Bypass Shunting MIG Arc Melt-Brazing Characteristics of Magnesium-Stainless Steel Dissimilar Metal Joints

Literature Overview

This study published in the Welding Journal (Vol. 35, No. 1, 2014, pp. 25–28) by Miao Yugang and colleagues from the College of Shipbuilding Engineering at Harbin Engineering University investigates the bypass shunting MIG arc melt-brazing process for joining AZ31 magnesium alloy to Q235 galvanized steel. The research was supported by the National Natural Science Foundation of China (Project 51005049) and addresses a critical need in lightweight structural applications where magnesium and steel components must be joined.

Technical Background

Magnesium alloys are the lightest structural metals with practical applications, offering a density of approximately 1.74 g/cm³ compared to 7.85 g/cm³ for steel. The AZ31 magnesium alloy, with its good combination of strength, formability, and cost-effectiveness, is widely used in automotive and aerospace lightweight structures. However, joining magnesium to steel is extremely challenging due to:

• A melting point difference of approximately 1000 °C (AZ31: 450 °C; Q235 steel: 1510 °C)
• Thermodynamic incompatibility leading to intermetallic compound formation
• Different thermal expansion coefficients causing residual stress
• The galvanic corrosion potential difference between magnesium and steel

The bypass shunting MIG arc melt-brazing process is designed to address these challenges by using a modified MIG arc configuration where a portion of the welding current is shunted through a bypass path, reducing the effective current density at the arc root. This modification affects the droplet transfer characteristics and heat input distribution, which are critical for achieving stable melt-brazing.

Process Description and Mechanism

In conventional MIG welding, the entire welding current flows through the electrode wire and arc. In the bypass shunting configuration, a portion of the current is diverted through an external path, effectively reducing the current density in the arc. This has several important consequences:

• Reduced arc force, which decreases the risk of excessive penetration and burn-through on the magnesium side
• Modified droplet transfer dynamics, as the electromagnetic pinch force on the molten droplets is reduced
• Lower heat input at the arc root, which limits the thermal damage to the magnesium alloy

The study specifically examines the droplet transfer process using high-speed camera imaging. The bypass shunting creates a repulsive droplet transfer mode, where the droplets are pushed away from the arc axis rather than being pulled toward the workpiece. This repulsive transfer increases the droplet transfer time, allowing for more uniform wetting and spreading of the molten metal across the joint interface.

Key Technical Findings

The study achieved stable and reliable welding with aesthetically pleasing bead formation. The key results are summarized below:

The tensile strength of 133 MPa represents 70% of the parent material strength, which is a significant achievement for a magnesium-steel dissimilar joint. The fracture occurring in the magnesium weld zone rather than at the interface indicates that the joint strength is limited by the magnesium alloy properties rather than by interfacial weakness. The ductile fracture morphology further confirms that the joint retains adequate toughness.

The high-speed camera analysis revealed that the repulsive droplet transfer mode increases the droplet transfer time, which allows for more uniform wetting and spreading of the molten metal across the joint interface. This is critical for achieving a continuous metallurgical bond without gaps or lack of fusion. The extended transfer time also promotes sufficient elemental diffusion at the interface, ensuring good atomic-level bonding between the magnesium and steel.

Engineering Practice Implications

The bypass shunting MIG arc melt-brazing process is particularly applicable to shipbuilding and marine engineering, where the authors' institutional affiliation (College of Shipbuilding Engineering) suggests practical relevance. In ship structures, magnesium components may be used for lightweight deck structures, superstructures, or auxiliary equipment, while steel remains the primary structural material. The ability to directly join these materials without transition elements or mechanical fasteners simplifies fabrication and reduces weight.

For production implementation, the following considerations are important:

• Electrode wire selection: A magnesium-based filler wire with appropriate composition is required to ensure good wetting and minimize IMC formation.
• Shielding gas: A high-purity argon or helium shielding gas is essential to prevent oxidation of the magnesium, which is highly reactive at elevated temperatures.
• Surface preparation: The galvanized steel surface must be cleaned to remove zinc coating and oxide layers to ensure proper wetting.
• Joint design: A lap joint configuration with appropriate overlap is recommended to maximize the joint area and minimize stress concentration.

The 70% joint efficiency is acceptable for non-critical structural applications but may be insufficient for primary load-bearing structures. Engineers should evaluate the joint strength requirements against the achieved joint efficiency and consider supplemental reinforcement (mechanical fasteners, adhesives) if higher strength is required.

Study Insights and Reflections

The bypass shunting concept is elegant in its simplicity—a modification to the current path that fundamentally changes the arc behavior without requiring complex equipment changes. The repulsive droplet transfer mode is a particularly interesting finding, as it demonstrates that the process can be tuned to achieve specific droplet transfer characteristics that are advantageous for melt-brazing.

The 70% joint efficiency is notable but should be viewed in context. For magnesium-steel joints, achieving 70% of the parent material strength (limited by the magnesium side) is a significant improvement over conventional fusion welding, which typically produces joints with 30–50% efficiency due to excessive IMC formation and cracking. The ductile fracture morphology further distinguishes this process from conventional approaches, where brittle interfacial fracture is common.

I would note that the study does not address the corrosion behavior of the joint. Magnesium-steel joints are subject to severe galvanic corrosion due to the large potential difference between the two metals (magnesium is approximately -1.6 V vs. SCE, while steel is approximately -0.6 V vs. SCE). In marine or corrosive environments, the magnesium side would corrode preferentially, potentially leading to premature joint failure. Corrosion protection strategies—such as coating, cathodic protection, or barrier layers—must be implemented in conjunction with the welding process.

Reference Value and Outlook

This paper demonstrates a practical approach to joining magnesium and steel that achieves acceptable strength and process stability. The bypass shunting MIG arc melt-brazing process is particularly relevant for lightweight structural applications in automotive, aerospace, and marine industries where weight reduction is a primary design driver.

Future research should focus on improving the joint efficiency beyond 70% through process optimization, filler material development, and joint design modifications. The corrosion resistance of the joint under various environmental conditions should be systematically evaluated, as this is a critical limitation for real-world applications. Additionally, the process should be investigated for other magnesium alloy grades (AZ91, AZ92, ZK60) and steel grades (stainless steel, high-strength low-alloy steel) to establish a comprehensive process window.

The combination of high-speed imaging, metallographic analysis, and mechanical testing provides a thorough characterization of the process and joint quality. This methodology serves as a template for future studies on dissimilar metal joining processes, where the interplay between process parameters, microstructure, and mechanical properties must be systematically understood.