Mechanical Properties of Magnesium-Steel Dissimilar Material TIG Brazing Joint

Literature Overview

This paper published in the Journal of Nanchang Hangkong University (Natural Science Edition) in 2014 by Cheng Dongxu, Peng Chi, Cheng Donghai, Chen Yiping, and Zhao Chao investigates the TIG brazing of AZ31B magnesium alloy to galvanized steel using AC TIG welding. The authors systematically varied welding parameters to optimize joint formation and measured the lap shear strength under different conditions. The study demonstrates that TIG welding is an effective method for joining magnesium alloy to galvanized steel, with the zinc coating facilitating wetting and bonding.

Core Technical Points

Dissimilar Material Joining Challenge

The joining of magnesium alloy to steel presents unique challenges due to:

• Large difference in thermal expansion coefficients (AZ31B: approximately 26 × 10⁻⁶/K; steel: approximately 12 × 10⁻⁶/K).
• Significant difference in melting points (AZ31B: approximately 450°C; steel: approximately 1500°C).
• Chemical incompatibility leading to intermetallic compound formation at the interface.
• Risk of excessive melting of the magnesium alloy when using conventional welding processes.

The TIG brazing approach addresses these challenges by using the zinc coating on galvanized steel as a brazing alloy. The zinc melts at 420°C, well below the melting point of both base metals, allowing the joint to be formed by melting only the zinc and a small amount of magnesium alloy without significantly affecting the steel substrate.

Optimal Welding Parameters

Interface Chemistry and Bonding Mechanism

The zinc coating on galvanized steel serves a dual function:

1. Wetting agent: Molten zinc wets the magnesium alloy surface, promoting capillary action and intimate contact between the two dissimilar materials.
2. Reaction layer formation: At the interface, zinc reacts with magnesium to form Mg-Zn intermetallic compounds (such as MgZn₂ and MgZn₄), which provide mechanical interlocking and chemical bonding between the two materials.

The thickness and composition of this intermetallic layer critically influence joint strength. Excessive intermetallic growth leads to a brittle interface that fails under load, while insufficient reaction results in weak bonding. The optimal welding parameters identified in this study produce an intermetallic layer of appropriate thickness that balances ductility and strength.

Mechanical Performance Analysis

The lap shear strength of 154.73 MPa achieved at optimal parameters is notable given the low melting point of the magnesium alloy and the inherent brittleness of Mg-Zn intermetallic compounds. The strength is primarily derived from:

• The ductile Mg-Zn eutectic phase that accommodates deformation at the interface.
• The mechanical interlocking between the molten magnesium alloy and the zinc coating.
• The controlled heat input that prevents excessive intermetallic growth.

Engineering Practice Implications

The TIG brazing of magnesium alloy to galvanized steel has potential applications in lightweight structural assemblies where weight reduction is critical, such as automotive, aerospace, and consumer electronics. The use of galvanized steel is advantageous because the zinc coating is readily available and provides inherent corrosion protection.

For engineering implementation:

• The zinc coating thickness on galvanized steel should be within the range of 20-50 μm to ensure adequate brazing alloy supply without excessive melting.
• Pre-weld cleaning of the magnesium alloy surface is essential to remove oxide layers that inhibit wetting.
• The shielding gas flow must be carefully controlled to prevent oxidation of the molten magnesium, which is highly reactive at elevated temperatures.
• Joint design should account for the differential thermal expansion between magnesium and steel, particularly in applications involving temperature cycling.

Study Insights and Reflections

This work demonstrates that the zinc coating on galvanized steel can serve as an in-situ brazing alloy for joining to magnesium alloy, eliminating the need for separate brazing filler metal application. This simplification of the process is a significant practical advantage. The AC TIG welding mode was chosen because the AC balance provides cathodic cleaning action on the magnesium alloy surface, removing the refractory magnesium oxide layer that would otherwise inhibit wetting.

The key limitation of this approach is the relatively low service temperature of the joint, limited by the melting point of the Mg-Zn eutectic (approximately 324°C). For applications requiring higher service temperatures, alternative joining methods such as friction stir welding or adhesive bonding may be more appropriate. Nevertheless, for room-temperature and moderately elevated temperature applications, this TIG brazing technique offers a simple, cost-effective, and reliable joining solution for magnesium-steel dissimilar material assemblies.