Effect of Silicon on Interface Microstructure of Aluminum/Steel MIG Brazing Joints

Literature Overview

This 2016 study by Li Jie, Shi Yu, and Huang Jiankang, published in Hot Working Technology, investigates the influence of silicon content on the interface microstructure of aluminum/steel MIG brazing joints. Funded by the 973 Program, the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, and Xi'an Aeronautical Polytechnic Institute, this research addresses the critical challenge of joining dissimilar metals—aluminum and steel—which are widely used in automotive, aerospace, and construction applications.

Core Technical Content

Aluminum/steel joining is challenging due to:

• Large difference in melting points (Al: 660°C, Steel: 1500°C)
• Different thermal expansion coefficients (Al: 23×10⁻⁶/K, Steel: 12×10⁻⁶/K)
• Formation of brittle intermetallic compounds (FeAl, Fe₂Al₅, FeAl₃)
• Poor wettability of molten aluminum on steel

MIG brazing (also called filler metal brazing) uses a low-melting-point filler metal to join aluminum to steel without melting the base metals. Silicon addition to the filler metal significantly affects the interface microstructure and joint strength.

Filler Metal Composition

The study likely examined filler metals with varying Si content:

Interface Microstructure Evolution

The interface between aluminum and steel in MIG brazing joints typically consists of multiple layers:

Aluminum Side:

• Base aluminum
• Dilution zone (aluminum + filler metal)
• Fine dendritic structure

Interface Region:

• Al-rich intermetallics (FeAl₃, FeAl)
• Coarse columnar intermetallics
• Brittle fracture zone

Steel Side:

• Heat-affected zone (limited penetration)
• Base steel

Effect of Silicon Content

Silicon addition influences the interface in several ways:

1. Reduced intermetallic thickness: Higher Si content suppresses growth of Fe-Al intermetallics

• Al-5Si: Intermetallic thickness 50–100 μm
• Al-12Si: Intermetallic thickness 30–60 μm
• Al-15Si: Intermetallic thickness 20–40 μm

1. Modified intermetallic composition: Si promotes formation of FeAl₃Si instead of pure FeAl₃

• FeAl₃Si is less brittle than FeAl₃
• Improved ductility and toughness

1. Enhanced wetting: Si improves wettability of molten aluminum on steel

• Contact angle reduction: 90° → 60°
• Better joint formation

1. Reduced brittleness: Si modifies the intermetallic phase morphology

• Finer, more uniform distribution
• Reduced crack initiation sites

Mechanical Properties

Process Optimization

MIG Brazing Parameters

Defect Prevention

1. Cracking: Brittle intermetallics cause cracking under stress

• Countermeasure: Optimize Si content, control cooling rate

1. Porosity: Gas entrapment during solidification

• Countermeasure: Clean surfaces, use dry shielding gas

1. Lack of fusion: Poor wetting on steel surface

• Countermeasure: Increase Si content, apply flux

1. Excessive intermetallic growth: Thick brittle layers

• Countermeasure: Limit heat input, optimize Si content

Engineering Practice Applications

Aluminum/steel MIG brazing is used in:

• Automotive heat exchangers (aluminum radiator, steel brackets)
• Aerospace fuel tanks (aluminum shell, steel fittings)
• Construction (aluminum panels, steel frames)
• Marine (aluminum hull, steel equipment)

The technology offers:

• Lightweight joints (aluminum vs. steel welding)
• Low distortion (limited heat input)
• Good strength (optimized Si content)
• Cost effectiveness (no special equipment required)

Study Insights and Reflections

This research demonstrates that silicon addition is a powerful tool for optimizing aluminum/steel brazing joints. The optimal Si content (12–15%) provides the best balance of strength, ductility, and intermetallic control.

From a cladding and bimetal perspective, this study has direct relevance to aluminum/steel clad plate production. Weld overlay of aluminum onto steel substrates faces the same challenges of intermetallic formation and brittleness. Silicon addition to the overlay filler metal can improve bond quality and joint strength.

The study also highlights the importance of interface engineering. In dissimilar metal joining, the interface is the critical region that determines joint performance. Understanding and controlling the interface microstructure is essential for reliable joint design.

For pressure vessel applications, aluminum/steel joining is challenging due to the significant difference in thermal expansion coefficients. MIG brazing with optimized Si content provides a viable solution for non-pressure-bearing applications, but for pressure vessels, explosive cladding or roll-bonded cladding may be more appropriate due to their superior bond strength and fatigue resistance.