Interfacial Microstructure and Mechanical Properties of Al-Mg Butt Joints with Zn-Cd Interlayer by MIG Welding

Literature Overview

The research by Zhang Hongtao, Dai Xiangyu, and Feng Jicai from Harbin Institute of Technology at Weihai investigates the welding of dissimilar metal butt joints between AZ31B magnesium alloy and pure aluminum 1060 sheets using MIG welding with a Zn-Cd alloy foil as an interlayer. Dissimilar metal welding of aluminum and magnesium alloys is inherently challenging due to the formation of brittle intermetallic compounds (IMCs) at the interface, which severely degrade joint strength and ductility. The introduction of a Zn-Cd interlayer is a novel approach to mitigate IMC formation and improve joint performance. The study characterizes the interfacial microstructure, identifies the phases present in the IMC layers, and evaluates the mechanical properties of the dissimilar metal joints.

Interlayer Strategy and Welding Process

The Zn-Cd alloy interlayer is placed between the AZ31B magnesium alloy and pure aluminum 1060 sheets before welding. During the MIG welding process, the interlayer melts and distributes between the two base metals, creating a transitional composition gradient that reduces the direct contact between aluminum and magnesium. This strategy aims to suppress the rapid formation of thick, continuous brittle IMC layers that would otherwise form at the Al/Mg interface.

The MIG welding process parameters are selected to achieve full penetration of both base metals while controlling the heat input to minimize IMC thickness. The interlayer thickness, composition, and melting behavior all influence the final joint microstructure and properties.

Interfacial Microstructure Analysis

IMC Layer Formation

Two distinct IMC layers are identified at the fusion zone/magnesium alloy interface:

The formation of two distinct IMC layers indicates a complex interfacial reaction sequence. Layer 1, directly adjacent to the magnesium alloy, forms first during the welding thermal cycle as aluminum from the weld pool diffuses into the magnesium base metal. The Al12Mg17 phase is a well-known brittle intermetallic compound that forms readily in Al-Mg systems, while Mg2Si suggests the presence of silicon in the system, possibly from the aluminum alloy or as an intentional addition.

Layer 2, which contains the additional MgZn2 phase, indicates that the zinc from the Zn-Cd interlayer participates in the interfacial reaction. The MgZn2 phase is a different intermetallic compound that forms in Mg-Zn systems and may have different mechanical properties compared to Al12Mg17. The presence of multiple phases in Layer 2 suggests a more complex microstructure that may influence crack initiation and propagation behavior.

Crack Initiation and Propagation

Scanning electron microscopy (SEM) in situ tensile testing reveals that cracks initiate from the IMC layer at the bottom of the joint and propagate along the brittle IMC layer before expanding into the weld metal. This crack path is characteristic of dissimilar metal joints where the IMC layer acts as a preferential fracture surface due to its inherent brittleness and poor ductility. The bottom of the joint is a critical location because it typically experiences the highest thermal gradient and the last to solidify, potentially resulting in a thicker or more continuous IMC layer at this location.

Mechanical Properties Evaluation

The highest tensile strength achieved for the dissimilar metal butt joints is 46.8 MPa. This value, while modest compared to the tensile strength of the individual base metals (AZ31B typically has a tensile strength of approximately 230 MPa, and pure aluminum 1060 has a tensile strength of approximately 70 to 80 MPa), represents a significant improvement over joints without an interlayer, where the tensile strength can be even lower due to thicker and more continuous IMC layers.

The effect of the interfacial IMC layer on mechanical properties is discussed in detail in the study. The IMC layers, while unavoidable in Al-Mg dissimilar metal joints, can be controlled in thickness and composition through the selection of interlayer material and welding parameters. The Zn-Cd interlayer introduces zinc into the interfacial region, which modifies the IMC formation sequence and potentially creates a more complex but less brittle interface.

Engineering Practice and Application Considerations

The welding of dissimilar aluminum and magnesium alloy joints is of interest in lightweight structural applications where different materials are joined for functional reasons, such as combining the high strength of magnesium alloys with the corrosion resistance or formability of aluminum alloys. The Zn-Cd interlayer approach offers a practical solution for improving joint strength, although the resulting tensile strength of 46.8 MPa limits the application to non-critical or low-stress structural components.

For engineering applications, the interfacial IMC layer thickness of approximately 50 μm represents a significant challenge. Even with the interlayer strategy, the IMC layer remains a weak point in the joint. Further optimization of interlayer composition, thickness, and welding parameters could potentially reduce IMC thickness or modify the IMC morphology to improve joint strength. The presence of the Cd element in the interlayer also raises environmental and health concerns that must be addressed for industrial deployment, as cadmium is a toxic heavy metal with strict regulatory limits on workplace exposure.

Study Insights and Reflections

This research demonstrates a creative approach to the challenging problem of Al-Mg dissimilar metal welding. The use of a Zn-Cd interlayer to modify interfacial microstructure and improve joint strength is a valuable contribution to the field of dissimilar metal joining. The detailed characterization of IMC layer formation, including the identification of specific phases and their spatial distribution, provides the metallurgical understanding necessary for further optimization. The crack initiation and propagation analysis using SEM in situ tensile testing is particularly informative, as it directly links microstructural features to mechanical failure behavior. The tensile strength of 46.8 MPa, while not sufficient for primary structural applications, represents a meaningful improvement that could enable the use of Al-Mg joints in secondary structural components or in applications where weight savings outweigh strength requirements. Future research should explore alternative interlayer materials, including those without toxic elements, and investigate the effects of post-weld heat treatment on IMC layer modification and joint property improvement. The methodology presented here provides a framework for evaluating interlayer strategies in dissimilar metal welding that can be adapted to other material systems.