Effect of External Longitudinal Magnetic Field on Plasma Arc Surfacing Layer Microstructure and Properties

Literature Overview

This study by Liu Zhengjun, Song Xingkui, Shao Dawei, and Zhao Qian from Shenyang University of Technology explores an innovative approach to controlling surfacing layer microstructure through the application of an external longitudinal magnetic field during plasma arc surfacing. Published in the Journal of Welding (Vol. 31, No. 1, 2010, pp. 71-74), this research represents a novel process parameter that can be used to tailor the morphology and distribution of hard phases in iron-based alloy overlay deposits.

Core Technical Approach

The researchers applied a longitudinal magnetic field during plasma arc surfacing of an iron-based alloy designated Fe5. The magnetic field was controlled through an external coil system, with the magnetic current varied to study its effect on deposit properties. The alloy Fe5 was selected as a representative iron-based surfacing alloy known for its potential to form hard carbide phases such as Cr7C3 and CrB.

The fundamental principle behind this approach is that the magnetic field exerts Lorentz forces on charged particles and influences the solidification behavior of the molten pool. In the context of surfacing, the magnetic field can affect dendrite growth direction, particle nucleation, and the morphology of intermetallic compounds formed during solidification.

Microstructural and Property Results

The study reveals that the application of a longitudinal magnetic field produces deposits with significantly higher hardness and improved wear resistance compared to deposits made without a magnetic field. The optimal magnetic current was found to be 3 A, at which the surfacing layer exhibited the best combination of hardness and wear resistance.

The microstructural analysis shows that the α and γ solid solution phases are significantly refined under the influence of the magnetic field. The hard phases Cr7C3 and CrB are obtained in ideal morphology and distribution when the optimal magnetic current is applied. This refinement and redistribution of hard phases directly contributes to the improved mechanical properties.

Mechanism of Magnetic Field Effect

The external longitudinal magnetic field influences the surfacing microstructure through several mechanisms. First, the Lorentz force acting on the moving molten pool enhances convective heat transfer, resulting in faster cooling rates and finer grain structures. Second, the magnetic field exerts a force on magnetic particles and carbide nuclei, promoting their uniform distribution rather than segregation at dendrite boundaries or grain boundaries. Third, the field can influence the crystallographic orientation of growing dendrites, favoring orientations that produce more uniform hard phase distribution.

At the optimal magnetic current of 3 A, these effects are balanced to produce the finest microstructure with the most uniform hard phase distribution. At higher magnetic currents, the enhanced convection may become too vigorous, leading to incomplete melting or uneven deposition, which degrades the deposit quality.

Engineering Practice Considerations

The application of magnetic fields during surfacing represents an advanced process control technique that could be particularly valuable for high-performance overlay applications. However, the practical implementation requires additional equipment (magnetic coils and power supplies) and process control considerations. Engineers considering this approach should evaluate whether the improved deposit properties justify the additional equipment and process complexity.

For applications where deposit performance is critical and conventional surfacing parameters cannot achieve the required properties, magnetic field-assisted surfacing offers a promising solution. The technique is particularly suitable for laboratory-scale production of high-performance coatings and for specialized industrial applications where the investment in magnetic field equipment can be justified by the value of the coated components.

Study Insights and Reflections

This research demonstrates that process physics can be leveraged to achieve microstructural control beyond what is possible through alloy composition alone. The magnetic field acts as a non-invasive tool that modifies solidification conditions without altering the chemical composition of the surfacing material. For engineers working on advanced surfacing applications, this approach opens new possibilities for property optimization. The key insight is that the optimal magnetic current is not simply the maximum achievable field strength but rather a specific value that balances competing solidification effects. Future research should explore the application of magnetic fields in combination with other process parameters such as cooling rate, feed rate, and substrate preparation to achieve synergistic microstructural improvements.