TIG Brazing of Magnesium Alloy and Stainless Steel Using Brass Interlayer

Literature Overview

The study by Kong Lingming and colleagues from Xi'an University of Science and Technology, published in Hot Working Technology in 2017, investigates a novel TIG brazing approach for joining AZ31B magnesium alloy to 304 stainless steel using a 0.3 mm thick H62 brass sheet as an intermediate layer. This work addresses the fundamental metallurgical challenge of joining two highly dissimilar materials that are not directly weldable due to vast differences in melting point, thermal expansion coefficient, and chemical affinity. The lap joint configuration was selected for its geometric simplicity and practical relevance in lightweight structural applications.

Metallurgical Challenges and Solution Strategy

The direct welding of magnesium alloys to stainless steel is generally considered impractical due to several inherent incompatibilities:

• Melting point disparity: AZ31B magnesium alloy melts at approximately 450 °C, while 304 stainless steel melts at approximately 1400 °C.
• Thermal expansion mismatch: Magnesium alloys have a thermal expansion coefficient roughly three times that of austenitic stainless steel, leading to severe residual stresses and potential joint failure during cooling.
• Chemical incompatibility: Direct contact between molten magnesium and molten steel leads to the formation of brittle intermetallic compounds that severely degrade joint strength.

The introduction of a brass interlayer serves as a metallurgical buffer. Brass (H62, approximately 62% Cu and 38% Zn) has a melting point of approximately 900 °C, which is intermediate between the two base metals. During TIG welding, the brass melts first, creating a liquid pool that partially wets both the magnesium and steel surfaces, enabling a brazing-like joint formation without requiring complete melting of either base metal.

Experimental Results and Microstructural Analysis

The optimal welding parameters identified in the study are summarized below:

The microstructural analysis reveals that the brazed joint consists of three distinct regions: the weld zone, the brazing seam zone, and the fusion zone. In the brazing seam zone on the stainless steel side, irregular dissolution of the steel into the molten brass occurred, forming an embedded irregular microstructure. The brazing seam microstructure comprises α-Fe, α-Mg, and small amounts of Mg-Cu-Al eutectic distributed along α-Mg grain boundaries.

The irregular embedded microstructure on the steel side of the brazing seam is particularly significant. This morphology indicates that the brass melted and partially dissolved into the steel surface, creating mechanical interlocking features that substantially enhance the interfacial bond strength. This is analogous to the mechanism observed in diffusion bonding and mechanical interlocking in high-temperature brazing applications.

Engineering Implications and Practical Assessment

The achieved tensile strength of 130 MPa is a significant result for a dissimilar metal joint involving magnesium alloy. For context, the ultimate tensile strength of AZ31B magnesium alloy in the extruded condition is approximately 230 MPa, and the strength of 304 stainless steel is approximately 520 MPa. A joint strength of 130 MPa represents approximately 57% of the magnesium alloy's strength, which is considered acceptable for many non-critical structural applications.

From a quality control perspective, several considerations arise:

1. Fracture location analysis. The fracture occurring in the weld zone rather than at the interface suggests that the interfacial bond strength exceeds the weld zone strength. This is a favorable outcome, as it indicates the brass interlayer successfully created a metallurgical bridge between the two dissimilar materials.
2. Brittle intermetallic concern. The presence of Mg-Cu-Al eutectic along grain boundaries raises concerns about long-term joint reliability, particularly under thermal cycling or creep conditions. In engineering practice, such intermetallic phases are associated with reduced ductility and potential crack initiation sites.
3. Process reproducibility. The use of AC current at 70 A with a welding speed of 100 mm/min represents a narrow process window. Minor deviations in parameter settings could lead to incomplete wetting, excessive melting of the magnesium alloy, or insufficient bonding. Robust process parameter optimization and statistical process control would be essential for production implementation.

Study Insights and Forward Looking Perspective

This research demonstrates a creative approach to the dissimilar metal joining problem by leveraging a brass interlayer as a metallurgical compatibility bridge. The concept of using intermediate layers to mediate between incompatible base metals is well-established in brazing technology (e.g., nickel foil interlayers for aluminum-to-steel joints), and this work extends the principle to TIG welding. For engineers working on lightweight vehicle or aerospace structures where magnesium-to-steel joints are required, this approach offers a viable alternative to mechanical fastening, particularly where weight reduction and leak-tight joints are priorities. The key area for further investigation would be the long-term mechanical and corrosion performance of the joint under realistic service conditions, including thermal cycling, vibration fatigue, and exposure to corrosive environments.