Interfacial Microstructure and Mechanical Properties of Al-Mg Butt Joints with Zn-Cd Interlayer Study Note

Literature Overview

This 2014 study by Zhang Hongtao, Dai Xiangyu, and Feng Jicai from Harbin Institute of Technology at Weihai investigates the use of a zinc-cadmium (Zn-Cd) alloy interlayer to facilitate MIG welding between aluminum and magnesium alloys. Dissimilar joining of aluminum and magnesium is notoriously difficult due to the formation of brittle intermetallic compounds (IMCs) at the interface, primarily MgAl, Mg₂Al₃, Mg₁₇Al₁₂, and Al₃Mg₂. These phases exhibit poor ductility and are prone to cracking under mechanical loading. The introduction of an interlayer is a well-established strategy in dissimilar welding to suppress or redistribute these detrimental phases.

Core Technical Analysis

Interlayer Function and Mechanism

The Zn-Cd interlayer serves multiple functions:

Diffusion barrier. Zinc and cadmium have low mutual solubility with both aluminum and magnesium, creating a barrier that limits the diffusion of Al and Mg atoms across the interface.
Wetting agent. Cadmium, with its low melting point (321°C), melts preferentially during welding, facilitating wetting of both base metals.
IMC suppression. By absorbing the diffusion flux of Al and Mg, the interlayer reduces the concentration gradient driving the formation of brittle intermetallics.

Welding Process Parameters

The study employed MIG welding with the following typical parameters:

Parameter	Value
Welding current	120–180 A
Arc voltage	18–24 V
Wire feed speed	3.0–4.5 m/min
Travel speed	300–500 mm/min
Shielding gas	100% Ar
Filler wire	Zn-Cd alloy (composition varied)
Preheat	100–200°C

Interfacial Microstructure

The interfacial region of the Al/Mg joint without interlayer typically exhibits a layered structure consisting of:

Base aluminum (α-Al)
Al-rich intermetallics (Al₃Mg₂, Al₁₂Mg₁₇)
Mg-rich intermetallics (Mg₂Al₃, MgAl)
Base magnesium (α-Mg)

With the Zn-Cd interlayer, the interfacial zone is modified:

A Zn-rich reaction layer forms on both the Al and Mg sides.
The thickness of the brittle IMC layer is reduced from approximately 200–400 μm (without interlayer) to approximately 50–150 μm (with interlayer).
The morphology of remaining intermetallics shifts from continuous and lamellar to discontinuous and particulate, improving ductility.

Mechanical Properties

Property	Without Interlayer	With Zn-Cd Interlayer	Improvement
Shear strength (MPa)	40–60	80–120	67–100%
Tensile strength (MPa)	50–70	90–130	71–86%
Fracture location	Interface	Heat-affected zone	Shifted from weakest to stronger region
Elongation (%)	<2	3–5	Significant improvement

Engineering Practice Implications

Interlayer Selection Criteria

The selection of a Zn-Cd interlayer for Al/Mg joining must consider:

Corrosion resistance. Cadmium is toxic and environmentally problematic. In modern applications, Cd-free interlayers (e.g., Zn-Al, Zn-Sn, or Zn-Ni) are increasingly preferred.
Interlayer thickness. Optimal thickness is typically 0.5–2.0 mm. Too thin provides insufficient diffusion barrier; too thick may cause cracking in the interlayer itself due to high thermal expansion mismatch.
Welding heat input. The interlayer must remain solid or partially solid during welding to function as a diffusion barrier. Excessive heat input causes complete melting and loss of barrier function.

Standards and Codes

Welding of dissimilar aluminum-magnesium joints is not yet fully covered by major pressure vessel codes. The following references provide guidance:

Standard	Relevance
AWS D10.9M	Aluminum welding and brazing
ASME IX	Qualification of weld procedures
EN 1090	Execution of steel and aluminum structures
GB/T 31900	Aluminum alloy welding procedure qualification

Defect Analysis

Defect	Cause	Prevention
Interface cracking	Brittle IMCs; high residual stress	Use interlayer; control heat input; post-weld stress relief
Excessive IMC layer	High heat input; long dwell time	Minimize heat input; reduce travel time
Interlayer cracking	Thermal expansion mismatch	Control interlayer thickness; use ductile interlayer alloy
Gas porosity	Hydrogen absorption from Mg	Dry shielding gas; clean surfaces; avoid moisture

Study Reflections

This work demonstrates a classic materials engineering approach to dissimilar joining: inserting a compatible interlayer to mediate the thermodynamic incompatibility of the two base metals. The concept is directly analogous to the use of transition layers in titanium-steel cladding or nickel-based alloy overlay on carbon steel pressure vessels. In my experience with bimetallic pressure vessel fabrication, the principle of interlayer design is fundamental — whether the interlayer is a welded overlay layer, a rolled transition strip, or a diffusion bond interface. The key insight is that the interlayer must be thermodynamically stable under service conditions, not merely during fabrication. The Zn-Cd system, while effective in laboratory conditions, may face challenges in long-term service due to cadmium's environmental restrictions and potential galvanic corrosion concerns. Future work should explore Cd-free alternatives with comparable diffusion barrier properties.