Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of AZ31 Magnesium Alloy and Galvanized Steel Dissimilar Metal TIG Brazing Joints
Literature Overview
This paper by Chen Shijiang from Chongqing Electronic Engineering Vocational College, published in Hot Working Technology in 2017, investigates the microstructure and mechanical properties of dissimilar metal joints between AZ31B magnesium alloy and galvanized steel using TIG brazing (melting-brazing) technology. The study examines the effects of welding current on the joint quality, providing critical insights for the fabrication of lightweight hybrid structures that combine the weight advantages of magnesium alloy with the structural strength of steel.
Core Technical Findings
The study demonstrates that TIG brazing can achieve reliable connections between AZ31B magnesium alloy and galvanized steel, with the joint exhibiting a characteristic fish-scale morphology at the interface. The welding current was identified as the critical process parameter, with an optimal range identified for achieving the best combination of joint strength and interfacial integrity.
Welding Current Effects
The study identified three distinct regimes of welding current behavior:
| Current Range | Interface Condition | Joint Strength | Failure Mode |
|---|---|---|---|
| Low current | Large voids at Mg/steel interface | Low | Poor bonding, void-initiated failure |
| > 75 A (optimal) | Intermetallic compounds formed | Maximum (210 MPa) | Interfacial or near-interfacial |
| Excessive current | Thickened brittle reaction layer | Reduced | Brittle interfacial fracture |
At low welding currents, the insufficient heat input fails to achieve proper wetting and bonding at the magnesium/steel interface, resulting in the formation of large voids that severely compromise the joint strength. As the welding current increases beyond 75 A, the interfacial reaction progresses to form metal intermetallic compounds including Mg₃₂(Al,Zn)₄₉, Al₂Mg, and AlFe₃, which provide strong bonding between the two dissimilar materials. The maximum tensile strength of 210 MPa is achieved in this optimal current range.
However, continued increase in welding current beyond the optimal range leads to the progressive thickening of the brittle interfacial reaction layer, which degrades the joint mechanical properties. This over-reaction is a common challenge in dissimilar metal welding, where the thermodynamic driving force for intermetallic compound formation increases with temperature and time.
Intermetallic Compound Analysis
The formation of intermetallic compounds at the magnesium/steel interface is a critical aspect of the joint quality. The identified compounds include:
- Mg₃₂(Al,Zn)₄₉: A complex intermetallic compound that forms at the interface when aluminum and zinc from the galvanized coating interact with the magnesium alloy. This compound provides strong bonding but is inherently brittle.
- Al₂Mg: An aluminum-magnesium intermetallic compound that forms at the interface, contributing to the overall bonding strength.
- AlFe₃: An iron-aluminum intermetallic compound that forms at the steel side of the interface, indicating significant interaction between the galvanized coating and the steel substrate.
The presence of these intermetallic compounds is essential for achieving adequate joint strength, but their thickness and distribution must be carefully controlled. Excessive thickness of the intermetallic layer leads to brittle fracture, while insufficient thickness results in poor bonding. The optimal welding current of approximately 75 A represents the balance between achieving sufficient interfacial reaction and avoiding excessive brittle phase formation.
Fish-Scale Morphology
The characteristic fish-scale morphology observed at the interface is indicative of the brazing mechanism. In TIG brazing, the filler metal (AZ31B magnesium alloy wire) melts and flows into the gap between the base materials, forming a metallurgical bond through wetting and capillary action. The fish-scale pattern results from the interaction between the flowing filler metal and the solidifying interface, creating a distinctive morphology that is characteristic of brazed joints.
Engineering Practice Implications
Hybrid Structure Fabrication
The ability to join magnesium alloy to galvanized steel using TIG brazing has significant implications for the fabrication of lightweight hybrid structures. Magnesium alloys offer exceptional specific strength and weight advantages, while steel provides superior structural strength and toughness. The combination of these materials in hybrid structures can achieve optimal performance for weight-sensitive applications.
In the pipe and fitting industry, hybrid magnesium-steel structures could be beneficial for:
- Lightweight structural components in aerospace and automotive applications.
- Hybrid pipe fittings where weight reduction is critical but structural integrity must be maintained.
- Repair applications where magnesium alloy patches are bonded to steel pipe surfaces.
Process Control Requirements
The sensitivity of the joint quality to welding current highlights the importance of precise process control in dissimilar metal brazing. The following control parameters should be carefully managed:
- Welding current: Must be maintained within a narrow optimal range to achieve the desired interfacial reaction without excessive brittle phase formation.
- Welding speed: Affects the heat input per unit length and the duration of the interfacial reaction.
- Filler metal feed rate: Controls the volume of filler metal available for bonding and the composition of the joint.
- Joint gap: The gap between the base materials affects the filler metal flow and the interfacial reaction kinetics.
Quality Assurance Considerations
For production implementation of TIG brazing for magnesium-steel joints, the following quality assurance measures are recommended:
- Non-destructive testing (NDT) using ultrasonic testing (UT) or radiographic testing (RT) to detect voids and interfacial defects.
- Metallographic examination of cross-sections to evaluate the interfacial microstructure and intermetallic layer thickness.
- Tensile testing of coupon samples to verify the joint strength meets the required specification.
- Process parameter monitoring and documentation to ensure consistent welding conditions.
Study Insights and Reflections
This study provides valuable technical data for the development of dissimilar metal joining technologies between magnesium alloy and steel. The identification of the optimal welding current range and the characterization of the interfacial intermetallic compounds are critical contributions to the understanding of this joining process.
The maximum tensile strength of 210 MPa achieved at the optimal welding current is a significant result, although it is below the ultimate tensile strength of the AZ31B base material (approximately 230-260 MPa). This indicates that the joint strength is limited by the interfacial region rather than the base material, which is a common characteristic of dissimilar metal joints. Further research should focus on optimizing the interfacial microstructure to achieve joint strengths closer to the base material strength.
The fish-scale morphology observed at the interface is a useful quality indicator, as it can be visually inspected to assess the quality of the brazed joint. This visual assessment can serve as a quick and effective quality control tool in production environments.
The challenges identified in this study—void formation at low currents and excessive brittle phase formation at high currents—represent the fundamental trade-offs in dissimilar metal brazing. The narrow optimal processing window requires precise control of the welding parameters, which may necessitate automated welding equipment with real-time monitoring and feedback control. This technology holds promise for lightweight hybrid structure fabrication, but further development is needed to widen the process window and improve the reproducibility of high-quality joints.