Zhuojin Pipe Fitting Co., LtdEffect of Welding Current on Microstructure and Properties of AZ31B Magnesium Alloy MIG Welds
Literature Overview
Published in Light Alloy Fabrication Technology (2020, Vol. 48, No. 8, pp. 54–60), this study by Jin Yuting, Lin Guanglei, Lin Maochun, Ji Lianzhen, and Zhang Fuhui investigates the influence of welding current on the microstructure and mechanical properties of AZ31B magnesium alloy butt joints produced by MIG welding. The research was funded by a Fujian Provincial Industry-Academia Cooperation Project (2019H6021) and conducted in collaboration with Fujian Kunfu Co., Ltd., an industrial partner with practical manufacturing needs. This work is particularly relevant to the lightweighting trend in automotive and aerospace applications where magnesium alloy components are increasingly specified.
Experimental Design and Parameters
The experimental matrix covered welding currents from 70 A to 100 A, with AZ61A magnesium alloy filler wire selected to match the base metal composition. Characterization methods included micro-Vickers hardness testing, universal tensile testing, optical microscopy, and scanning electron microscopy (SEM) for microstructural analysis.
| Welding Current | Weld Width | Weld Reinforcement | Tensile Strength | Elongation |
|---|---|---|---|---|
| 70 A | Baseline | Highest | Lower | Moderate |
| 80 A | Moderate increase | Moderate | 227.5 MPa | 5.5% (maximum) |
| 90 A | Further increase | Decreasing | Slightly lower | Decreasing |
| 100 A | Maximum | Lowest | Reduced | Lowest |
Microstructural Analysis
The study reveals that within the tested current range (70–100 A), the weld width increases only modestly with rising current, while the weld reinforcement (convexity) exhibits a clear decreasing trend. This geometric behavior is attributed to increased heat input promoting lateral heat flow into the base metal, which reduces the vertical reinforcement while slightly broadening the fusion zone.
The selection of AZ61A filler wire is a critical process variable. The higher aluminum content in AZ61A (6.0–7.0 wt% Al) compared to AZ31B (2.5–3.5 wt% Al) results in a greater volume fraction of β-Mg₁₇Al₁₂ intermetallic phase within the weld metal. This strengthening phase is thermodynamically stable during solidification and provides significant precipitation hardening contribution. The SEM observations confirm that the β-phase particles are distributed within the dendritic grain structure of the weld metal, acting as effective strengthening agents.
The optimal performance was achieved at 80 A, where the tensile strength reached 227.5 MPa with an elongation of 5.5%. This represents the best compromise between strength and ductility, satisfying the requirements of GB/T 5156 for magnesium alloy structural applications. However, the authors note that both strength and ductility remain significantly lower than the base material, indicating that the weld zone remains the weakest link in the joint.
Standards Compliance and Engineering Assessment
The tensile strength requirement for AZ31B magnesium alloy per GB/T 5156 specifies a minimum of 180 MPa for the base material. The weld joint achieving 227.5 MPa exceeds this threshold, which is somewhat surprising given that the weld metal typically exhibits lower strength than the base material in magnesium alloys. This favorable outcome is attributed to the higher aluminum content of the AZ61A filler wire promoting more β-Mg₁₇Al₁₂ formation in the weld zone.
| Criterion | Requirement | Achieved | Status |
|---|---|---|---|
| Tensile strength (GB/T 5156) | ≥180 MPa | 227.5 MPa | Pass |
| Elongation | Not explicitly specified | 5.5% | Adequate |
| Weld geometry | Visual acceptance | Good profile | Pass |
Key Reflections and Engineering Implications
This study addresses a practical manufacturing challenge: the reliable joining of AZ31B magnesium alloy components using a cost-effective, high-productivity MIG welding process. The findings are particularly relevant for non-critical structural applications where the primary design consideration is static load-bearing capacity rather than fatigue or impact resistance.
The observation that welding current has a relatively modest effect on weld width but a pronounced effect on reinforcement height provides useful guidance for procedure development. In production welding of magnesium alloy pipe fittings or thin-walled structural components, maintaining current at approximately 80 A (for typical wire diameters of 1.0–1.2 mm) offers the best balance of weld geometry and mechanical properties. Higher currents risk excessive reinforcement reduction and potential under-cutting, while lower currents may produce insufficient fusion and porosity.
A critical limitation noted by the authors is the significant loss of ductility relative to the base material. For applications involving dynamic loading, impact, or crashworthiness requirements, MIG welding of AZ31B may not be suitable without additional post-weld treatments such as solution heat treatment and aging. The recommendation to use MIG welding for "non-rigid collision" components is prudent and should guide specification writers.
From a quality control perspective, the hardness gradient across the weld zone should be monitored. Magnesium alloy welds are susceptible to stress corrosion cracking, particularly in the HAZ where grain coarsening occurs. Engineers specifying MIG welded magnesium alloy joints should mandate appropriate surface treatments and corrosion protection systems.
This research provides a solid foundation for developing welding procedure specifications (WPS) for AZ31B magnesium alloy components, particularly in the automotive lightweighting sector where production efficiency is paramount.