Laser-MIG Brazing of Magnesium AZ31B and Copper-Plated Q235 Steel Joints

Literature Overview

This 2019 paper by Wu Shan, Shi Fangchang, and Zhang Lei from Jiangsu University, published in Hot Working Technology (Vol. 48, No. 11, pp. 53-56), investigates the laser-MIG brazing connection of AZ31B magnesium alloy and Q235 steel with a 20 μm electroplated copper interlayer. The study examines the macroscopic morphology, microstructure, and mechanical properties of the brazed joints under various welding parameters, with particular attention to the interfacial reaction layer formation.

Core Technical Content and Metallurgical Analysis

The laser-MIG hybrid brazing process combines the deep, narrow penetration of laser welding with the deposition capability of MIG welding. In this application, the laser provides the primary heat source for melting the magnesium alloy and the copper interlayer, while the MIG arc deposits AZ31B magnesium alloy wire as filler material. The Q235 steel base metal remains in a solid state throughout the process, with the copper interlayer serving to control the interfacial reaction and improve wettability.

Interfacial Microstructure Analysis

Zone	Location	Microstructure	Key Phases
Steel substrate	Q235 steel surface	Ferrite-pearlite (unaffected)	α-Fe, Fe₃C
Copper interlayer	Steel surface	Dissolved into joint	Cu dissolved in Al-Mg matrix
Reaction layer (outer)	Adjacent to steel	3-5 μm, dual-layer structure	AlFe₃, Al₂Cu₃
Reaction layer (inner)	Adjacent to weld metal	Fine grain transition zone	α-Mg solid solution, Mg₂Cu, AlCuMg
Weld zone (near reaction)	Transition to weld center	Mixed microstructure	α-Mg solid solution, Mg₂Cu, AlCuMg
Weld zone (center)	Filler metal region	Coarse grain structure	α-Mg solid solution, Al₁₂Mg₁₇

The formation of a 3-5 μm interfacial reaction layer at 6 mm/s welding speed is critical to the joint's mechanical integrity. The dual-layer structure, consisting of AlFe₃ and Al₂Cu₃ compounds, represents a controlled reaction between the aluminum-rich weld metal and the iron and copper from the steel and interlayer respectively.

Process Parameter Optimization

The optimal welding parameters identified in the study produce a tensile strength of 188.97 MPa, which represents a substantial fraction of the AZ31B base metal strength (approximately 230 MPa ultimate tensile strength). The parameter optimization reveals important relationships between heat input and joint quality.

Parameter	Optimal Value	Effect of Increase	Effect of Decrease
Laser power	1.5 - 2.0 kW	Excessive reaction layer, steel melting	Incomplete bonding
Welding speed	6 mm/s	Thinner reaction layer, weaker bonding	Thicker reaction layer, brittle fracture
MIG current	120 - 180 A	More filler deposition, wider joint	Insufficient filler, lack of fusion
Wire feed speed	4 - 6 m/min	Excess filler, porosity	Insufficient filler, undercut
Shielding gas	100% Ar	—	Oxidation if CO₂ introduced
Gap between parts	0.5 - 1.0 mm	Incomplete wetting	Excessive penetration

Engineering Practice and Application Considerations

The magnesium-steel dissimilar metal joint is of significant practical interest in lightweight vehicle design, where magnesium alloy components are increasingly used for weight reduction while steel remains the dominant structural material. The laser-MIG brazing approach offers a practical solution to the fundamental challenge of joining these materials, which have vastly different melting points (650°C for Mg vs. 1510°C for steel) and thermal expansion coefficients.

The copper interlayer serves a dual purpose: it prevents the direct reaction between magnesium and iron, which would form extremely brittle intermetallic compounds (such as Mg₂Fe and MgFe), and it improves the wettability of the aluminum-rich filler metal on the steel surface. The electroplating method of applying the copper interlayer is practical for production environments, though the thickness control (20 μm in this study) must be maintained within tight tolerances.

FMEA Analysis of Potential Failure Modes

Failure Mode	Cause	Detection Method	Prevention/Countermeasure
Excessive reaction layer	High heat input, slow speed	Metallographic examination	Optimize laser power and speed
Incomplete bonding	Low heat input, large gap	UT, tensile testing	Reduce gap, increase power
Steel melting	Excessive laser power	Visual inspection, RT	Reduce laser power, increase speed
Porosity	Contamination, improper shielding	RT, PT	Ensure clean surfaces, proper gas flow
Hydrogen embrittlement	Residual moisture	Delayed cracking observation	Dry all materials, control environment

Key Questions and Reflections

The study provides valuable data on interfacial reaction layer formation, but raises questions about long-term joint durability. The AlFe₃ and Al₂Cu₃ phases at the interface are thermodynamically stable but may be susceptible to corrosion in aggressive environments. Additionally, the thermal cycling behavior of the joint, where the significant difference in thermal expansion between magnesium and steel creates cyclic stresses, warrants further investigation for applications involving temperature fluctuations.

The tensile strength of 188.97 MPa, while impressive for a dissimilar metal brazed joint, should be evaluated in the context of the joint's ductility and fracture mode. The brittle intermetallic phases at the interface suggest that fracture would likely occur at the interface with minimal plastic deformation, which is a concern for impact loading and fatigue applications.

Study Insights and Practical Recommendations

This research demonstrates that laser-MIG brazing with a copper interlayer is a viable approach for joining magnesium alloy to steel, with the interfacial reaction layer being the critical factor controlling joint strength. For production applications, the following recommendations emerge: maintain copper interlayer thickness between 15-25 μm for optimal reaction control; optimize welding speed to balance reaction layer thickness against bonding quality; and implement rigorous surface preparation protocols to ensure clean interfaces. The approach offers a practical pathway for incorporating magnesium alloy components into steel structures without the need for extensive design changes, making it particularly relevant for automotive and transportation applications where weight reduction is a primary design driver.