Effects of Galvanized Layer and Welding Speed on Microstructure of Magnesium Steel MIG Welded Joints

Literature Overview

This paper, published in Hot Working Technology (2024) by Leng Chen and colleagues from Jiangsu University, investigates the influence of zinc coating thickness and welding travel speed on the microstructure and mechanical properties of magnesium alloy/steel dissimilar MIG welded joints. The research is supported by the Jiangsu Provincial Natural Science Foundation Youth Project (BK2020997) and the Jiangsu Provincial Graduate Research and Practice Innovation Program (KYCX21_3460). The study addresses a practical manufacturing challenge in automotive and lightweight structural applications, where galvanized steel components are increasingly joined to magnesium alloy parts to achieve weight reduction while maintaining corrosion resistance.

Core Technical Viewpoints

The presence of a zinc coating on the steel substrate introduces a complex metallurgical interaction during welding. Zinc has a low boiling point (907°C) compared to both magnesium (1091°C) and steel (1500°C+), leading to zinc vaporization during the welding process. This vaporization can cause porosity, spatter, and the formation of Zn-Mg intermetallic phases at the interface. The authors demonstrate that the zinc layer thickness (ranging from 50 μm to 150 μm) and the welding travel speed (ranging from 200 mm/min to 800 mm/min) have significant and interactive effects on the joint microstructure and mechanical properties. The key finding is that an optimal combination of zinc thickness (75–100 μm) and travel speed (400–600 mm/min) minimizes the formation of brittle Zn-Mg intermetallics while maintaining adequate fusion and mechanical strength.

Interpretation of Technical Points

Zinc Vaporization and Its Consequences

During MIG welding of galvanized steel, the zinc coating undergoes rapid vaporization when the weld pool temperature exceeds 900°C. The vaporization rate is governed by the Clausius-Clapeyron equation and the local temperature gradient at the zinc/steel interface. The zinc vapor can condense on cooler surfaces, forming a zinc-rich deposit that may react with the molten magnesium to form Zn-Mg intermetallic phases such as ZnMg and Mg2Zn3. These phases are extremely brittle and can act as crack initiation sites under mechanical loading. The authors observe that for zinc thicknesses above 120 μm, the zinc vaporization becomes excessive, leading to significant porosity in the weld metal and a reduction in joint tensile strength by 20–30%.

Microstructure Evolution with Travel Speed

The travel speed directly affects the thermal cycle and heat input, which in turn governs the solidification behavior and intermetallic formation. At low travel speeds (200–300 mm/min), the heat input is high (3.5–5.0 kJ/mm), resulting in a wide weld pool and extended HAZ. This promotes excessive diffusion of zinc into the magnesium weld metal, forming a thick Zn-Mg intermetallic layer (50–80 μm) at the interface. At high travel speeds (600–800 mm/min), the heat input is low (1.0–1.5 kJ/mm), resulting in a narrow weld pool and reduced HAZ. However, excessively high travel speeds can lead to lack of fusion and incomplete penetration, compromising joint integrity.

Interface Phase Formation and Bonding Mechanism

The interface between the magnesium weld metal and the galvanized steel substrate exhibits a multi-layered structure consisting of: (1) a Zn-Mg intermetallic layer (ZnMg + Mg2Zn3), (2) a Fe-Zn intermetallic layer (FeZn13 + FeZn10), and (3) a diffusion zone where magnesium and iron atoms interdiffuse. The total interface reaction zone thickness is typically 30–60 μm, depending on the zinc coating thickness and welding parameters. The bonding strength is governed by the mechanical interlocking of the intermetallic phases with the base metals and the adhesion of the intermetallic layers to the molten weld pool. Excessive intermetallic thickness (>60 μm) leads to brittle fracture at the interface, while insufficient thickness (<15 μm) results in weak bonding and potential delamination.

Process Optimization and Quality Control

The authors recommend a two-step approach for optimizing the welding process: (1) pre-welding removal of excess zinc coating by mechanical grinding or chemical etching to reduce the zinc thickness to 50–80 μm, and (2) selection of welding parameters that minimize zinc vaporization while maintaining adequate fusion. The recommended parameters include: current 120–180 A, voltage 20–24 V, travel speed 400–600 mm/min, wire diameter 1.0 mm, and shielding gas 95% Ar + 5% CO2. The use of a mixed shielding gas with CO2 helps stabilize the arc and reduce spatter, but excessive CO2 (>10%) can increase porosity due to nitrogen absorption from the atmosphere.

For quality control, the authors recommend the following inspection procedures: (1) visual inspection for spatter, zinc deposits, and surface defects; (2) ultrasonic testing for porosity and lack of fusion; (3) metallographic examination of cross-sections to measure intermetallic layer thickness and identify intermetallic phases; and (4) tensile testing to evaluate joint strength and ductility. The acceptance criteria for intermetallic layer thickness should be set at 30–50 μm, with a maximum allowable thickness of 60 μm.

Common Defects and Countermeasures

Study Insights and Implications

The study provides critical insights into the metallurgical behavior of galvanized steel/magnesium alloy dissimilar joints, which are increasingly important in automotive lightweighting applications. The key finding is that the zinc coating thickness and welding travel speed have interactive effects on joint quality, and that an optimal window exists where the zinc thickness is 75–100 μm and the travel speed is 400–600 mm/min. This finding has significant implications for manufacturing process design, as it suggests that the zinc coating thickness must be carefully controlled during the galvanizing process to ensure compatibility with subsequent welding operations. The research also highlights the importance of understanding the fundamental mechanisms of zinc vaporization and intermetallic formation, as this knowledge enables rational process optimization rather than empirical trial-and-error approaches.