Zhuojin Pipe Fitting Co., LtdEffect of Magnetic Field Frequency on TIG Welding of AZ91 Magnesium Alloy
Literature Overview
Published in Ordnance Materials Science and Engineering (2014, Vol. 37, No. 3, pp. 99-101), this paper by Qi Xiuling et al. from Shenyang University of Technology and Liaoning Technical University investigates the influence of externally applied longitudinal alternating magnetic field frequency on the TIG welding of AZ91 magnesium alloy plates. The study examines how varying magnetic field frequencies affect weld microstructure and mechanical properties, and proposes a mechanism for the magnetic field's influence on weld pool dynamics and solidification behavior.
AZ91 Magnesium Alloy Welding Challenges
AZ91 is a widely used wrought and cast magnesium alloy containing approximately 9% aluminum and 1% zinc. Its welding presents several challenges: high reactivity with oxygen and nitrogen leading to porosity and oxide formation; low thermal conductivity requiring careful heat input management; susceptibility to hot cracking due to the presence of β-Mg17Al12 phase; and grain coarsening in the heat-affected zone. The β-Mg17Al12 phase, which precipitates along grain boundaries in a continuous network, is particularly detrimental as it acts as a crack propagation path and significantly reduces ductility.
Magnetic Field Frequency Effects
The key finding is that an applied longitudinal alternating magnetic field at 20 Hz frequency produces optimal results: weld grain refinement, disruption of the continuous β-Mg17Al12 network distribution, and simultaneous improvement in strength, hardness, and plasticity of both the weld metal and HAZ. However, when the magnetic field frequency is excessively high, the weld microstructure becomes coarse and mechanical properties deteriorate.
| Magnetic Field Frequency | Microstructure | Mechanical Properties | Assessment |
|---|---|---|---|
| No magnetic field | Baseline (coarse grains, continuous β-network) | Baseline | Poor |
| 20 Hz | Fine grains, disrupted β-network | Improved strength, hardness, plasticity | Optimal |
| Excessively high | Coarse grains | Decreased strength and plasticity | Degraded |
Mechanism Analysis
The magnetic field exerts Lorentz forces on the electrically conductive molten weld pool. At 20 Hz, the oscillating magnetic field generates electromagnetic stirring that promotes convection within the weld pool, enhancing heat transfer and promoting grain refinement. The stirring action disrupts the directional solidification pattern, encouraging equiaxed grain formation. Simultaneously, the improved mixing helps dissolve and redistribute the β-Mg17Al12 phase, preventing its continuous network formation along grain boundaries.
At excessively high frequencies, the rapid oscillation of the magnetic field may cause turbulent flow in the weld pool that leads to increased entrainment of atmospheric gases, promoting porosity. Additionally, the high-frequency electromagnetic forces may create unstable arc conditions and excessive turbulence that promotes grain coarsening through enhanced grain growth during solidification. The optimal frequency of 20 Hz represents a balance between effective stirring for grain refinement and maintaining stable welding conditions.
Engineering Practice Implications
The application of magnetic fields during welding is not a new concept, but systematic investigation of frequency effects on magnesium alloys is valuable for practical implementation. Engineers considering this technique should evaluate: the cost and complexity of magnetic field generation equipment; the geometric constraints imposed by the magnetic field apparatus on weld access; the consistency and repeatability of results across different production conditions; and the scalability from laboratory specimens to production-scale components. The 20 Hz optimal frequency should be validated across different welding parameters (current, travel speed, shielding gas flow rate) to establish robust process windows for industrial application.