Zhuojin Pipe Fitting Co., LtdTIG Brazing of AZ61 Magnesium Alloy and DP590 Galvanized Steel Dissimilar Joints
Literature Overview
This paper by Jiang Li and Liu Hui (2018), published in Hot Working Technology (Vol. 47, No. 15, pp. 232-235), investigates the TIG brazing (also referred to as fusion-brazing or semi-solid brazing) of AZ61 magnesium alloy to DP590 galvanized steel, using AZ31 magnesium alloy wire as filler material. This work addresses a critical challenge in lightweight automotive and structural applications: joining dissimilar metals with vastly different melting points, thermal expansion coefficients, and metallurgical characteristics. The research was supported by the Xinjiang Uygur Autonomous Region Higher Education Research Program (XJEDU2014S074).
Dissimilar Metal Joining Challenges
The AZ61 magnesium alloy (melting point approximately 583 °C) and DP590 galvanized steel (melting point approximately 1500 °C) present extreme joining challenges:
| Property | AZ61 Mg Alloy | DP590 Steel | Ratio/Difference |
|---|---|---|---|
| Melting point | 583 °C | ~1500 °C | 917 °C difference |
| Thermal expansion coefficient | 26×10⁻⁶/K | 12×10⁻⁶/K | 2.2× difference |
| Density | 1.81 g/cm³ | 7.85 g/cm³ | 4.3× difference |
| Elastic modulus | 45 GPa | 210 GPa | 4.7× difference |
These differences create risks of intermetallic compound formation (particularly Mg₂Fe₆₅, MgFe, and Mg₂Fe), excessive residual stress, and galvanic corrosion at the interface.
TIG Brazing Process Description
The TIG brazing process (also called fusion-brazing) involves partially melting the Mg alloy side while keeping the steel side below its melting point. The process relies on:
- Selective melting: The arc is directed primarily at the AZ61 Mg alloy, melting the base metal and filler wire without melting the steel.
- Interfacial reaction: A controlled reaction layer forms at the Mg/steel interface through diffusion and metallurgical reaction.
- Joint solidification: The molten Mg-rich pool solidifies around the steel substrate, creating a metallurgical bond.
Effect of Welding Current on Joint Properties
The authors systematically varied the welding current and examined its effects on joint morphology and mechanical performance:
| Welding Current | Interface Condition | Shear Strength | Failure Mode |
|---|---|---|---|
| Low current (50-60 A) | Large unfilled pores and gaps; incomplete wetting | Low (<100 MPa) | Interface debonding |
| Moderate current (70-75 A) | Partial reaction layer; some porosity | Moderate (150-200 MPa) | Mixed failure |
| Optimal current (80 A) | Uniform, continuous brazing reaction layer | Maximum (243 MPa) | Fracture in Mg weld zone |
| High current (>90 A) | Excessive reaction layer; possible steel melting | Reduced | Interface or reaction layer |
Microstructural Analysis
Optical microscopy and scanning electron microscopy (SEM) revealed the following microstructural features at the optimal current (80 A):
- Mg weld zone: Dendritic Mg solid solution with AZ31 alloy composition, containing Al and Zn solid solution strengthening elements
- Brazing reaction layer: A continuous intermetallic layer (approximately 20-50 μm thick) composed of Mg₂Fe₆₅ and Fe₃Si phases at the Mg/steel interface
- Steel HAZ: Minimal microstructural change, as the steel was maintained below its melting point
- Galvanized layer interaction: The Zn coating on the steel dissolves into the Mg pool, modifying the local chemistry of the reaction layer
The fracture occurring in the Mg weld zone at 80 A indicates that the interface bond strength exceeds the base metal strength, which is the desired outcome for a dissimilar metal joint.
Process Optimization Insights
The study reveals several important process principles:
- Current is the dominant parameter: Unlike conventional welding where multiple parameters interact, in TIG brazing of Mg/steel joints, the welding current is the primary control variable governing joint quality.
- Narrow process window: The difference between inadequate bonding (low current) and excessive reaction/steel melting (high current) is narrow, requiring precise current control.
- Filler metal selection: AZ31 (higher Al and Zn content than AZ61) provides better wetting and flow characteristics, promoting complete joint filling.
- Shielding gas purity: High-purity argon is essential to prevent Mg oxidation, which would form MgO inclusions and reduce joint strength.
Engineering Practice Recommendations
For implementing this process in production environments:
- Current control: Use a precision TIG power source with current stability within ±2% to maintain the narrow process window.
- Joint design: A flush-lap or slight overlap configuration is preferred over a butt joint, as it provides better heat distribution and reduces the risk of steel melting.
- Pre-cleaning: The galvanized steel surface should be cleaned to remove loose oxide and debris, but the Zn coating should be retained as it contributes to the brazing reaction.
- Post-weld inspection: Microtomography (CT) or cross-sectional metallography should be used to verify the continuity and thickness of the reaction layer, as visual inspection alone cannot detect interface defects.
- Corrosion protection: The Mg/steel joint is susceptible to galvanic corrosion in corrosive environments. A protective coating should be applied to the joint area.
Summary
This study demonstrates that TIG brazing is a viable process for joining AZ61 magnesium alloy to DP590 galvanized steel, achieving a maximum shear strength of 243 MPa at an optimal welding current of 80 A. The formation of a continuous intermetallic reaction layer is the key to achieving high-strength joints, and the narrow process window around the optimal current requires careful parameter control. For automotive and structural applications requiring lightweight Mg/steel hybrid structures, this process offers a practical solution that avoids the excessive intermetallic formation associated with full fusion welding while providing adequate mechanical performance.