Fundamental Characteristics of Reverse Polarity Weak Plasma Arc Cladding

Literature Overview

This 2009 study by Liu Zhengjun, Yang Yang, Zhao Qian, and Zhang Sixin from the School of Materials Science and Engineering, Shenyang University of Technology, published in the Journal of Welding, presents a systematic analysis of the fundamental characteristics of reverse polarity weak plasma arc cladding. This is a specialized plasma arc welding configuration where the electrode is connected to the negative terminal and the workpiece to the positive terminal, with the plasma arc operated in a "weak" mode characterized by lower current density and reduced energy concentration. The study provides valuable insights into the arc physics, thermal behavior, and metallurgical characteristics of this process configuration.

Core Technical Content

Plasma arc cladding has gained increasing attention for its ability to produce high-quality overlay layers with low dilution, good surface finish, and precise geometric control. The reverse polarity (electrode negative, workpiece positive) configuration is less commonly studied than the conventional direct polarity (electrode positive) but offers distinct advantages for certain cladding applications.

Arc Physics and Thermal Characteristics

In the reverse polarity configuration, the workpiece acts as the anode, and the plasma arc concentrates its energy on the electrode surface rather than the workpiece. This results in:

Characteristic	Direct Polarity (EP)	Reverse Polarity (WP)
Arc spot location	Workpiece	Electrode
Heat input to workpiece	High	Low
Penetration depth	Deep	Shallow
Dilution rate	High	Low
Surface quality	Requires dressing	Smooth
Deposition rate	High	Lower
Heat-affected zone	Large	Small

The "weak" plasma mode refers to a lower current setting (typically 30–80 A) where the plasma arc has a lower energy density and a wider arc spot. This configuration is particularly suitable for:

Cladding of thin-walled components where excessive heat input would cause distortion
Applications requiring minimal dilution with the substrate
Cladding of heat-sensitive materials such as aluminum alloys and austenitic stainless steels
Repair of precision components where dimensional accuracy is critical

Process Parameters and Their Effects

The study systematically investigated the effects of key process parameters on cladding quality:

Parameter	Range Studied	Effect on Dilution	Effect on Deposition Rate	Effect on Surface Quality
Plasma current	30–80 A	Decreases with increase	Increases with increase	Improves with moderate increase
Travel speed	100–400 mm/min	Decreases with increase	Decreases with increase	Improves with increase
Shielding gas flow	5–20 L/min	Minimal effect	Minimal effect	Improves with increase
Transferred powder flow	20–100 g/min	Increases with increase	Increases with increase	Deteriorates with excess
Standoff distance	3–8 mm	Increases with distance	Decreases with distance	Deteriorates with distance

Metallurgical Characteristics

The reverse polarity weak plasma arc produces a distinctive metallurgical structure in the overlay layer:

Grain structure — Columnar grains growing from the overlay/base interface, with a finer grain size than conventional plasma cladding due to the lower heat input
Phase composition — Depends on the filler material, but the lower dilution ensures the overlay composition closely matches the filler composition
Microhardness — Typically 10–20% higher than conventional plasma cladding due to the reduced dilution and faster cooling rate
Bond strength — Adequate bond strength despite the lower heat input, as the process still achieves sufficient melting of the substrate surface

Engineering Practice Implications

The reverse polarity weak plasma arc cladding process is particularly well-suited for the following applications:

Turbine blade repair — Restoration of worn blade surfaces with minimal heat input to preserve the substrate microstructure
Medical implant cladding — Application of bio-compatible coatings on titanium implants with controlled dilution
Electronics component protection — Cladding of heat-sensitive electronic components with conductive or protective coatings
Nuclear component repair — Cladding of reactor components where dilution must be minimized to maintain material properties

The process offers several advantages over conventional cladding methods:

Low dilution — Typically 5–15%, compared to 20–40% for SAW or GMAW cladding
Minimal HAZ — The heat-affected zone is limited to a few hundred micrometers
Good surface finish — Ra values of 0.5–1.5 μm achievable without post-processing
Precise geometry control — Bead width and height can be controlled within ±0.1 mm
Low distortion — Suitable for thin-walled and precision components

Study Reflections

This study contributes significantly to the fundamental understanding of plasma arc physics in the reverse polarity configuration. The systematic investigation of process parameters and their effects on cladding quality provides a solid foundation for process optimization. The key insight is that the reverse polarity weak plasma arc represents a fundamentally different heat input regime compared to conventional cladding processes, with implications for both the thermal cycle experienced by the substrate and the solidification behavior of the overlay. The practical significance of the low dilution capability cannot be overstated, as it enables the use of overlay materials with compositions that would be compromised by excessive substrate dilution in conventional processes.