIG Arc Brazing with Bypass Shunt Configuration
Literature Overview
This 2014 publication in the Welding Journal, supported by the National Natural Science Foundation of China (grant 51005049) and Harbin Municipal High-Tech Industry Special Fund (ZX2012ZC006015), investigates the microstructure evolution and mechanical performance of dissimilar metal joints produced by MIG arc brazing between aluminum and galvanized steel using a novel bypass shunt configuration. The research addresses a critical industrial challenge in lightweight vehicle manufacturing, where joining aluminum body panels to galvanized steel structural components is increasingly common.
Technical Challenge and Process Innovation
The fundamental challenge in joining aluminum to galvanized steel lies in the formation of brittle intermetallic compounds (IMCs) at the interface, which severely degrade joint strength and ductility. The bypass shunt configuration represents an innovative process modification that redirects part of the welding current through a conductive shunt path, effectively reducing the heat input at the critical interface zone.
| Process Parameter | Conventional MIG Brazing | Bypass Shunt Configuration | Effect |
|---|---|---|---|
| Heat input at interface | 0.8–1.2 kJ/mm | 0.5–0.7 kJ/mm | 40–50% reduction |
| Maximum interface temperature | 750–820 °C | 650–720 °C | 100–120 °C reduction |
| IMC layer thickness | 25–45 μm | 8–18 μm | 60–70% reduction |
| Welding current | 160–200 A | 140–180 A | Moderate reduction |
| Travel speed | 400–600 mm/min | 350–500 mm/min | Comparable range |
Interface Microstructure Analysis
Metallographic examination of the bypass shunt joints revealed a distinctly different interfacial microstructure compared to conventional MIG brazing:
- IMC layer composition – The primary intermetallic compounds formed at the Al/steel interface are FeAl₃ and Fe₂Al₅, with FeAl₃ forming first at the steel side and Fe₂Al₅ forming at the aluminum side. The bypass shunt configuration reduces the total IMC thickness to 8–18 μm compared to 25–45 μm in conventional joints.
- IMC morphology – The IMC layer in bypass shunt joints exhibits a more uniform, continuous morphology without the irregular, dendritic growth patterns observed in over-heated conventional joints.
- Brazing alloy flow – The reduced heat input allows better control of the brazing alloy (typically Al-Si based, such as 4043 or AlSi5) flow and wetting behavior on the galvanized steel surface.
- Galvanized layer interaction – The zinc-rich coating on galvanized steel reacts with the molten aluminum to form Zn-Al intermetallics, which can either promote or inhibit wetting depending on their thickness and morphology.
- Heat affected zone – The steel HAZ shows minimal microstructural change, with retained ferrite-pearlite structure and no evidence of grain coarsening or phase transformation.
Mechanical Performance Evaluation
Tensile and shear testing of the dissimilar joints demonstrated significant improvements with the bypass shunt configuration:
| Test Type | Conventional Joint | Bypass Shunt Joint | Improvement |
|---|---|---|---|
| Shear strength (MPa) | 45–65 | 72–95 | 45–50% increase |
| Tensile strength (MPa) | 35–55 | 60–82 | 55–60% increase |
| Fracture location | Interface (IMC) | Brazing alloy zone | Fundamental improvement |
| Elongation (%) | <2 | 5–12 | Significant ductility gain |
| Fatigue life (cycles to 10⁶) | 1.2–2.5×10⁵ | 3.8–6.2×10⁵ | 2–3× improvement |
Failure Analysis and Fractography
Scanning electron microscopy (SEM) of fractured specimens revealed:
- Conventional joints – Fracture consistently occurred within the thick, brittle IMC layer, exhibiting intergranular fracture morphology characteristic of FeAl₃ and Fe₂Al₅. The fracture surface showed minimal plastic deformation.
- Bypass shunt joints – Fracture preferentially occurred within the brazing alloy zone rather than at the interface, indicating that the joint strength was governed by the brazing alloy properties rather than the weak IMC layer. Ductile dimples were visible on the fracture surface.
- Mixed mode fracture – Some bypass shunt joints exhibited mixed-mode fracture with partial interface separation, suggesting that further optimization could shift the fracture mode entirely to the brazing alloy.
Process Optimization and Parameter Sensitivity
The research identified the following critical process parameters and their sensitivities:
- Current level – Increasing current above 180 A with bypass shunt configuration begins to negate the benefits by elevating interface temperature above 750 °C, promoting excessive IMC growth.
- Travel speed – Optimal travel speed of 400–450 mm/min provides the best balance between adequate heat input for brazing alloy flow and limiting interface temperature.
- Shunt resistance – The bypass shunt resistance should be calibrated to divert 30–50% of the total current, with lower diversion ratios providing insufficient heat reduction.
- Wire feed speed – Maintaining a wire feed speed that ensures continuous but not excessive brazing alloy deposition (1.5–2.5 mm deposit thickness) is critical for joint quality.
- Joint fit-up – A gap of 0.5–1.5 mm between the aluminum and galvanized steel substrates is optimal, allowing adequate brazing alloy flow without excessive capillary action that could increase IMC formation.
Engineering Application and Future Directions
The bypass shunt MIG arc brazing technique represents a practical solution for the automotive and transportation industries where dissimilar metal joining is essential for lightweight design. The technology offers several advantages for production implementation:
- Compatible with existing MIG welding equipment with modification of the welding circuit to incorporate the bypass shunt.
- No special filler metals required beyond standard Al-Si brazing alloys.
- Applicable to a wide range of aluminum alloy grades (5xxx, 6xxx series) and galvanized steel grades (DC04, SPCC, etc.).
- Achieves joint strengths sufficient for non-load-bearing and semi-structural applications in vehicle bodies.
- Production speeds of 350–500 mm/min are compatible with automated robotic welding cells.
The research opens avenues for further investigation into multi-pulse current configurations, advanced shunt circuit designs with active control, and extension to other dissimilar material combinations including copper-aluminum and titanium-steel joints. The methodology demonstrates that intelligent current management, rather than exotic filler metals or surface treatments, can be the key to solving difficult dissimilar metal joining challenges. This approach is particularly relevant to the growing demand for lightweight, multi-material structures in transportation, aerospace, and energy sectors.