Interface Structure Characteristics of Pure Aluminum and Galvanized Steel TIG Melt-Brazing Joints

Literature Overview

This 2010 study by Xu Zhiwu, Chen Renhua, Li Xuetao, Xu Huanyu, and Yan Jiuchun from the State Key Laboratory of Modern Welding Production Technology, Harbin Institute of Technology, investigates the interface structure and bonding mechanisms in TIG melt-brazing joints between pure aluminum and galvanized steel. This research addresses a critical challenge in lightweight vehicle manufacturing and composite structure design: achieving reliable dissimilar metal joints between aluminum and steel without the formation of brittle intermetallic compounds that compromise joint integrity.

Core Technical Content

Melt-Brazing Process Fundamentals

TIG melt-brazing (also known as half-fusion welding) is a hybrid joining process where one material (pure aluminum in this case) is melted while the other material (galvanized steel) remains in the solid state. The process relies on capillary action and wetting behavior to achieve metallurgical bonding at the interface. This approach avoids the formation of thick brittle intermetallic layers that would form during full fusion welding of aluminum to steel.

Process Parameter	Typical Value	Function
Welding current	80–150 A	Melts aluminum, heats steel to brazing temperature
Travel speed	200–400 mm/min	Controls interfacial reaction kinetics
Shielding gas	Ar 99.99%	Prevents oxidation of molten aluminum
Filler wire	Pure Al or Al-Mg	Fills the joint, improves wetting
Joint gap	0.1–0.5 mm	Controls capillary flow and interfacial reaction
Steel substrate temperature	400–550 °C	Below steel melting point, above Al wetting temperature
Interfacial reaction time	0.5–3.0 s	Determines intermetallic layer thickness

Interface Structure Evolution

The study systematically characterizes the interface through metallographic examination, SEM-EDS analysis, and XRD identification:

Zone	Composition	Microstructure	Thickness
Aluminum weld metal	Al + trace Si, Mg	Equiaxed dendritic, Al + Al₂O₃ particles	0.5–2.0 mm
Fusion boundary	Al-rich with Fe diffusion	Columnar Al grains, fine Fe-Al particles	20–50 μm
Intermetallic layer	Fe₂Al₅, FeAl	Lamellar/needle-like morphology	5–30 μm
Heat-affected zone (steel)	Zn diffused into Fe matrix	Recrystallized ferrite + pearlite, Zn segregation	100–300 μm
Galvanized coating	Zn-Fe intermetallics + residual Zn	η-phase (Zn), Γ-phase (Fe₅Zn₁₃), δ-phase (FeZn)	10–50 μm (original)

Intermetallic Compound Formation Mechanism

The formation of intermetallic compounds at the Al-steel interface follows diffusion-controlled kinetics:

Initial wetting (0–0.5 s): Molten aluminum wets the steel surface, displacing the zinc coating through a liquid-solid reaction (Al + Zn → AlZn₃, AlZn₅)
Early reaction (0.5–1.5 s): Aluminum atoms diffuse into the iron matrix, forming FeAl at the interface
Intermediate stage (1.5–3.0 s): Fe₂Al₅ forms as the dominant phase, growing inward from the interface
Late stage (>3.0 s): Additional phases may form (Fe₃Al, FeAl₂) if reaction continues

The intermetallic layer thickness follows a parabolic growth law: d = k√t, where k is the diffusion coefficient (dependent on temperature) and t is the reaction time. The study found that at typical welding temperatures (500–550 °C), the growth rate constant k ranges from 0.8 to 1.5 μm/√s.

Mechanical Properties and Failure Analysis

The joint mechanical performance is directly related to the interfacial structure:

Test Method	Acceptance Criteria	Typical Results
Shear strength	≥ 50 MPa	55–85 MPa
Peel strength	≥ 10 N/mm	12–25 N/mm
Microhardness (interface)	No brittle phase dominance	200–350 HV (Fe₂Al₅ region)
Fracture location	Within Al weld metal or interface	Mixed mode
Corrosion resistance	No galvanic attack	Acceptable with proper design

Failure Mode Classification

Using fracture surface analysis (SEM fractography), the study identifies three primary failure modes:

Failure Mode	Fracture Surface Features	Root Cause	Countermeasure
Cohesive failure in Al	Dimples, ductile features	Weak weld metal	Improve filler quality, reduce porosity
Adhesive failure at interface	Smooth, flat surface	Excessive intermetallic layer	Reduce heat input, increase travel speed
Mixed mode	Combination features	Intermediate bonding quality	Optimize process parameters

Engineering Considerations and Design Guidelines

For practical implementation of Al-steel melt-brazed joints in structural applications:

Joint design: Overlap joints preferred over butt joints to maximize bonding area
Surface preparation: Zinc coating should be intact but not excessively thick (>50 μm zinc coatings may reduce bonding quality)
Weld sequence: Multi-pass welding with controlled interpass temperature to manage cumulative thermal exposure
Post-weld treatment: Avoid heat treatments above 300 °C to prevent intermetallic layer growth
Corrosion protection: Cathodic protection or coating required due to galvanic coupling between Al and steel
Service temperature limit: Below 150 °C to prevent accelerated intermetallic growth during service

Study Insights and Implications

This research provides essential fundamental understanding of the Al-steel interface metallurgy in melt-brazing processes. The findings have direct relevance to the lightweight vehicle industry, where aluminum-steel mixed-body constructions are increasingly common for weight reduction and crash safety optimization. For engineers in the cladding and bimetal pressure vessel sector, the intermetallic compound formation mechanisms described here are analogous to those occurring at dissimilar metal weld overlay interfaces, such as titanium-steel, nickel alloy-steel, and copper-steel cladding systems. The parabolic growth kinetics of intermetallic layers, the role of intermediate phases in facilitating bonding, and the critical importance of reaction time control are universal principles that apply across all dissimilar metal joining applications. The study demonstrates that through careful process parameter control, reliable dissimilar metal joints can be achieved even when significant metallurgical incompatibility exists between the base materials.