Zhuojin Pipe Fitting Co., LtdTIG Welding of Magnesium-Aluminum Dissimilar Metals Using Zinc Filler Wire
Literature Overview
This paper, published in Welding Journal (2011, Vol. 32, No. 10, pp. 49-52), was authored by Liu Fei, Zhang Zhaodong, and Liu Liming from the Liaoning Key Laboratory of Advanced Joining Technology at Dalian University of Technology. Funded by the Central University Basic Research Fund and the Liaoning Provincial Doctoral Startup Fund, this study investigates a novel approach to joining dissimilar magnesium and aluminum alloys using conventional TIG arc welding with zinc filler wire.
Technical Approach and Results
The researchers employed standard TIG arc as the heat source to weld AZ31B magnesium alloy to 6061 aluminum alloy, using zinc wire as the filler material. The joints were characterized using optical microscopy, electron probe microanalysis (EPMA), and universal tensile testing.
Weld Metal Microstructure
The resulting weld metal is a zinc-based alloy, consisting primarily of MgZn₂ intermetallic compound and minor solid solutions of aluminum and zinc. This is a remarkable finding, as the zinc filler wire acts as a metallurgical bridge between the two dissimilar base metals, forming an intermediate alloy phase rather than simply diluting with either parent metal.
Interface Characteristics
The interface between the weld metal and the aluminum alloy base metal shows no distinct transition layer, indicating good metallurgical compatibility in that direction. However, a transition layer approximately 20-100 μm thick exists between the weld metal and the magnesium alloy base metal. This asymmetry is attributed to the different diffusion rates and intermetallic formation tendencies of aluminum and magnesium in the zinc-rich environment.
Mechanical Properties
The weld metal hardness exceeds that of both the 6061 aluminum alloy and AZ31B magnesium alloy base metals, which is attributed to the formation of the hard MgZn₂ intermetallic phase. The tensile strength of the butt joint reaches 75 MPa, which, while modest in absolute terms, represents a functional connection between two metals that are traditionally considered extremely difficult to join by fusion welding.
| Characteristic | Observation |
|---|---|
| Weld Metal Composition | Zinc-based alloy (MgZn₂ + minor Al-Zn solid solution) |
| Weld-Aluminum Interface | No distinct transition layer |
| Weld-Magnesium Interface | Transition layer 20-100 μm thick |
| Weld Hardness | Higher than both base metals |
| Joint Tensile Strength | 75 MPa |
Discussion of Engineering Relevance
The challenge of joining magnesium and aluminum alloys by fusion welding has long been considered one of the most difficult problems in welding metallurgy. The extreme difference in melting points (AZ31B: ~650°C; 6061 Al: ~650°C but with different solidification behavior), the formation of brittle intermetallics (Mg-Al phases such as Mg₅Al₈), and the large thermal expansion coefficient mismatch all contribute to poor joint quality with conventional fusion welding approaches.
The use of zinc filler wire is an elegant solution because zinc (melting point 420°C) is sufficiently reactive with both magnesium and aluminum to form stable intermetallics without creating the brittle Mg-Al phases. The resulting MgZn₂ phase provides adequate strength while maintaining some degree of ductility at the interface.
From a pipe fitting engineering perspective, this technique could potentially be applied to lightweight structural joints where magnesium and aluminum components must be connected. However, the relatively low tensile strength (75 MPa) limits its application to non-load-bearing or secondary structural connections. For primary load-bearing pipe joints, alternative joining methods such as friction stir welding or brazing would still be preferred.
Study Insights and Outlook
This work demonstrates that unconventional filler metal selection can overcome fundamental metallurgical incompatibilities in dissimilar metal welding. The zinc-based approach opens a new design space for joining light alloys, and further optimization of filler wire composition, wire diameter, and welding parameters could potentially improve joint strength significantly. The transition layer thickness variation (20-100 μm) also suggests that welding parameters can be tuned to control the interfacial reaction, which is critical for achieving consistent production quality.