Pulsed DE-MIG Brazing of Aluminum to Galvanized Steel Dissimilar Metal Joints

Literature Overview

This paper by Gu Yufen, Shao Ling, Shi Yu, and Huang Jiankang, published in Materials Science and Technology (Vol. 22, Issue 4, 2014, pp. 118-123), presents a novel welding-brazing process for joining aluminum to galvanized steel: the pulsed dual-electrode MIG (Pulsed DE-MIG) method. The research was conducted at the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, supported by multiple National Natural Science Foundation of China grants. The work addresses a significant industrial challenge in the automotive and construction industries, where the combination of lightweight aluminum panels with galvanized steel structural components is increasingly common to reduce vehicle weight while maintaining corrosion resistance.

Core Technical Findings

The study proposes the Pulsed DE-MIG method as a means to precisely control welding heat input and thereby limit the growth of intermetallic compounds (IMCs) at the aluminum-steel interface. Using ER5356 aluminum filler wire in a surfacing experiment on galvanized steel, the researchers found that increasing the bypass current while maintaining constant total wire current significantly reduces the weld bead width and increases the weld buildup height. Microstructural analysis revealed the formation of Fe2Al5 and FeAl3 intermetallic compound layers at the interface. Critically, higher base metal heat input leads to more FeAl3 precipitation, while reducing heat input progressively decreases FeAl3 formation until it is completely eliminated.

Technical Interpretation

The Pulsed DE-MIG process is fundamentally a modification of the dual-electrode MIG concept, where two electrodes carry separate current paths that can be independently controlled. In this configuration, the total current delivered to the wire is the sum of the main arc current and the bypass current. By adjusting the ratio between these two currents, the operator can control the arc power (which determines the heat input to the base metal) independently of the wire melting rate (which determines the deposition rate). This decoupling of arc heat and wire feed is the key innovation that enables precise control of the thermal cycle at the aluminum-steel interface.

The formation of Fe2Al5 and FeAl3 intermetallic compounds at the aluminum-steel interface is a well-documented phenomenon. These IMCs are inherently brittle and can significantly reduce the joint strength and ductility. FeAl3 is particularly problematic because it forms at lower temperatures and is more brittle than Fe2Al5. The study's finding that reducing base metal heat input eliminates FeAl3 formation is metallurgically consistent with the phase diagram of the Fe-Al system, where FeAl3 forms at temperatures above approximately 600°C. By controlling the arc heat input to keep the base metal temperature below this threshold, the Pulsed DE-MIG process effectively suppresses the formation of the most detrimental IMC phase.

Process Engineering Considerations

The use of ER5356 filler wire is appropriate for this application because it is an Al-Mg-Si alloy with good fluidity and wetting characteristics on steel surfaces. The surfacing experiment approach, where the aluminum is deposited onto the galvanized steel substrate, is a practical method for evaluating the process parameters without the complexity of a full butt joint or lap joint configuration. However, engineers should recognize that the surfacing geometry may produce different thermal gradients and IMC formation characteristics compared to a lap joint or a butt joint with a backing material.

The galvanized coating on the steel substrate introduces an additional variable that is not fully addressed in this paper. The zinc coating melts at a much lower temperature than the steel substrate (419°C vs. approximately 1500°C), and the interaction between the molten zinc and the molten aluminum filler metal can affect the wetting behavior and the composition of the interfacial region. In practice, the zinc coating may partially evaporate during welding, creating a zinc vapor plume that can affect arc stability and weld quality. Engineers should consider pre-cleaning the galvanized surface or using a dedicated filler metal that is compatible with the zinc coating.

Connection to Engineering Practice

The Pulsed DE-MIG process is particularly relevant to the automotive industry, where the trend toward lightweight vehicles has driven the adoption of aluminum body panels on steel-based vehicle structures. The ability to join aluminum to galvanized steel without excessive IMC formation is critical for ensuring the structural integrity and durability of these mixed-metal assemblies. In my experience, the most common failure mode in aluminum-steel joints is interfacial fracture along the IMC layer, which can occur at stresses well below the yield strength of either parent metal.

For production implementation, the Pulsed DE-MIG process requires specialized equipment with dual-current control capability, which is not standard on all MIG welding machines. The process also requires careful parameter optimization for each specific joint configuration and material combination. However, the potential benefits in terms of joint quality and the ability to use lower heat input (which reduces distortion and residual stresses) make it an attractive option for high-quality dissimilar metal joints. The process is also compatible with automated and robotic welding systems, which is important for high-volume automotive production.

Key Questions and Reflections

The paper focuses on the surfacing experiment, which provides valuable fundamental data but does not directly address the mechanical properties of the joint. Engineers need to know not only the IMC thickness and composition but also the tensile strength, shear strength, and fatigue resistance of the completed joint. A follow-up study that includes mechanical testing of lap joints or butt joints produced by the Pulsed DE-MIG process would significantly enhance the practical applicability of these findings.

Another important consideration is the long-term corrosion resistance of the joint. The IMC layers at the aluminum-steel interface can act as galvanic couples, and the differential corrosion rates between the aluminum, the steel, and the IMC phases can lead to intergranular corrosion and eventual joint failure. The effect of the Pulsed DE-MIG process on the corrosion performance of the joint, particularly in salt spray environments, should be investigated.

Study Insights and Implications

This paper introduces a promising process innovation for dissimilar metal joining that addresses a fundamental challenge in the aluminum-steel welding field. The ability to independently control arc heat and wire feed through the Pulsed DE-MIG method provides a powerful tool for optimizing the thermal cycle and minimizing detrimental IMC formation. The finding that FeAl3 can be completely eliminated by reducing base metal heat input is particularly significant, as this IMC phase is the primary driver of joint embrittlement. Engineers should consider the Pulsed DE-MIG process as a viable option for high-quality aluminum-steel joints, particularly in applications where joint integrity is critical and the additional equipment investment can be justified. The paper represents a meaningful contribution to the development of advanced welding processes for dissimilar metal joining, and its findings should be further validated through mechanical and corrosion testing before widespread industrial adoption.