Zhuojin Pipe Fitting Co., LtdEffect of Galvanizing Layer and Welding Speed on Microstructure of Magnesium Steel MIG Weld Joints
Literature Overview
This paper by Leng Chen and colleagues from Jiangsu University of Science and Technology investigates the MIG arc welding of AZ31 magnesium alloy to both Q235 low-carbon steel and DX51D galvanized steel. The study systematically varies welding speed to examine its influence on weld bead geometry, interfacial microstructure, and hardness distribution. Published in the journal Thermal Processing Technology in 2024, this work addresses a critical engineering challenge: joining dissimilar lightweight materials where galvanization may serve as an unintended but beneficial metallurgical intermediary.
Core Technical Content
The research employs MIG arc as the heat source for connecting AZ31 Mg alloy to Q235 steel and DX51D galvanized steel substrates. The key finding is that the presence of a zinc galvanizing layer fundamentally alters the interfacial metallurgy during welding, promoting metallurgical bonding at the Mg/steel interface where direct AZ31/Q235 joints exhibit lack of fusion and cracking.
Key Technical Parameters
| Parameter | AZ31/Q235 Joint | AZ31/DX51D Joint |
|---|---|---|
| Optimal welding speed | 10 mm/s | 15 mm/s |
| Interface defects | Lack of fusion, cracking | Good metallurgical bonding |
| Maximum interface hardness | Below 250 HV | Up to 250 HV |
| Intermetallic phase | Limited interfacial reaction | β-Al₁₂Mg₁₇ phase enrichment |
| Interface layer thickness | Thin, discontinuous | Thickened with lower speed |
Microstructural Evolution with Welding Speed
As welding speed decreases, the AZ31/DX51D joint exhibits increased precipitation of β-Al₁₂Mg₁₇ intermetallic phase at the weld zone, accompanied by thickening of the interface layer. This behavior follows a classic diffusion-controlled growth mechanism where reduced cooling rates permit greater atomic interdiffusion between Mg, Al, and Zn species. The galvanizing layer acts as a reactive diffusion medium, reducing the thermodynamic barrier for intermetallic formation between the Mg alloy and the steel substrate.
Interpretation of the Galvanizing Layer Mechanism
The galvanizing layer serves multiple functions during the welding process. First, zinc has a significantly lower melting point (419°C) than both magnesium (650°C) and steel, meaning it melts first and creates a liquid film that wets both substrates simultaneously. Second, zinc acts as a surfactant, reducing surface tension at the Mg/steel interface and promoting capillary-driven wetting. Third, the Zn-rich liquid facilitates the formation of a graded intermetallic transition zone rather than a sharp interface, which is mechanically more robust.
The formation of β-Al₁₂Mg₁₇ phase is particularly significant because this intermetallic compound has a hexagonal close-packed structure with relatively good ductility compared to other Mg-Al intermetallics such as γ-Mg₁₇Al₁₂. The maximum hardness of 250 HV at the AZ31/DX51D interface indicates a well-bonded, metallurgically sound joint, whereas the AZ31/Q235 joint shows clear evidence of insufficient interfacial reaction.
Engineering Practice Implications
This research has direct relevance to lightweight structural applications where galvanized steel components must be joined to magnesium alloy parts. In automotive and aerospace industries, the use of pre-galvanized steel substrates for Mg/steel hybrid structures can be considered a process advantage rather than a contamination concern. Engineers should note that:
- Welding speed must be optimized differently for galvanized versus bare steel substrates
- Lower welding speeds promote thicker interfacial layers but may introduce brittleness if excessive
- The galvanizing layer thickness (typically 45-100 μm per side for DX51D) provides sufficient reactive material for interfacial bonding
- Post-weld heat treatment may be required to reduce intermetallic thickness if ductility is critical
Key Questions and Reflections
Several questions emerge from this study that warrant further investigation. The long-term stability of the Zn-containing intermetallic layer under thermal cycling or corrosive environments remains unaddressed. Additionally, the influence of galvanizing layer thickness on joint performance should be quantified systematically. The practical challenge of maintaining consistent galvanizing layer integrity during welding preparation (grinding, cleaning) also needs consideration in manufacturing settings.
Study Insights
This work demonstrates an elegant example of how a surface treatment, originally intended for corrosion protection, can inadvertently improve a welding process outcome. The galvanizing layer effectively solves the classic Mg/steel joining problem of poor wettability and insufficient interfacial bonding. For engineers working on dissimilar metal joining in lightweight structures, this finding suggests that substrate surface condition should be considered as a process variable rather than merely a quality control parameter to be eliminated. The approach of leveraging existing surface layers as metallurgical intermediaries represents a cost-effective solution to a persistent joining challenge.