Microstructure and Mechanical Properties of Mg/Steel Laser-MIG Brazing Joints

Literature Overview

The paper by Wu Shan, Shi Fangchang, and Zhang Lei (2019), published in Hot Working Technology, investigates the laser-MIG brazing of AZ31B magnesium alloy to copper-plated Q235 steel. This study addresses the challenging problem of joining dissimilar metals, specifically the combination of lightweight magnesium alloy with structural steel, which has potential applications in automotive and aerospace lightweighting strategies. The authors employed a laser-MIG hybrid process, using AZ31B magnesium alloy wire as filler material, to create brazed joints between AZ31B magnesium alloy and Q235 steel pre-treated with a 20 μm electroplated copper layer.

Core Technical Approach and Process Parameters

The laser-MIG hybrid brazing process combines the high energy density and deep penetration of laser welding with the high deposition rate of MIG welding. In this application, the process is configured as a brazing operation rather than a fusion weld, meaning that the base metals (magnesium alloy and steel) remain solid while the filler metal melts and flows to form the joint. The copper plating on the steel surface serves as a diffusion barrier to prevent excessive intermetallic compound formation at the steel interface, which would otherwise lead to brittle, thick reaction layers that compromise joint strength.

The key process parameter investigated was welding speed, with 6 mm/s identified as the optimal condition. At this speed, the steel-side interface reaction layer was 3-5 μm thick and exhibited a bilayer structure. The thermal input was carefully controlled to achieve sufficient wetting of the filler metal on both base metals while minimizing intermetallic compound growth.

Microstructural Analysis

The microstructural examination revealed a complex gradient of phases across the joint interface. On the steel side, the interface reaction layer was composed of AlFe3 and Al2Cu3 compounds, as confirmed by EDS and XRD analysis. Adjacent to the reaction layer, in the weld region, the microstructure consisted of α-Mg solid solution with Mg2Cu and AlCuMg compounds. The center of the weld zone contained α-Mg solid solution and Al12Mg17 compounds.

This microstructural gradient is characteristic of brazed joints and reflects the diffusion of elements between the base metals and the filler metal. The bilayer structure of the reaction layer on the steel side suggests that two distinct intermetallic phases formed sequentially during the brazing process, likely driven by different diffusion rates and thermodynamic driving forces.

The presence of Al2Cu3 in the reaction layer is notable because it indicates that copper from the plating layer diffused into the joint and reacted with aluminum from the filler metal. This is expected behavior in copper-plated steel brazing, but the thickness of the reaction layer must be controlled to prevent excessive brittleness.

Mechanical Performance

Under optimal welding parameters, the tensile strength of the Mg/steel laser-MIG brazing joint reached a maximum of 188.97 MPa. This value represents a significant achievement for a Mg/steel dissimilar metal joint, given the inherent challenges of joining these materials. The tensile strength is influenced by several factors including the thickness of the intermetallic compound layer, the quality of wetting at both interfaces, and the microstructural homogeneity of the weld zone.

The relationship between welding parameters and joint strength is nonlinear. Insufficient heat input results in poor wetting and incomplete joint formation, while excessive heat input promotes thick intermetallic compound formation, which reduces joint ductility and strength. The optimal parameter window is narrow, requiring precise control of laser power, MIG current, travel speed, and wire feed rate.

Engineering Practice and Application Potential

The Mg/steel laser-MIG brazing process offers a promising solution for lightweight structural applications where the combination of high-strength steel and lightweight magnesium alloy is desired. The copper plating on the steel surface is a critical pre-treatment step that enables controlled intermetallic formation and improves joint strength. However, the process requires careful parameter optimization and quality control to ensure consistent joint performance.

For industrial implementation, several challenges must be addressed. The copper plating process adds a manufacturing step and cost, and the plating thickness must be controlled to ensure uniform coverage. The laser-MIG hybrid equipment is more complex and expensive than conventional welding systems. Additionally, the narrow parameter window for optimal joint formation requires robust process monitoring and control systems to maintain consistency in production environments.

Study Insights

This research demonstrates that laser-MIG brazing, combined with appropriate surface preparation, can produce Mg/steel joints with acceptable mechanical properties. The key to success lies in controlling the intermetallic compound layer thickness through careful management of welding heat input. For engineers working on lightweight structural design, this approach offers a viable alternative to conventional fusion welding, which often produces thick, brittle intermetallic layers when joining magnesium alloy to steel.