Reverse Polarity Weak Plasma Arc Surfacing for Dissimilar Material Overlay

Literature Overview

Liu Zhengjun, Yang Yang, Zhao Qian, and Zhang Shixin from the School of Materials Science and Engineering at Shenyang University of Technology published a fundamental research paper in The Welding Journal of China (Vol. 30, No. 10, 2009, pp. 102-104) that investigates the basic characteristics of reverse polarity weak plasma arc surfacing welding. This work is significant because it systematically characterizes the arc energy density, arc pressure distribution, thermal behavior, combustion stability, and static characteristics of the reverse polarity weak plasma arc, establishing the physical basis for its application as an ideal heat source in dissimilar material surfacing.

Core Technical Characteristics

The reverse polarity configuration in plasma arc welding places the electrode as the cathode and the workpiece as the anode. This is fundamentally different from the conventional direct polarity configuration and produces distinctly different arc characteristics. The authors demonstrate that the reverse polarity weak plasma arc exhibits a more uniform arc pressure distribution compared to conventional configurations, which is advantageous for achieving consistent penetration and bead geometry in surfacing applications.

Arc Characterization Parameters

Metallurgical Implications of Reverse Polarity Arc

The most significant finding from this research is the demonstration that the cathode atomization effect of the reverse polarity arc effectively improves the bonding condition between the surfacing metal and the base metal. In conventional direct polarity plasma arc welding, the anode (workpiece) receives the majority of arc energy, resulting in deep penetration and significant dilution of the overlay alloy. The reverse polarity configuration, by contrast, concentrates energy at the cathode (electrode), producing a different interaction with the workpiece surface.

The cathode atomization phenomenon occurs because the intense cathode spot energy at the workpiece surface causes localized vaporization and atomization of both the base metal and the incoming filler material. This creates a highly active, atomized pool that promotes intimate mixing and metallurgical bonding at the interface. For dissimilar material surfacing, where achieving a sound metallurgical bond between chemically dissimilar materials is often the primary challenge, this effect is particularly valuable.

The concave distribution of voltage, current, and cathode spot location all being consistent indicates a highly stable and predictable arc behavior. This stability translates to consistent weld bead geometry and uniform heat input, which are critical for producing repeatable surfacing layers with controlled dilution rates. The ability to control energy density through shielding gas flow rate provides an additional process variable that can be optimized for specific surfacing applications.

Application to Dissimilar Material Surfacing

The reverse polarity weak plasma arc is identified as an ideal heat source for dissimilar material surfacing because it combines three essential capabilities: controlled and uniform energy input, enhanced interfacial bonding through cathode atomization, and the ability to minimize dilution while maintaining sound metallurgical fusion. These characteristics address the fundamental challenges of dissimilar material surfacing, which include preventing excessive mixing of the overlay and base metals, ensuring a crack-free interface, and achieving adequate bond strength.

For practical applications such as surfacing corrosion-resistant alloys onto carbon steel, or depositing hardfacing alloys onto ductile base materials, the reverse polarity weak plasma arc offers advantages over conventional welding processes. The focused energy input allows for precise control of the weld pool, reducing the risk of excessive penetration into the base metal and subsequent dilution. The enhanced cathode atomization effect promotes wetting and bonding between the overlay material and the base surface, reducing the likelihood of interfacial defects such as lack of fusion or microcracking.

Study Reflection and Process Development Implications

This fundamental research provides the physical understanding necessary for rational process development in dissimilar material surfacing applications. Engineers developing surfacing procedures for challenging material combinations, such as nickel-based alloys on carbon steel or high-alloy overlays on cast iron, should consider the reverse polarity weak plasma arc as a viable alternative to conventional processes. The key advantage lies in the combination of controllable energy input and enhanced interfacial bonding, which together address the two most common failure modes in dissimilar material surfacing: excessive dilution and inadequate bonding.

The finding that cathode spot energy constitutes approximately 50 percent of the effective arc energy has important implications for process parameter selection. This energy concentration means that even at relatively low total arc power, a significant portion of energy is directed to the workpiece surface, producing efficient melting without excessive heat input into the bulk material. For thin-walled components or heat-sensitive base materials, this characteristic allows for surfacing operations that would be difficult or impossible with conventional processes due to thermal distortion or base metal damage.