Plasma Arc Surfacing of Alloy Powders with Longitudinal Magnetic Field Effects on Mechanical Properties

Literature Overview

This study by Liu Zhengjun and colleagues from Shenyang University of Technology, published in Welding Technology (2007, Vol. 36, No. 2, pp. 23-25), investigates the effect of applying a longitudinal magnetic field during plasma arc surfacing of Fe3, Fe5, and Co-based alloy powders onto low-carbon steel substrates. The research was supported by the Liaoning Provincial Natural Science Foundation (Project No. 200412025). The work systematically examines how magnetic field intensity influences hardness and wear resistance of the deposited layers, providing valuable insights into electromagnetic stirring as a process optimization tool for surfacing operations.

Core Technical Content

The fundamental principle underlying this research is the application of an external longitudinal magnetic field to the plasma arc surfacing process. When a conductive molten pool exists in a magnetic field, electromagnetic forces are induced that create stirring effects within the melt. This electromagnetic stirring promotes more uniform heat distribution, reduces columnar grain growth, and refines the microstructure of the deposited layer. The study specifically tested three types of surfacing powders — Fe3 (iron-based with moderate alloy content), Fe5 (iron-based with higher alloy content), and Co-based (cobalt-based, typically used for high-temperature and severe wear applications) — all applied to low-carbon steel substrates under controlled plasma arc parameters.

The key experimental variable was the magnetic field current, which was varied to determine the optimal intensity for microstructure refinement. The study identified that at a magnetic field current of 3 A, the microstructure of all three alloy types achieved optimal grain refinement regardless of the specific powder composition. This represents a significant finding because it suggests a universal optimal magnetic field intensity across different alloy systems, simplifying process parameter selection in industrial applications.

Technical Parameter Analysis

The electromagnetic stirring mechanism operates through Lorentz forces generated by the interaction between the induced currents in the molten pool and the applied magnetic field. At 3 A magnetic field current, the stirring intensity is sufficient to disrupt dendritic growth patterns without causing excessive turbulence that might lead to oxide inclusions or porosity. This represents a critical process window that engineers must respect when implementing magnetic field-assisted surfacing in production environments.

Microstructure and Performance Interpretation

The grain refinement achieved through electromagnetic stirring directly translates to improved mechanical properties. Finer grains in the deposited layer result in higher hardness due to the Hall-Petch relationship, where yield strength increases with decreasing grain size. Additionally, the more uniform microstructure reduces directional properties and improves isotropic wear resistance. The study demonstrates that electromagnetic stirring enhances both the hardness and wear resistance of the surfacing layer, leading to superior comprehensive mechanical performance of the deposited metal.

From a metallurgical perspective, the columnar-to-equiaxed transition (CET) is promoted by the electromagnetic stirring. In conventional plasma arc surfacing without magnetic field assistance, columnar grains grow preferentially along the heat flow direction, creating a microstructure susceptible to cracking and exhibiting anisotropic properties. The magnetic stirring disrupts this preferential growth, promoting equiaxed grain formation that provides better mechanical integrity and resistance to cracking during subsequent service or thermal cycling.

Engineering Practice Implications

For engineers involved in surfacing operations for wear-resistant components, this research offers a practical approach to improving deposited layer quality without changing the fundamental process equipment. The addition of a longitudinal magnetic field coil around the surfacing area is a relatively straightforward modification that can yield significant improvements in deposit quality. This technology is particularly relevant for:

• Repair of worn pump impellers and hydraulic components where Co-based surfacing is common
• Hardfacing of mining equipment components using Fe-based alloys
• Surface treatment of steel pipe ends or coupling surfaces requiring enhanced wear resistance
• Maintenance welding on rotating equipment where dimensional accuracy and surface integrity are critical

The finding that 3 A represents an optimal magnetic field current across multiple alloy systems provides a convenient starting point for process development. However, engineers should note that this optimal value may shift slightly depending on the specific plasma arc parameters (current, voltage, travel speed, powder feed rate) and the geometry of the component being surfaced.

Key Reflections and Study Insights

This research highlights an important concept in modern surfacing technology: that external field applications can fundamentally alter the solidification behavior of deposited layers without requiring changes to the base process. The electromagnetic stirring approach is non-contact, non-invasive, and compatible with existing plasma arc surfacing equipment. The systematic approach taken — varying one parameter (magnetic field current) while maintaining other conditions — provides clear cause-and-effect relationships that are directly applicable to process optimization.

The practical significance extends beyond academic interest. In industrial settings where surfacing quality directly impacts component life and reliability, even modest improvements in microstructure refinement can translate to significant gains in service life. The universal applicability of the 3 A optimal magnetic field current across Fe3, Fe5, and Co-based powders suggests that this technology could be standardized and implemented across multiple surfacing applications without requiring separate process development for each alloy system.

This study contributes to the broader understanding of how electromagnetic effects can be harnessed to improve welding and surfacing processes, complementing other field-assisted welding technologies such as magnetic arc welding and electromagnetic arc welding. For practitioners in the steel pipe and fitting industry, the principles demonstrated here could potentially be adapted for surfacing of pipe couplings, flange faces, and other components requiring enhanced surface properties.

Summary

The research by Liu et al. demonstrates that applying a longitudinal magnetic field at 3 A current during plasma arc surfacing of Fe3, Fe5, and Co-based alloys onto low-carbon steel produces optimal microstructure refinement, enhanced hardness, and significantly improved wear resistance across all tested alloy systems. The electromagnetic stirring mechanism disrupts columnar grain growth and promotes equiaxed grain formation, resulting in isotropic mechanical properties and superior comprehensive performance of the deposited layer. This technology offers a practical, non-invasive enhancement to conventional plasma arc surfacing that can be implemented with minimal equipment modification, making it a valuable tool for industrial surfacing applications where deposit quality directly impacts component service life.