Zhuojin Pipe Fitting Co., LtdEffect of Galvanized Layer and Welding Speed on Microstructure of Magnesium-Steel MIG Hybrid Welds
Literature Overview
The paper authored by Leng Chen, Wang Xiaoyong, Peng Hongbing, and Chen Tianwei from Jiangsu University of Science and Technology investigates the influence of galvanizing layers and welding speed on the microstructure of AZ31 magnesium alloy to Q235 low-carbon steel and DX51D galvanized steel MIG hybrid-brazing joints. Published in Thermal Processing Technology (2024, Vol. 53, No. 19, pp. 44-49), this work is funded by the Jiangsu Provincial Natural Science Foundation Young Project (BK2020997) and the Jiangsu Provincial Graduate Research and Practice Innovation Plan Project (KYCX21_3460). The study addresses a critical challenge in dissimilar lightweight-material joining: achieving reliable metallurgical bonding between magnesium alloys and steels without interfacial cracking or lack of fusion.
Core Technical Findings
The researchers employed MIG arc as the heat source to join AZ31 magnesium alloy with both Q235 bare steel and DX51D galvanized steel, systematically varying the welding speed to examine its effects on weld bead geometry, microstructure, and hardness distribution. The key findings reveal that optimal weld bead formation was achieved at welding speeds of 10 mm/s and 15 mm/s for both joint configurations.
A particularly significant observation is that as welding speed decreases, the AZ31/DX51D joint exhibits more precipitation of the β-Al₁₂Mg₁₇ intermetallic phase at the weld zone, accompanied by an increase in interfacial layer thickness. The AZ31/DX51D joint demonstrates superior interfacial bonding quality, whereas the AZ31/Q235 joint exhibits lack of fusion and cracking at the interface. This finding indicates that the zinc galvanizing layer effectively promotes metallurgical bonding at the magnesium-steel interface. The maximum hardness at the AZ31/DX51D joint interface reaches 250 HV, which is higher than the corresponding hardness at the AZ31/Q235 joint interface.
Technical Analysis of the Galvanizing Layer Mechanism
The mechanism by which the zinc galvanizing layer promotes interfacial bonding is of considerable engineering significance. Zinc, being more reactive than iron, preferentially dissolves into the molten magnesium pool during welding, forming a Zn-Mg intermediate layer that acts as a diffusion bridge between the two dissimilar substrates. This intermediate layer reduces the direct contact between magnesium and iron, thereby suppressing the formation of brittle Mg-Fe intermetallic compounds that are known to be responsible for interfacial cracking.
| Parameter | AZ31/Q235 Joint | AZ31/DX51D Joint |
|---|---|---|
| Optimal Welding Speed | 10, 15 mm/s | 10, 15 mm/s |
| Interfacial Defects | Lack of fusion, cracking | Good bonding |
| Maximum Interface Hardness | Lower than 250 HV | Up to 250 HV |
| β-Al₁₂Mg₁₇ Precipitation | Less at low speed | More at low speed |
| Interfacial Layer Thickness | Relatively thin | Increases with lower speed |
The precipitation of β-Al₁₂Mg₁₇ phase is a direct consequence of the aluminum content in AZ31 alloy reacting with magnesium during the solidification process. At lower welding speeds, the extended heat input provides sufficient time for diffusion-controlled precipitation reactions, leading to a thicker intermetallic layer. While this layer enhances bonding integrity, excessive thickness could potentially compromise ductility, which warrants further investigation for structural applications.
Engineering Practice Implications
For engineers working on lightweight structural assemblies where magnesium alloy components must be joined to steel frames, this study provides actionable guidance. The use of galvanized steel substrates offers a practical solution to the well-known problem of poor Mg-Fe interface bonding without requiring additional interlayer materials or complex surface treatments. This approach is particularly relevant for automotive and aerospace applications where weight reduction is critical but steel structural elements remain necessary for crashworthiness or load-bearing requirements.
However, engineers must be mindful that the galvanizing layer thickness and composition can vary significantly between suppliers and specifications. The DX51D grade used in this study conforms to EN 10346 with a specific zinc coating mass, and deviations in coating thickness could affect the interfacial reaction dynamics. Process windows should be established through qualification welding for each specific galvanizing specification encountered in production.
Key Reflections and Study Insights
The most compelling insight from this work is the demonstration that a commercially available galvanizing layer can serve as a functional interlayer in Mg-steel hybrid joining, eliminating the need for expensive diffusion bonding or brazing filler metals. This finding opens a pathway toward cost-effective dissimilar material joining in mass production environments. The observation that welding speed directly controls interfacial reaction extent suggests that process parameter optimization is a viable lever for tuning joint properties without altering material specifications. Future work should investigate the mechanical properties under cyclic loading and the long-term corrosion resistance of these hybrid joints, as the intermetallic layers may introduce galvanic corrosion concerns in service environments.