Effect of Transverse AC Pulsed Magnetic Field on Microstructure and Properties of Surfacing Metal

Scientific Background and Research Motivation

The application of external magnetic fields to welding and surfacing processes has attracted increasing attention as a non-invasive method to control solidification microstructure without altering consumable chemistry or process parameters. Among various magnetic field configurations, transverse AC pulsed magnetic fields offer unique advantages: they can be applied without contact, they induce electromagnetic stirring in the molten pool, and they can be modulated to target specific solidification regimes. This study investigates how a transverse AC pulsed magnetic field (frequency: 50 Hz, peak intensity: 0.5–2.0 T) affects the microstructure and mechanical properties of surfacing metal deposited by submerged arc welding (SAW).

Mechanism of Magnetic Field Interaction with Molten Pool

The interaction between a transverse AC pulsed magnetic field and the molten weld pool involves three primary mechanisms:

1. Electromagnetic stirring: The time-varying magnetic field induces eddy currents in the conductive molten pool. The interaction between these eddy currents and the magnetic field generates Lorentz forces that drive fluid flow within the pool. This stirring effect promotes uniform temperature distribution and enhances heat and mass transport.
2. Grain refinement: The electromagnetic stirring disrupts the natural dendritic growth pattern by creating temperature gradients that favor equiaxed grain formation. The increased nucleation rate and reduced grain size are direct consequences of the enhanced fluid flow.
3. Dendrite fragmentation: The Lorentz forces can mechanically fragment growing dendrites, creating additional nucleation sites. These fragments can also be transported by the fluid flow to regions of the pool where they act as heterogeneous nucleation substrates.

The study confirms that the optimal magnetic field intensity falls in the range of 1.0–1.5 T. Below 0.5 T, the stirring effect is too weak to significantly influence solidification. Above 2.0 T, the magnetic force begins to distort the arc column, leading to arc instability and potential porosity formation. The 50 Hz frequency was selected because it corresponds to the industrial power frequency and provides sufficient temporal variation to induce effective eddy currents without introducing high-frequency effects that could disrupt the welding process.

Microstructure Characterization

Grain Size and Morphology

Without magnetic field application, the surfacing deposit exhibits a columnar grain structure with an average grain size of 120–150 μm. The columnar grains grow perpendicular to the fusion line, following the thermal gradient direction. With the application of a 1.0 T transverse AC pulsed magnetic field, the grain structure transforms to a predominantly equiaxed morphology with an average grain size of 70–85 μm. This represents a 40% reduction in grain size and a complete transition from columnar to equiaxed growth.

The mechanism behind this transformation is well established: the electromagnetic stirring creates local temperature fluctuations that promote the detachment of dendrite tips, which then act as nucleation sites for equiaxed grains. The increased fluid flow also disrupts the thermal gradient that favors columnar growth, allowing equiaxed grains to compete effectively with columnar grains.

Phase Distribution and Inclusion Morphology

The magnetic field also influences the distribution and morphology of inclusions within the surfacing deposit. Without magnetic field, elongated oxide inclusions aligned with the solidification direction are common. With magnetic field application, the inclusions become more spherical and uniformly distributed. This is attributed to the electromagnetic stirring, which breaks up large inclusions and distributes them throughout the pool.

Mechanical Property Enhancement

The microstructural improvements translate directly into enhanced mechanical properties. The study measured hardness, tensile strength, elongation, and impact energy for surfacing deposits fabricated with and without magnetic field application.

The results demonstrate a clear trend: mechanical properties improve with increasing magnetic field intensity up to 1.5 T, beyond which the improvements plateau or slightly decline. The impact energy improvement is particularly significant—increasing from 18 J to 55 J at -40°C represents a 206% improvement, which is critical for low-temperature pressure vessel applications. This enhancement is attributed to the finer grain size, which increases the crack propagation resistance according to the Hall-Petch relationship.

Hardness Uniformity

One of the most practical benefits of magnetic field application is the improvement in hardness uniformity across the surfacing layer. Without magnetic field, the hardness varies from 170 HV at the fusion line to 200 HV at the surface, with a standard deviation of ±12 HV. With 1.0 T magnetic field application, the hardness range narrows to 195–210 HV with a standard deviation of ±6 HV. This uniformity is critical for applications where consistent wear resistance is required across the entire surfacing surface.

Process Integration and Practical Considerations

Implementing a transverse AC pulsed magnetic field in a production environment requires careful consideration of several factors:

• Magnet design: Permanent magnets or electromagnets must be positioned to create a transverse field relative to the welding direction. The magnet geometry must be optimized to maximize field intensity at the molten pool while minimizing interference with the welding arc.
• Field modulation: The pulsed nature of the field is essential. A continuous DC field would induce steady-state eddy currents that reach equilibrium and cease to stir the pool. The pulsing maintains dynamic stirring throughout the solidification process.
• Safety and shielding: High-intensity magnetic fields can interfere with nearby electronic equipment and pose safety risks to personnel. Proper shielding and safety protocols must be established.
• Cost-benefit analysis: The capital cost of magnetic field equipment must be weighed against the benefits of improved mechanical properties, reduced defect rates, and potential elimination of post-weld heat treatment.

Comparison with Alternative Grain Refinement Methods

The transverse AC pulsed magnetic field offers a unique combination of effectiveness and process compatibility. Unlike grain refiners, it does not require consumable modification. Unlike oscillating welding, it does not alter the fundamental welding process. This makes it particularly attractive for retrofitting existing production lines.

Study Insights and Engineering Implications

This study provides compelling evidence that electromagnetic stirring is a powerful tool for microstructure control in surfacing applications. The key insight is that the magnetic field acts as a process variable that can be optimized independently of consumable selection and welding parameters. This adds a new dimension to the process optimization space, allowing engineers to achieve property targets that would otherwise require more expensive consumables or more complex procedures.

For pressure vessel fabrication, the impact energy improvement is particularly significant. Many pressure vessel codes require minimum impact energy values at specific temperatures, and achieving these values often requires expensive post-weld heat treatment or the use of premium consumables. The magnetic field approach offers a path to meeting these requirements during the welding process itself, potentially reducing fabrication costs while improving quality.

Future research should focus on scaling this technology to industrial production environments, optimizing field parameters for specific alloy systems, and developing welding procedure specifications that incorporate magnetic field application as a standard process variable.