CLADDING TECHNOLOGY SHANXI CO., LTDResearch Status and Progress of Plasma Transferred Arc Powder Cladding Processes
Literature Overview
The review article by Lin Zhenlie, Cheng Huichao, Zhang Xin, and Qian Cheng, published in Cemented Carbides in 2023, provides a comprehensive survey of plasma transferred arc (PTA) powder cladding technology. Authored by researchers from Zhongjin Lingnan Nonferrous Metals Co., Ltd. and the State Key Laboratory of Powder Metallurgy at Central South University, this work represents a timely synthesis of decades of accumulated knowledge in a technology that has become indispensable in the surface engineering of critical industrial components.
PTA cladding occupies a unique position in the cladding technology spectrum, offering superior metallurgical bonding, low dilution rates, and excellent process control compared to conventional arc welding methods, while maintaining the productivity advantages of automated powder feeding systems. This review is particularly relevant for engineers working in the cemented carbide industry, where PTA is routinely employed to apply nickel-based and cobalt-based alloy cladding layers on tungsten carbide tools and wear parts.
Core Technical Content
Process Parameters and Their Interactions
PTA cladding is governed by a complex set of interdependent process parameters that determine the final clad layer quality:
| Parameter | Typical Range | Effect on Clad Layer |
|---|---|---|
| Arc current | 200-600 A | Higher current increases deposition rate but also dilution |
| Travel speed | 100-500 mm/min | Affects layer width, penetration, and dilution |
| Powder feed rate | 200-1500 g/min | Controls deposition efficiency and layer thickness |
| Gas flow rate (Ar) | 15-30 L/min | Shields molten pool from atmospheric contamination |
| Powder-to-gas ratio | 1:1 to 3:1 | Determines powder delivery stability and arc stability |
| Torch stand-off distance | 5-15 mm | Influences arc focusing and heat input |
The interplay between these parameters creates a multi-dimensional process window. For instance, increasing the powder feed rate without adjusting the travel speed leads to excessive heat input, resulting in a wider dilution zone and potential base material melting. Conversely, too rapid a travel speed with insufficient powder feed produces a thin, discontinuous layer with poor surface quality.
Clad Layer Quality Characteristics
The review emphasizes several quality metrics that distinguish PTA from other cladding processes:
- Dilution rate: Typically 5-15% for nickel-based alloys on carbon steel substrates, compared to 30-50% for conventional SAW overlay. This low dilution preserves the alloying elements and carbide-forming elements in the clad layer.
- Microstructure: PTA deposits exhibit fine, equiaxed dendritic structures with minimal macrosegregation, owing to the rapid solidification rates (10-100 K/s) achieved during the process.
- Surface quality: As-cast PTA surfaces typically exhibit roughness values of Ra 10-50 μm, which can be further improved through machining or shot peening.
- Bond strength: Metallurgical bonding is achieved with bond strengths generally exceeding 200 MPa for properly qualified procedures.
Comparison with Alternative Cladding Technologies
| Feature | PTA Cladding | Laser Cladding | SAW Overlay | ESW Overlay |
|---|---|---|---|---|
| Dilution rate | 5-15% | 2-10% | 30-50% | 20-40% |
| Deposition rate | High | Low-Medium | Very High | Very High |
| Equipment cost | Medium | High | Low | Medium |
| Scalability | Large to small | Small to medium | Large only | Large only |
| Process flexibility | High | Very High | Low | Low |
| Heat input | Medium | Low | High | Very High |
| Typical application | Wear parts, tools | Precision repair, biomedical | Pressure vessels, tanks | Large pressure vessels |
The review notes that while laser cladding offers even lower dilution and finer microstructures, PTA remains the preferred choice for high-volume industrial applications due to its superior deposition rates and lower equipment costs.
Standards and Qualification Requirements
PTA cladding procedures for pressure vessel applications must comply with stringent qualification requirements:
- NB/T 47014 and ASME IX require that the welding procedure qualification coupon demonstrate the required bond strength, hardness, and chemical composition of the clad layer.
- For nickel-based alloy cladding on carbon steel, the clad layer thickness must typically be at least 6 mm to provide adequate corrosion or wear protection, with a minimum of 3 mm remaining after machining.
- Non-destructive testing (NDT) requirements include magnetic particle inspection (MT) for surface defects, ultrasonic testing (UT) for subsurface porosity and lack of fusion, and radiographic testing (RT) for volumetric defect detection.
- The clad layer must pass intergranular corrosion testing per ASTM A263 or equivalent standards when used in corrosive environments.
Engineering Practice Insights
From my experience in specifying PTA cladding for industrial components, several practical considerations emerge:
- Powder characterization is critical. The particle size distribution, sphericity, and chemical homogeneity of the powder directly affect arc stability and clad layer quality. Powders with a particle size distribution of 45-150 μm (ASTM E1131) and sphericity greater than 75% are generally recommended for stable PTA operation.
- Substrate preparation significantly influences bond quality. The substrate surface should be machined to a flatness of ±0.5 mm/m and cleaned of contaminants such as oil, rust, and oxide. For thin-walled components, preheating to 150-250°C can reduce thermal stress and minimize distortion.
- Layer thickness control is essential. Multi-layer cladding is typically employed, with individual layer thicknesses of 0.5-2 mm. The total clad thickness must account for machining allowance, typically 1.5-3 mm per side, to achieve the specified final thickness.
- Distortion control is a major concern for thin-walled components. Implementing back-up cooling, symmetric welding sequences, and fixture design can minimize angular and bow distortion.
Key Challenges and Future Directions
The review identifies several areas requiring continued research and development:
- High-dilution-resistant powder development: Novel powder compositions that maintain functional properties even at elevated dilution levels would expand the range of applicable substrates.
- Process monitoring and control: Real-time monitoring of arc voltage, current, and powder feed rate, coupled with automated parameter adjustment, can improve process consistency and reduce operator dependence.
- Multi-material cladding: The ability to deposit gradient or functionally graded layers using PTA, transitioning from a tough base-compatible alloy to a hard wear-resistant surface layer, represents a promising direction.
- Integration with additive manufacturing: PTA technology is increasingly being adapted for directed energy deposition (DED) applications, enabling the fabrication of complex-shaped components with tailored material properties.
Summary and Conclusions
The review by Lin Zhenlie and colleagues provides a thorough and well-organized assessment of PTA powder cladding technology, covering process fundamentals, parameter interactions, quality characteristics, and industrial applications. For practicing engineers, the key takeaway is that PTA represents a mature, versatile, and economically viable cladding technology that bridges the gap between conventional welding overlay and advanced laser-based surface engineering. The continued evolution of powder metallurgy, process automation, and real-time monitoring technologies will further extend the capabilities of PTA cladding, making it an increasingly important tool in the surface engineering arsenal for critical industrial components. Engineers should invest in thorough process qualification and ongoing quality control to fully exploit the potential of this technology.