Effect of Welding Current on Microstructure and Mechanical Properties of Magnesium and Galvanized Steel TIG Brazing Joints

Literature Overview

This paper, published in the Journal of Engineering Sciences in 2016 by researchers from Chongqing University School of Materials Science and Engineering, investigates the influence of welding current parameters on the microstructure evolution and mechanical performance of magnesium-to-galvanized steel dissimilar joints produced via TIG brazing. The work was supported by the National Natural Science Foundation of China and central university basic research funding. The study is particularly significant in the context of lightweight structural applications where joining dissimilar metals—such as magnesium alloys to steel substrates—is essential for weight reduction without compromising structural integrity.

Core Technical Content

The researchers employed gas tungsten arc brazing (GTAB) to create dissimilar joints between magnesium alloy and hot-dip galvanized steel. The galvanization layer, typically composed of an iron-zinc intermetallic compound (Fe-Zn) with a zinc coating thickness ranging from 40 to 100 micrometers, introduces additional metallurgical complexity to the joint interface. The study systematically varied welding current from approximately 60 A to 140 A and examined the resulting microstructural changes across the brazed zone, heat-affected zone (HAZ), and base metal regions.

Microstructural Analysis

At lower current settings, the joint interface exhibits a relatively thin reaction layer with limited intermetallic formation. As current increases, the thermal input rises, promoting greater diffusion of iron and zinc into the magnesium matrix. The key intermetallic phases identified include MgZn₂ (β phase), MgZn (γ phase), and Mg₄₁Zn₄₉ (η phase). These phases form a gradient structure at the interface, with the composition transitioning from pure zinc near the galvanized coating to magnesium-rich phases near the base magnesium alloy.

Mechanical Performance

The tensile strength of the joints shows a non-monotonic relationship with welding current. At the optimal current window of 90–110 A, the joint achieves maximum tensile strength approaching 80–90 percent of the base magnesium alloy strength. Beyond this range, excessive intermetallic growth leads to brittle fracture at the interface. The fracture morphology transitions from ductile dimple rupture at optimal parameters to intergranular and quasi-cleavage fracture at high current levels.

Engineering Practice Implications

For engineers working in bimetal product manufacturing, this study offers critical insights into managing dissimilar metal joints where galvanization layers are present. The galvanization layer, while providing corrosion protection, acts as a diffusion barrier and source of intermetallic-forming elements. In pressure vessel fabrication, where cladding layers must maintain both mechanical integrity and corrosion resistance, understanding the current-dependent microstructural evolution is essential for process qualification.

Key Process Control Points

1. Current selection: The optimal welding current must balance sufficient heat input for complete wetting and bonding against excessive thermal input that drives brittle intermetallic formation.
2. Pre-heating considerations: For thicker galvanized substrates, controlled pre-heating to 200–250°C can reduce the required welding current while maintaining adequate joint formation.
3. Filler metal selection: The study implies that zinc-rich filler alloys facilitate the brazing process but require careful control to prevent excessive intermetallic thickening.

Defect Analysis and Countermeasures

The primary defects observed at excessive current levels include interface cracking, void formation at the brazed zone, and spalling of the galvanized layer. These defects arise from thermal stresses induced by the coefficient of thermal expansion mismatch between magnesium (approximately 26 × 10⁻⁶/K) and steel (approximately 12 × 10⁻⁶/K). The countermeasures include:

• Reducing welding current to stay within the optimal thermal window
• Employing pulsed TIG to control instantaneous heat input
• Applying flux or protective atmospheres to minimize oxidation during brazing
• Implementing post-weld heat treatment to relieve residual stresses

Study Insights and Reflections

This research highlights a fundamental challenge in dissimilar metal joining: the interplay between thermal input and intermetallic formation. In the context of cladding and bimetal pressure vessel fabrication, analogous challenges arise when overlaying dissimilar materials where diffusion-driven phase formation can compromise joint integrity. The systematic approach of varying a single parameter (welding current) while maintaining other conditions constant provides a clear methodology that can be adapted for process development in other dissimilar metal systems, such as nickel-based alloy cladding on carbon steel substrates.

The galvanization layer introduces an additional variable not always considered in standard cladding practice. In industrial settings, galvanized steel components are sometimes used as substrates for cladding operations, and the presence of zinc can significantly alter the weld pool chemistry and resulting microstructure. Engineers must account for this during process qualification and ensure that the galvanization layer is either removed or its influence is incorporated into the welding procedure specification.

This work reinforces the importance of microstructural characterization in understanding joint performance. The correlation between intermetallic phase morphology, thickness, and mechanical properties provides a direct link between process parameters and final product quality, which is essential for establishing robust welding procedure specifications in accordance with standards such as ASME IX or NB/T 47014.