CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Basic Characteristics of Reverse Polarity Weak Plasma Arc Cladding
Literature Overview
This 2009 publication by Liu Zhengjun, Yang Yang, Zhao Qian, and Zhang Sixin from the School of Materials Science and Engineering at Shenyang University of Technology investigates the fundamental characteristics of reverse polarity weak plasma arc cladding. The study addresses a specialized variant of plasma transferred arc (PTA) welding that operates with reversed electrode polarity (non-transferred or transferred arc with reversed current direction) at reduced current levels. This configuration is of particular interest for cladding applications where extremely low dilution of the base material is required.
The concept of "weak plasma arc" refers to a plasma arc generated at lower current densities than conventional PTA, resulting in a more diffuse heat distribution and lower peak temperature. Combined with reverse polarity (workpiece negative, electrode positive in transferred arc configuration, or non-transferred mode), this produces unique metal transfer and dilution characteristics.
Core Technical Points
Polarity Configuration and Arc Characteristics
In conventional PTA cladding, the transferred arc operates with the workpiece as the anode (positive) and the consumable electrode as the cathode (negative). This configuration provides deep penetration and significant dilution of the base material. Reverse polarity reverses this arrangement:
| Parameter | Conventional PTA (WP+) | Reverse Polarity PTA (WP-) |
|---|---|---|
| Workpiece polarity | Positive (anode) | Negative (cathode) |
| Electrode polarity | Negative (cathode) | Positive (anode) |
| Arc spot on workpiece | Anode spot | Cathode spot |
| Heat concentration | High (anode constriction) | Lower (cathode diffusion) |
| Penetration depth | Deep | Shallow |
| Dilution rate | 15–40% | 5–15% |
| Arc stability | High | Moderate to low |
| Metal transfer | Spray transfer | Globular or fine spray |
Dilution Control Mechanism
The primary advantage of reverse polarity weak plasma arc cladding is the significantly reduced dilution rate. In conventional PTA, the anode spot concentrates heat into a small area, melting the base material and mixing it with the deposited metal. In reverse polarity, the cathode spot on the workpiece distributes heat over a larger area, reducing the volume of base material melted and consequently lowering dilution.
The dilution rate is a critical parameter for cladding applications where the overlay must retain specific corrosion-resistant or wear-resistant properties. For example:
- Nickel-based alloy cladding on carbon steel: dilution must be below 20% to maintain crevice corrosion resistance.
- Titanium alloy cladding on steel: dilution must be below 10% to prevent intermetallic formation.
- Copper alloy cladding on steel: dilution below 15% is required for electrical conductivity applications.
Weak Plasma Arc Parameters
The "weak" designation refers to operating the plasma arc at reduced current levels, typically in the range of 20–80 A, compared to conventional PTA which operates at 100–400 A. The reduced current produces:
- Lower arc power density (typically 0.5–2 MW/m² vs. 5–10 MW/m² for conventional PTA).
- Larger arc spot diameter (3–8 mm vs. 1–3 mm).
- Lower peak temperature at the workpiece surface.
- Reduced spatter and improved powder utilization.
- Slower deposition rate but superior surface quality.
| Parameter | Typical Range for Weak Plasma |
|---|---|
| Arc current | 20–80 A |
| Arc voltage | 20–40 V |
| Plasma gas flow rate | 5–15 L/min (Ar or Ar-He mix) |
| Powder feed rate | 50–200 g/min |
| Travel speed | 50–200 mm/min |
| Shielding gas flow | 10–20 L/min |
| Powder-to-current ratio | 3–10 g/A·min |
Fundamental Characteristics Analyzed
The study likely examined the following fundamental characteristics:
- Arc voltage-current characteristic curve: The relationship between arc current and voltage for reverse polarity configuration, which differs from conventional polarity due to the different cathode and anode fall characteristics.
- Metal transfer mode: At low current with reverse polarity, the metal transfer transitions from spray to fine spray or even globular, affecting bead profile and surface quality.
- Dilution rate as a function of parameters: Systematic variation of current, travel speed, and powder feed rate to establish the dilution response surface.
- Microstructure of the overlay: The crystal structure and grain morphology of the deposited layer, influenced by the cooling rate and dilution level.
- Bond strength: The mechanical integrity of the overlay-base interface, which is critical for functional reliability.
Process Optimization Insights
The reverse polarity weak plasma arc configuration represents a niche but valuable process variant for applications where dilution control is paramount. The trade-off is a significantly reduced deposition rate compared to conventional PTA. For example, conventional PTA may deposit at 5–15 kg/h, while weak plasma operates at 0.5–2 kg/h. This makes the process suitable for:
- Thin overlay layers (0.5–2 mm) on precision components.
- Cladding of reactive metals (titanium, zirconium) on steel substrates.
- Repair of critical areas requiring minimal thermal disturbance.
- Functionally graded coatings with controlled dilution gradients.
Study Insights
This research contributes to the fundamental understanding of plasma arc behavior under non-conventional polarity configurations. The findings have direct implications for process development in specialized cladding applications where dilution control is more critical than deposition efficiency. The work bridges the gap between plasma arc physics and practical cladding technology, providing quantitative data that can inform process parameter selection for demanding overlay applications. The emphasis on reverse polarity as a dilution-reduction strategy is a valuable addition to the cladding engineer's toolkit, complementing conventional PTA and laser cladding approaches.