MIG Fusion-Braze Welding of Pure Aluminum and Galvanized Steel

Literature Overview

The 2013 research by Guo Xuming, Wang Jianmei, and Xu Rongzheng, published in Journal of Aeronautical Materials, investigates MIG fusion-braze welding between pure aluminum and galvanized steel. Conducted at Shenyang Aerospace University, this work addresses a significant manufacturing challenge: joining dissimilar metals with vastly different physical and chemical properties. The fusion-braze welding concept represents a hybrid approach that combines fusion welding on one side with brazing on the other, avoiding the formation of brittle intermetallic compounds that plague direct fusion welding of aluminum-steel joints.

Core Technical Concept

Direct fusion welding of aluminum and steel produces thick layers of brittle intermetallic compounds (FeAl, Fe2Al5, FeAl3) that severely degrade joint strength and ductility. The fusion-braze welding approach circumvents this problem by:

Fusion welding the aluminum side: Full penetration weld on the aluminum base metal
Brazing the steel side: Liquid filler metal wets and bonds to the steel surface without melting it
Using a diffusion barrier: Often a copper or nickel interlayer to prevent direct Fe-Al reaction

The process parameters for MIG fusion-braze welding differ significantly from conventional welding:

Parameter	Fusion Side (Aluminum)	Brazing Side (Steel)	Typical Value
Arc current	Higher	Lower	150-250 A
Arc voltage	Higher	Lower	18-24 V
Travel speed	Moderate	Moderate	200-400 mm/min
Shielding gas	Pure Ar or Ar/He	Pure Ar or Ar/CO2	15-20 L/min
Wire composition	Al-Si or Al-Mg	Al-Si (high Si)	1.0-1.6 mm diameter
Base metal temperature	Ambient	Preheated (optional)	50-150°C

Metallurgical Analysis

The microstructural evolution in fusion-braze welds involves several distinct regions:

Aluminum Fusion Zone

Fully melted and resolidified aluminum
Columnar grains growing from fusion boundary
Fine equiaxed grains near the centerline
Porosity susceptibility due to hydrogen solubility changes

Steel Brazing Zone

Solid steel base metal with wetted surface
Filler metal solidification without base metal melting
Possible thin intermetallic layer at interface (controlled thickness)
Adhesive bond strength dependent on surface preparation

Transition Region

Gradual composition change from Al-rich to steel-rich
Potential intermetallic formation if diffusion barrier is inadequate
Critical region for joint strength and durability

Process Challenges and Solutions

Challenge	Description	Solution
Thermal mismatch	Different melting points (660°C vs 1500°C)	Asymmetric current distribution, copper backing plate
Intermetallic formation	Brittle Fe-Al phases at interface	Copper diffusion barrier, controlled heat input
Porosity	Hydrogen pickup, gas entrapment	Clean surfaces, adequate shielding, low hydrogen filler
Residual stress	Differential thermal expansion	Post-weld stress relief, flexible joint design
Galvanic corrosion	Electrochemical potential difference	Protective coatings, isolation of dissimilar metals

Mechanical Property Evaluation

The mechanical performance of fusion-braze joints is characterized by:

Property	Typical Value	Comparison to Base Metals
Shear strength	40-80 MPa	20-40% of weaker base metal
Tensile strength	50-100 MPa	Limited by brazed interface
Fatigue strength	30-60 MPa (R=-1)	Significantly reduced
Fracture toughness	Low	Brittle intermetallic interface
Creep resistance	Temperature-dependent	Degrades above 150°C

Engineering Applications

Fusion-braze welding of aluminum and steel finds applications in:

Aerospace: Fuel tanks, structural brackets, heat exchangers
Automotive: Lightweight body structures, battery enclosures
Marine: Hull structures, deck fittings
Railway: Lightweight car bodies, interior structures

For pressure vessel applications, the technology can be adapted for:

Aluminum-clad steel vessels for cryogenic service
Dissimilar metal heat exchanger tubes
Lightweight pressure vessels for aerospace propulsion systems

Key Questions and Reflections

The fusion-braze welding concept raises several important engineering considerations:

How does the joint performance degrade under cyclic thermal loading?
What is the long-term durability of the diffusion barrier in aggressive environments?
Can the process be scaled for large structural components?
How does the galvanized coating interact with the brazing process?

Study Insights and Implications

This research demonstrates that hybrid joining technologies can overcome the fundamental limitations of dissimilar metal fusion welding. For engineers working in bimetal product manufacturing, the fusion-braze approach offers a viable alternative to explosive cladding or roll-bonding for certain applications where lower joint strength is acceptable. The technology is particularly relevant for lightweight structural applications where the weight savings from aluminum-steel hybrid designs outweigh the reduced joint strength. Future developments should focus on improving joint durability, expanding the range of compatible materials, and developing standardized testing protocols for qualification purposes.