Properties of Magnesium-Aluminum Dissimilar Metal TIG Welded Joints

Literature Overview

This 2015 paper by Liu Zhengjun, Gong Ying, and Su Yunhai from Shenyang University of Technology, published in the journal Materials Engineering and supported by the Liaoning Provincial Natural Science Foundation (20072041), investigates the properties of TIG-welded joints between magnesium and aluminum alloys. Dissimilar metal welding of magnesium and aluminum is of growing interest in lightweight structural applications, particularly in automotive and aerospace industries where weight reduction is a primary design driver.

Core Technical Points

Material Incompatibility Challenges

Magnesium and aluminum alloys present fundamental challenges for welding due to:

Large difference in melting points – Mg alloys melt at 450-650 °C, Al alloys at 560-660 °C
Significant difference in thermal conductivity – Mg: 150-200 W/(m·K), Al: 200-240 W/(m·K)
Chemical incompatibility – formation of brittle intermetallic compounds (Mg₂Al₃, MgAl₂, Mg₅Al₈)
Large difference in coefficients of thermal expansion – Mg: 26×10⁻⁶/K, Al: 23×10⁻⁶/K
Galvanic corrosion – significant potential difference in chloride environments

Welded Joint Microstructure

The microstructure of the Mg-Al TIG welded joint exhibits distinct zones:

Base metal zones – retain original microstructure with minimal changes
Heat-affected zones – grain coarsening, precipitate dissolution
Weld metal – complex microstructure with intermetallic compounds
Interfacial reaction zone – layer of intermetallic compounds at the Mg/Al interface

The intermetallic layer thickness is strongly dependent on welding parameters:

Parameter	Effect on IMC Layer
Higher current	Thicker IMC layer
Faster travel speed	Thinner IMC layer
Higher preheat	Thicker IMC layer
Lower shielding gas flow	Possible oxidation

Mechanical Properties

Property	Mg Side HAZ	Weld Metal	Al Side HAZ
Tensile strength (MPa)	180-220	150-200	200-250
Elongation (%)	5-8	3-5	8-12
Hardness (HV)	60-80	50-70	70-90

The joint efficiency (weld strength / base metal strength) typically ranges from 50-70%, significantly lower than similar-metal joints. The fracture usually initiates at the Mg/Al interface or within the IMC-rich zone.

Corrosion Behavior

The galvanic coupling between Mg and Al creates a significant corrosion risk:

Open circuit potential difference – 300-500 mV
Preferential corrosion – Mg alloy acts as the anode and corrodes preferentially
Pitting initiation – at the weld interface and IMC boundaries
Hydrogen evolution – during Mg corrosion accelerates degradation

Defect Analysis

Defect	Cause	Mitigation
Intermetallic embrittlement	Excessive thermal input	Reduce current, increase speed
Cracking at interface	Thermal stress mismatch	Preheat, controlled cooling
Porosity	Gas absorption, oxidation	High-purity shielding, clean surfaces
Corrosion at interface	Galvanic coupling	Coating, isolation
Lack of fusion	Insufficient heat input	Increase current, optimize fit-up

Integration with Engineering Practice

The practical application of Mg-Al welded joints is limited but growing. Current applications include:

Automotive structural brackets – where Mg-Al hybrid structures reduce weight
Aerospace interior components – non-load-bearing panels
Electronic housings – where thermal management and weight are critical

The key engineering challenge is ensuring long-term durability in corrosive environments. Surface treatments (anodizing, conversion coatings, organic coatings) are essential to mitigate galvanic corrosion. The weld zone itself requires special attention as the disrupted microstructure provides preferential corrosion paths.

Key Reflections

The most significant finding from this study is that while Mg-Al dissimilar welding is technically achievable using TIG, the resulting joints have limited mechanical performance and poor corrosion resistance. The intermetallic layer, while providing metallurgical bonding, is inherently brittle and acts as a crack initiation site. The thickness of this layer must be controlled to less than 50 μm to maintain acceptable joint properties.

From a practical standpoint, the Mg-Al welding process requires careful parameter optimization to minimize the intermetallic layer while ensuring adequate fusion. A recommended approach is:

Use pulsed TIG to control heat input precisely
Employ a filler metal containing Al (e.g., Al-5Mg) to reduce intermetallic formation
Apply post-weld heat treatment to relieve stresses and coarsen intermetallics
Implement comprehensive surface protection for corrosion resistance

The study highlights the need for continued research into dissimilar metal welding technologies. Future approaches may include:

Friction stir welding (FSW) for solid-state joining without melting
Diffusion bonding for thin-layer bonding without intermetallic formation
Intermediate bonding layers (e.g., Zn, Ti) to prevent direct Mg-Al contact

Reference Value and Outlook

This research provides valuable baseline data for engineers considering Mg-Al hybrid structures. The findings emphasize that dissimilar metal welding is not merely a process challenge but a materials science challenge requiring holistic consideration of microstructure, mechanics, and corrosion. The work should be viewed as a stepping stone toward more advanced dissimilar joining technologies rather than a complete solution for production applications.